Severe Weather Discussion for March 28 – 29, 2007.
– Severe thunderstorms are possible across the region on Wednesday and Thursday.
12z models are consistent with the evolution of our next system, progged into the western U.S. late Monday. Unlike the prior system this one will remain progressive, but it will also have to deal with the eastern U.S. ridge. Ensemble runs are in very good agreement regarding the progression of this system. The 00z ECMWF has shifted its solution some and now projects a closed low over Colorado, keeping the majority of the energy in the southern stream. While this solution may not be wrong, none of the other models are following track. For now, I'm staying with the 12z NAM/GFS/Ensemble solutions and this discussion is based on them.
Pesky closed low over the southwest U.S. has lifted into South Dakota this afternoon as an open wave. The northward movement of this system resulted in little to no airmass change across the region. As such, 60F dewpoints are common across Oklahoma and Texas. Along the coast line, mid/upper 60s dewpoints can be found. Surface pressure remains around 1023mb along the coast as the large surface high continues off the eastern U.S. coast. This ridge has circulated modified Canadian/Polar air into the region for the past week and resulted in high octane moisture return being limited. However, this has not been a problem, as confirmed by the 13 tornadoes on Friday in west Texas/N.M. The 850mb level has dried out some and this is to be expected.
A southern stream closed low has develop very near where the prior system was. This system should move slowly across Texas on Monday and Tuesday as the next trof moves into the western U.S. Thunderstorms are possible with this system, but not expecting a repeat of last Friday. This system will also cause some drying in the low levels as it progresses out of the region Tuesday afternoon. However, the surface airmass should remain unchanged.
The large ridge out east is the big caveat for significant severe this week. This ridge, which is deep layer and strong, will continue to circulate dry air into the region. However, it does move east as the longwave trof moves into the western U.S. Both the NAM and GFS agree that deep layer moisture return will occur by Wednesday afternoon in western OK/TX. PW values go from 2 to 2.5 between Wed morning and the late afternoon. This may actually work in our favor as it keeps activity from going too soon. Model soundings are not that impressive and do not compare while to the map progs. Many of these issues are common this far out and why forecasting skill is so low. These trends will be monitored the next few days.
The moisture concerns aren't that great at this point given both models are progging CAPE values between 2000 and 2500 J/Kg along with LIs to -6. I think by late Wednesday there will be deep layer moisture return across the threat area, along with a large warm sector into at least Nebraska. The models consistently prog the dryline to be near the OK/TX border when convection initiates Wednesday afternoon. Models also develop QPF across western OK/TX during the afternoon. I do think the dryline will be a little west of the progged location but some of that will depend on how fast moisture makes it up here.
The upper system is progged to progress rapidly east into the region on Wednesday and then wrap up over the northern U.S. into a closed low as it runs into the longwave ridge to the east. The resulting flow across this region will be a rather narrow axis of south-southwest to north-northeast jet streak at 500mb. Directional shear isn't as great as it can be, but certainly not limited either. 0-1km shear looks good and depending on which model you chose the 0-3km is marginal to good. Now, the 0-6km shear isn't that great beyond 3km. Lots of things to watch for this event, but none are odd for late March.
This event compares strongly to March 27, 2004, with that system being only slightly farther east. Currently the upcoming event is expected to have stronger thermodynamics associated with it. Reference the Hook-echo.com events page for details on the past event.
SPC Day 4-8 looks good and I certainly expect a slight risk on the morning Day 3. I'm not thinking they'll do a moderate right now, because I'd wait another day first. However, I would go with a 30% area from southwest OK into Nebraksa. It will be interesting to see what happens. Regarding the chase status, it is not normal to raise it right now and I'm not inclined to do so. However, I am closely watching Wednesday for a chase with Sayre the initial target area.
GFS and ECMWF agree, in different ways that part of the energy will hang back to the west on Thursday. This may provide a severe weather threat for eastern Oklahoma on Thursday but more likely for much of eastern Texas.
Risk area for any part of OK/TX (valid: 03/26 Day 3):
Level 1 – Normal
For more information, weather news, weather blog, and chase summaries go to http://www.hook-echo.com.