Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 182325
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
625 PM CDT Mon Mar 18 2024

...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Elevated fire weather concerns Tuesday and Wednesday, possibly
  continuing into Thursday.

- Chances for rain/snow late Thursday into Thursday night.
  Highest chances for light snow accumulations in far northern
  Iowa.

- High chances for rain/snow late in the weekend into early next
  week. Considerable uncertainty with potential snowfall amounts
  at this time.

&&

.DISCUSSION...
Issued at 241 PM CDT Mon Mar 18 2024

Stratus deck that dropped south into the state overnight has
steadily eroded over the last few hours. The sunshine has helped put
forecast highs back on track, although still running 10 to 15
degrees below average for mid March and feels quite chilly when
combined with the blustery northwest wind. Winds tail off a bit and
become more westerly this evening into tonight as a sfc ridge axis
passes through. Winds on the backside of the ridge ramp back up
quickly and should be gusty out of the northwest again by
morning. Switch to a warm air advection regime will push
afternoon temps back into the 50s and 60s. Fire weather concerns
crop up as both wind speeds and min RH values will not be far
removed from critical thresholds. NBM RH is marginally improved
compared to today, however the output running near or above
most bias corrected guidance which may better capture the recent
general trend of overly moist dewpoint guidance.

Upper pattern energy really picks up toward the end of the week as a
deep low is carved out over the eastern two-thirds of Canada. A
low amplitude but robust shortwave rounds the base of the upper
low and pushes through the region Thursday night, akin to a
traditional `clipper` type system. Bulk of the QPF tied to this
wave is focused on an intense mid-level fgen zone sweeping
south across the Dakotas and into southern Minnesota. This band
may clip our far northern counties with the NBM advertising a
20-30% chance for 1" of snowfall. Further south, precip
potential is limited by weaker forcing and delayed moisture
return. Models activate a strong surface front crashing through
the state late Thursday night into Friday morning and provides a
brief opportunity for more widespread precip. Model soundings
suggest temp profiles will still be warm enough to support
mostly rain from about Hwy 30, but wintry/mixed ptypes could be
in play for points northward.

Attention then turns to the weekend into early next week when most
of the model guidance spectrum advertises increasing potential for a
prolonged period of precip over the region. Phased kinematic and
particularly strong thermodynamic forcing contributions ahead of a
wave diving south of Canada will set the stage for precip
development Saturday night and into Sunday. Deep synoptic lift
continues into Sunday night and Monday as an upper low emerges from
the western CONUS. Ensemble output shows remarkable agreement wrt
QPF at that time range with both GEFS/ECENS camps posting 80%+ probs
of 1" by 00z Tuesday. Where stark differences arise is within the
low-level thermal fields between the two models and the fallout on
ptypes through the event. Euro is much warmer at the onset with
850mb 0c isotherm north toward I-90. The GFS is much more aggressive
with the colder air and slower to pull the warm sector back
northward, which favors wintry ptypes for the initial wave of
precipitation. Euro is more delayed with the arrival of cold
air until northerly winds push it south on the backside of the
departing low late Monday into Tuesday. At this same time, the
GFS is much more progressive and already drying out the area.
Not surprisingly, ensemble data sets show a massive range in
potential snowfall amounts which considerably hampers forecaster
confidence at this point in time. The possible scenarios are
likely to change and evolve over the coming days, so those with
travel interests early next week will want to pay attention for
forecast updates in the near future.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 618 PM CDT Mon Mar 18 2024

As far as flight categories go, VFR conditions remain expected
through the TAF period. There are a couple items of note though
with strong low-mid level winds overnight resulting in a period
of LLWS, as well as generally elevated winds and increasing
winds as they shift from southwesterly to northwesterly through
the period. LLWS window is primarily around 6z through 13z with
potential for near 50 kt westerly winds at all TAF sites. At
the surface, winds turn northwesterly and gusty by mid-morning,
with strongest winds in/around KMCW/KALO with potential 20G30+
kts. Elsewhere, gusts into the 20s kts remain.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Martin
AVIATION...Curtis


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