Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 230926
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
326 AM CST Mon Jan 23 2017

.SHORT TERM.../Today through Tonight/
Issued at 326 AM CST Mon Jan 23 2017

09z Water vapor depicts well-defined upper low spinning offshore in
the Pacific and another low spinning across the southeastern US.
Meanwhile, Iowa is sandwiched in between with weak ridging taking
place. This is a break from the synoptic setup the past few days
which generally kept IA in southerly flow, reinforcing low-level
moisture which aided in fog formation the past several mornings.
With now northwesterly winds failing to decouple and near-sfc
generally failing to saturate as with the past few mornings, have
kept vsby reduction to 3-5 mile range...well above Dense Fog
Advisory criteria.

IR imagery showing stratus deck extending from SE IA northwestward
through the Dakotas and into Canada. Ultimately, this means another
cloudy day for Iowa. 00z Mon GFS initializing too fast/east with
clouds and as a result, is a warm outlier for temps today. Hi-Res
models initializing and handling evolution better, so have leaned
towards them- cooling temperatures today relative to guidance. Left
-dz mention out of fcst as it appears clouds not thick enough to
support precip and sfc dwpt depression ranges around 5 to 10 degrees
in central to southern CWA.  Mason City and far NE CWA best chance
for -dz and fog.

Skies to remain cloudy overnight in advance of aforementioned
negatively-tilted upper low/sfc reflection begin to propagate into
the Midwest. This feature will be discussed below.

.LONG TERM.../Tuesday through Sunday/
Issued at 326 AM CST Mon Jan 23 2017

Broad upper trough in place across the western/central US with
southwest flow across the region with an associated area of low
pressure situated across the western high plains. Focus still on
this upper trough and low pressure system as it lefts ENE through
the state into Tuesday night/Wednesday. Strong southerly flow off
the gulf will push a large amount of moisture northward Tuesday with
strong WAA and theta-e advection in place across the state. This
should result in areas of increasing precipitation through the day
across the CWA. Thermal profile warm enough across the
central/southern CWA where rain is mostly expected with rain/snow
mix expected further north, and colder air in place across the far
north to still see all snow. Some very weak instability aloft, so
could see a convective nature/more showery precip especially Tuesday
afternoon across the central/south. Any instability doesn`t look
enough for outright thunder, but that can`t be completely ruled out
either with MUCAPE values pushing close to the far southern CWA.

SFC low to be situated across the far southeastern CWA by late
Tuesday evening, and may see some stronger showers push into the SE
CWA late Tuesday as the sfc warm front lifts into the state. Colder
air to work its way into the area Tuesday night into Wednesday with
precipitation changing over to all snow. With the track of the low a
bit slower, expect changeover across central Iowa to be a bit later
and closer to late evening/overnight hours Tuesday night. Sfc low
lifts into ern WI by Wednesday morning with def zone snow across
much of the CWA Wednesday morning. Snow is then expected to taper
off into the afternoon/early evening hours as the system moves away
from the area. Initially expect lower snow rations around 7-9:1 with
abundant moisture and warmer temps. As the colder air works in some
snow rations closer to 12:1 expected. This yields around a trace to
2 inches across the southeastern half of the CWA where warmer air
will keep heavier precipitation mainly rain. Areas along and just
north of a line from Denison to Waterloo can expected to see amounts
closer to 2-4 inches. The far northern tiers which will see snow the
entire time can expect to see 6-8 inches. Watch area still looks
pretty decent given the current forecast amounts and will stress
that there will be a fairly tight gradient on the south/southeast
side of this system between the areas that see more snow than rain
and the areas that see mostly rain. Winds still to pick up on the
backside of the system with bufkit showing around 30-35kts in the
mixed layer. Therefore could see some blowing snow mainly Wednesday
across the north/west. Kept the blowing snow wording confined to
sites that are getting at least a few inches as don`t expect enough
snow in other areas to have it be much of a factor. Watch area also
included portions of the CWA that are getting 4+ inches of snow with
the blowing snow potential.

Little other changes expected beyond the storm for the going
forecast with northwesterly upper level flow continuing through the
period. This will keep temps cooler and bring through a few weak
disturbances that could bring some very light snow to mainly
northern Iowa. Little additionally accumulations are expected with
any of these weak systems.

&&

.AVIATION.../For the 06Z TAFS through 06Z Monday evening/
Issued at 1120 PM CST Sun Jan 22 2017

LIFR cigs will remain in place through 16Z before improving. Fog
should not be as dense tonight through pockets of deeper moisture
may be responsible for an occasional 1/2sm to 1/4sm.  In general
vsbys of 1sm or better are anticipated.  Patchy drizzle is also
likely but should not impact aviation as it is expected during a
time when temps should remain above freezing.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Tuesday afternoon through Wednesday
afternoon for IAZ004>007-015>017-023>028-033>038.

&&

$$

SHORT TERM...Kotenberg
LONG TERM...Beerends
AVIATION...FAB


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