The more I looked at the NSSL WRF forecast from yesterday the more interesting it was.
While the model initiated storms off the warm advection cloud band in Iowa after 0100 UTC, which did turn out to be correct, it was not severe nor was it the major player. Observations indicate this was part of an alto-cumulus castellanus area.
The area in NE where storms initiated in the real world along the warm front sharpened until 23-00 UTC where in model land it weakened considerably. There was an indication in the model that reflectivity was small but non-zero when the convergence was strong. This is a good albeit weak signal.
Another area under consideration for convection initiation was along the dryline in KS and northern Oklahoma where one storm formed around 23 UTC. The model had little in the way in convection here until 2 hours later, along the dryline.
Another area was in SW OK, where observations indicated a small, weak storm developed around 01 UTC and quickly died off. The closest model storm was at 05 UTC. Of course, the whole forecast goes awry in these latter two isolated storms as the model initiates convection all along the dryline from KS through Southern Oklahoma. The last 6 hours of the forecast looks little like what happened in terms of storms. I would caution that the model is not entirely wrong, just very aggressive. The cloud fields in the model develop into convection but closely resemble the cloud fields observed.
We are just beginning to harvest the wealth of information contained within such forecasts. I believe we will learn a lot more about these types of forecasts when we get down to looking at cloud fields (not at the grid scale but over substantive areas) and use these to compare the model with observations. This perspective should give forecasters more confidence in the overall appearance, and solidify what to look for when examining fine resolution forecasts. I think information extraction will be much more successful than reflectivity alone.
http://www.nssl.noaa.gov/wrf/110409/
Showing posts with label convection initiation. Show all posts
Showing posts with label convection initiation. Show all posts
Sunday, April 10, 2011
Saturday, April 9, 2011
Supercell buffet
It appears that parts of Iowa, near Mapleton and points east and southeast were hit hard by 3/4 mile wide tornado (according to the storm reports). The storms took a while to develop and mature but sure enough once that cell crossed into Iowa it was going nuts. Here's why:
1. The instability was high, up around 3600 J/kg and moisture was 13.6 g/kg through a deep layer.
2. The low level shear was 27 knots and the bulk deep layer shear was 62 knots.
3. The hodograph is a classic quarter circle up to 6 km.
Of course the other storms in Nebraska were prolific splitting supercells, with the left mover racing north. A very impressive supercell buffet. This area was highlighted well by SPC, despite the NSSL WRF having nothing in this area today. This was also despite a relatively good forecast by the same model for events last night in the Ohio River Valley. The cloud simulation looked good too, but perhaps the resolution was just too coarse to really capture the boundary layer processes responsible for convective initiation of relatively small thunderstorms.
UPDATE: Sounding image obtained from SPC, and radar image from College of DuPage (http://cod.weather.edu)
2. The low level shear was 27 knots and the bulk deep layer shear was 62 knots.
3. The hodograph is a classic quarter circle up to 6 km.
Of course the other storms in Nebraska were prolific splitting supercells, with the left mover racing north. A very impressive supercell buffet. This area was highlighted well by SPC, despite the NSSL WRF having nothing in this area today. This was also despite a relatively good forecast by the same model for events last night in the Ohio River Valley. The cloud simulation looked good too, but perhaps the resolution was just too coarse to really capture the boundary layer processes responsible for convective initiation of relatively small thunderstorms.
UPDATE: Sounding image obtained from SPC, and radar image from College of DuPage (http://cod.weather.edu)
Friday, April 8, 2011
Chasing supercells
Pretty good chase day today. Any chase day is a great day, but this was good because it ended my absence on the Plains after roughly 4 years. It culminated in a splitting supercell with near tennis ball size hail in a marginal low level shear environment, but favorable bulk shear environment.
Hard to say what the issue with convection initiation was, but most of the updrafts that initiated at least marginal vertical cumulus growth seemed to tilt and then result in small turrets on the top. The clusters of clouds that did manage to grow finally did so after 2200 UTC, and by 2300 UTC we had a nice storm to chase at least from its radar presentation. The base was elevated a bit but a clear updraft bell had formed, and when we drove north on I35 to stay ahead of it, noticed pretty good size hail on the ground. Maybe 2-3 inches in diameter, low-density. I even found some that had split in half so you could see the rings. Most of the hail in the grass was unbroken, but the stuff that had hit the pavement was easily smashed.
The model scenario played out fairly well. A moisture pool was left over from the late afternoon mixing of the dryline further south. Last nights NSSL WRF simulation had 1 storm form west of Tulsa and I took this to be a positive sign that CI was probable. Of course the model had some pretty high moisture values so that was a concern. However, afternoon observations indicated that dew points would hold near 64 F. Of course the low level shear was weak as was the SRH, but the bulk shear was up around 20 m/s. Plenty good for supercells, but not so good for tornados.
