Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 212349
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
649 PM CDT Thu Mar 21 2024

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

.KEY MESSAGES...

- Snow tonight north/northeast with snow rates of 0.5-2" per
  hour at times, notably around the Friday morning commute, that
  could create a difficult morning rush hour, especially east of
  I-35 and north of about US Highway 20 where a Winter Weather
  Advisory exists.

- Precipitation timing tonight is the overnight hours into
  Friday morning with a wintry mix possible between the snow
  north/northeast and the rain expected for areas south and
  west.

- Large, prolonged system this weekend into early next week.
  Accumulating snow likely north.

&&

.DISCUSSION...
Issued at 305 PM CDT Thu Mar 21 2024

Plenty of weather to talk about over the next several days with
a small break in between systems to end the week and start the
weekend. Although we do bring the return of wintry precipitation
to the forecast, the positive is some much-needed moisture!


Tonight into Friday:

Minimal improvement from our dry air mass has occurred today with
relative humidities remaining generally sub 30% from this morning
all the way into early this afternoon. Given this stubborn dry air
in place, any radar returns thus far have resulted in little to no
precipitation reaching the ground though a few flurries have been
seen at times north or northeast. Through the rest of the afternoon
into evening, a few sprinkles or flurries may occur at times as we
continue to erode our very dry low levels. Forecast soundings are in
pretty good agreement that it will be near to after midnight when
saturation is achieved which is the last piece to come together for
our precipitation chances tonight into early tomorrow. Lift in the
mid to upper levels will continue to increase through the evening
with theta-e advection notably ballooning over much of the state
through the overnight hours. The result is widespread precipitation
chances that begin north late tonight before translating south and
east with time as a frontogenetic band moves northwest to southeast
through the area into tomorrow morning. Although confidence in
precipitation occurring is pretty high, precipitation type and
amounts are a bit more nuanced. For areas south and west, rain will
prevail with the bulk of the precipitation occurring from the
late overnight through the morning hours Friday and rain amounts
of a tenth of an inch to nearing a half inch or more towards
portions of central Iowa. Meanwhile, the gauntlet of
precipitation types remains plausible across northern into
northeastern Iowa. With no questions in the forcing, which
notably maximizes near the morning rush hour time frame,
profiles change considerably hour to hour or location to
location with a loss of ice introduction at times or falling
through an around 1 km warm layer that could help melt the snow
towards 11/12Z in portions of north central to east central
Iowa. Thus, the forecast for north/northeast areas continues to
reflect a messy convective wintry mix with elevated instability
helping to lead to increased precipitation rates at times
translating to 0.5" to upwards of 2" per hour snow rates which
could absolutely overcome the antecedent warm ground
temperatures to accumulate snow on roads in addition to grassy
surfaces. The area most likely for this accumulating snow and
higher snow rates remains east of I-35 and north of about US
Highway 20 but could create a messy morning rush as the highest
snow rates will occur right through the morning commute or
around 10-13Z, maximizing right about 12Z or 7am. Snow amounts
remain similar to the previous forecast, from 1-4"
north/northeast but hi-res guidance continues to indicate a
narrow, smaller than county-wide banded snow potential that
could lead to higher snow amounts in some locations given the
significant forcing through the DGZ and elevated instability all
leading to those higher snow rates for a few hours as just
discussed. The predictability of the exact location of the
highest banded snow accumulations remains low due to the small
spatial scale of occurrence but did add one county to the Winter
Weather Advisory with some hints of a southwestward trend in
some of the midday model runs. Will need to continue to monitor
trends tonight closely in case a further southward trend
continues. Although any ice accumulations or impacts will
generally be minimal, for those that see a wintry mix, the snow
itself will be wet creating sloppy, slushy conditions. In
addition, given the snow rates, visibilities may drop to 1/2
mile or less at times adding further impacts to the Friday
morning commute.

Friday night into Saturday:

A temporary lull and return to quiet conditions will occur from
Friday night into Saturday morning though temperatures will be cool
with lows on Friday night/Saturday morning in the teens and highs on
Saturday still in the 30s to 40s, similar to Friday.

Saturday night into Tuesday:

Precipitation returns to the forecast later Saturday into Sunday
with multiple waves occurring between late Saturday and Tuesday
making for a prolonged precipitation event as a much larger system
moves towards and through the region through the later half of the
weekend and start of next week. Theta-e advection increases on
Saturday night and remains robust into Monday with precipitation
type ranging from rain to a wintry mix to snow with significant snow
still expected, but some uncertainty remains in terms of where. Will
continue to monitor the trends over the next several days with some
convective potential as well on Monday as the system lifts
northeast.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 649 PM CDT Thu Mar 21 2024

VFR conditions remain this evening with mid level clouds passing
through across the region with light winds. Late this evening,
a system passing through is expected to bring increasing chances
for precipitation across the state. This looks to be in the form
of rain/snow across the northern terminals around 06z, turning
into snow through 12z, which will allow for ceiling and
visibility restrictions. Further south at KDSM and KOTM, mainly
rain is expected after midnight and remaining through the
morning, with MVFR conditions expected. Precipitation chances
end into Friday afternoon, with winds turning breezy out of the
north.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 1 AM to 10 AM CDT Friday for
IAZ006-007-016-017-026>028-038-039.

&&

$$

DISCUSSION...KCM
AVIATION...Bury


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