Vermillion, South Dakota 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

293
FXUS63 KFSD 221119
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
519 AM CST Sat Feb 22 2025

.KEY MESSAGES...

- Well above normal temperatures are expected this weekend into
  next week. The degree of warmth may at least partially depend
  on snow melt/potential for resultant fog development. However
  highs in the 50s will be possible Sunday through Tuesday with
  some 60s in central SD.

- Watch for refreeze of melted snow causing scattered slick
  spots as nighttime temperatures fall back below freezing.

- Predominately dry conditions will persist through next week.
  Low (20%-30%) chances for light rain Monday (mainly north of
  I-90) and again Tuesday night through Wednesday.

&&

.DISCUSSION...
Issued at 359 AM CST Sat Feb 22 2025

Focus for much of this forecast period will be on temperatures.
However, unlike last week when we were seeing temperatures 25 to
35 degrees below seasonal normals, this weekend into at least
the middle of next week will feature readings 15 to 25 degrees
above normal!

The arctic high pressure responsible for the recent cold snap
has weakened as it moved into the southeast CONUS. This has been
replaced across the northern Plains by a much more mild westerly
low level flow which looks to remain in place through at least
Tuesday. 850-925mb temperatures warm into the single digits above
zero across the forecast area by this afternoon with moderate
west-southwest winds 10-20 mph. Temperatures the past couple of
days have been exceeding the NBM by 2-5F, perhaps indicative of
a lingering cold-bias in the NBM coming out of our recent deep
freeze. Thus have nudged temperatures a bit more toward the 75th
percentile, which looks close to mixing to around 925mb. This
yields highs generally in the 30s near/east of I-29 today with
40s to the west.

The warmest low level air settles over the forecast area Sunday
through Tuesday. This along with deep westerly flow which favors
deeper mixing, supports continuing to push our high temperature
forecast a few degrees above the NBM. Areas along/east of Hwy 81
(Arlington to Yankton SD) may still have lingering snow cover
Sunday which could inhibit warming slightly, but 40s still seem
achievable east of I-29, while areas west of the James River
stand a good shot at seeing 60 degrees. As we continue to lose
snow, highs in the 50s are expected to expand eastward for Monday
and Tuesday.

Two nighttime concerns crop up as we melt our snow over the next
few days. First, adding this moisture into the boundary layer
could result in late night/morning fog development. This would be
more favored where we see mostly clear skies and light winds to
optimize radiational cooling and would expect to see it mainly in
low lying/valley areas. Westerly flow is not really supportive of
the fog persisting through the entire day, though, so if fog does
develop, expect it to burn off by mid-late morning. Second, any
snowmelt running across untreated roads/sidewalks/driveways could
refreeze as temperatures fall back below freezing at night. Keep
this in mind if you have early morning travel plans in areas where
there is currently ample snow to melt in the days ahead.

Meanwhile the mid-upper levels will be dominated by a broad but
relatively flat ridge over the Rockies, placing our region in weak
northwest flow through which various weak waves will move through
the first half of next week. The first such wave slides through
tonight, with limited low level moisture leading to just a modest
increase in mid-high level clouds. Wave #2 arrives on Monday and
again moisture seems to be lacking. However, this wave is slightly
stronger and could produce some sprinkles/very light showers,
with a low (<20%) chance of measurable rain north of I-90.

Perhaps of greater note on Monday will be the increasing low-mid
level winds accompanying the wave. Both the GFS/NAM continue to
show 40+kt west-northwest winds at 850mb, though the strongest
winds don`t reach our area until late Monday afternoon. This is
in line with latest ensembles indicating 30-60% probability of
850mb winds exceeding 40kt by 00Z Tuesday, but much lower (<20%)
probability for most areas by 18Z Monday. That said, even 30+kt
winds at this level, which could be tapped into with expected
mixing during peak heating in the afternoon could result in wind
gusts pushing 35 mph Monday afternoon. NBM was a little shy of
this, so blended some NBM90th percentile into our winds Monday
afternoon. With snow cover in our far western counties likely to
be gone by then, this will result in areas of elevated grassland
fire danger west of the James River.

A third mid-upper level wave arrives Tuesday night, moving quickly
off to the southeast by midday Wednesday. Lower level moisture
with this wave appears more sufficient, however latest models are
now weaker with mid-level moisture. That combined with the speed
of the wave will limit rainfall potential. Ensembles are showing
moderate (50-60%) probabilities for exceeding 0.01" rainfall over
the 24-hour period ending 18Z Wednesday. However, the probability
for exceeding 0.10" over this same period is less than 20%, so any
rainfall should be very light.

The latter half of next week will see the upper level pattern
shift slightly toward a more dominant northwest flow as the upper
ridge builds further over the West Coast. This will allow cooler
air to filter back into the northern Plains at times from Thursday
onward, though not seeing any hints of temperatures anywhere near
as cold as this past week. More likely just "not as warm", but
still a potential for above normal temperatures as we head toward
the end of February.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 519 AM CST Sat Feb 22 2025

VFR conditions are expected to dominate through the TAF period.
Hi-res ensembles indicate a 20-30% probability for fog development
during the latter half of this TAF period, mainly over northwest
Iowa after 23/03Z. Confidence is low given an expected increase in
mid-high level cloud cover overnight, so have not included any fog
or sub-VFR visibility in the forecast at this time. However, may
have to watch for spotty development toward daybreak if the higher
clouds clear early enough. South/southwesterly winds will be on
the lighter side through the period.

&&

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

&&

$$

DISCUSSION...JH
AVIATION...JH