587
FXUS63 KFSD 070944
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
344 AM CST Thu Nov 7 2024

.KEY MESSAGES...

- Weather remains benign into Friday.

- Warm on Friday with highs 5-10 deg F above normal in the upper 50s.

- The next risk for widespread light rain arrives late Friday
  into Saturday, exiting by Sunday morning. Projected rainfall
  amounts have been raised slightly with this cycle, though 00Z
  ensemble guidance shows high (80-90%) probability for 0.25" or
  more south of I-90. No snow is expected.

&&

.DISCUSSION...
Issued at 342 AM CST Thu Nov 7 2024

Morning water vapor imagery shows the region is under convergent
flow aloft as a strong upper level low forms over the southwestern
CONUS, with surface observations revealing a weak surface high
pressure sliding eastwards towards the region from our west.
Portions of NW Iowa are seeing some patchy fog as of 2 am, though as
the surface high continues to drift eastwards most guidance suggests
the fog will be dissipating or gone completely by sunrise.

As the surface high pressure slides by to our south before further
moving eastwards throughout the afternoon hours, southwesterly
return flow sets up across the region. This will bring marginally
warmer temperatures back into the region for Thursday, with
afternoon highs in the upper 40s to mid 50s. Areas along and north
of I-90 will likely see some elevated winds, though gusts should
only peak up into the teens and lower 20s. Upper level ridging will
continue to push the surface high off to our east overnight into
Friday, which aside from some possible river valley fog, will
continue the benign conditions across the region. Overnight lows
into Friday in the mid 20s to lower 30s.

The wave ejects out of the base of the trough on Friday, sending a
surface low pressure onto the central plains throughout the day.
With mid-level heights rising before the upper level low reaches the
area, Friday will start off with mostly clear skies and afternoon
highs in the mid to upper 50s before mostly cloudy skies are
expected to move in throughout the afternoon hours. Light rain will
begin moving in from south to north for areas west of I-29 during
the evening, continuing throughout the day on Saturday before ending
overnight hours into Sunday. While ensembles continue to show
rainfall amounts will remain on the lighter side, 00Z ensembles have
increased chances for at least a quarter of an inch (50% for areas
along highway 14, 70-80% along I-90, and 90+ along highway 20),
though probabilities for a half an inch remain lower in the 20-40%
range for areas south of I-90. They also remain in good agreement
that only rain will fall, with less than a 10% chance for any snow.

However, as the former forecaster mentioned, deterministic models and
ensembles continue to show a pronounced mid-level dry slot getting
ingested into the low as it moves into the area on Saturday.
Generally that points towards a period of drizzle as the mid-level
moisture depletes, possibly switching back to regular rain as mid-
level moisture increases again. Looking at how the system
progresses, most models are suggesting the majority of rainfall
occurs before the dry slot moves in, with the drizzle only adding a
few hundredths over time. So, this forecaster is cautiously
optimistic that the rainfall probabilities are representative.

As for temperatures, expect a cooler and rainy day on Saturday with
highs in the upper 40s to lower 50s. Sunday is a touch warmer as
upper level ridging takes place throughout the day, with highs in
the lower to mid 50s. Agreement amongst the deterministic models and
ensembles continues to decrease on Monday, due to timing/amplitude
details of the upper level trough moving ashore the west coast over
the weekend into early next week. Regardless of which solution,
upper level flow remains amplified into next week.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1035 PM CST Wed Nov 6 2024

A few areas of stratus remain over northwest IA this evening,
though this should continue to decrease overnight. Winds will
be light tonight, though with more of a drier westerly
component, fog is not expected to be as widespread as last
night. Even so, there will be some potential for at least patchy
fog development, with latest ensembles indicating a 20-30%
probability of visibilities less than 1 mile from east central
SD into southwestern MN and northwest IA. Because of the patchy
nature of the fog, did not include in the TAF sites at this
time.

&&

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

&&

$$

DISCUSSION...APT
AVIATION...JM