Storm chances enter our forecast on Monday, and each day this week but Wednesday could see severe weather. I believe the risk profile for Tuesday is higher than in the previous briefing. Models have converged on an area from southwest Oklahoma into Central Kansas, along a sharpening dryline, with ample moisture and shear, followed shortly after sunset by a healthy low-level jet. Historically conditions such as these are tornado producers.
Monday, May 15
Monday’s risk area is in western Kansas, pretty much centered on US-83. The main body of the system is still back in the mountains, so the storms will be driven by the day’s heating and should be hail and wind producers.
I think the best area for storms will be within 40 miles of US-83, south of Garden City and north of Perryton, TX. The air temperature will be higher to the west and north, but the moisture won’t have time to make it to where temps are hottest. In southwest Kansas, air temps will be lower and dew point temperatures will be higher, giving a more potent atmosphere. Wind profiles along Highway 83 support hail storms, as the southeast surface winds (circled in the image below) are displaced from the better dynamics.
Tuesday, May 16
As we get closer to this event I am starting to become concerned about the tornado potential. Models have converged on a solution that has Kansas in its sights, along with western Oklahoma. Here are some comparisons:
Dry Line Position, Moisture
Supercell Composite Index
The biggest takeaway from the comparison graphics: for three days ahead of the event (model time is Saturday at 7pm CDT), they can be considered essentially the same. There are some subtle differences — and one that’s not-so-subtle. The glaring difference is maximum CAPE the difference between 3000-3200 in the GFS and 4250 or so in the NAM is fairly substantial.
Factors Enhancing Storm Risk
Surface to 3km Helicity
Helicity is a measure that indicates how prone to twisting an air parcel is as it rises into the sky. GFS forecasts a small bubble of >250 helicity from southeast of Doge City to northwest of Woodward. A value greater than 150 favors the development of twisting in the mid-levels of a storm.
As a result of both of its factors being relatively high, the Energy-Helicity Index from Mullinville south approaches 7. EHI combines CAPE (as seen above, running around 3000) and helicity. EHI of above 5 is correlated with strong to violent tornadoes. In an environment where storms are expected to break the cap, NWS says “By itself, [EHI] is the best index available for tornado prediction”
Shortwave trough entering the area at 700 mbar
A ripple in the wind flow at 700 mb (about 7,500 to 10,000 feet MSL, or 5,500 to 8,000 feet above the surface in that area) can enhance any rotation in place as it arrives.
Strong Low-Level Jet Arriving after sunset
The low-level jet is something that occurs due to the heating of the atmosphere each day, about 3,000 feet above ground. On a day where storms are ongoing when the LLJ arrives, those storms already rotating often get a strong boost. It’s one of the best explanation why tornadoes often strengthen around 9 or 10pm. I’m showing the 10pm forecast values because the LLJ has only begun to form and is well south of Kansas at 7pm.
The biggest limiting factor I can find in the models is surface to 1km helicity. That value is pretty low, in spite of surface to 3km helicity being relatively high.
SPC is not including mention of tornadoes in its outlook for Tuesday at this time. The main reason is because of some lingering concerns about moisture return. If we get moisture of the amount forecast by the NAM and the placement forecast by the GFS, it could be a very interesting night.
Wednesday, May 17
A good portion of Wednesday’s severe weather chances and evolution depend on what happens Tuesday night and where. I’ve not dug into the models for Wednesday in much detail due to the conditional nature of the risk. At this point NWS Wichita is calling Wednesday dry.
Thursday, May 18
The next substantial severe weather risk I see good model agreement on is for Thursday. This system is looking to affect some of the more populous areas of Kansas and Oklahoma.
More moisture in the warm sector
Substantial CAPE over a large part of the state
A strong low-level jet
The slight negative to Thursday’s setup as I see it is that the low level jet is potentially west of the storms by the time it pushes through. As this forecast is still five days out, I’m not pushing really hard on Thursday yet.
SPC 15% risk area for Thursday
This is pretty simple: models diverge too much to make any real call. Given time of year it’s pretty safe to assume some risk of severe weather, but SPC is pegging it right now as low predictability due to the model spread.