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FXUS63 KLSX 040108 AFDLSX

Area Forecast Discussion...Updated National Weather Service Saint Louis MO 708 PM CST Sun Dec 3 2023

UPDATE
Issued at 639 PM CST Sun Dec 3 2023

A potent shortwave over northeast Kansas will track east- southeast through Missouri late tonight. Hi-res guidance began hinting at the potential for a narrow band of frontogenetically- driven wet snow late tonight into early Monday morning. HRRR/RAP began hinting at the potential around 20z this afternoon and aside from slight north/south adjustments in placement, the idea has been consistent with wet snow impacting areas along and just north of I-70 in central Missouri, running east-southeast skimming the St. Louis Metro area and moving east along I-70 into Illinois. The best timeframe is from approximately 06z-09z over central Missouri to around 09z-12z over the metro area and 12z-14z into eastern sections of the CWA.

The main challenge is assessing the overall impact with surface temperatures in the mid-30s during the event. However, some indications show a brief period (1-2 hours) of heavy, wet snowfall approaching rates of 1 inch per hour. If this occurs, timing will impact the morning rush hour around the metro and points north.
Despite road temperatures, should the brief period of heavy, wet snow occur, it could overcome road temperatures to result in a quick hit of slushy, minor accumulations. The current forecast includes accumulations of 1/2 an inch with locally up to an inch not out of the question, primarily from eastern Audrain County eastward through Macoupin and Montgomery Counties in Illinois. Be advised that if trends continue and/or temperatures cool even a couple degrees from the current forecast, additional updates may be needed.

Maples

KEY MESSAGES

- Light rain likely (up to 75%) across the CWA tonight with a few snowflakes mixed in, mainly over portions of northeast Missouri and west central Illinois.

- Another round of light rain (up to 40% chance) Monday night.

- Near normal temperatures are expected through Wednesday with high confidence (90% +) in above normal temperatures Thursday and Friday.



SHORT TERM
(Through Late Monday Night)
Issued at 259 PM CST Sun Dec 3 2023

The next in the series of shortwaves will move through the area tonight. The latest water vapor imagery indicates that the shortwave is over the central Plains as of 20z. This shortwave, as well as it's associated surface low, will continue to slide southeast into the area this evening. Mid and high clouds are already beginning to stream into Missouri ahead of it. There are still some minor differences among the latest CAM models of the location/track of the surface low as it moves through the region.
However, all are a bit further south than previous runs, thus shifted the axis of highest POPs a bit further to the southwest, from Moberly, MO through the St. Louis metro area and into south central Illinois.

As far as precipitation type, the majority of the region will only get rain. Some of the latest CAMs are indicating some weak MU CAPES (100-250 J/kg) mainly along the track of the surface low. So we could hear a few claps of thunder as the rain moves through, but not mentioning it in the grids at this time. A few of the CAMs and ensembles are indicating that the surface low will deepen with stronger dynamics aloft. Thus as precipitation falls through the column could see wet bulb temperatures lower enough for rain to transition to mostly snow in portions of northeast Missouri, but confidence is low with this scenario as surface temperatures and dewpoints will remain above freezing.

The rain will taper off by mid morning on Monday. Since we remain in northwest flow aloft, temperatures will be near normal with highs in the low 40s to low 50s. Then the last in a series of shortwaves will slide southeast through the region, though the best dynamics with it will be to our northeast. So just kept chances of light rain (up to 40%) across portions of northeast Missouri and west central/ southwest Illinois Monday night.

Byrd

LONG TERM
(Tuesday through Next Sunday)
Issued at 259 PM CST Sun Dec 3 2023

Even with northwesterly flow lingering aloft, CAA advecting into the region will be rather meager, thus temperatures will remain near normal Tuesday and Wednesday. In the meantime, as the upper level trough slides off to the east, ridging will build aloft with warmer conditions moving into the area once again. The latest ensembles still indicate that the 10th percentile of ensemble guidance for surface temperatures remains above seasonal normals, leading to continuing high confidence (90% +) in above normal temperatures Thursday and Friday.

Beyond that, upper level ridge breaks down and flow becomes zonal.
The next chance for precipitation will be next weekend. However, there are timing, location and strength differences among the latest deterministic and ensembles, so stuck with the latest NBM solution which has chances (up to 50%) of rain Friday night through Sunday.

Byrd

AVIATION
(For the 00z TAFs through 00z Monday Evening)
Issued at 549 PM CST Sun Dec 3 2023

There has been considerable change to the potential impacts late tonight into early Monday morning. Hi-res guidance has been signaling the potential for a narrow band of wet, heavy snow developing at the northern periphery of a strengthening area of low pressure as it pivots through central and east-central Missouri and continues into southwest Illinois. Given the data that supports this potential is only coming to light over the last few hours, rain has been maintained in the current update.
However, if this trend continues, later updates will need to incorporate -SN and/or -RASN with IFR ceilings and visibility reductions lower than current forecasts. Should this come to fruition, the prime time would be somewhere in the 08z-12z window with snow potential around 2-3 hours around the metro terminals with updates provides as confidence increases.

Should -RA be the primary precipitation type, MVFR/IFR ceilings will remain the main impact with minor visibility restrictions in pockets of more persistent rainfall. Beyond that, conditions improve through MVFR and eventually VFR Monday afternoon/evening.

Maples

PRELIMINARY POINT TEMPS/POPS
Saint Louis 36 48 37 47 / 90 70 10 5 Quincy 33 44 35 42 / 50 20 40 0 Columbia 34 47 35 45 / 60 10 10 0 Jefferson City 34 49 36 47 / 40 10 10 0 Salem 34 47 34 48 / 80 80 30 10 Farmington 36 49 33 50 / 40 20 5 0

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

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