Impressive rain totals

A ton of rain has fallen in the last few days but the image above shows the last 7 days. Is it any wonder that a levee in MO is about to burst? From Central AR through TN, and across MS and AL will also fill in as more severe storms and rain affect the region tomorrow and Wednesday. Tonight it appears OK will add to its catchup rainfall.  Below is the 5 day precip forecast from HPC:

Certainly the human impact will be a bit more wild as all the rain floods some areas and puts them at risk for the severe weather outbreak. Today was the first time that the Storm Prediction Center issued Moderate Risk areas for Day 1, 2, and 3. Regardless of the tornado impacts, it is going to pour down rain on already wet ground tomorrow. Add some tornadoes and wind, and trees are going down. Add more rain and certain locales will experience flash floods and your regular run-of-the-mill flooding.

And if that isn't enough, a nasty bow echo with tornado warnings is currently going through Little Rock. A quick glimpse at the storm total rainfall product from Little Rock shows 15"+ rainfall in the last few days over a non-trivial area. The bow echo should add even more to those totals.

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.

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.

More storm discussion

The plot thickens. Not only are subsequent model forecasts increasing the snow for central OK, but the global model has backed way off. The SREF was going crazy with anywhere from 1inch to 22 inches of snow for Norman as of 2100 UTC. The operational NAM was going for 24 inches, and the  GFS was going for 1 inch. Talk about uncertainty. The funny thing is that the NAM gets its boundary conditions from the previous GFS model run.

I speculate that the width and depth of the cold air helped get the GFS a good initialization and as a result a good depiction for the NAM. But maybe that cold air depiction was projected onto too large scale such that the ingredients for this event were misaligned. Also, the NAM ingredients may be too well aligned such that convective feedback may have been playing a role in creating a lot of precipitation but was able to simulate the banded features well.

As of 9:25pm there is the lovely, heart pumping sound of thunder, big lightning streaking across the sky, and ice pellets! Some of which could count as hail. I had to take a break so I could enter some observations for the W-PING project at NSSL. On a related note station KPVJ in OK reported 0.25" of liquid equivalent in an hour (6.3 mm/hr). Impressive rate. If only we had data for the precipitation rates at all mesonet stations!

It looks like the main issues of the day were (in no particular order):
1. where would cold air be, its depth, and the depth and magnitude of the warm air layer aloft,
2. where would thunderstorms initiate and then travel over,
3. as a result of the position of the cold air, where would the surface low be,
4. and related to 3 where would the maximum divergence aloft be (horizontally coupled jets).

I looked at the SREF placement of the Upper Level Jet (ULJ) and looked at the diagnostics from SUNYA* (irrotational and non-divergent wind at 300 hPa; the 850 q vectors with the along and cross front components of the vertical motion). This event looks very similar to east coast snow storms with coupled jets. In this case the irrotational wind center was intense over OK nearly tripling in value over a 6 hour period (18 to 24 hr forecast) from this mornings NAM. The along front forcing for ascent was forecast to come through in 2 waves ... the leading one much weaker than the 2nd. Aloft the forcing for ascent is forecast to be more widespread and last longer into tomorrow late morning.

There were hints that a coupled jet structure was developing at 0000 UTC this evening. But closer inspection of the soundings suggested that a the true jet was near 200 hPa as evidenced by the 150 knot winds near 210 hPa at LZK and the 120 kts at SGF and 135 at LMN and OUN at about 145 kts. Only 0600 UTC balloons at TOP and LMN could help identify if a coupled jet feature did develop.


*Thanks to Kevin Tyle for trying to keep SUNYPAK alive!

Impressive Jet streak off the Pac coast

The new radiosondes with GPS winds really have made a difference, though there has been little talk or even research on this. The difference has really been the availability of upper tropospheric winds in strong jets. I did a little work in this area to look into where and when the old radiosondes would lose the wind data. I will have to dig it up at some point.

The inspiration for this post came as I checked my email this evening and was made aware that the RUC model failed in its initial attempt at a forecast today. The reason was a strong jet entering the Pac Coast. Here is the 250 hPa map:

And the sounding for MFR:

167 knots or 86 m s-1! And there was still wind data all the way to 7 hPa! These speeds are not too much higher than this morning with 157 and 149 knots at SLE and MFR.

Upon further reflection though, October 500 hPa temperatures at -25 C seem a little extreme. And thats not even in the core of the extratropical cyclone! Maybe it will come ashore tomorrow near UIL.

