Spencer, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

391
FXUS63 KFSD 150108
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
708 PM CST Thu Nov 14 2024

.KEY MESSAGES...

- Dense fog appears likely tonight, and may form quickly after
  sunset. Advisories may be needed. Fog may linger well into
  Friday morning.

- Dry with above normal temperatures continuing for Friday
  through Sunday.

- Widespread rain likely Monday night into Tuesday. Conditions
  likely become cold enough for mostly snow later on Tuesday
  into Wednesday. Better chances for accumulating snow will be
  west of I-29 and north of I-90, but still 6-7 days out so
  confidence in details is still fairly low.

&&

.UPDATE...
Issued at 704 PM CST Thu Nov 14 2024

As expected, fog is rapidly beginning to develop after sunset as
the low-lvl inversion settles. Latest nighttime microphysics
RGB provides a good view of the moisture discontinuity across
the CWA produced by a stalled frontal boundary. Currently
western edge of shallow boundary layer moisture resides nearly
along and James River from Huron to Mitchell to Yankton with
surface dew points east of this area holding in the middle to
upper 30s. A light southeast wind should hold this moisture in
place, as as temperatures cool at the surface fog should
continue to expand and/or existing stratus should lower to the
ground.

Will start with a more narrow dense fog advisory where current
development has taken place and fog will advect. However,
further expansion will be likely this evening, and will watch
trends.

&&

.DISCUSSION...
Issued at 248 PM CST Thu Nov 14 2024

A broad area of stratus has held tight in areas just to the east of
the I-29 corridor through the morning, though is beginning to show
signs of eroding along the western edge this afternoon. Latest HREF
would indicate these clouds continuing to shift to the east through
the remainder of the afternoon and evening. Otherwise, our area will
be under the influence of weak surface high pressure oriented
through MN and IA overnight. In the weak surface pressure gradient
winds will become very light tonight, and with model soundings
indicating a strong inversion overhead there will be a potential for
fog development across the area. Latest HREF would show moderate to
high probabilities (40-70%) of visibilities less than 2 miles by late
tonight through Friday morning - with the highest probabilities
residing east of I-29. Soundings indicate the inversion holding
strong through late morning/early afternoon on Friday, so any fog
that develops may persist well into this time period. It will be
another cool night with lows in the upper 20s to lower 30s.

Fog and potential stratus is expected to decrease by Friday
afternoon (though like today, may hold longest in areas to the east
of I-29). This will keep highs a little cooler over that area - in
the upper 40s to lower 50s, while to the west highs will top out in
the mid and upper 50s. Southeasterly winds will increase on Friday
in a steepening surface pressure gradient as a surface trough
deepens over the western High Plains. This will result in gusts of
20 to 25 mph in the afternoon.

The aforementioned surface trough will slide eastward across the
region on Saturday. Out ahead of this feature, strong warm air
advection will bring warmer temperatures, though mixing will be
somewhat inhibited by a lingering inversion so highs will remain in
the mid 50s to lower 60s. With cold air advection behind the system,
Sunday will be a littler cooler with highs in the upper 40s to mid
50s.

Still looking at the potential for a possibly impactful storm to
affect the region by the middle of next week as low pressure over
the Desert Southwest lifts into the Central and Northern Plains on
Monday and Tuesday - possibly lingering over the region even into
Wednesday and Thursday. Although models are converging on the track
of the system, differences remain on the evolution of the system as
it pushes through the Northern Plains and Upper Midwest on Tuesday
through Thursday. The current model trends would suggest rain
developing across the area on Monday afternoon and Monday night,
then decreasing on Tuesday with the potential for rain changing to
light snow sometime on Tuesday as colder air feeds into the area.
This light snow may then linger through midweek depending on how the
system evolves. Ensembles would suggest a decent rainfall is
possible for Monday into early Tuesday, with a 50-80% probability of
0.5" of rainfall and a 30-60% of receiving 1" through the period.
Any snowfall amounts later in the period remain uncertain due to
model differences. Another concern with the system may be the
potential for strong winds as the system wraps up. Latest ensembles
suggest a 30-50% probability of gusts greater than 40 mph west of I-
29 by Wednesday. The system does pull cooler air into the region
with highs in the 40s on Tuesday dropping to mostly 30s by Wednesday
and Thursday.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 521 PM CST Thu Nov 14 2024

Fog appears likely to develop later this evening and persist
deep into the morning hours of Friday. Stratus that struggled to
dissipate Thursday is already showing signs of lowering as the
inversion overhead settles lower.

Additional fog will develop as temperatures cool with dew points
in the upper 30s to lower 40s. Current expectations are fairly
widespread dense fog is likely through at least mid-morning
Friday, with slow improvement by mid-day.

Winds remain light to nearly calm at times which should allow
fog to stay in place longer.

Once fog and stratus burns off mid-day Friday, VFR conditions
are then likely.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Dense Fog Advisory until 10 AM CST Friday for SDZ039-040-055-
     056-062.
MN...Dense Fog Advisory until 10 AM CST Friday for MNZ071-089-097-
     098.
IA...Dense Fog Advisory until 10 AM CST Friday for IAZ001-002-012-
     013.
NE...None.

&&

$$

UPDATE...Dux
DISCUSSION...JM
AVIATION...Dux