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FXUS63 KARX 072345 AFDARX

Area Forecast Discussion National Weather Service La Crosse WI 545 PM CST Thu Dec 7 2023

KEY MESSAGES

- Continued unseasonably warm for Friday.

- Precipitation overspreads the area Friday night into Saturday.
Rain is expected initially. With above freezing temps ahead of the first round of precip Friday night/Saturday morning...snow may mix in or change over to all snow for some Saturday morning. Accumulations will be less than a half of an inch.

- Precipitation will change over to all snow Saturday afternoon with the possibility of up to an inch of accumulation over parts of southeast Minnesota/western Wisconsin.

DISCUSSION
Issued at 306 PM CST Thu Dec 7 2023

Overview:

Water vapor satellite imagery/heights showed a broad 500mb ridge over the Mississippi River Valley with a trough over the Pacific Northwest and the northern Rockies. At 12Z, the MPX precipitable water value was 0.67" or 200% of normal for this time of year. The moisture. Morning temperatures were starting out mild in the mid 20s to lower 30s. Surface low pressure was over western North Dakota with a warm front trailing southeast toward the local area.
The 850m thermal ridge was centered over the Plains with the +12 deg C air making it into the forecast area. Temperatures at 19Z were in the mid 40s to the mid 50s with the warmer highs toward Rochester, Dodge Center, and Austin.

Continued unseasonably warm for Friday:

Through Friday, the mid-tropospheric trough over Montana with swing east and deepen over the northern Plains into northern Minnesota/ Ontario. We remain in the warm sector, however through today and tonight, we see cooling of 850mb temperatures from +12 deg C to +6 deg C Friday morning and +3 deg C by 09.00Z with the lowering heights. The 07.00Z EFI again has 50 to 60% of the members above the M-Climate maximum for Friday, with no Shift of Tails. The records for Friday are 60 (1946) for LSE and 58 degrees (1946) at RST. Again, lows will start out mild in the 30s with highs in the upper 40s and lower 50s with no records forecast.

Friday night through Sunday

Upper level ridge breaks down and trough digs over the Northern Plain States/Upper Great Lakes Region Friday night into Sunday.
Pieces of energy embedded in the southwesterly flow aloft will track over the forecast area Friday night into Saturday evening.
The first piece of energy moves over the forecast area...mainly after 00z Saturday. Decent isentropic lift/forcing is associated with this piece of energy. Main issue will be change over of precipitation from rain to snow Friday night into early Saturday morning. With bufkit soundings showing above freezing temps near the surface and just aloft...precipitation type over the southeastern half of the forecast area should be all rain initially. There are hints from the 07.12z NAM/HRRR indicating diabatic cooling through the atmosphere column and allow snow to mix in or change over to all snow after 09z Saturday. Though with the warm ground and temps in the lower to middle 30s over this area...snowfall amounts will be less than a half of an inch through 12z Saturday if this occurs.

Next focus is the upper level trough begins to close off over western Wisconsin during the day Saturday. This will filter colder air over the forecast area and bufkit soundings indicate all precipitation will be snow Saturday afternoon into the evening.
The stronger lift/forcing is expected to be over southeast Minnesota/western Wisconsin under the upper level trough. Snowfall amounts of up to 1 inch is possible over these areas with lesser amounts elsewhere.

Drier and colder temperatures are expected for Sunday with highs mainly in the upper 20s to lower/middle 30s across the forecast area.

Main forecast concerns Sunday night through Thursday are temperatures. Upper level trough over the northeast United States/Upper Great Lakes region will allow for northerly flow aloft over the forecast area through much of the period. This should advect a cooler airmass over the Upper Great Lakes region with both the latest 07.12z GFS/ECMWF suggesting 925mb temperatures dropping to minus 5 to minus 10 degrees celsius by early next week. The deterministic models indicate an upper level ridge over the Rocky Mountains to build into the Upper Midwest/ Northern Plain States late in the period. However, ensemble members from the latest GEFS/ENS does show issues on the strength of the upper level trough/ridge over the Northern Plains States/Upper Great Lakes region late in the period. This could potentially delay warmer temperatures filtering into the forecast area late in the period/late next week. Temperatures Monday through Wednesday are expected to be near or slightly below normal with highs mainly in the upper 20s to 30s. Warmer than normal temperatures possible Thursday with highs in the 30s to lower/middle 40s.

AVIATION /00Z TAFS THROUGH 00Z SATURDAY/
Issued at 545 PM CST Thu Dec 7 2023

VFR conditions are expected through the TAF period with few to sct high clouds. Opted to hang onto a sct050 group at KLSE for the overnight hours with probabilistic guidance still showing some low probabilities of some low clouds sneaking in but confidence in this solution continues to remain low (under 30% chance). Otherwise, some LLWS will be present early in the TAF period at KRST with model soundings showing winds nearing 40 kts at 2kft for a few hours this evening. Winds will remain fairly blustery at around 10-15 kts from the south shifting to southwesterly overnight and into the morning hours.

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

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