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FXUS63 KLSX 040925 AFDLSX

Area Forecast Discussion National Weather Service Saint Louis MO 325 AM CST Mon Dec 4 2023

KEY MESSAGES

- Precipitation is expected to be primarily rain this morning, with a low chance (10%) of some snow mixing in. Accumulations and impacts are no longer expected.

- Confidence has increased in a band of light rain (up to a 65% chance) moving through portions of the area Monday night.

- Confidence remains high (90% +) in above normal temperatures by the end the week that will come to an end via our next weather system Saturday.



SHORT TERM
(Through Late Tuesday Night)
Issued at 321 AM CST Mon Dec 4 2023

A compact vort max can be seen in current water vapor imagery moving into southeastern Missouri and southwestern Illinois, its surface reflection seen in surface observations over southeastern Missouri.
A band of precipitation roughly ahead and north of the the low associated with low-level frontogenesis and a deformation zone, respectively, is pivoting through the CWA. Yesterday evening's hi- res guidance was showing that snow rates may overcome just above freezing ground temperatures to produce accumulating snowfall in a corridor from roughly Audrain County, MO east-southeastward toward Fayette County, IL, passing just north of the St. Louis metro.
Observed temperatures over the CWA have trended a couple of degrees warmer overnight than what was forecast, leading to this precipitation falling as all rain thus far. With both temperatures and dewpoints looking to remain just above freezing across the CWA per recent hi-res guidance initializations and a lack of sub- freezing air advecting into the CWA behind the low, the chance of snow appears to be very low (10% or less). Additionally, model soundings show drying in the dendritic growth layer as precipitation winds down, leading to a lack of ice crystals aloft and preventing the development of snow later this morning as precipitation exits the area around 9am. I can't rule out some snow flakes mixing in with the rain at any point this morning, but accumulations (if any)
will be limited to grassy and elevated surfaces at worst, and impacts are not expected.

Low-level flow in the wake of this morning's system will only briefly be northwesterly, limiting the potential for cold air advecting into the CWA. However, cloud cover over a majority of the CWA through the day will help keep temperatures right around climatological normals.

Tonight, a vorticity maximum currently seen in water vapor imagery over Montana will dive southeastward into the Midwest, with its associated surface low passing just north of the CWA and a weak front through the CWA. Spotty, light rainfall is possible (up to 65%) along the front as it quickly swings through the CWA late this evening into the overnight hours. The track of the low northeast of the CWA is unfavorable for wintry weather, and temperatures are expected to remain above freezing overnight. Therefore, only rain is expected with this next round of precipitation.

The axis of an upper-level trough will pass overhead on Tuesday, with deep northwesterly flow and gusty surface winds advecting colder air into the CWA. This colder air is not expected to have much impact on Tuesday's highs, but will push Tuesday night's lows into the 20s, making this the coldest night in the forecast period.

Elmore

LONG TERM
(Wednesday through Sunday)
Issued at 321 AM CST Mon Dec 4 2023

At the start of the period on Wednesday, guidance consensus is that the upper-level trough will be digging southeastward away from the area, leaving the CWA beneath northwesterly flow aloft. At the surface, an area of high pressure will pass through the Middle Mississippi Valley, gradually causing winds to become southerly through the day. This will begin advecting warmer air into the CWA, marking the beginning of the end of our near-normal temperatures we've been experiencing as of late.

On Thursday, upper-level ridging will build into the CWA, with ensemble clusters converging on the phasing of this feature. Mid- level temperatures will warm in turn, combining with the low level southerly flow to push afternoon highs 10-20 degrees above climatological normals. These above normal temperatures will continue into Friday, as ensemble consensus is that the CWA will be beneath southwesterly flow aloft ahead of trough digging southward over the western CONUS.

This period of above normal temperatures will come to an end on Saturday, as a cold front associated with the trough passes through the CWA. The timing of this front is uncertain, as temperature spread on Saturday among ensemble members is about 15 degrees between the 25th and 75th percentile. Additionally, a majority of ensemble members have a surface low traversing the front sometime Saturday into Sunday. The exact timing and track of this low varies among ensemble members, leading to differences in the placement of best rain chances, though recent ensemble initializations consistently place rainfall at least across southeastern Missouri and southwestern Illinois. Additionally, depending on the timing and placement of the low as it passes by or through the CWA, wintry precipitation could mix in with the rain, though a minority of ensemble members (15% or less) have snow falling in the CWA with this system.

Elmore

AVIATION
(For the 06z TAFs through 06z Monday Night)
Issued at 1050 PM CST Sun Dec 3 2023

A vigorous shortwave is tracking through Missouri late this evening with an increase in precipitation showing up on radar mosaics over central Missouri. The leading edge of the precipitation is not making it to the ground just yet, but moisture will increase and eventually lead to light rain. While ceilings are initially VFR, conditions will lower into MVFR as rainfall develops over the next few hours over central Missouri with MVFR favored over KCOU northeast to KUIN. Slightly further south, KJEF may get into a brief period of IFR through early morning.

The greatest impacts will be over metro terminals as temperatures cool overnight into the morning. The system rotates overhead with ceilings dropping to IFR with the potential for rain to mix with or even briefing change to snow between 09z-12z. However, uncertainty in precipitation type remains rather high with the borderline temperatures. Should temperatures cool enough and snow materializes, a few hours of visibility restriction will accompany IFR ceilings.

The system exits to the east between 14z-16z or so with condition gradually improving back to MVFR and VFR later in the afternoon/evening Monday.

Maples

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

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