Monday, May 21, 2012

5/21 - 11am CDT - A Bit of a Surprise This Morning!

Yesterday evening we saw a wonderful view of the annular solar eclipse just south of Lubbock, Texas. This was my first solar eclipse and even though clouds obscured our view of the maximum "ring of fire" portion of the eclipse, it was a spectacular sight. It was good that we had an eclipse because the storm threat that we were targeting in that area was never realized. A good convergence boundary formed and intensified quite a bit, but it seems the "cap" (warm layer of air aloft) was too strong to get any good storms going. There was one small storm that formed to our east, but it quickly dissipated since it was moving into cooler air.

This morning we're waking up to some very good news. Last night's model runs, particularly of the Rapid Refresh, were quite pessimistic about today's setup in Northeast New Mexico and the western Texas Panhandle but early this morning things changed around considerably. The main difference here that I've noticed is the amount of moisture that will be available in this area. For example, the 4z run of the RAP model late last night had dew points near 50 degrees in Northeast New Mexico. The 13z run of the same run this morning has dew points from 60 to 65 degrees. That's a huge difference! Right now the dew points in the northeastern section of New Mexico are in the mid to upper 50's, so the newer runs of the RAP model with the increased moisture are looking good since higher dew points will move in this afternoon.

Having these higher dew points is so important because this moisture is pure thunderstorm fuel. Putting more moisture into the air near the surface increases instability because you have more latent heat contained in the  air as it rises. As such, the CAPE (instability) forecast on the RAP model has responded very positively to the model's higher "correction" in dew point this morning and we're looking at a situation where instability should be quite good at just under 3000 J/kg in most spots around Northeast New Mexico, parts of Southeast Colorado, and the western Texas Panhandle. This is pretty good!

Now that we've cleared moisture and instability for take-off on this event, let's go over what the winds will be doing. The change in the surface wind forecast between last night and this morning hasn't been as great as the change in dew point, but we're still seeing some differences. Winds today will mostly be out of the south-southeast or southeast in the region, with some places especially in Northeast New Mexico seeing a more easterly wind vector than what models were showing yesterday. Storms today will be forming in what we call upslope flow, so having more easterly winds blowing up the higher elevations in this region should create an ample source of lift. This should also increase wind shear a bit due to a contrast with the northwesterly winds aloft at 500 mb (18,000 ft) and create the potential for more organized, isolated storms. When you couple helicity, which is spinning in the atmosphere that takes wind shear to create, with the increased instability that we'll be seeing in a mathematical formula, you get an index called the Energy Helicity Index (EHI). I usually don't look at indexes and parameters too much since they get in the way of looking at the basics behind a storm setup, but EHI is one I do like because it's simple and gives you a decent idea where a tornado threat may set up since helicity and instability are so important. Values last night were slim to none on the models for EHI in our target region, but today with the increased moisture creating more instability we're seeing a marked upswing in forecast values to 2 or more in places.

Even though these values are higher than previously thought for today, we're still not looking at any sort of severe weather or tornado outbreak here. What we are seeing is increased potential for an isolated, organized chance for severe weather in this region that may carry a bit of a tornado threat with it. Right now we're monitoring current conditions across the area and picking out where storm formation may first occur. We drove through some storms this morning on our way from Lubbock to Amarillo as you see on radar there to the right, but those are clearing and the majority of the western Texas Panhandle and Northeast New Mexico is now clear from the rain. A pretty shallow cloud deck is hanging around in Northeast New Mexico at the moment, but there should be time to burn that off and create instability from the heating of the day this afternoon.

Taking all of these factors into account, the Storm Prediction Center has issued a Slight Risk for severe weather in portions of New Mexico, Colorado, Texas, and even the extreme western Oklahoma Panhandle. The Slight Risk includes a 2% tornado risk due to the increased shear and instability that's expected this afternoon that could create some supercell storms. I'm not a big fan of the northwesterly winds aloft at 18,000 feet today because you generally want southwesterly winds up there to bring in drier air aloft and enhance wind shear a little bit more, but this will have to do today. We've seen decent storms with this kind of upper-level wind situation before so it's not enough to rule anything out. It's nice to be able to chase on a day when we thought we would not be able to, so anything we see today will be icing on that cake!

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