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342
FXUS63 KDMX 180754
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
254 AM CDT Sun Apr 18 2021

.DISCUSSION.../Today through Saturday/
Issued at 254 AM CDT Sun Apr 18 2021

The forecast remains on track with no major changes since previous
updates. Highlights:

- Partly cloudy and mild again today.
- Much cooler Monday through Wednesday, with freezing temperatures
  possible on multiple mornings.
- Light snowfall at times late tonight into Tuesday.

Clouds have filled back in across much of Iowa overnight, but are
slowly clearing out from northwest to southeast and should
continue to do so through the morning, leading to mostly sunny
skies by this afternoon. A few sprinkles may be possible at times,
mainly over the southeastern half of Iowa, but would be so
isolated and light that they are not included in the forecast.
Meanwhile as winds turn to west and pick up this afternoon,
concurrent with increasing insolation, temperatures should once
again climb into the 60s across the area making for a beautiful
spring day. Enjoy it while it lasts, because...

By this evening a sharp cold front will be approaching from South
Dakota and Minnesota, heralding a significant weather regime
change as it ushers in several days of much cooler, and at times
wet weather. As the surface boundary moves southward over Iowa
tonight and early Monday it will be accompanied by light
precipitation, mostly in the form of a cold rain but with forecast
soundings and surface temperatures supporting potential for light
snowfall, especially in our northern and northwestern counties
where a dusting of accumulation is forecast, perhaps up to near an
inch in a few spots. Monday will be a cool and dreary day, with
temperatures steady or falling through the 40s in the afternoon,
mostly cloudy skies, and areas of light rain.

Things really get interesting from Monday night into Tuesday as
an energetic mid- level trough rounds the base of cyclonic flow
aloft, sharpening and deepening as it moves over Iowa by Tuesday.
Along the leading flank of this trough, broad and strengthening
forcing will promote the development of a swath of precipitation
north of the old surface front, spreading across much of Kansas,
Missouri and southern Iowa on Tuesday morning. Temperatures in our
southern areas will have fallen to near or below freezing by late
Monday night, with profiles supporting a change to all snow
before sunrise and into late Tuesday morning. The primary
uncertainties in this forecast are the northward extent of the
precipitation, and the amount of QPF. Most of the prognostic
models have been slowly trending southward with the northern edge
of the precipitation shield, but still keep it within our service
area and roughly between the Highway 34 and Interstate 80
corridors or so, with most snow accumulation south of that. The
NAM is further north and paints light snow amounts all the way up
to Highway 30 or so, but the farther south ECMWF/GEFS appear more
likely at this time. While the highest QPF is over northern
Missouri, it appears our southern counties, especially south of
Highway 34, could receive perhaps 2-3 inches of snowfall on
Tuesday morning, leading to minor travel impacts. The silver
lining is that with temperatures warming into the lower 40s or so
on Tuesday afternoon, any snow that does accumulate on roadways
should melt fairly quickly. Nevertheless, it is possible a Winter
Weather Advisory may become warranted for these areas in the next
day or two.

The precipitation will end by Tuesday afternoon/evening as the
strong mid-level trough moves off to the east. A second impulse
will move over the region Tuesday night and Wednesday however,
providing an opportunity for perhaps a few light showers but also
helping push temperatures down even further, with Wednesday
morning lows forecast in the 20s across all of Iowa and making for
a late season hard freeze in many areas. This will not last long
however, as by Thursday morning deep layer ridging will be moving
in quickly from the west, allowing for warm air advection and
pushing temperatures up to near 60 degrees by Thursday afternoon.
A weak impulse may then provide a chance for light rain or
sprinkles from Thursday night into Friday, before another cold
front approaches Iowa at the end of the week.

&&

.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1138 PM CDT Sat Apr 17 2021

Ceilings remain VFR for duration of the forecast with unrestricted
visibilities. Surface winds will be northwest overnight becoming
westerly on Sunday. A cold front enters northern Iowa on Sunday
evening with winds becoming north behind the boundary across
northern Iowa during the evening. Little threat of precipitation
during the forecast period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

DISCUSSION...Lee
AVIATION...Cogil