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FXUS63 KARX 022345 AFDARX

Area Forecast Discussion National Weather Service La Crosse WI 545 PM CST Sat Dec 2 2023

DISCUSSION
(This evening through Saturday)
Issued at 310 PM CST Sat Dec 2 2023

Key Messages:

- Forecast remains on track, with a system expected to bring snow accumulations to the area tonight. Snowfall amounts of 1 to 2 inches are possible over southwest/central Wisconsin with lesser amounts northwest of these areas.

- Another system moves through Monday night into Tuesday bringing light snow chances.

- Continue to see a trend towards increasing temperatures into the 40s for the end of the week.

Tonight Into Monday:

Latest water vapor satellite imagery shows upper level trough over the High/Northern Plains states and shortwave ridge over the southeast United States. The upper level trough will dig into the Central Plains States tonight into Sunday. First piece of energy ejects out of the upper level trough and tracks into the Upper Great Lakes region tonight. Decent forcing and lift is in association with this piece of energy with the better forcing/lift mainly over southwest/central Wisconsin. Temperatures at onset of precipitation over southwest Wisconsin will be above freezing per bufkit soundings and latest 20z metars over this area. Initial precipitation type may be rain
Then the rain will quickly change over to all snow
as cooler air from aloft mixes down to the surface.

02.12z GFS/NAM indicate precipitable water values near a half of an inch tonight/Sunday over the far southern forecast area. With the amount of lift/forcing...QPF amounts of a tenth to two tenths are expected over southeast/central Wisconsin. These QPF amounts are slightly higher by a couple of hundredths from previous shifts...as the latest hi-resolution models and deterministic 02.12z NAM showed an uptick in qpf. Given the qpf amounts and snow ratio at 10 to near 8:1. This would give snowfall amounts of 1 to 2 inches over southwest/central Wisconsin with lesser amounts northwest of these areas.

The other caveat with this piece of energy is the potential for freezing drizzle across parts of the area this evening. A few bufkit soundings suggest no ice in the cloud late this evening across parts of the north central Wisconsin and far southwest Wisconsin.
However...there is weak lift through much of the atmosphere column.
This lift may allow for ice to be generated into the cloud and be all snow or rain depending on temperatures near the surface. For now...will leave mention of freezing drizzle out and let later shifts to monitor.

Monday Night Into Tuesday:

Ensemble model guidance from the GEFS and ECMWF have been consistent in showing an upper level shortwave trough dropping from Canada and southeast across the Upper Mississippi River Valley Monday night.
The most noted difference from previous forecasts is the 02.12Z runs of the GEFS and ECMWF ens show a slight trend southward in the track of the surface low pressure and in turn a slight shift southward in the associated precipitation (though still remaining across the forecast area). Ensemble solutions still suggest fairly light QPF, with the Grand Ensemble made up of the GEFS, ECMWF, and CMC ens showing probabilities of near 25-45% for greater than or equal to 0.05 inches across the area. With that, chances for snowfall greater than or equal to 1 inch continues to remain a bit lower in the current ensemble guidance (< 20%), with the CMC ens being the most excited (mainly across southern WI). As the previous discussions have noted, the lighter QPF has resulted in lower precipitation chances in the NBM in the longer term. Thus, have maintained previous thinking and increased snow chances (20-50% W of MS River, 40-60% E) Monday night.

Tuesday Night Through The End Of The Week:

The aforementioned system looks to make its exit southeast early Tuesday, with upper level ridging building over the Plains. Though, the upper level flow pattern then looks to become more zonal by Thursday. Though spread begins to increase a bit towards the end of the week, a fairly consistent signal among guidance suggests an increasing trend in temperatures towards the 40s for the area and maybe even low 50s for a few locations to the south. Otherwise, an upper level shortwave trough is currently forecast to pass to the north of the area on late Thursday into Friday. Right now, highest precipitation chances look to remain north of the area with only small chances (10-20%) clipping our far northern counties. Some indication for a western U.S. upper level trough to move inland through the weekend. However, there remain a few details to still work out this far out, but will continue to monitor in the coming days.

AVIATION
(For the 00Z TAFS through 00Z Sunday evening)
Issued at 544 PM CST Sat Dec 2 2023

Conditions steadily deteriorate through the evening with widespread IFR/LIFR levels by 06Z areawide owing to a combination of low ceilings, snow (east of the Mississippi River mainly), and fog. These conditions linger through the morning and then slowly improve through the afternoon, though confidence is lower on just how quickly conditions will improve. Winds will be light and variable tonight, backing more towards the north on Sunday.

ARX WATCHES/WARNINGS/ADVISORIES
WI...NONE.
MN...NONE.
IA...NONE.

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