The focus for initiation was along a convergence zone in Northern Oklahoma that would eventually redevelop rapidly north, at least in model land. I will have to investigate if this actually occurred. I think its worth investigating from the modeling standpoint, especially the structure of the boundary layer that led to the mixing out event further south but kept enough moisture further north.
The storm we were on is to the east-northeast of WDG on the image below at the time tennis ball hail was falling while we were on I-35.
Glad to be back chasing storms...
Hard to say what the issue with convection initiation was, but most of the updrafts that initiated at least marginal vertical cumulus growth seemed to tilt and then result in small turrets on the top. The clusters of clouds that did manage to grow finally did so after 2200 UTC, and by 2300 UTC we had a nice storm to chase at least from its radar presentation. The base was elevated a bit but a clear updraft bell had formed, and when we drove north on I35 to stay ahead of it, noticed pretty good size hail on the ground. Maybe 2-3 inches in diameter, low-density. I even found some that had split in half so you could see the rings. Most of the hail in the grass was unbroken, but the stuff that had hit the pavement was easily smashed.
The model scenario played out fairly well. A moisture pool was left over from the late afternoon mixing of the dryline further south. Last nights NSSL WRF simulation had 1 storm form west of Tulsa and I took this to be a positive sign that CI was probable. Of course the model had some pretty high moisture values so that was a concern. However, afternoon observations indicated that dew points would hold near 64 F. Of course the low level shear was weak as was the SRH, but the bulk shear was up around 20 m/s. Plenty good for supercells, but not so good for tornados.
The focus for initiation was along a convergence zone in Northern Oklahoma that would eventually redevelop rapidly north, at least in model land. I will have to investigate if this actually occurred. I think its worth investigating from the modeling standpoint, especially the structure of the boundary layer that led to the mixing out event further south but kept enough moisture further north.
The storm we were on is to the east-northeast of WDG on the image below at the time tennis ball hail was falling while we were on I-35.
Glad to be back chasing storms...
Sunday, February 27, 2011
Uncertainty cont.
I have been able to examine the NSSL-WRF, 00 UTC and 12 UTC NAM this morning.
With regard to the trough that is forecast to be the major player, it is likely that significant severe will break out colocated with the strong forcing along the triple point, in the dry punch into MO, and then another round even later in AR.
The main uncertainty lies in OK, where my hopecast suggests storms could try to break out along the dryline. The problem is that it occurs just around 22-23 UTC when the wind profile might be considered terrible for tornadoes. The wind profiles in general become more favorable further east and later on putting the tornado threat into AR but the window for discrete supercells appears to be small. Rather a squall line of some type will form with probably the chance for embedded supercell structures.
Further south however, there are better wind profiles, but the cap is somewhat stronger. The 00 UTC NSSL-WRF forms a squall line there as indicated by the synthetic satellite imagery.
I don't have a good intuitive feel for what may occur given some of the wind profiles I have seen. I do think the overnight models will struggle as they are typically too far east with any convection. They also struggle to produce individual storms ... will only produce storms in stronger forcing. The resolution of the models, the tendency to produce weaker lapse rates, etc all contribute to the storm bias. That said, the main threat appears to Normans east (Tulsa area and north), northwest (along the triple point), and southeast (secondary dry punch). There is still a chance for central OK. It all depends on if any storms attempt to go up along the dryline, which ultimately depends on the relative balance between the depth of moisture in the warm sector and the cap strength.
I am hoping that LMN and OUN will launch 21 UTC soundings, and maybe 18 UTC soundings so we can really examine the wind profile evolution as well as the cap. This will be a good case for analysis either way as it is a strong forcing case that is highly dependent on mesoscale details that our models may not get correct.
With regard to the trough that is forecast to be the major player, it is likely that significant severe will break out colocated with the strong forcing along the triple point, in the dry punch into MO, and then another round even later in AR.
The main uncertainty lies in OK, where my hopecast suggests storms could try to break out along the dryline. The problem is that it occurs just around 22-23 UTC when the wind profile might be considered terrible for tornadoes. The wind profiles in general become more favorable further east and later on putting the tornado threat into AR but the window for discrete supercells appears to be small. Rather a squall line of some type will form with probably the chance for embedded supercell structures.
Further south however, there are better wind profiles, but the cap is somewhat stronger. The 00 UTC NSSL-WRF forms a squall line there as indicated by the synthetic satellite imagery.
I don't have a good intuitive feel for what may occur given some of the wind profiles I have seen. I do think the overnight models will struggle as they are typically too far east with any convection. They also struggle to produce individual storms ... will only produce storms in stronger forcing. The resolution of the models, the tendency to produce weaker lapse rates, etc all contribute to the storm bias. That said, the main threat appears to Normans east (Tulsa area and north), northwest (along the triple point), and southeast (secondary dry punch). There is still a chance for central OK. It all depends on if any storms attempt to go up along the dryline, which ultimately depends on the relative balance between the depth of moisture in the warm sector and the cap strength.
