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 Predictability. Show all posts
Showing posts with label Predictability. 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, March 4, 2011
Uncertainty and skill
I was reading about what the National Hurricane Center will be doing this year. It was quite interesting:
1. Watch lead time to 48 hours; warning lead time to 36 hours
2. The size of the cones will be based on 5 year running mean of 2/3 of track error. The cone size radius varies in the Atlantic basin from 36 to 59 miles from 12 to 24 hours and grows about 35-40 miles per 24 hours thereafter.*
This means they have skill out to 48 hours in advance. They will be communicating actively their uncertainty via the graphical product (cones).
My foolish expectation would be to shy away from any sort of climatological cone of uncertainty and use ensemble guidance. This may not be the best option since it could provoke the so-called meteorological cancer i.e. over-dependence on models which have little value, not necessarily little skill. On the plus side it allows forecasts to be naturally consistent ... fairly certain in their ability to track where storms are, where they are going in the immediate future. To be fair the cone widens for a reason: tropical storms and hurricanes can encounter harsh or favorable environments quickly and these types of environments are hard to recognize over the ocean at longer lead times. Of course these environments can bring about changes to the inner workings of tropical storms in which case certain status quo forecasting rules may not work so well, and of course models also tend to not be spot on with hurricane intensity changes.
If nothing else, just seeing the products and how they are discussed should be interesting. It will be worth paying attention to see how the "public" reacts to be under "threat" for longer periods of time.
* I have not seen what this will look like but it will certainly be interesting!
1. Watch lead time to 48 hours; warning lead time to 36 hours
2. The size of the cones will be based on 5 year running mean of 2/3 of track error. The cone size radius varies in the Atlantic basin from 36 to 59 miles from 12 to 24 hours and grows about 35-40 miles per 24 hours thereafter.*
This means they have skill out to 48 hours in advance. They will be communicating actively their uncertainty via the graphical product (cones).
My foolish expectation would be to shy away from any sort of climatological cone of uncertainty and use ensemble guidance. This may not be the best option since it could provoke the so-called meteorological cancer i.e. over-dependence on models which have little value, not necessarily little skill. On the plus side it allows forecasts to be naturally consistent ... fairly certain in their ability to track where storms are, where they are going in the immediate future. To be fair the cone widens for a reason: tropical storms and hurricanes can encounter harsh or favorable environments quickly and these types of environments are hard to recognize over the ocean at longer lead times. Of course these environments can bring about changes to the inner workings of tropical storms in which case certain status quo forecasting rules may not work so well, and of course models also tend to not be spot on with hurricane intensity changes.
If nothing else, just seeing the products and how they are discussed should be interesting. It will be worth paying attention to see how the "public" reacts to be under "threat" for longer periods of time.
* I have not seen what this will look like but it will certainly be interesting!
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
Uncertainty foci
1. The trough in question is still only half over the upper air network, thus there is uncertainty in its exact structure. It appears to be currently moving more south than east but that trend may be shifting to more east.
2. I found a website to scrutinize the 3 hourly soundings from the NAM. It turns out the cold air advection over OK around 00 UTC may actually be a reflection of a boundary layer deepening up to 550 hPa at KGAG! Thus while it is getting cooler aloft, it does not necessarily imply a forcing mechanism for ascent. Note that 3 hours later, descent is implied from the inversion yielding net warming in the same layer while the boundary layer significantly cools. The exact role this feature plays remains uncertain.
3. Soundings from central OK show a very shallow moist layer and cap aloft which although weakens some remains strong until after 03 UTC. This is saying a lot since the previously mentioned odd double low level jet is clearly playing a big role in the development of deep moisture. Its even difficult to get moisture into KLZK under this scenario.
2. I found a website to scrutinize the 3 hourly soundings from the NAM. It turns out the cold air advection over OK around 00 UTC may actually be a reflection of a boundary layer deepening up to 550 hPa at KGAG! Thus while it is getting cooler aloft, it does not necessarily imply a forcing mechanism for ascent. Note that 3 hours later, descent is implied from the inversion yielding net warming in the same layer while the boundary layer significantly cools. The exact role this feature plays remains uncertain.
