Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 170508
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1204 AM CDT Tue Apr 17 2018

.DISCUSSION.../Tonight through Monday/
Issued at 359 PM CDT Mon Apr 16 2018

It has been a very cold, mid-April day with gusty breezes from the
northwest accentuating the chill. Highs will end up being 20 to 25
degrees below normal. GOES-16 Day Cloud Phase Distinction shows
scattered to broken clouds on the back side of the departing low
pressure over parts of central Iowa and more numerous over eastern
Iowa. Skies will continue to clear and winds will decrease this
evening. This will set the stage for another very cold night with
lows in the low to middle 20s. Record lows in a few places will be
challenged as well. Also of concern is the low probability of
patchy fog development, especially in those areas that saw snow
melt focused over southern Iowa. For now, have left out mention in
the forecast, but something to monitor this evening.

Main focus of the rest of the forecast was on the fast-moving,
vigorous shortwave trough that will bring wintry precipitation
once again to mainly northern Iowa. Mid-level ridging will pass
over the state with sunshine initially Tuesday morning giving way
to increasing clouds. As the shortwave trough moves across the
Rockies, low pressure will develop over eastern Colorado and
deepen to around 992mb as it moves into western Kansas. Initially
over Iowa, soundings show some low level saturation with no ice
introduction aloft. With temperatures falling below freezing, will
have a short period of freezing drizzle or freezing rain (once
ice introduction occurs) across northern Iowa. As cooling aloft
removes the warm nose and saturation occurs within the ice
introduction layer, will see a transition to all snow toward
Wednesday 12z over the far north. As the low pressure weakens to
around 1000mb as it moves east-northeast over southern Iowa or
northern Missouri Wednesday, winds from the northwest will wrap
colder air into the state and make for a fast transition to all
snow north by mid-morning Wednesday. Cross sections from the NAM
and GFS both show a very strong area of omega within the dendritic
growth zone over north central Iowa Wednesday morning with rates
likely at or in excess of 1 inch per hour. QPF amounts are still a
challenge as Superblend smoothes out the maximum values. With the
exception of the NAM, the GFS, EC, CMC, and WPC all have near or
more than an inch of liquid precipitation over northern Iowa.
Therefore, still blended in WPC QPF with Superblend to get some
higher values, though may still be underdone with this blend. This
results in the highest snow amounts just north of the Iowa
border. However, the exact placement of the QPF remains in
question as the Euro came in a bit farther south compared to the
WPC/CMC/GFS. Therefore, will continue Winter Storm Watch as
already issued given the uncertainty that remains in the location
of the highest amounts. An added challenge to snow amounts is
whether there may be any horizontal convective rolls as NAM and
GFS soundings show elevated instability late Wednesday morning
into the afternoon. This will certainly be something to watch in
the coming 36 to 48 hours. All the precipitation will end
Wednesday evening quickly from west to east as the low pressure
moves into the Ohio Valley.

Along and south of I-80, a chilly rain is expected through the
event. With warm air advection aloft ahead of the low, may even
see enough instability for some thunder Tuesday night into early
Wednesday, especially for this area. However, thunder may extend
into the wintry precipitation as well. Other items of note on
Wednesday are that temperatures may be a bit on the high side in
the area of highest precipitation rates would likely result in
further cooling at the surface. Winds were once again boosted
several knots over southern Iowa as the low tracks across the
area.

Beyond the midweek winter storm, ridging will move across the
state on Thursday allowing for more sunshine with temperatures
mainly in the 40s, though temperatures may be held into the upper
30s over far northern Iowa with the new, additional snowpack. The
next shortwave trough will move from the Southwest US, but the
trend continues of this system generally staying south of the
state. Southwestern half or so of the state may be brushed by some
precipitation Friday night into Saturday before drier weather is
expected to arrive to end the weekend.

&&

.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1204 AM CDT Tue Apr 17 2018

VFR conditions are in place across the forecast area and are pretty
much expected to remain so through the entire period. The only
concern would be brief MVFR radiation fog toward daybreak at all
sites except KDSM. Precip may also approach KFOD late in the period,
but chances and confidence are not there for inclusion at the
moment, more likely with 12z issuance and later.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late Tuesday night through late
Wednesday night for IAZ004>007-015>017-023>027.

&&

$$

DISCUSSION...Ansorge
AVIATION...Small



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