Monday, April 25, 2011

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.

Sunday, April 10, 2011

NSSL WRF performance

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.

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 (

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...