National Weather Service Raw Text Product

Displaying AFOS PIL: AFDDMX Received: 2018-04-17 05:04 UTC


318 
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