I am hoping that LMN and OUN will launch 21 UTC soundings, and maybe 18 UTC soundings so we can really examine the wind profile evolution as well as the cap. This will be a good case for analysis either way as it is a strong forcing case that is highly dependent on mesoscale details that our models may not get correct.
Saturday, February 26, 2011
Severe weather potential
Interesting forecast shaping up for tomorrow evening. Somewhat strong trough will move into OK tomorrow bringing with i the chance for significant severe weather to central and eastern OK and continuing eastward overnight.
The more certain portion of the forecast is the later period late Sunday night when it appears probable that a severe MCS/squall line will tear up parts MO, AR, LA again. I think its likely as more time will allow for more moisture to advect northward in what amounts to me to be an odd linkage between a pre-existing low level jet well east of the effects of the trough and the trough. The apparent phasing of the two will occur later sunday evening. These highly dynamic environments lead to some interesting MCSs.
The early part of the threat in central OK is less certain. To me, at the moment, the most interesting part of the forecast is that the features of interest appear to slow down before entering it the Plains, then speed up once they do. This makes for a confusing scenario. The prominent features I see this morning are the dryline in western OK, the dryline warm front intersection in NW OK, and the rather strong 700 hPa cold advection over the dryline all at 00 UTC. Now, I have no idea what to expect in this type of slow then fast large scale regime. This fact alone adds uncertainty for my limited experience of significant severe weather in OK during Feb.
What is not clear is if the cold advection aloft will be enough to remove the cap, and if the instability will be large enough to be realized. We just had some rain followed by cold temperatures, and soil water fractions are high apparently. The last 10 and 30 day rainfall maps show that western OK is dry and eastern OK has seen around 2+ inches. So it is possible that strong sensible heating will take place on Sunday in areas that are drier ahead of the dryline. Will this actually matter? I will tell you on Monday.
The other major issue is how the dryline and shear align. At the moment the mean wind for sunday evening appears to be from the SW and the dryline is oriented more north-south. This angle difference will allow storms to move off the dryline. The shear vector should also orient itself across the dryline but the magnitude of that angle will be very important to storm mode.
I guess my inclination at this moment are that all the necessary ingredients can be found, it will be a matter of how they come together and when that will occur and for how long. For now, at least, it is important to get that moisture screaming northward. I could use a good storm chase.
A lot will change. But I think there is a chance for supercells. I am uncertain where convective initiation will occur, and where severe storms will get going, and what that initial mode will be. I will evaluate the Short range ensemble Forecast (SREF) when the 15z run becomes available later this afternoon. Really this is just me getting my thoughts ramped up ... otherwise known as situational awareness.
The more certain portion of the forecast is the later period late Sunday night when it appears probable that a severe MCS/squall line will tear up parts MO, AR, LA again. I think its likely as more time will allow for more moisture to advect northward in what amounts to me to be an odd linkage between a pre-existing low level jet well east of the effects of the trough and the trough. The apparent phasing of the two will occur later sunday evening. These highly dynamic environments lead to some interesting MCSs.
The early part of the threat in central OK is less certain. To me, at the moment, the most interesting part of the forecast is that the features of interest appear to slow down before entering it the Plains, then speed up once they do. This makes for a confusing scenario. The prominent features I see this morning are the dryline in western OK, the dryline warm front intersection in NW OK, and the rather strong 700 hPa cold advection over the dryline all at 00 UTC. Now, I have no idea what to expect in this type of slow then fast large scale regime. This fact alone adds uncertainty for my limited experience of significant severe weather in OK during Feb.
What is not clear is if the cold advection aloft will be enough to remove the cap, and if the instability will be large enough to be realized. We just had some rain followed by cold temperatures, and soil water fractions are high apparently. The last 10 and 30 day rainfall maps show that western OK is dry and eastern OK has seen around 2+ inches. So it is possible that strong sensible heating will take place on Sunday in areas that are drier ahead of the dryline. Will this actually matter? I will tell you on Monday.
The other major issue is how the dryline and shear align. At the moment the mean wind for sunday evening appears to be from the SW and the dryline is oriented more north-south. This angle difference will allow storms to move off the dryline. The shear vector should also orient itself across the dryline but the magnitude of that angle will be very important to storm mode.
I guess my inclination at this moment are that all the necessary ingredients can be found, it will be a matter of how they come together and when that will occur and for how long. For now, at least, it is important to get that moisture screaming northward. I could use a good storm chase.
A lot will change. But I think there is a chance for supercells. I am uncertain where convective initiation will occur, and where severe storms will get going, and what that initial mode will be. I will evaluate the Short range ensemble Forecast (SREF) when the 15z run becomes available later this afternoon. Really this is just me getting my thoughts ramped up ... otherwise known as situational awareness.
Saturday, October 23, 2010
Interesting Null tornado event
Yet, despite having supercell like qualities they faded into obscurity after 2-3 hours.
Missing from the closest observed sounding (taken AFTER the storms and somewhat well removed from the INITIAL area, but was later just south of an area of again small storms) was a layer of steep lapse rates, and a cap. Seemingly the near isothermal area around 500 hPa was a CAPE robber.
