Severe Weather Discussion for March 28 - 29, 2007. Valid: 03/28/2007. Highlights: - Active severe weather day will occur with potential for significant severe weather from west Texas northward into Nebraska. Primary threats throughout the western parts of the region are tornadoes, hail to 3 inches, and damaging winds. - Severe thunderstorms are possible on Thursday. - Chase Condition is raised to Level 4, leaving at 13:00. The webcam and tracking devices will be online, provided we're near Internet. I hope to blog from the road, but we'll see. Discussion: No change to prior forecast. Upper system evolving as expected and for the record the ECMWF hit this one pretty well. The dryline is much farther west than most models indicated, although the ECMWF had it a few day ago. Part of this is extensive cloud cover, which continues across parts of the threat area. Recent satellite imagery does show the clouds breaking and would expect better clearing within the next two hours. Temperatures have responded in areas where the clouds have cleared. CAPE values are around 2200 J/Kg along the Red River with LIs to -5. The dryline is expected to start moving east and be in the central Texas panhandle at convective initiation. Storms should then move northeast into Oklahoma. Initially and through late afternoon storms should be isolated with more widespread activity tonight. There is a damaging wind threat for central Oklahoma as storms merge into a large complex. The forecast for Thursday is rather muddied given model progs of widespread precip Thursday morning. I kept the threat in, but think it can be removed tomorrow. Probabilities: Risk area for any part of OK/TX (valid: 03/29 Day 1, Day 2): Slight: 100%, 50% Moderate: 40%, 0% High: 0%, N/A Chase Status: Level 4 - Heading to Hollis around 13:00. Chase Discussion for March 28, 2007. Valid: 03/27/2007. Target Location: Hollis Departure Time: 13:00 Discussion: 00z NAM increases the potential significance for Wednesday. It is now progging CAPE values AOA 3000 J/Kg along the dryline and LIs AOB -7. The NAM has backed the dryline up a little bit with it solidly in the Texas panhandle by 7pm. However, that shouldn't matter much as storm are likely to initiate by 4pm. NAM-WRF soundings places EHI values near 3.5 to 4.0 and the Thompson index around 40. The soundings are alike from Childress to Elk City. Farther north the thermodynamic parameters are decent but directional shear isn't as good. This may be compensated for, but for now I think that area is too far north. The sweet spot for this event appears to be in the southwest quarter of Oklahoma. If surface winds back more than progged, it may increase the significance of the event. I'm struggling between Hollis and Sayre, but will likely go with the former. Time to Hollis isn't too bad and believe that we can fall in behind any storms that have already developed. I think this warrant a moderate risk with at least a 15% on the tornado graphic in western OK. We'll see what SPC does, but I've gone on record with my opinion. Severe Weather Discussion for March 28 - 29, 2007. Valid: 03/27/2007. Highlights: - Active severe weather day is likely on Wednesday with the potential for significant activity across western sections of TX/OK. Primary threats are tornadoes, large hail, and damaging winds. - Severe thunderstorms are possible on Thursday across central/eastern parts of the region. - Chase condition increased to Level 3, looking to go tomorrow. Discussion: No change to prior forecasts or reasoning. The ECMWF will likely verify with this system as the 12z runs have drifted south and a tad slower. However, all the models have done varying levels of good/bad during this event. There is better confidence on the evolution of this system, although the exact processes expected on Wednesday can not yet be determined. Upper system is still progged to race eastward into Colorado where it will stall and close off. At the surface a dryline is progged to extend from an occluded surface low in northeast Colorado. This feature has been progged in a variety of location, but I prefer the GFS's farther west location about two row of counties west into the Texas panhandle. The 12z NAM-WRF has the dryline on the OK/TX border but think this may be too far east. Models agree that CAPE values should be at least 1500 J/Kg with LIs to -6. The NAM-WRF is much more aggressive with 3300 J/Kg CAPE at 1pm, decreasing it to 2000 J/Kg at 7pm. This seems mainly due to the continued odd dewpoint sink during the afternoon. The GFS does not hint at this and deep layer moisture would negate such. Some of it may be due to the shallow air the NAM has across central Texas, but once again I'm suspicious of this solution. Speed shear looks good across the threat area with directional shear remaining a concern. Model soundings indicate that supercell potential will be AOA 70% in southwest Oklahoma and if the EML can hold, then storms should remain discrete. All models trigger convection along the dryline during the afternoon, most likely between 3-5 pm. This is why I'm putting 1pm as the departure time. There is something about southwest Oklahoma that the models keep developing QPF in this area first. The NAM/GFS/HIRESW all do this. It will be interesting to see what actually occurs tomorrow. The afternoon issuance of the Day 2 is good and I like the 10% hatched area across western OK. I'm not high on NW OK since the flow will be unidirectional until the warm front in Nebraska where better turning will be found. Other than that the better turning will be from southwest Oklahoma southward into Texas. PRIND: The dryline will be in the eastern Texas panhandle Wednesday afternoon with convective initiation expected by 5pm. Storms may try to line out, but the EML/cap may keep them discrete in the early stages. This situation warrants a moderate risk and I'm continuing with 100%. Little time to talk about Thursday but with the upper system hanging back west there is a severe weather threat for central/eastern parts of OK/TX on Thursday. Much of that will depend on atmospheric recovery, a rather difficult thing in March if there is a lot of convection on Wednesday. Potential certainly exists for another active severe weather day on Thursday. Heavy rain may become a problem, but not going to make the same mistake I did last week. Probabilities: Risk area for any part of OK/TX (valid: 03/28 Day 1, Day 2, Day 3): Slight: 100%, 100%, 70% Moderate: 100%, 50%, 0% High: 0%, 0%, N/A Chase Status: Level 3 - Wednesday is the current target day. Looking to leave around 13:00 for Altus to Sayre. I'm not able to isolate it beyond those two places. Will also have to monitor Thursday for a local chase. Severe Weather Discussion for March 28 - 29, 2007. Valid: 03/26/2007. Highlights: - Active severe weather days expected Wednesday and Thursday. - Chase Condition increased to Level 2. Discussion: 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... Probabilities: 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 Chase Status: Level 2 - Monitoring Wednesday. Target region is Altus to Hobart to Sayre. Severe Weather Discussion for March 28 - 29, 2007. Valid: 03/25/2007. Highlights: - Severe thunderstorms are possible across the region on Wednesday and Thursday. Discussion: 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. Probabilities: Risk area for any part of OK/TX (valid: 03/26 Day 3): Slight: 100% Moderate: 40% High: N/A Chase Status: Level 1 - Normal