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FXUS63 KFSD 040128 AFDFSD

Area Forecast Discussion National Weather Service Sioux Falls SD 728 PM CST Sun Dec 3 2023

KEY MESSAGES
-Areas of dense fog expected overnight into early Monday, mainly east of the James River, and possibly expanding east into the I-29 corridor and portions southwest Minnesota/northwest Iowa.

-More light snow chances (30%-40%) return Monday afternoon mostly along the Hwy-14 corridor and Buffalo Ridge. However, accumulation amounts should range from a trace to a few tenths.

-Elevated fire weather concerns return Monday afternoon mostly along and west of the James River.

-Above normal temperatures will continue for the remainder of the week with the warmest conditions expected on Wednesday & Thursday.

UPDATE
Issued at 725 PM CST Sun Dec 3 2023

While we still have a widespread low cloud deck east of the James River this evening, web cam imagery shows the coverage of dense fog has increased quickly after sunset along and west of these low clouds. Have issued a Dense Fog Advisory for a wedge from roughly the James River Valley into the Highway 81 corridor through early Monday morning.

Expect coverage of the lowest visibility to change through the overnight as a light westerly flow becomes more prevalent, and additions/subtractions to the advisory area are possible.

DISCUSSION
Issued at 235 PM CST Sun Dec 3 2023

Rest of the Evening/Tonight:

A dreary day continues! Taking a look at satellite imagery, a combination of low-level stratus and lingering patchy to dense fog have limited our mixing and kept things on the drearier side for most of the day. Expect the drearier conditions to continue for the remainder of the afternoon before things clear our overnight with the passage of a weak surface boundary this evening. With lighter winds expected overnight with the arrival of a weak ridge, expect another round of fog to develop in areas along the Hwy-81 corridor and eastwards. Similar to today, some fog could be locally dense with visibilities between 1-2 miles or less possible at times especially in open areas and along the our river valleys. Otherwise, depending on how soon things can clear out; temperatures will gradually drop into the low to mid 20s for the night.

The Extended Forecast (Monday-Saturday):

Heading into the start of the work week, more light snow chances return as a clipper system moves into the region from the northwest along with a surface cold front. Looking aloft, mid-level warm air advection (WAA) begins to strengthen ahead of the shortwave as more moisture advects into region along the cold front. This along with strong PVA and 850:750 frontogenesis should be enough to get trigger a few flurries to light snow showers Monday afternoon and evening along the Hwy-14 corridor and Buffalo Ridge. As far as amounts go, the GEFS, Euro, and Grand ensemble members show medium to high confidence that amounts should varying between a trace to a few tenths with the higher amounts expected along a Brookings to Windom, MN line. Nonetheless, snowfall amounts will remain below an inch for most of the area with most of the activity clearing out before sunset.

Otherwise, the SPG will tighten behind the cold front causing northwesterly winds to become breezy to strong especially west of the James where wind gusts between 35-45 mph will be possible. After further discussion with neighboring offices, we've decided to hold off on any wind headlines for now at least until the next round of guidance arrives. Lastly, the combination of mild and breezy conditions will lead to some elevated fire weather concerns west of the James Monday afternoon. However, with the cold front expected to pull through in the afternoon; there is some uncertainty as to if we can get clear enough to get to those milder conditions in the previously outlined areas. Unfortunately, its just one of those situations where we'll just have to wait and see. Shifting gears to temperatures, expect daily highs to peak in the upper 30s to low 50s with the warmest conditions expected in our far western counties.

As cooler air continues to funnel into the area on Tuesday, a pattern shift is on the horizon as a strengthening upper level ridge arrives to the region bringing a much needed dose of mid-level WAA.
This will work to bring our 850mb temperatures back into the 8-12 degree C range on both Wednesday and Thursday which is between the 97.5th and 99th percentile of climatology. With this in mind, a small warming trend will set up as daily highs shift from upper 30s to lower 40s on Tuesday to the low 50s to mid 60s by Wednesday and Thursday which is unseasonable warm for this time of year! Depending on if these temperatures actually come to fruition, there is a decent chance that we could tie or break a few new record across the area on Thursday and Friday. Otherwise, a cold front will pull through Thursday night shifting our temperatures back towards the colder side as we head towards the weekend.

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/
Issued at 553 PM CST Sun Dec 3 2023

IFR-LIFR stratus and/or fog persists across much of the forecast area to start this period, especially along and east of the James River Valley. With loss of heating/mixing in the stratus-free areas farther west, have seen rapid development of fog near/west of the James River Valley. This could lead to a period of VLIFR visibility at KHON this evening, and overall low confidence in expected trends at KHON until westerly winds become more prevalent and increase later tonight/early Monday.

Farther east, have shifted to a more pessimistic outlook for much of this TAF period, as 1recent RAP runs and HRRR ensemble show moderate to high probabilities of IFR stratus persisting through at least daybreak Monday, with greater potential for improvement by late morning as stronger mixing commences.

Locations west of the James River toward south central SD could see increasing west-northwest winds after 04/18Z, with gusts in excess of 30-35kt possible. These stronger wind gusts are not expected to impact TAF locations within this TAF period.

FSD WATCHES/WARNINGS/ADVISORIES
SD...Dense Fog Advisory until 9 AM CST Monday for SDZ038>040- 053>055-059>061-065-068.
MN...None.
IA...None.
NE...None.

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