This sounding is intriguing only when we look at the overall 12 hour changes in the area of interest. The 850, 700, 500 temperatures had cooled at OUN. At FWD, 850 and 700 T was down 1 degree C, but the 500 T had warmed by 3 C while at 601 T had cooled by 2.6C! The upper low which showed a jetlet rounding the base of the trough at 500 hPa had weakened by 5 m s-1.
Water vapor imagery doesn't show this 500-600 presumed frontal zone as the upper levels were quite moist. It appears a subtropical jet/ shortwave was influencing the action over Texas. Maybe this "front" was a small scale circulation. I found it about the same pressure level, same potential temperature at DRT 12 hours prior albeit weaker.
Nothing at the surface really jumps out. Dew points held in the 59-62 F range in advance of the storms. Something mesoscale was happening for sure but its difficult to understand what. Model simulations anyone?
Sidenote: I did happen to see a wave-like feature that had passed over the CI area just prior to CI on IR imagery that interacted with some small storms in OK. Easily seen on Visible as cirrus.
Saturday, July 31, 2010
Forecasting thoughts
I read Chuck Doswell's essay on project Vortex 2 and a few folks have commented who are in the know on some things. I found it to be quite interesting.
Chuck wrote:
Consensus wins by virtue of avoiding extremes. In the process, it is consistently mediocre. A really good forecaster can beat consensus consistently.
In my mind, consensus is essentially a human ensemble of forecasts. Take the average of all forecasts issued by the forecasters and you get consensus. I don't think consensus wins by avoiding extremes, actually it does well no matter how (and maybe even because) individual forecasters go extreme (in both directions sometimes). It covers the phase space.
I am hard pressed to say that consensus is mediocre, though it is possible. Rather consensus tends to do quite well over the long term. (Wishing I had access to my old UAlbany forecast contest results). That being said, a good forecaster knows WHEN to beat a model that forecasters rely on and knows HOW to beat it. Playing the game you can easily make good bets or poor bets consistently, but to make big leaps in "skill" required risk taking. This comment applies mostly to the numbers (Max/ Min temperature, POP, precip amount).
Consensus on thunderstorm forecasting might be a different story, and supercells would be in another league entirely.
I too am a big fan of the dropsonde and I was very much disappointed that learjet dropsonde was not going to be used. (I tried to keep up with the mobile upsonde reports to see there progress. I really don't have much to say on if they succeeded in their mission or not, publications will have to tell that story.) I think what is most missed though will be a tremendous variety of environmental soundings. Not that the upsonde teams failed in any way. I think they have already published a paper on an MCS ...clearly not the goal of V2 alone.
But I think an opportunity was missed to really explore the environment around supercells which would have required a lot more forecasting precision (initiation location and time, relevant mechanisms teased out for sample flying patterns (across fronts, boundaries, moisture discontinuities, etc)).
The stated goals that were outlined in the essay:
I am less certain they can answer why a tornado was short lived (since we have no proof it should have lasted longer) or long lived (since we have no proof it should have been shorter lived). Thats where sample size becomes an issue (and where the project will prove its worth). Are 40 tornado's enough? Do you need to have 40 similar tornado's paired with 40 similar storms to draw some conclusions? Only time will tell.
The last goal sets the bar very high and is another reason why a dropsonde platform would have been so key. Researchers have opted to use the RUC model to circumvent this. It is a decent quality model that has proved its worth but it is still a model. It is always desirable to have observations since it is the small details that tend to matter most. And more soundings over a larger area is exactly what would help. Spatial and temporal coverage needs to improve, but when V2 is over and the mobile radars go home we are left with the "old" network to work with. It would be nice to have perspective and quantification on how good or uncertain some proximity soundings are, and if these proximity soundings tell us something more than tornadoes are possible. We still have no clue why some storms produce them, why some storms don't, and why some storms don't rotate when we think rotation is possible. They should be able to differentiate whether storm scale or environmental processes are dominating with the help of model sensitivity studies.
Has anyone showed how poor or how good we already forecast tornado's? I believe there is one study showing the lead time of warnings but I am unaware of anything else that truly shows skill at tornado forecasting. I will have to check into that.
Chuck wrote:
Consensus wins by virtue of avoiding extremes. In the process, it is consistently mediocre. A really good forecaster can beat consensus consistently.
In my mind, consensus is essentially a human ensemble of forecasts. Take the average of all forecasts issued by the forecasters and you get consensus. I don't think consensus wins by avoiding extremes, actually it does well no matter how (and maybe even because) individual forecasters go extreme (in both directions sometimes). It covers the phase space.
I am hard pressed to say that consensus is mediocre, though it is possible. Rather consensus tends to do quite well over the long term. (Wishing I had access to my old UAlbany forecast contest results). That being said, a good forecaster knows WHEN to beat a model that forecasters rely on and knows HOW to beat it. Playing the game you can easily make good bets or poor bets consistently, but to make big leaps in "skill" required risk taking. This comment applies mostly to the numbers (Max/ Min temperature, POP, precip amount).
