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FXUS63 KFSD 081125 AFDFSD

Area Forecast Discussion National Weather Service Sioux Falls SD 525 AM CST Fri Dec 8 2023

KEY MESSAGES

- Not as warm, but still well above normal temperatures today.

- Periods of gusty west to northwest winds in excess of 30-35 mph today through Saturday. Wind gusts will remain below advisory levels (45 mph) today, but some areas could approach those speeds early Saturday.

- Quick-moving band of light snow late tonight into Saturday morning. Localized areas greater than an inch are possible, and strong winds with the falling snow may create difficult travel at times. Some uncertainty in how far southeast light accumulations will extend.

- Dry conditions return Saturday night through much of next week, with temperatures near to slightly above normal.

DISCUSSION
Issued at 352 AM CST Fri Dec 8 2023

Gusty west winds will prevail across the region today, ushering in the first shot of cooler air, though high temperatures in the upper 40s to mid 50s today are still well above normal for early December.

HRRR/RAP forecast soundings do show ~40kt winds at the top of the mixed layer this morning, mainly west through north of Sioux Falls.
Cannot rule out an isolated gust that approaches advisory criteria in these areas. However, gusts to 45 mph are not expected to be widespread or persistent, so opted to hold off on any wind headlines for today. These winds, combined with dry vegetation will lead to another day of High to Very High grassland fire danger, despite moderate humidity levels only falling into the 40-50% range.

The gusty winds really do not subside much tonight as secondary shot of cold advection this evening enhances low level mixing, and could even see spotty sprinkles work into the Highway 14 corridor this evening as a shallow unstable layer develops above 850mb.

Main focus for precipitation chances, though, will be on approaching upper level trough associated with the aforementioned secondary cold front, which swings through the northern Plains tonight-Saturday.
With column fully below freezing before precipitation begins, should see only light snow with this system, with the main question being "how much?". Latest ensembles have trended lower with QPF amounts from the previous forecast, with ECENS/GEFS probabilities of 0.10" or more liquid equivalent have decreased to 20-40% across most of the Highway 14 corridor and extending south toward I-90 in areas west of Sioux Falls (with much higher probabilities farther north toward the Sisseton Hills). HRRR probabilities are similar, perhaps a bit higher in the 40-50% range. This leads to a slight decrease in snowfall amounts from previous forecast, though pockets of an inch or more are still possible in portions of southeast SD/southwest MN by midday Saturday. Another trend seen in hi-res models tonight is for the band of snow to break up/dissipate as it drifts east into northwest Iowa Saturday morning, and as a result have decreased precipitation chances a little more quickly through the late morning to early afternoon.

Despite the decrease in expected snowfall amounts, soundings still show 40+kt atop the mixed layer while the snow is falling west of the I-29 corridor. Ensembles support the strong winds with better than 50% probability of gusts in excess of 40 mph, also focused west of I-29 Saturday morning. With no existing snow cover and new snow generally less than an inch, anticipate that any blowing snow will primarily be a concern during periods of falling snow Saturday morning, but anyone with travel plans late tonight into Saturday, especially west and north of Sioux Falls, will want to monitor the latest road and weather conditions and forecast before heading out.

Northern Plains remains in northwest flow through midweek, which should provide for dry weather with near to slightly above normal temperatures, though this may be dependent on amount of new snow cover in some locations. By Thursday, seeing a trend toward an upper ridge building back into the Plains, which should push temperatures a bit farther above normal by the end of the week.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/
Issued at 525 AM CST Fri Dec 8 2023

Stratus has expanded southward across east central SD/southwest MN early this morning, with a period of MVFR ceilings expected for KHON/KFSD before the stratus layer thins and becomes SCT after 14-16Z.

Gusty west winds are expected throughout the TAF period, turning to the northwest with the passage of a cold front after 09/03Z/ Daytime gusts as high as 25-30kt east of the James River will diminish only slightly after sunset. Near and west of the James River Valley, stronger gusts as high as 35kt will be more likely.

An area of light snow will also follow the cold front, producing MVFR-IFR visibility at times after 09/06Z.

FSD WATCHES/WARNINGS/ADVISORIES
SD...None.
MN...None.
IA...None.
NE...None.

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