Severe Weather Discussion for February 19 – February 25, 2007.
– Above normal temperatures this week, with Sunday being the only day below normal. Thursday and Friday will likely be much above normal.
– Severe thunderstorms are possible Friday and/or Saturday across central/eastern Oklahoma/Texas.
– Snow is possible across northern Oklahoma on Saturday.
– Severe Thunderstorms in Oklahoma: 40%
– Storm Chase: 15%
– Winter Precip: 15%
Days until storm season: 23.
Mid-level pattern will amplify somewhat this week, making for an active end of week for Oklahoma. Temperatures should remain above normal, 55F in Oklahoma City, this week as only weak Pacific cold fronts are expected.
The first few days of the week are rather uneventful as a weak southern stream shortwave trof moves across the region on Wednesday. GFS shears this system east and results in little return flow for the region. 00z NAM is much more aggressive with moisture return and hence precip for parts of the region. I’m not going with this solution as 50F dewpoints were pushed to the Yucatan and Cuba with the last cold front intrusion. While said intrusion happened last week, we’re still in the Gulf recovery period and do not see how we can get sufficient moisture up here for lots of rainfall. The more likely solution is for mid-level clouds and maybe some light shower activity in Texas. This agrees well with the 00/06z GFS and 06z NAM.
More interesting situation develops as a longwave trof moves into the western U.S. on Thursday. Synoptic scale pattern will change with progressive flow across the lower 48. It certainly appears the models are phasing the jet streams with this system. As with any event there are lots of timing differences in the models. The 00z GFS/ECMWF agreed nicely on the timing of the progressive longwave trof while the 06z GFS slowed the system down. I agree with OUN regarding the progressive nature of the pattern and upstream kicker. As such, I’m going with the 00z GFS/ECMWF on timing, which brings the upper system into the region on Friday. The probabilities for this week are all set for Friday-Saturday; Monday – Thursday should be rather uneventful, except for wildfire potential Mon/Tue.
SPC Day 4-8 is reasonable for today, although rather vague. Not a shocker at this range, but makes this forecast rather tough. These Day 5/6 events are nutcrackers. Going with above reasoning, I expect the longwave trof to start influencing the region on Thursday with strong southerly flow across the region. Moisture will be poised at the Coast to return north as a function of the system on Wednesday. However, the Gulf will still be recovering and do not expect rich moisture with this event. Despite the obvious moisture concerns, the GFS progs 60F dewpoints moving into southern Oklahoma Friday noon. GFS is way too fast with the dryline, pushing it out of Oklahoma by Friday evening. Even factoring in time of year, the model is too fast. Part of this is the shallow nature of the 850mb flow with the GFS progging a closed low in South Dakota Friday noon, this produces strongly veered flow at 850mb across the region and shifts the moisture east. Again, I do not agree with this
solution and it is another GFS bias. Location of the 850mb/700mb lows should be farther south and we’ll probably see that adjustment in future model runs.
All of that said, dewpionts around 60F should be in Oklahoma by early afternoon Friday with the dryline approaching central sections. The base of the upper system will be near AZ/NM with vorticity moving into central KS/NE. The GFS lifts the surface low out with this initial push, resulting in an unrealistic dryline punch into Arkansas late Friday. It appears that most NWS offices are also ignoring this prog. Unfortunately I cannot reach the SREF, so will have to modify the 00z GFS for expected conditions late in the week. For now, CAPE values around 1000 – 1300 J/Kg are expected in central/eastern Oklahoma on Friday with near 2000 J/Kg expected in adjacent parts of Texas. LIs of -2 to -4 in Oklahoma and to -6 in Texas seem a good bet. These parameters are favorable for isolated to scattered severe thunderstorms across the region. Current model progs have very impressive 0-1km shear, but believe this will change as the models get a better handle on the 850mb low placement.
Historic events and system placing yield de facto shear and see no reason to disagree. Widespread showers and thunderstorms should occur in eastern Oklahoma as the system lifts across the area late Friday. Any storms that can form ahead of the dryline Friday afternoon in Texas or Oklahoma will have the potential to be supercells.
I’m rather hesitant to go with 40% on the severe level and especially the 15% chase. However, the setup does warrant going above climatology as noted by SPC’s risk area. I do think 40% is a good balance between coverage and significance for the Friday into Saturday event. Some of this has to be balanced with the excitement of the approaching storm season. The 15% snow is a CYA as the cold core moves over the region on Saturday. Thickness values will be less than 540 dm across northern parts of Oklahoma. Lack of a strong Arctic push at the surface should limit the amount of moisture removal. As such, expect snowfall across parts of the region. It is not expected to be significant in terms of accumulations with 1-3 inches possible in northern Oklahoma. Discussions will likely be needed starting tomorrow for the weekend event.
Warm conditions are expected to continue for the next two weeks as the Arctic air remains confined to the north. I see nothing in the models that will bring any of this south. If anything, periodic Pacific cold fronts should be the rule as we enter late winter.by