Consensus on thunderstorm forecasting might be a different story, and supercells would be in another league entirely.
I too am a big fan of the dropsonde and I was very much disappointed that learjet dropsonde was not going to be used. (I tried to keep up with the mobile upsonde reports to see there progress. I really don't have much to say on if they succeeded in their mission or not, publications will have to tell that story.) I think what is most missed though will be a tremendous variety of environmental soundings. Not that the upsonde teams failed in any way. I think they have already published a paper on an MCS ...clearly not the goal of V2 alone.
But I think an opportunity was missed to really explore the environment around supercells which would have required a lot more forecasting precision (initiation location and time, relevant mechanisms teased out for sample flying patterns (across fronts, boundaries, moisture discontinuities, etc)).
The stated goals that were outlined in the essay:
- How, when, and why do tornadoes form? Why some are violent and long lasting while others are weak and short lived?
- What is the structure of tornadoes? How strong are the winds near the ground? How exactly do they do damage?
- How can we learn to forecast tornadoes better?
I am less certain they can answer why a tornado was short lived (since we have no proof it should have lasted longer) or long lived (since we have no proof it should have been shorter lived). Thats where sample size becomes an issue (and where the project will prove its worth). Are 40 tornado's enough? Do you need to have 40 similar tornado's paired with 40 similar storms to draw some conclusions? Only time will tell.
The last goal sets the bar very high and is another reason why a dropsonde platform would have been so key. Researchers have opted to use the RUC model to circumvent this. It is a decent quality model that has proved its worth but it is still a model. It is always desirable to have observations since it is the small details that tend to matter most. And more soundings over a larger area is exactly what would help. Spatial and temporal coverage needs to improve, but when V2 is over and the mobile radars go home we are left with the "old" network to work with. It would be nice to have perspective and quantification on how good or uncertain some proximity soundings are, and if these proximity soundings tell us something more than tornadoes are possible. We still have no clue why some storms produce them, why some storms don't, and why some storms don't rotate when we think rotation is possible. They should be able to differentiate whether storm scale or environmental processes are dominating with the help of model sensitivity studies.
Has anyone showed how poor or how good we already forecast tornado's? I believe there is one study showing the lead time of warnings but I am unaware of anything else that truly shows skill at tornado forecasting. I will have to check into that.
Nocturnal storms in the Tri Cities!
If only they could have occurred when I was awake. 1013 UTC thunderstorms developed over Southern WA. We have had altocumulus in the skies the last two days, morning and afternoon. Last night there was some well to the east and I figured perhaps the threat was gone and didnt even bother to pay attention to dessert weather. Surprise! at 315 I awoke to "plink!" "plink!" on the fireplace. A sign that it was raining big drops, but not pouring down. I peaked outside to verify and quickly went back to sleep.
Radar loops show this very nicely. I will have to make a movie of it since it is indeed rare (maybe 1 or 2 nocturnal events in the summertime). I saw very little on the surface chart, perhaps local flows were more dominant given the proximity of the hills around here. Area soundings showed 2C decreases at 500 hPa from 00 to 12 UTC, and a 3C increase at 700 hPa at Spokane. We happen to be on the cold side of the jet and it did not appear to be related to any jet streak circulation especially with anticyclonic curvature.
The dew point has been higher than usual, especially with the missed rain of the other day. It rained south of here 2 days ago, but yesterday was dry and the dew points dipped down into the mid-50s only, then came back up to 60. Of course our weather has been marked decidedly by the regular passage of midlatitude cyclones, which I must say has had a profound effect on Seattle, but not apparently us.
Day 2 update: more TS obs ! 1230-4am, and again this am from 9-11am!
They seem to initiate off the the hills to the south of the basin. Looks similar to the cap rock area.
This afternoon there is a TS playground out by Yakima which is awesome from my westward facing balcony. Just a wall of cumulonimbus:
Radar loops show this very nicely. I will have to make a movie of it since it is indeed rare (maybe 1 or 2 nocturnal events in the summertime). I saw very little on the surface chart, perhaps local flows were more dominant given the proximity of the hills around here. Area soundings showed 2C decreases at 500 hPa from 00 to 12 UTC, and a 3C increase at 700 hPa at Spokane. We happen to be on the cold side of the jet and it did not appear to be related to any jet streak circulation especially with anticyclonic curvature.
The dew point has been higher than usual, especially with the missed rain of the other day. It rained south of here 2 days ago, but yesterday was dry and the dew points dipped down into the mid-50s only, then came back up to 60. Of course our weather has been marked decidedly by the regular passage of midlatitude cyclones, which I must say has had a profound effect on Seattle, but not apparently us.
Day 2 update: more TS obs ! 1230-4am, and again this am from 9-11am!
