Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 221149
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
549 AM CST Thu Feb 22 2018

...Updated for 12z Aviation Discussion...

.SHORT TERM.../Today through Tonight/
Issued at 412 AM CST Thu Feb 22 2018

GOES-East Nighttime Microphysics shows higher level clouds moving
off to the northeast revealing low level clouds over much of the
state. KDMX radar shows a band of snow over from near Charles City
extending west-southwestward toward Fort Dodge and then Denison.
Several sites are also reporting light precipitation including -SN
at KDSM, though recent manual observations here have not noted
anything out our door. The forcing mechanism for this precipitation
is 850mb warm air advection. Forecast soundings show that saturation
within the ice introduction layer will be lost from south to north
this morning into this afternoon. In addition, warmer air will begin
to arrive. Both of these changes to the atmospheric profile will
result in a transition from mostly snow initially to freezing rain.
As modest Q-vector divergence lifts north and drier air arrives
around 850mb, a transition to more freezing drizzle over southern
Iowa is anticipated. Over our southeast forecast area, temperatures
just above freezing within the boundary layer should allow from some
of the freezing drizzle to change to liquid drizzle this afternoon
and evening.

By tonight, a second wave of stronger Q-vector divergence lifts
across the state along with a surge of warm air advection. This
better lift will likely mean precipitation that is more than drizzle
so a change back to freezing rain or rain is possible over the
southern 2/3rds of the state. Over far northern Iowa where ice
introduction is more likely, snow is favored, but could still mix
with freezing rain at times. The Q-vector divergence moves quickly
across the state and by midnight, should begin to see an end to the
precipitation over western areas with precipitation out of the
forecast area by around 5 or 6am Friday morning.

Ice accumulations are tricky as was observed earlier this week. With
this winter system, the convective element does not look in play,
which allowed for sleet instead of freezing rain at times Tuesday
night. In addition, rates were high Tuesday night and this allowed
for less accretion. Current forecast for today and tonight has ice
accumulations of 0.10 to near 0.20 inches over parts of central
Iowa. However, the time spent as freezing rain or freezing drizzle
will dictate the amounts. More time as freezing drizzle or light
freezing rain will find ice accumulations toward the higher end and
perhaps a wider area while a moderate freezing rain may have lower
values. For snow amounts, generally have around 1 inch around the
Highway 3 corridor to 2 to near 3 inches closer to the Minnesota
border.

.LONG TERM.../Friday through Wednesday/
Issued at 412 AM CST Thu Feb 22 2018

Southwest flow pattern in place aloft with an upper trough digging
into the desert southwest initially Friday. High pressure in place
act the surface through the day Friday across the region, with the
SW US upper trough to lift into the CO/NM region by early Saturday.
A shortwave trough embedded in the southwesterly flow aloft will
impact areas just to the south of the state and may brush far
southeastern portions of the forecast area late Friday night.
However many of the models drier until Saturday, so trend was to
lower pops from what was in there previously. Upper trough over CO
then to lift ENE through the central US Saturday and moving across
portions of IA into Saturday night. An associated surface low
expected to track from western Kansas Saturday and into northeastern
MO by 00z Sunday, then lift northeastward through southeastern/east
central Iowa Sunday evening before lifting into western WI by early
Sunday morning. Strong WAA and moisture advection off the Gulf to
lift into the state Saturday with a TROWAL feature setting up across
portions of northern IA late Saturday. This system should spread
precipitation across much of the state through the day Saturday,
with a large area of def zone precip area moving across western and
northern Iowa into Saturday night. With the warmer air moving into
the state, will have some changing precipitation types and ice
introduction issues aloft especially in the southeastern half to
2/3rds of the state. Expect a wintry mix early Saturday across the
south with a chance of snow north. As the warmer air moves in some
of the wintry mix will changeover to all rain across the southeast
half of the forecast area from Atlantic to Des Moines to Waterloo
Saturday afternoon, with the wintry mix just north of that line.
Still anticipating snow from mainly Denison to Fort Dodge to Mason
City. Initially expect lower snow ratios into the 5 to 10 to 1 range,
and as colder air moves in to the state on the backside with the def
zone snow, ratios increase to 10-15 to 1 by  Saturday night. Some
uncertainty still with the track of the system as the GFS brings
the sfc low a bit further west which would bring warmer air into
the state and decrease wintry mix chances some. However all
solutions pointing toward moderate to heavy snow accumulations
across west central into north central IA. Initial thoughts given
where the lift lines up with the dendritic zone and expected
thermal profiles indicate around 4 to 8 inches across the
northwest third of the forecast area with the highest amounts
expected toward northwest IA and the Estherville/Algona/Emmetsburg
areas.

System to exit the area Sunday with dry conditions into Monday as
high pressure builds into the state and have nearly zonal flow
aloft. GFS/EC try to hint at another system impacting the region
toward mid week next week, however timing issues remain with the GFS
about 24 hours faster than the EC. Therefore have stuck with the
blended grids for now that maintain pops across much of the area for
Wednesday. Overall temperatures near seasonal values or warmer to
begin the period, then warm through the period next week with above
average for late February temps expected.

&&

.AVIATION.../For the 12Z TAFS through 12Z Friday morning/
Issued at 549 AM CST Thu Feb 22 2018

Primary concerns this period are restrictions from the wintry
precipitation set to move across the state today into tonight.
Will see conditions drop down to IFR or LIFR in the snow and
freezing rain/drizzle later this morning into this afternoon. The
exception may be KOTM, which may miss on the heavier
precipitation passing to its northeast until this evening.
Precipitation will end from west to east around and after
midnight allowing for visibilities to improve, but low clouds may
keep ceiling restrictions until the end of the TAF period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until midnight CST tonight for
IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097.

&&

$$

SHORT TERM...Ansorge
LONG TERM...Beerends
AVIATION...Ansorge


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