Portland has thunderstorms in the forecast and there is lightning along the coast.

I will have to dig into how extreme these temperatures are for the Pacific Coast in October.

UPDATE: 195 knots from MFR this morning!

Interesting Null tornado event

The tornado watch appeared promising. Plenty of shear with a bulk wind difference near 25 m s-1, CAPE was solid at 1000-1500 J kg-1 and storms had initiated in proximity to the dry line outflow boundary intersection. The storms stayed small and very early on began to split with small left movers that dissipated rapidly north of the outflow boundary.

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.

Pondering perspective in ensemble modeling

When forecasting anything, one must always consider the perspective one has. This is not easily achieved since our point of view is necessarily biased, either by a previous forecast, previous experience, analogs, or instinct.

Perspective "is the choice of a context or a reference (or the result of this choice) from which to sense, categorize, measure or codify experience, cohesively forming a coherent belief, typically for comparing with another." - From wikipedia.

Note the implied bias: "typically for comparing with another".

This is why ensembles are so neat in modeling the weather. The whole point of an ensemble is provide perspective or perhaps more appropriately predictability and by extension certainty (or uncertainty). This is particularly true even if the range of solutions does not cover the phase space of what is possible. In most instances, the mean of the ensemble is better than any individual member.

In the case where the outlier has the most value (no matter how wrong it is), the forecasters perspective may be the only real clue that it is even remotely likely for it to be correct. That is the value of the human forecaster and their experience is most likely to recognize the value of an outlier. There is significant risk associated with favoring an outlier. For one, you are going against what all the other members of the ensemble are trying to convey. So, you have to have good reasoning and great perspective on why so many members could be wrong.

In my opinion, this is why so many BIG forecast failures have occurred. It is difficult to trust an outlier, in a timely manner, because it takes a long time to discount a lot of members AND analyze the outlier in question in great detail such that you trust the solution. This is a major issue in severe storms research since the forecast period is short, the lifetime of some storms and their hazards are even shorter, and the models we use are shrouded in uncertainty (initial data, model spin up time, resolution, physics, dynamics).

Let us not forget that even ensembles have difficulty in predicting the certainty. Just because all the members are similar does not mean the forecast is certain. The issues we face now are just as much technical as they are scientific. Navigating the world of coarse ensembles and fine resolution ensembles will be fascinating.

back to modeling

The return to modeling. I have worked extensively with cumulus parameterization schemes in mesoscale and now regional climate models. No matter how good they do, there are serious known limitations to the physical schemes. The problem is that most people do not do a very good analysis on the known unknowns (the things we know we don't know) or at least the characteristics that have not been examined yet.

Cumulus schemes transport heat and moisture (vapor, microphysical species, and rain) from the lower levels to the upper levels of the atmosphere. They do not typically involve momentum transfer though there are always exceptions.

Cumulus schemes do NOT have:
1. sufficient variability to handle tropical or intense midlatitude convective heating,
2. sufficient resolution to handle MCS stratiform cloud shields including the associated coupled heating-cooling vertical dipole,
3. universal trigger function that works in the topics and midlatitudes,
4. can not generate significant heavy rainfall unless numerous non-physical scheme activations occur.

Furthermore, my work clearly shows that PBL temperature and moisture biases exist depsite resolution (4 vs 8-50 km). Vertical velocity is DAMPED when a cumulus scheme is used, not because the instability is being removed but because the cumulus scheme barely counters the destabilization, thus the vertical velocity distribution does not expand.

The cumulus scheme's are heavily untested at fine grid spacing. Good simulations with a cumulus scheme do not mean the cumulus scheme was either the cause of success or failure.
I think we need to ask the following:
1. What unique properties of a cumulus scheme contribute to the succesful components of a good forecast?
2. How does the cumulus scheme interact with other physics?
3. How does a change to the cumulus scheme impact the simulation?
4. What is the tendency of a particular series of changes to a cumulus scheme?

I would also urge that anyone using a cumulus scheme that wants to nest to fine resolution: use it to drive a seperate model run. You dont want cumulus tendencies going through multiple domains in a nested two-ay or one-way feedback.

Nesting in my opinion is not a proven concept until some more work on physics is completed. Early nesting used simple CPSs on the outer coarse grid. CPSs have been known to corrupt daughter nests. User beware.