They seem to initiate off the the hills to the south of the basin. Looks similar to the cap rock area.
This afternoon there is a TS playground out by Yakima which is awesome from my westward facing balcony. Just a wall of cumulonimbus:
I watched the failed attempts at CI along the boundary as it moved in, then behind the boundary attached to the weakening storms. Some interesting cloud features for around these parts.
Thursday, July 29, 2010
multiscale convection initiation 29 July 2010
Tis the season for CI close to the mountains. I have been saving the water vapor image for 0645 UTC so I can look at the monsoon season over AZ and MCSs over the midwest for the season in its entirety. The other evening I saw that the WV imagery depicted some weak mountain generated convection in Northern CO and Southern WY. This is a late season hot spot for convection (proof required, so this is anecdotal at the moment). The story unfolded at the surface as simply southeasterly flow with dew points in the upper 60's. At 850 hPa the situation could be described as a moisture pool along a warm front.
In any case, it didnt look dynamic until 300 hPa, where an upper level anticyclonic shear zone was situated along the NE SD border, with an anticyclonically curved jet streak approaching. So the situation unfolded as short-lived convection in a pretty humid pouch failed ... drifted eastward and by 0845 UTC CI was underway in this pouch. It was not a big event but did drop copious amounts of rain in the FSD area in eastern SD over into Storm Lake in Iowa.
This appears to be an established corridor for storms this evening. Some could argue it is intimately tied to the developing monsoon moisture stream. Tonights storms initiated over the Rapid City area (mountain generated). They have currently split with a slow moving portion of the high cloud tops over NE and drifting slowly SE, while further to the east in SD the storms are just slightly north of last nights storms.
Storm reports are absent in this corridor with the exception of tonight, so this type of event would only stand out as being different in terms of the precip. The NSSL model run handled this quite nicely though the second diurnal cycle appears to be extreme ... this case could be examined quite well.
Hopefully these types of events don't get ignored since the storm report numbers were low. How was convection initiated? What were the dynamical controls, and how much did the upslope, moist flow at the surface contribute, and what role did pre-existing convection play and how did it manifest itself (humidification or boundary development)?
In any case, it didnt look dynamic until 300 hPa, where an upper level anticyclonic shear zone was situated along the NE SD border, with an anticyclonically curved jet streak approaching. So the situation unfolded as short-lived convection in a pretty humid pouch failed ... drifted eastward and by 0845 UTC CI was underway in this pouch. It was not a big event but did drop copious amounts of rain in the FSD area in eastern SD over into Storm Lake in Iowa.
This appears to be an established corridor for storms this evening. Some could argue it is intimately tied to the developing monsoon moisture stream. Tonights storms initiated over the Rapid City area (mountain generated). They have currently split with a slow moving portion of the high cloud tops over NE and drifting slowly SE, while further to the east in SD the storms are just slightly north of last nights storms.
Storm reports are absent in this corridor with the exception of tonight, so this type of event would only stand out as being different in terms of the precip. The NSSL model run handled this quite nicely though the second diurnal cycle appears to be extreme ... this case could be examined quite well.
Hopefully these types of events don't get ignored since the storm report numbers were low. How was convection initiated? What were the dynamical controls, and how much did the upslope, moist flow at the surface contribute, and what role did pre-existing convection play and how did it manifest itself (humidification or boundary development)?
Thursday, July 15, 2010
Predictability
Tough forecast day for SPC in light of the disappearance of the spring forecast experiment model suite. I know I felt the loss of NCARs model when I was making my forecast for Minnesota and Iowa.
The story starts with obscenely high values of dew points across the midwest ... 80+ dew points in Iowa and above 75 just about everywhere else. A potent little low was forecast to move through the area around the Twin Cities with a moderate risk of tornadoes given the extreme instability and large vertical shear.
What was more obvious today was that the hodograph curvature would be decreasing through the day as the cold front advanced eastward. A remnant MCS was forecast to depart the area and another was forecast to erupt in its wake. The NSSL model had a hard time with that component of the forecast, instead developing the convection along the cold front and slightly ahead.
The hard part was utilizing this model run to determine what role the remnant MCS would play in terms of any leftover boundaries, the role of the associated cloud field to the west, and the cold front even further to the west. As it turned out, the departing MCS gave way to another MCS accompanied by damaging winds and tornadoes. The cold front lit up in sections appearing to be linear but isolated enough to warrant tornado warnings before collapsing. About 5 hours later as the front passed the Twin Cities it lit up again.
All in all it was a hard forecast because the model simulation was not quite capturing the episodic nature of the convection. This is one forecast where the model was working against the forecasters despite the somewhat strong forcing for ascent. The details were absolutely lacking in the model forecast and this case will be a prime candidate for further research into why this happened.
This radar image sums it up nicely with the western most convection marking the cold front and accompanied by TOR warnings, Then the cloud field marked by the 25-30 dBz echo, then the isolated convection which exploded into the MCS over the Twin Cities, and the remnant MCS is off to the east.
