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FXUS63 KLSX 292359 AFDLSX

Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 559 PM CST Wed Nov 29 2023

KEY MESSAGES

- A widespread beneficial rainfall is on track for Thursday into Friday.
For most of us this will be the most significant rainfall since October, with a large portion of the region having a strong chance (50+ percent) of seeing an inch or more of rain.

- Forecast uncertainty increases from Saturday onward.
Probabilities favor near to above normal temperatures especially as we go into next week. While we can't rule out precipitation at some point confidence is low due to low predictability in the pattern.



SHORT TERM
(Through Late Thursday Night)
Issued at 240 PM CST Wed Nov 29 2023

With surface high pressure off to our east and low amplitude ridging over the region, we've seen a rapid warm up of the temperatures.
We'll drop back into the 30s tonight, but a gradient southerly flow will keep the low levels at least somewhat mixed and prevent temperatures from getting much colder.

Moisture return begins across the Plains tonight in advance of the next trough emerging out of the Southwest US. By Thursday morning some of this moisture will begin to move into our region from the southwest being driven in the 850-700MB layer. We'll see increasing clouds and the chance for some showers. These warm advection showers will be light if they occur as the more widespread significant rainfall arrives Thursday night. Despite still being in the warm sector of the developing system on Thursday, increased clouds are likely to lead to cooler temperatures. NBM interquartile range is rather high at this late stage reflecting the influence that cloud cover will have on temperatures tomorrow. If clouds are less extensive and more sun gets through, we could see upper 50s to near 60 degrees again. But if clouds are thicker and showers more numerous then we may not make it much above 50.

Kimble

LONG TERM
(Friday through Next Wednesday)
Issued at 240 PM CST Wed Nov 29 2023

Thursday night, surface low pressure ejects out of the lee of the Rockies and tracks northeast along a developing frontal boundary that will extend through northeast Missouri into central Illinois.
The next slug of moisture will pull northeastward ahead of it and this will bring our most widespread and significant rainfall we've seen since October. The moisture available with this is high as reflected by 60s dewpoints being ingested via the lower Mississippi Valley and PWATs in our area rising to near or over 1 inch. The bulk of the rain to the south of the low track will fall in a 6-12 hour period Thursday night before the mid level dry slot comes in and shuts things down. In this warm sector area south of the low track probabilities for 1+ inches of rain are higher. NBM probabilities of 1+ inches in this area are between 50 and 90 percent, although the 00Z ensemble probabilities are lower and displaced a little to the south of the NBM. The uncertainty in this region is in part due to whether convection will occur and achieve a deeper release of moisture. Some weak and elevated instability could exist above the low level moist advection so some locally heavier amounts are possible. However, the trajectory of this system is such that any convective elements will not linger and should limit overall precipitation potential. Either way, this will be a good soaking rainfall but not excessive to the point of flooding.

To the north of the low track across northeast Missouri into central Illinois, precipitation will be more persistent as moisture wraps into and around the low as it exits. Here precipitation is more likely to continue into Friday morning and perhaps into Friday night. Precipitation type is still expected to be rain in this region as temperatures start off well above freezing. However, models are showing low level cold air undercutting the warmer air above to the north of the surface front. This leads to a transition to freezing rain and eventually snow from north to south. At this time it looks like this wintry transition is likely to stay north of our forecast area, however, parts of Knox County could be impacted by this if the colder air moves in a bit quicker than forecast.

We continue in the southwest flow aloft this weekend and another weak wave takes a similar track on Saturday. This time there's not as much moisture to work with so precipitation chances are lower and focused more to our northeast. A strong temperature gradient remains across the area through the weekend as the surface front keeps the colder air in the north while southern areas stay warmer. Lingering clouds could also dampen temperatures especially Friday into Saturday.

Beyond this we transition into northwest flow for next week as an upper trough develops in the East and a ridge develops in the West.
A weak wave on Tuesday will send a cold front toward the area, but the bulk of the cold air with this will stay to our north and east.
While we can't rule out precipitation with this front, the moisture connection looks weak at this point. By the middle to later part of next week the western ridge will become more dominant and temperatures more confidently move back above normal.

Kimble

AVIATION
(For the 00z TAFs through 00z Thursday Evening)
Issued at 546 PM CST Wed Nov 29 2023

South-southwest surface flow will accompany VFR conditions tonight into early Thursday. The primary impacts will stem from the gradual lowering of cloud bases and development of rain from southwest to northeast late Thursday morning into early Thursday afternoon. Guidance has been fairly consistent on timing the onset of precipitation between 16z-18z around KJEF/KCOU, 18z-21z around the metro terminals and just after 21z around KUIN.

VFR conditions will give way to MVFR ceilings, which quickly sink to borderline MVFR/IFR levels around 1,000 feet. As Thursday evening progresses, it is anticipated that rainfall will broaden and become steadier in nature. A few pockets of moderate rainfall are likely with visibility reductions in the 3SM-5SM range.
Persistent MVFR is expected through the end of the period with the likelihood that IFR ceilings settle in after 00z Thursday evening.

Maples

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

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