Severe Weather Discussion 2007-9-3

Severe Weather Discussion for May 4 – 8, 2007.
Valid: 05/04/2007.

Highlights:

– Active severe weather day is likely in western KS into NW OK/SW Nebraska. Destructive hail and tornadoes are the main threats. Isolated severe thunderstorms are possible farther south into Oklahoma and the TX/OK panhandles; however, this threat is conditional.

– Active severe weather day is expected on Saturday, this is a potentially dangerous weather situation with numerous supercell thunderstorms expected across Nebraska, Kansas, and Oklahoma.

– Severe thunderstorms are possible Sunday – Tuesday across much of the region. It remains difficult to isolate a particular day or location that has higher potential than the other days. Forecast details are quite muddied by expected activity today and tomorrow.

– Chase Status is increased to Level 3.

Discussion:

Very exciting and potentially significant severe weather situation is developing for the region today through the weekend. First off, I know better than to not trust my forecasting skills. I had today nailed a few day ago. Such as it is, we're all on board at this point.

12z NAM/GFS runs continue prior trends of developing extreme instability across the region today with CAPE values above 4000 J/Kg expected. The dryline is located along the TX/OK border at 17:40z per the Oklahoma Mesonet. 17z CAPE values are 2500 J/Kg and higher across the region and this should increase during the day. LIs are -5 to -8 with dewpoints in the mid/upper 60s. Storm initiation remains iffy today due to a strong cap across the region. CIN values are 25 J/Kg and higher with near 100 J/Kg along the dryline. The strongest surface heating is occurring away from the dryline ATTM. As the day progresses, the convective inhabitation along the dryline should decrease and the question becomes can storms develop. Little in the way of upper support will be available in this area, so it is a tough call. Additionally, the cap should strengthen today with the NAM showing 10C 700mb temperatures at 00z in NW Texas. OUN is doing a 19z sounding to assess cap strength and
thermodynamics. Wind fields aloft are more than favorable for supercell thunderstorms should any develop. Storms will remain discrete and likely produce high-end severe weather across western OK. More numerous activity may occur in Kansas and Colorado up to Nebraska. SPC Day 1 has this will covered.

Elevated thunderstorms are possible tonight across parts of the region as 40-60kt low-level jet develops. How widespread and intense will how an impact on Saturday. Many of us forecasters are still licking our wounds from the 24th.

The new Day 2 is somewhat of a disappointment and I'll say I disagree with it. The moderate risk should cover into northwest Texas, why Goss pulled it north to I-40 is very odd. There is little new model data that argues changing the risk area. Additionally, I'm not sure what he's looking at regarding meridional flow. I do agree that mid-level and especially upper-level flow will be more backed than usual, but I don't see anything limiting rotation potential. In fact the 12z NAM helicity values are much higher than I typically expect from this model. 12z GS has 3500 J/Kg CAPE along the dryline Saturday afternoon and this looks good. NAM instability is a little off mainly due to it tracking a supercell east across southern Oklahoma tonight into tomorrow. I'm not saying it won't happen, just saying I'm not convinced of it. And, that it will have that big of an affect on the thermodynamics. As with last week, if storms develop in the morning, it will greatly reduce the severe
weather threat. Models are not as aggressive with widespread activity as they did last week and this would make sense given the location of the upper system, as compared to the system last week that was lifting northeast into the region in the morning. Another note about Day 2, there is meridional flow in KS/Nebraska but I see little impact here. The best winds are in NW OK, but the extreme instability farther south should compensate for marginally reduced wind fields. There isn't that big of a difference.

My chase location is Sayre/Elk City area right now. I like the north-south options and this allows coverage in both quadrants. This is not a tail-end Charlie day or to avoid such, T/Td spreads will be quite low along with LFC's and LCL's. If the atmosphere is not worked over today, there should be plenty of thermodynamics for widespread severe thunderstorms (supercells) Saturday afternoon. Another consideration for a southern location is potential for anvil seeding. Given 300-200mb flow less than 60kts across much of the area, this could be a problem. The southern storms however should not have that issue.

Sunday is technically within the forecast range, but very tough at this point. 12z GFS/NAM agree that a large chunk of the upper system will have lifted out by Sunday afternoon. Wind fields will remain favorable for severe thunderstorms. The question remains how unstable can the atmosphere get. SPC has a slight risk up and this looks good. The threat looks to shift to eastern Oklahoma and points east on Monday. It does not look like a moderate or high-end day, more on the high side of a slight risk. I'll put probabilities in for it at this point given the potential.

Tomorrow is one of those almost classic outbreak days. Whether or not SPC does a high risk will depend on forecaster bias and especially if convection occurs in the morning. I'm going with 50% mainly due to the latter issue, otherwise I'd give it a 75% rating.

Probabilities:

Risk area for any part of OK/TX (Valid: 05/05 Day 1, Day 2, Day 3):

Slight: 100%, 100%, 75%
Moderate: 100%, 50%, 0%
High: 50%, 0%, N/A

Chase Status:

Level 3 – Looking to go Saturday, location Elk City/Sayre, target time 13:30 CDT.


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About Putnam Reiter

Putnam has been storm chasing since 1990 and is a co-founder of Hook-Echo.com. For his day job, Putnam works in emergency management for information technology.
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