Severe Weather Discussion for March 28 – 29, 2007.
– Active severe weather days expected Wednesday and Thursday.
– Chase Condition increased to Level 2.
Little change in the over all picture for Wednesday and no change to prior reasoning. However, the models are now trending slower with the upper system by Thursday. This is caused by the longwave ridge to the east blocking the flow. As the upper system moves rapidly into the central U.S. on Wednesday, it is progged to wrap up into a closed low. The models have different solutions regarding how said closed low will develop. The ECMWF is the south solution with a closed low over Colorado by Wednesday evening, keeping the majority of the energy south. The NAM and GFS runs tend farther north with the northern stream energy with a closed low over South Dakota. Southern stream system develops back west a little on Thursday and moves east during the day.
These model differences lead to a difficult problem. The NAM-WRF/GFS agree with each other and their prior runs while the UKMET/ECMWF tend to agree. Although the UKMET has flipped around a little bit. There are rather serious implications to this forecast given the difference in location and speed. Should the ECMWF et al verify, then the threat would shift to Thursday, conversely the NAM/GFS would be a Wednesday threat. As with yesterday, I'm staying with the NAM/GFS solutions.
Given the above, where are we today? Well, Wednesday continues to show signs of an active severe weather day. I've used a blend of the NAM/GFS models for the most part, although I do favor the GFS dewpoints for Wednesday afternoon. The NAM has an odd dewpoint sink over parts of Texas. This happened last week and did not verify. The GFS has a more uniform moisture field and this appear more representative of the overall pattern. As such, I expect CAPE values to be AOA 2500 J/Kg and LIs AOB -7 across western Oklahoma. Model soundings for Wednesday look representative of what one would expect for such a day. Models prog that the 850mb level will initially be quite dry Wednesday morning with rapid moistening between the surface and 850mb during the day. Additionally, wind fields will increase at all levels. Directional shear is progged to range from marginal to decent depending on which model you choose.
Many of the parameters appear to be in-place for an active severe weather day on Wednesday. Confidence is reduced a little due to the ongoing disagreement among the models. For now I've raised the chase condition to 2 and will continue to monitor Wednesday. The models have backed the dryline up a little and this is not a surprise. They also continue to initiate convection during the afternoon, say 3-4pm. I think the cap (EML) will hold down activity across southwest OK into Texas, especially since moisture will be lacking until mid-afternoon. This looks like an event that will come together at the perfect time.
I'm happy I got the SPC Day 3 outlook and it looked great to me. I'm not sure they'll go with a moderate on the morning Day 2, but think we'll get it for the afternoon one. Although, if the ECMWF verifies, then Wednesday may get pushed back to Thursday. Time will tell…
Risk area for any part of OK/TX (valid: 03/27 Day 2, Day 3):
Slight: 100%, 100%
Moderate: 100%, 30%
High: 0%, N/A
Level 2 – Monitoring Wednesday. Target region is Altus to Hobart to Sayre.
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