The story starts with obscenely high values of dew points across the midwest ... 80+ dew points in Iowa and above 75 just about everywhere else. A potent little low was forecast to move through the area around the Twin Cities with a moderate risk of tornadoes given the extreme instability and large vertical shear.
What was more obvious today was that the hodograph curvature would be decreasing through the day as the cold front advanced eastward. A remnant MCS was forecast to depart the area and another was forecast to erupt in its wake. The NSSL model had a hard time with that component of the forecast, instead developing the convection along the cold front and slightly ahead.
The hard part was utilizing this model run to determine what role the remnant MCS would play in terms of any leftover boundaries, the role of the associated cloud field to the west, and the cold front even further to the west. As it turned out, the departing MCS gave way to another MCS accompanied by damaging winds and tornadoes. The cold front lit up in sections appearing to be linear but isolated enough to warrant tornado warnings before collapsing. About 5 hours later as the front passed the Twin Cities it lit up again.
All in all it was a hard forecast because the model simulation was not quite capturing the episodic nature of the convection. This is one forecast where the model was working against the forecasters despite the somewhat strong forcing for ascent. The details were absolutely lacking in the model forecast and this case will be a prime candidate for further research into why this happened.
Monday, July 5, 2010
Updated: MCS
Here is the update of the MCSs that occurred over the last 5 weeks (May 28 - July 4) at 0645 UTC:
If I counted correctly, something like 26 systems (of various sizes) over that span,
not including multiple MCS events, or the fact that multiple systems could be identified on radar under one cloud shield.
The precipitation anomaly for the last 30 days still indicate 8" swaths from NE through IA. Lets recall the radical flooding in OK City on 14-15 June (which also has their 30 day anomaly above 5"). KS also participated in the events but it was much less a focus for extreme precipitation totals.
60 day anomalies were significantly less revealing the reduced May rainfall in Ia, NE and northern KS.
Compared to 2008 and 2009, June 2010 was literally the summation of the last two Junes. In 2008, 8" anomalies were common in IA and northern MO and a small part of eastern KS. In 2009, 8" anomalies were common in western NE and a sliver in KS, NE and MO (1 small corridor). 2006 and 2007 were very different from these last 3 years, but 2005 was similar to 2009.
Iowa is currently in flood mode with 35 of 99 counties under some kind flood watch or warning. Interestingly, the Iowa Environmental Mesonet reported that DSM had 22 days of measurable rain and more is on the way this week. up until the 22nd the IEM reported the highest mixing ratio average for the state since 1902!
If I counted correctly, something like 26 systems (of various sizes) over that span,
not including multiple MCS events, or the fact that multiple systems could be identified on radar under one cloud shield.
The precipitation anomaly for the last 30 days still indicate 8" swaths from NE through IA. Lets recall the radical flooding in OK City on 14-15 June (which also has their 30 day anomaly above 5"). KS also participated in the events but it was much less a focus for extreme precipitation totals.
60 day anomalies were significantly less revealing the reduced May rainfall in Ia, NE and northern KS.
Compared to 2008 and 2009, June 2010 was literally the summation of the last two Junes. In 2008, 8" anomalies were common in IA and northern MO and a small part of eastern KS. In 2009, 8" anomalies were common in western NE and a sliver in KS, NE and MO (1 small corridor). 2006 and 2007 were very different from these last 3 years, but 2005 was similar to 2009.
Iowa is currently in flood mode with 35 of 99 counties under some kind flood watch or warning. Interestingly, the Iowa Environmental Mesonet reported that DSM had 22 days of measurable rain and more is on the way this week. up until the 22nd the IEM reported the highest mixing ratio average for the state since 1902!
Sunday, June 6, 2010
Convection initiation
The event on the June 3rd was an interesting case of CI. Apologies in advance for the 1D depiction of the maps. I think I have decided I want a blog with a little more functionality ... like manipulating imagery more than just size, left or right...anyway.
This was a Vortex2 day where the shear was sufficient with plenty of moisture. The hodograph curvature was good enough and even made it into the outlook discussion:
However, only 1 tornado developed despite multiple storms developing. The storms appeared to stay small. The few that were able to grow larger became severe with 1 tornado produced.
What was more intriguing to me was the model forecasts that I had perused. The night before NSSL WRF run and NCARs WRF run from the night before and the morning of the event. I saved some imagery for this event for some crude verification.
Here are some (side by side apparently not possible ... a little help blogger?!) images of the NSSL vs NCAR 0000 UTC run:
Now here is the morning run from NCAR:
The discussion is as follows:
NSSL's run shows CI in western NE by 21 UTC from an area of low reflectivity which looks like noise. By 2300 UTC two areas stand out: southern SD and an area to the south in central NE. by 0100 there is a secondary band of broken convection that merges with the easternmost convection.
