Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 171713
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1213 PM CDT Tue Apr 17 2018

...Updated for 18z Aviation Discussion...

.SHORT TERM.../Today through Wednesday Night/
Issued at 332 AM CDT Tue Apr 17 2018

Confidence: Medium

Today will be pleasant compared to later tonight into Wednesday.
Clouds will be on the increase today ahead of the storm with
cloudy skies tonight. Highs today will recover to the mid to
upper 30s north and the upper 40s to lower 50s over the south.
Tonight with the low passing southeast of the region temperatures
will fall into the upper 20s north and in the mid 30s across the
south. The freezing line will be near the US 30 corridor.

Storm still on track to reach Iowa later tonight and bring a quick
hit of snow/freezing rain/and wind.  Models tonight are tracking
farther south and colder than earlier runs.  The GFS/Euro/NAM all
suggest abundant moisture available for snow production across the
north. Deep lift in the H700-H600 layer with potential for
elevated instability over the east may lead to more widespread
heavy snow farther south. H850 inflow into the storm of 40 to 45
kts from 09z through 17z will aid in 1 inch snowfall rates per
hour across the north during that time. Though saturation may take
until about 07z across the northwest...went ahead and began
warnings/adv over the northwest. Once the precip begins it will
quickly cause some impacts with some light icing/sleet and a quick
changeover to all snow by 09z as rates pick up over the northwest
for the remainder of the day. Over the northeast rain/freezing
rain will being initially with a changeover taking place nearer
09z to 12z. None the less...morning commutes will be impacted over
most of the north. Farther south a mixture of light icing and
light snow will be possible. Though road temperatures may limit
any road icing along the US Highway corridor north toward US
20...decided to add a row of counties to for chance that some
bridges/overpasses may become slick or briefly ice up in the
morning hours. If impacts lessen during the day Wednesday...this
portion of the advisory may end earlier. This storm has a lot
going for it...possible convection along with strong forcing. Some
uncertainty still remains regarding the amount of qpf and how far
south the storm may still trend which will affect snowfall
amounts. Generally looking for a 6 to 8 inch area of snow north in
the warning areas with 2 to 5 inches in the advisory areas along
with a glazing of ice there. Day shift will need to carefully
evaluate trends and again later tonight. Highs tomorrow will warm
into the mid to upper 30s south of US 30 - at best with
temperatures holding in the lower 30s over the north. With the
snowfall rates quite high...this event will support snow below 35
degrees and likely some compaction will take place but with high
snow:water ratios up to 15 to 17 due to tremendous forcing...
should be able to see more than 6 inches in the warning areas.

.LONG TERM.../Thursday through Monday/
Issued at 332 AM CDT Tue Apr 17 2018

Finally some quiet and spring-like weather in store through the
weekend and into the early part of next week. The main focus of
the extended was on a another upper level low to pass to the
south of the state Friday night into Saturday. Models continue to
shift this system south from run to run and confidence is
increasing with the better chances of remaining dry across much of
the forecast area. Hence the good news, no major winter storm for
the weekend for central Iowa and much of the forecast area looks
to be near or even slightly above normal Saturday and Sunday.

Thursday through Saturday...Surface high pressure builds into the
region and becomes centered over Iowa b/t 03-09z Friday. There is
plenty of dry and cool air with this airmass and expecting a good
radiational cooling set up by Friday morning. Record or near record
lows are likely, especially over the northern half of the forecast
area with the anticipated fresh snow pack in place.  Something to
mention in forecast after Wednesday`s system. The strong surface
high shifts to the Great Lakes area and looks to keep the next 500mb
low tracking across Arkansas and Missouri. Ahead of the low, a weak
ribbon of vorticity provides enough forcing to keep the mention of
rain and snow going Friday night into Saturday. Soundings suggest
either rain or snow during the day Saturday with ice introduction
present, but there remains a deep layer of dry air to contend
with below 800mb to the surface. Thus, QPF and any snow amounts
look to be light attm.

Sunday into Tuesday...GFS and ECMWF have come into agreement with
warmer temperatures by Sunday and tweaked highs up slightly and may
need to continue to trend warmer if models persist with this signal.
Only caveat is some mid-level clouds across the state. Certainly
Monday and Tuesday are forecast to be around normal with highs
rising into the upper 50s to 60s, but with the latest MOS guidance
from the ECMWF and GFS, highs looks to be even warmer.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1214 PM CDT Tue Apr 17 2018

High confidence in VFR conditions through at least early this
evening and then confidence decreases as a spring storm with
wintry precipitation moves into the state. Will see conditions
dropping into IFR at all sites but OTM a few hours either side of
12z. Did include a period of LIFR visibilities at FOD and ALO
Wednesday morning where confidence is highest in a period of
heavy snow, though fine tuning of timing and overall impacts at
all terminals is expected. Finally, still going over the
preponderance of incoming data so not included in this set of
TAFs, but wintry mix may slip a bit farther south into the DSM
terminal.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 4 AM Wednesday to 1 AM CDT Thursday
for IAZ036>039-047>050.

Winter Storm Warning from 1 AM to 10 PM CDT Wednesday for IAZ004-
005-015-024.

Winter Storm Warning from 4 AM Wednesday to 1 AM CDT Thursday
for IAZ006-007-016-017-025>028.

Winter Weather Advisory from 1 AM to 10 PM CDT Wednesday for
IAZ023-033>035-044>046.

&&

$$

SHORT TERM...REV
LONG TERM...Podrazik
AVIATION...Ansorge



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