3. Soundings from central OK show a very shallow moist layer and cap aloft which although weakens some remains strong until after 03 UTC. This is saying a lot since the previously mentioned odd double low level jet is clearly playing a big role in the development of deep moisture. Its even difficult to get moisture into KLZK under this scenario.
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.
Sunday, February 6, 2011
NSSL WRF performance over OK
I grabbed some 2 meter temperature forecasts from NSSL's WRF 4 km model simulation from 00 UTC 5 February 24 hour forecast to compare with what happened on Saturday.
The model did not do terribly well but it highlighted the issue with snowpack. Lets go to the pictures:
The images depict the 2m temperature in 2 hour increments from 18 to 22 UTC; with the 22 UTC image being the warmest time of this day. What stands out is the warm air over the TX panhandle which does not expand rapidly into OK. Just east of the OK panhandle it warms rapidly, but it does not expand and penetrate eastward. Over OK, the cold patch, which aligns perfectly with the storm total snowfall and thus snow pack, does not appreciably change shape but it does warm a bit. Clearly the model had a poor representation of the snow cover, both in areal extent and depth.
From the previous post, the high temperatures even over the deep snow pack near Tulsa in the core of the model cold patch, got to near 40F ... a difference of 20 F!
The situation eases later as night arrives by 00 UTC. Below are the 24 hour forecast from the model and the initialization from the next cycle.
The differences between these 2 images is difficult to discern but they are still large, because of the eastward shift of the OK cold patch and the eastward extent of warmer air. This is an interesting case where I would expect this type of model to perform better. Diagnosing the evolution of the snow pack and the low level temperature tendencies both aloft and from within the model physics (boundary layer scheme) should shed some light on why the model performed poorly.
The model did not do terribly well but it highlighted the issue with snowpack. Lets go to the pictures:
The images depict the 2m temperature in 2 hour increments from 18 to 22 UTC; with the 22 UTC image being the warmest time of this day. What stands out is the warm air over the TX panhandle which does not expand rapidly into OK. Just east of the OK panhandle it warms rapidly, but it does not expand and penetrate eastward. Over OK, the cold patch, which aligns perfectly with the storm total snowfall and thus snow pack, does not appreciably change shape but it does warm a bit. Clearly the model had a poor representation of the snow cover, both in areal extent and depth.
From the previous post, the high temperatures even over the deep snow pack near Tulsa in the core of the model cold patch, got to near 40F ... a difference of 20 F!
The situation eases later as night arrives by 00 UTC. Below are the 24 hour forecast from the model and the initialization from the next cycle.
Saturday, December 25, 2010
Scales of uncertainty
The latest fuss in meteorological circles has focused rather intently on the Storm of Christmas, then the day after, and now the day after the day after Christmas storm. The storm has been "delayed" by 2 days over the course of the last week, and has shifted position numerous times in the forecast (gonna miss far to the east, bounce back to the west for a hit). We call this behavior uncertainty.
So what is uncertainty? "not confirmed", "still undecided", a "lack in confidence"
In modeling terms, the forecast solution has not yet converged. Our current capabilities, however, are quite robust at short range, meaning we expect the solution (over many initialization cycles) to converge. This time period is usually between 48 and 72 hours depending on the modeling system, the scale of the features that need to be resolved well, and the overall scale of the dynamical system which we seek to predict.
Over the years, despite many advances in satellite data, data assimilation systems, and numerical models and their increasingly sophisticated methods ... we still need to wait for the disturbances to enter the well instrumented radiosonde network*. Alas this too can be deceiving as was the case for the Surprise snowstorm of January 2000. You see a classic nor'easter can have origins from the Pacific Northwest which travels in amplified flow towards the gulf coast. The system itself may never leave the well instrumented but the warm gulf, cool temperatures aloft, and strong wind shear may foster convection ... thunderstorms ... or organized noise. This noise can then feedback from the large scale to the small scale ... outside the network and remain outside the network as the coastal low forms and deepens as it travels up the coast.
So, you can see how a "converged" solution of where the storm will be, can help us have confidence on predicting the other aspects of the storms. But uncertainty remains in where the precipitation will be, what precipitation type will fall, and for how long.
This uncertainty about snowfall placement and amount is very similar to the summer forecasting of thunderstorms. We may have a converged solution of the larger scale details but the smaller scale details can have a large effect on where, when, which storms will form and how severe they might be.