The NCAR run shows a large area of low reflectivity enter in western NE that gradually and then suddenly diminishes into an of small cells by 1900 UTC. By 2000 UTC these cells consolidate and CI commences in an arc. The arc of convection never intensifies except for the southernmost cell. But eventually the line falls apart and only two areas remain: southern NE and southeastern SD.
The morning run from NCAR had CI in western NE west of LBF by 1900 UTC, again from what appear to be remnants of a low reflectivity band. Again two areas develop, with the southern most cell in NE becoming the largest and breaking away southeastward. The cells in Northern NE and southern SD stay cellular through 0000 UTC.
Largely, the forecasts had a few things right. Convection would develop between 1900-2100 UTC, with two main areas of concern in NE and SD. The threats for supercells appear to be justified, given that models tend not to develop isolated cells in general, and the NCAR trend of not developing a squall line.
Water vapor imagery shows that CI occurred between 2200 and 2300 UTC in central NE first, followed by SD by 2315 UTC. The convection becomes washed out, by satellite perspective after 0600 UTC.
I also show the OAX soundings for the two NCAR runs versus the observation. The 24 hr forecast OAX sounding has evidence of anvil cloudiness and a stronger wind field aloft. Despite this the thermodynamic structure is similar enough in shape minus the ability to resolve the low level inversion. Much can be said of the 12 hour forecast for OAX. Though the inversion structure is different, the hodograph is much more comparable.
This leaves me with a few thoughts:
It would appear the kinematic fields can determine the location of CI despite similar evolution of the thermodynamic fields.
Why do the 24 hour forecasts show dissipation of the convection even though the cells form a line the NSSL run and an arc in the NCAR run? The formation of a squall line should build a cold pool that helps sustain convection.
It is interesting that if you directly compare the 00 vs 12 UTC runs one gets a sense of consistency or reliability. The differences might appear to be large, but if we don't take the NWP output too literally and add more uncertainty to the 00 UTC forecast I believe the models achieve a relative consistency and thus reliability. It would be very interesting to vary the boundary layer physics to see what variety of CI scenario's emerge in this context.
This was a Vortex2 day where the shear was sufficient with plenty of moisture. The hodograph curvature was good enough and even made it into the outlook discussion:
However, only 1 tornado developed despite multiple storms developing. The storms appeared to stay small. The few that were able to grow larger became severe with 1 tornado produced.
What was more intriguing to me was the model forecasts that I had perused. The night before NSSL WRF run and NCARs WRF run from the night before and the morning of the event. I saved some imagery for this event for some crude verification.
Here are some (side by side apparently not possible ... a little help blogger?!) images of the NSSL vs NCAR 0000 UTC run:
Now here is the morning run from NCAR:
The discussion is as follows:
NSSL's run shows CI in western NE by 21 UTC from an area of low reflectivity which looks like noise. By 2300 UTC two areas stand out: southern SD and an area to the south in central NE. by 0100 there is a secondary band of broken convection that merges with the easternmost convection.
The NCAR run shows a large area of low reflectivity enter in western NE that gradually and then suddenly diminishes into an of small cells by 1900 UTC. By 2000 UTC these cells consolidate and CI commences in an arc. The arc of convection never intensifies except for the southernmost cell. But eventually the line falls apart and only two areas remain: southern NE and southeastern SD.
The morning run from NCAR had CI in western NE west of LBF by 1900 UTC, again from what appear to be remnants of a low reflectivity band. Again two areas develop, with the southern most cell in NE becoming the largest and breaking away southeastward. The cells in Northern NE and southern SD stay cellular through 0000 UTC.
Largely, the forecasts had a few things right. Convection would develop between 1900-2100 UTC, with two main areas of concern in NE and SD. The threats for supercells appear to be justified, given that models tend not to develop isolated cells in general, and the NCAR trend of not developing a squall line.
Water vapor imagery shows that CI occurred between 2200 and 2300 UTC in central NE first, followed by SD by 2315 UTC. The convection becomes washed out, by satellite perspective after 0600 UTC.
I also show the OAX soundings for the two NCAR runs versus the observation. The 24 hr forecast OAX sounding has evidence of anvil cloudiness and a stronger wind field aloft. Despite this the thermodynamic structure is similar enough in shape minus the ability to resolve the low level inversion. Much can be said of the 12 hour forecast for OAX. Though the inversion structure is different, the hodograph is much more comparable.
This leaves me with a few thoughts:
It would appear the kinematic fields can determine the location of CI despite similar evolution of the thermodynamic fields.
Why do the 24 hour forecasts show dissipation of the convection even though the cells form a line the NSSL run and an arc in the NCAR run? The formation of a squall line should build a cold pool that helps sustain convection.
It is interesting that if you directly compare the 00 vs 12 UTC runs one gets a sense of consistency or reliability. The differences might appear to be large, but if we don't take the NWP output too literally and add more uncertainty to the 00 UTC forecast I believe the models achieve a relative consistency and thus reliability. It would be very interesting to vary the boundary layer physics to see what variety of CI scenario's emerge in this context.
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