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FXUS63 KLSX 211731 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1231 PM CDT Tue May 21 2024

KEY MESSAGES

- It's going to be a hot one today, with temperatures reaching the low 90s, near record territory at St Louis. Heat and humidity will fuel a line of severe thunderstorms which will move in from the northwest this evening. Damaging winds will be the most widespread threat, but large hail and a few tornadoes will also be possible depending on storm mode.

- The severe weather threat Wednesday shifted southeast and is more limited locally. Wednesday is likely to be the coolest day of the forecast for most.

- The rest of the forecast will have near to above normal temperatures with frequent chances for showers and thunderstorms, although no obvious severe weather episodes are evident at this range.



SHORT TERM
(Through Late Wednesday Afternoon)
Issued at 249 AM CDT Tue May 21 2024

Our warm and humid air mass continues today and actually reaches its peak in terms of temperatures. We'll see a fair amount of 90+ degree readings, which will be near the record for St Louis (93 set in 1941). A warm front to our north across Iowa will be the primary focus for thunderstorms this morning and into the day being fueled by the very warm and humid air mass. Our area should remain dry for most of the day Tuesday as at least some capping of the air mass inhibits thunderstorm formation and forcing remains focused to our north. That changes this evening, though.

The primary severe weather episode of Tuesday is expected to commence in the late afternoon to early evening across northwest Missouri into central Iowa. Surface low pressure will be forming and pushing off to the northeast while a cold front develops and pushes southeast behind it. Thunderstorm initiation seems most likely near where the cold front reaches the dryline in northeast Kansas into northwest Missouri as what capping exists will be eroded just as the next shortwave trough moves out into the northern Plains. With strong deep layer shear, supercells will be the initial storm mode in an environment very favorable for large hail and damaging winds as instability in the hail growth zone aloft is quite strong while a deep near-surface mixed layer will enable the development of strong downdrafts. There will also be a threat for tornadoes, although with the deep boundary layer the low level shear will be somewhat limited and LCL heights relatively high. These factors will get somewhat more favorable for tornadoes during the early evening.

Relatively widespread initiation is expected in a 1-2 hour window early this evening with storms likely to form quickly into a broken line with QLCS structure and embedded supercells. This is the likely storm mode as they enter our forecast area around 6PM this evening.
Damaging winds will be the primary threat, though local mesovortices could also develop where the low level shear lines up favorably. As the line pushes southeast it will begin to outrun the better instability and shear, gradually losing its punch and eventually falling apart altogether. Most of our area will see rain tonight, though, and POPs are high due to the expected widespread coverage this evening. The line should be exiting to the southeast near or shortly after midnight.

Models are in pretty strong agreement that the storms tonight will push the effective surface front southeastward with a significant drop in dewpoint noted behind it. This will push Wednesday's severe weather threat southeast as well in the remaining warm sector. We could still see rain and a few thunderstorms north of the front in the southern half of our forecast area Wednesday afternoon as a shortwave trough moves by, but the threat for severe weather is significantly lower. Temperatures will also be significantly cooler Wednesday behind the front, with most areas staying in the 70s, the coolest day of the forecast.

Kimble

LONG TERM
(Wednesday Night through Monday)
Issued at 249 AM CDT Tue May 21 2024

For the extended portion of the forecast, the upper flow pattern should be westerly to southwesterly right through the holiday weekend with Gulf of Mexico moisture still readily available. In general this spells temperatures near or above normal with frequent opportunities for rain as nearly daily shortwave troughs move by.

While the front does push south of us for Wednesday, renewed convection along it Wednesday afternoon helps to wash out the boundary. Surface flow turns back to the south on Thursday with those higher dewpoints oozing back northward because of it. It looks like the best shortwave energy is to our south on Thursday and to our north on Friday, but we can't rule out some showers and thunderstorms developing both days. Friday's trough drags a cold front in behind it which stalls near our area Saturday. The next wave tracks along this stalled boundary on Sunday with a potentially deeper trough developing behind this wave.

With so many potential rounds of showers and thunderstorms to track, it is difficult to have much confidence on the timing of any individual rounds considering the impact prior rounds will tend to have on the subsequent rounds. As a result, there are POPs in every period of the forecast with only small fluctuations each period.
This is not meant to say the whole weekend will be a washout. It just means we can't clearly identify any periods where it definitely won't rain. Each round of showers and thunderstorms may only bring a couple of hours of wet weather with it allowing lots of dry time in between. Temperatures will be seasonably warm for the most part but not excessively hot, in the 80s. But there could be some cooler weather as well if clouds and showers limit heating more or if we get a more solid cold front pushing through the area. There isn't a strong signal for severe weather in any period, but we couldn't rule out a few stronger storms depending on how each day plays out. Sunday's wave may prove the best opportunity, but confidence is low on all the variables lining up.

Kimble

AVIATION
(For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1226 PM CDT Tue May 21 2024

Southerly winds gusting to 25-30+ kts and scattered-broken VFR cumulus clouds will prevail across the region this afternoon. We are still anticipating a line of severe thunderstorms to develop across western MO later this afternoon, with this line moving quickly to the east-southeast across central/eastern MO and western IL this evening. The TAFs reflect a 2 hour window with IFR flight conditions and strong winds when the line of storms is expected to move through the respective terminals. Following the passage of the line of storms there should be a westerly wind shift and return to VFR flight conditions.

Glass

LSX WATCHES/WARNINGS/ADVISORIES
MO...None.
IL...None.

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