983
FXUS63 KFSD 121122
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
622 AM CDT Wed Mar 12 2025

.KEY MESSAGES...

- Well above average temperatures return today through Friday.
  Near record highs are possible Thursday, with record warm lows
  likely for some areas Friday.

- Warm, very dry, and breezy conditions will produce elevated
  fire danger Thursday and Friday. Near-critical to critical
  conditions are likely Thursday in areas west of I-29 where
  humidity levels will be the lowest, and a Fire Weather Watch
  will be in effect Thursday afternoon.

- Well-advertised Spring storm is still on track to bring
  impactful weather to the region Friday through Saturday.
  Multiple hazards will include a potential for thunderstorms
  (some strong), high winds, snow and perhaps a brief period of
  mixed precipitation.

- Greatest travel impacts with the upcoming storm will be late
  Friday night and Saturday with a combination of high winds and
  falling snow. Low-moderate (30-50%) probability of snowfall
  exceeding 2 inches (highest across eastern SD/western MN)
  Saturday, coincident with 30-50% probability of wind gusts
  exceeding 50 mph.

&&

.DISCUSSION...
Issued at 433 AM CDT Wed Mar 12 2025

TODAY-THURSDAY: After our brief cooldown yesterday, warmer air
makes a return to the region today, with temperatures peaking on
Thursday as the low level thermal ridge builds into Nebraska and
portions of South Dakota. Deep south-southwest flow and the dry
air mass favor strong mixing. Thus have favored models such as
the HRRR/RAP and the greater extremes of the NBM, which tend to
perform well in these scenarios, for temperatures/dew points and
winds on Thursday afternoon. This results in near-critical to
critical winds and humidity levels in many areas west of I-29
and a Fire Weather Watch will be in effect for the afternoon and
early evening hours. More details in the Fire Weather section of
this discussion.

FRIDAY-FRIDAY EVENING: This will be phase one, the "Spring"
portion of our well-advertised storm system. The thermal ridge
shifts east, which will mean another very warm day across most
of the forecast area. Even with similar low level temperatures,
we may not be quite as warm as Thursday, with increasing cloud
cover/moisture and chances for afternoon precipitation limiting
our full mixing potential. The main focus through this period
will be our convective chances during the afternoon/evening
ahead of the approaching storm system. Still some differences in
timing/track of the low which result in uncertainty regarding
how far west sufficient moisture/instability will spread to
support surface-based storms. However, seeing at least low
(20-30%) joint probabilities of MUCAPE > 500 J/kg and modest
deep layer shear >30kt across portions of northwest Iowa and
southwest Minnesota Friday afternoon/evening. Thus cannot argue
with our far eastern areas being included in a Marginal Risk in
the latest SPC Day 3 outlook for an elevated hail risk. Storms
would likely move north as a broken west-east line along the
leading edge of the dry slot which is currently projected to
move into northwest Iowa after 00Z. At the same time, a well
defined TroWAL will be setting up from from northern Minnesota,
southwestward into eastern South Dakota, and this leads into
phase two, the "Winter" side of the system.

LATE FRIDAY NIGHT-SATURDAY: Transition from Spring back to
Winter may not be completely smooth, with ensembles showing a
low (10-20%) probability for mixed precipitation during the
transition Saturday morning. These probabilities may be on the
low side of reality due to model differences in the system track
and associated low level temperatures, with any undercutting of
the warm air aloft by a shallow cold layer producing a risk for
mixed precipitation. Will hopefully be able to refine the timing
and extent of a freezing rain or sleet risk in the coming days,
but for now will include a mention of the mixed precipitation
and possible icing in our messaging, and expand the NBM`s low
freezing rain probability a bit farther south given the 12/00Z
NAM forecast soundings.

The main concern with phase two will be the combination of very
strong winds (gusts possibly exceeding 50 mph) and any falling
snow which may occur on the cold side of the system, mainly
during the day Saturday. ECMWF ensemble EFI (Extreme Forecast
Index) continues to show a high (80%) potential for gusts near
the extremes of seasonal climatology across the forecast area
Saturday and the broader NAEFS/ECMWF grand ensemble concurs with
a moderate-high (60-80%) probability of 850mb winds exceeding
50kts in areas west of I-29 Saturday morning. The main question
will be how much snow occurs concurrently with these strong
winds and ensembles are still showing a very wide range of
possibilities in this regard. We are seeing low to moderate
(30-50%) probabilities of snowfall exceeding 2 inches across a
good portion of southeast SD/southwest MN, but similar to the
mixed precipitation potential mentioned above, the probabilities
could be muted by differing placement of a relatively narrow
band of heavier snow, so do not discount the potential for some
higher end amounts (25% chance of exceeding 3-4 inches).

While we are still working out the finer details, the important
takeaway for Saturday will be any falling snow with the expected
strong winds will lead to difficult travel, and near blizzard
to blizzard conditions are possible. Please consider adjusting
travel plans accordingly.

SUNDAY-TUESDAY: Little time to examine the outer periods of the
forecast this morning, but it does look like early next week
will feature a brief warming trend with no significant systems
expected at least through Tuesday.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 620 AM CDT Wed Mar 12 2025

VFR conditions and variable winds at or below 10kt are expected
through this TAF period. Very low probability of patchy valley
fog during the early morning hours, and opted to leave mention
out of TAFs/gridded forecast due to low confidence. However,
very localized impacts of industrial steam trapped by a strong
inversion may produce a thin layer of very low "cloud" or minor
visibility restrictions around KFSD early this morning.

&&

.FIRE WEATHER...
Issued at 433 AM CDT Wed Mar 12 2025

Much warmer air returns to the region by Thursday, with strong
mixing in a very dry air mass resulting in near-critical to
critical fire weather conditions in areas west of I-29 Thursday
afternoon. A deep south to southwest wind profile will promote
mixing as high as 8000ft AGL, especially in areas west of the
James River Valley toward south central South Dakota. The deep
mixing could drop dew points into the 20s as temperatures warm
to near 80 degrees. This yields relative humidity values as low
as 15-20 percent west of the James River Valley, and as low as
25 percent as far east as Salem South Dakota to Ponca Nebraska.
Add to this gusty southerly winds, with gusts as high as 30 mph
during peak mixing, and near-critical to critical fire behavior
will be possible. A Fire Weather Watch has been issued from 1 PM
to 8 PM Thursday, roughly along and west of a line from Iroquois
to Salem to Vermillion SD and Ponca NE. It should be noted that
while humidity levels will be slightly higher farther to the
east, similar to slightly stronger winds will still result in
Very High grassland fire danger.

Friday will be another warm and breezy day, but humidity levels
will be on the increase ahead of an approaching storm system.
That said, we are still looking at elevated (High to Very High)
fire danger across the region Friday.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Fire Weather Watch from Thursday afternoon through Thursday
     evening for SDZ038-050-052>054-057>061-063>066-068>070.
MN...None.
IA...None.
NE...Fire Weather Watch from Thursday afternoon through Thursday
     evening for NEZ013.

&&

$$

DISCUSSION...JH
AVIATION...JH
FIRE WEATHER...JH