This where the art of forecasting kicks in. Where the analog experience of forecasters contributes. Knowing how they were fooled last time, or how they picked up on certain observed details which caused them to do better than the models. The forecaster has the ability to understand these scales of uncertainty. Only in the last few years have methods been developed, like the Ensemble Kalman Filter, which can show us where the uncertainty is for a specified region at 3,5,7 day lead time. In fact the Winter Weather Reconaissance program is designed to use these methods, then collect data in the uncertain regions to see how that can change the forecast and its uncertainty!
So what is uncertainty? "not confirmed", "still undecided", a "lack in confidence"
In modeling terms, the forecast solution has not yet converged. Our current capabilities, however, are quite robust at short range, meaning we expect the solution (over many initialization cycles) to converge. This time period is usually between 48 and 72 hours depending on the modeling system, the scale of the features that need to be resolved well, and the overall scale of the dynamical system which we seek to predict.
Over the years, despite many advances in satellite data, data assimilation systems, and numerical models and their increasingly sophisticated methods ... we still need to wait for the disturbances to enter the well instrumented radiosonde network*. Alas this too can be deceiving as was the case for the Surprise snowstorm of January 2000. You see a classic nor'easter can have origins from the Pacific Northwest which travels in amplified flow towards the gulf coast. The system itself may never leave the well instrumented but the warm gulf, cool temperatures aloft, and strong wind shear may foster convection ... thunderstorms ... or organized noise. This noise can then feedback from the large scale to the small scale ... outside the network and remain outside the network as the coastal low forms and deepens as it travels up the coast.
So, you can see how a "converged" solution of where the storm will be, can help us have confidence on predicting the other aspects of the storms. But uncertainty remains in where the precipitation will be, what precipitation type will fall, and for how long.
This uncertainty about snowfall placement and amount is very similar to the summer forecasting of thunderstorms. We may have a converged solution of the larger scale details but the smaller scale details can have a large effect on where, when, which storms will form and how severe they might be.
This where the art of forecasting kicks in. Where the analog experience of forecasters contributes. Knowing how they were fooled last time, or how they picked up on certain observed details which caused them to do better than the models. The forecaster has the ability to understand these scales of uncertainty. Only in the last few years have methods been developed, like the Ensemble Kalman Filter, which can show us where the uncertainty is for a specified region at 3,5,7 day lead time. In fact the Winter Weather Reconaissance program is designed to use these methods, then collect data in the uncertain regions to see how that can change the forecast and its uncertainty!
Monday, November 1, 2010
From hurricane to weak storm: Tomas
Tomas weakened rapidly as "shear" tore it apart. It became obvious this wasnt upper level shear but rather midlevel shear as the mid-level vortex decoupled from the low-level vortex. Animations of this period show a very strong band of convection to the southeast actually develop its own outflow signature. This area of convection persisted even as Tomas' CDO decayed to reveal the low level swirl this morning. The upper level shear probably did change as Tomas excellent outflow was virtually absent in the southern quadrant. Deep convection refired late in the morning but was pretty ragged and the HH aircraft found Tomas significantly weakened and disorganized.
hard to know what may have actually happened, but I am stuck wondering what role the SE area of convection played in Tomas demise, more specifically how that area of convection established its own outflow and how that interacted with Tomas. Also, what was the decoupling mechanism and how vertically confined was it to shear the vortex apart and only briefly stall deep convection?
Anywho, this is not my area of expertise. I am interested in where Tomas goes and how it interacts with the cutoff cyclone in the Gulf by Saturday.
The GFS takes the upper level low into the gulf where it rapidly weakens in 18 hours, though the effects of the low may be felt very far south as a cold surge. Though cold might be a relative term. The GFS is rampant with parameterized convective rainfall and that could be one mechanism for the destruction of PV aloft. The other mechanism for not leaving the cutoff over the gulf is another disturbance on the dynamic tropopause that crosses the ridge and helps keep moving things along.
Either way how things play out with Tomas and the cutoff low is rather interesting from a predictability standpoint, for all the individual elements (TC, cutoff low, cold surge, and heavy rainfall for Florida, Cuba and Haiti). Expect some impacts from this entanglement. In the meantime I need to look closely at the GFS ensemble.
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