May 3, 2020 MCS Review

May 3, 2020 MCS Review (central/eastern KS)

As a general rule, convection is a tough, and humbling, phenomena to forecast. But, elevated convection? Well, that's a level above (pun intended). I don't always have time to review every event I work, but I try to when possible. The note with any review I do is it's simply a way to think back through an event and see what I can learn. If others have insights, I'm all ears. Let's learn together.

Before going any further, let me tell you what I didn't do well. I forecast quarter size hail and no wind of any significance. Our CWA ended up with 2" hail and 70-80 mph wind gusts. This is why I do reviews of events (in case you were wondering).

Early in the morning, thunderstorms developed over central Kansas and quickly became severe. Those storms eventually congealed, forming the beginning stages of a long-track MCS (as shown by Sean Ash) that produced severe weather well into the TN Valley.

Days in advance, this event carried a lot of uncertainty with it. Zonal flow with embedded shortwaves; questions about moisture depth/quality (especially following a more amplified upper level pattern that sent drier air well into the northern Gulf of Mexico); and less-than-stellar model agreement.

From a forecast standpoint, the part that gave me some pause (for initiation, coverage, and severity in KS) was questions surrounding moisture return and what impact the warm temps at the base of the EML plume would have on capping. 23 hours out, the RAP was forecasting some decent moistening around 850 mb, but even that wasn't enough to fully erode the elevated cap in place (we've had some very steep lapse rates recently, by the way).

23 hr RAP Forecast Sounding near Wichita, KS (valid 08z May 3)

23 hr RAP Forecast Environmental Parameters near Wichita, KS (valid 08z May 3)

MUCINH was around -40 j/kg, so not insurmountable, especially with an embedded wave forecast to move through and convergence along a west-east baroclinic zone, but not a guarantee of development either.

23 hr RAP 700 mb Temperature Forecast (valid 08z May 3)

Jumping ahead in time closer to initiation, the 2 hr RAP forecast sounding for Wichita showed a bit more moistening in and around 850 mb, but potentially more importantly, it also showed slightly cooler 700 mb temps, effectively removing any inhibition.

2 hr RAP Forecast Sounding near Wichita, KS (valid 08z May 3)

2 hr RAP Forecast Environmental Parameters near Wichita, KS (valid 08z May 3)

Weaker inhibition, modest large-scale forcing for ascent from the shortwave, and a mid-level convergence zone, and I'm assuming that is what gave rise to the fairly rapid development in/around the greater Wichita metro. As far as why storms formed where they did, it appears initiation held off until the shortwave/mid-level convergence zone interacted with instability axis oriented SW to NE across southern KS.

2 hr RAP Forecast Instability and 0-6 km shear (valid 08z May 3)

Sometimes it seems like elevated convection is more sensitive to subtle features (moistening, temperatures aloft, etc). Maybe that's the case with all convection, but it certainly seems important in this one. One of the forecasters at my office (WFO ICT) keyed in on one potential subtlety.  When he came on shift, he noticed a more backed flow in the low-levels on the ICT VWP compared to what models were forecasting the night before. That piqued my interest, and was something I didn't even think of looking at. It appears it was an important find.

The 23 hr RAP forecast sounding (valid near initiation time) showed westerly winds around 5 kt at 850 mb, while the 2 hr forecast sounding (valid near initiation time) showed slightly stronger 850 mb winds (15 kt) that were slightly south of due west. In fact, the 2 hr RAP forecast showed slightly better veering in the 850-700 mb layer. The end result was effective SRH of 96 m2/s2 (23 hr forecast) vs 144 m2/s2 (2 hr forecast). Both soundings had similar shear and instability, but that added SRH might have contributed to several of the updrafts being more efficient at acquiring low-mid level rotation.

It appears the increased potential for rotating updrafts aided in some larger hail sizes being realized and might have contributed to downdrafts being able to reach the ground more efficiently than would otherwise have been expected with a more stable SFC/near-SFC layer. That second possibility is pure speculation.

On the topic, though, I was very surprised to get a 76 mph wind gust so early in the event. It pays to look at more than just one forecast sounding prior to an event. Lesson learned! Take a look at the RAP forecast sounding for southeast Kansas (where the strongest wind gusts in our CWA occurred).

4 hr RAP Forecast Sounding over SE KS (valid 10z May 3)

4 hr RAP Forecast Environmental Parameters over SE KS (valid 10z May 3)

Despite southeast KS being post-frontal, it appears the drier low-level air, that was observed further west near Wichita, was not fully realized to the east. So, perhaps the better low-level moisture further east of I-35 allowed storms to be rooted a bit closer to the SFC compared to further west? If so, this might also explain the noticeable increase in SFC wind gust speeds as storms progressed further east.

Thoughts? Observations? Let me know!

Credits

Forecast soundings and analysis maps courtesy College of DuPage.
Storm reports courtesy SPC.