Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 181754

Area Forecast Discussion
National Weather Service Des Moines IA
1154 AM CST Sun Feb 18 2018

.SHORT TERM.../Today through Tonight/
Issued at 405 AM CST Sun Feb 18 2018

A low pressure area is developing from eastern Wyoming into central
South Dakota, while broad high pressure moves away from our area
over the Ohio River valley. As the low pressure trough approaches
today our winds will turn south and increase, then tonight as broad
forcing aloft and low-level moisture advection increase we will see
development of light precipitation across much of the forecast area.
Multiple potential hazards are present in the short term forecast
period and will be outlined individually below.

Early this morning areas of fog have developed across eastern and
northeastern Iowa, due to nearly calm winds and clear skies
associated with the aforementioned departing high pressure center.
Thus far dense fog has remained just east of our forecast area, with
a few stations from Waterloo over to Webster City reporting brief
visibility below 1/4 SM before rebounding. However, in the next few
hours is is likely that these dense fog observations may become a
bit more numerous. Given their isolated and transitory nature, have
handled this threat with a special weather statement rather than a
very short dense fog advisory. Conditions should improve rapidly
around sunrise as daytime warming begins and southerly winds

As the high moves away and the trough approaches from the northwest
today we will see a significant tightening of the low-level pressure
gradient. Forecast soundings at 18Z today show top of the mixed
layer winds around 40 to 45 knots across most of the area, excepting
the southeast, and all signs point toward windy conditions for a
relatively brief period. The tightening of the pressure gradient
will be more or less concurrent with enhanced daytime mixing,
supporting strong winds by late morning. The winds should then
decrease noticeably around mid to late afternoon as the trough
approaches and the gradient begins to slacken. Have issued a
targeted wind advisory from 9 am to 2 pm to account for this hazard.

Finally, late tonight we will see development of light precipitation
as the column gradually saturates from the surface upward and broad
lift slowly increases ahead of the approaching system. Forecast
soundings after 06Z tonight are unanimous in their depiction of a
shallow saturated layer around 5-8 kft deep and entirely without ice
crystal introduction. This will lead to a drizzle/rain scenario,
with northwestern areas below freezing experiencing ice
accumulations. The predicted freezing line placement has been pretty
consistent over the last couple of days and confidence in the area
of freezing rain/drizzle is high. Therefore, a winter weather
advisory has been issued for our northwestern counties beginning at
midnight tonight. Icing may not begin until more like 2 or 3 am, but
once it does start impacts may be almost instantaneous, so prefer to
play it safe with a slightly early start time at midnight. More
freezing rain is expected from Monday into Tuesday as discussed in
the long term section below, and additional headline considerations
exist in that period as a result.

.LONG TERM.../Monday through Saturday/
Issued at 405 AM CST Sun Feb 18 2018

Complex meteorological situation through Tuesday evening with
broad southwest upper flow across the central United States. A
surface front nearly parallel with this flow will move only
gradually from Monday into Tuesday as ample moisture flows north
into the boundary with large elevated warm layer above a shallow
cold layer leading to extensive freezing precipitation over a good
portions of central Iowa.

Light precipitation will be ongoing Monday morning as surface wave
passes through the state as moisture surges northward. The
surface wave is forecast along a boundary which will stretch from
near Bedford to Des Moines and Waterloo around daybreak. Will see
freezing drizzle/light freezing rain northwest of the front with
little ice introduction and a robust warm layer above the shallow
polar air. There will be a very sharp temperatures gradient along
the boundary with readings climbing from the mid 20s to mid 40
over a short distance and light rain is forecast near the front
and to the south/east. A dry slow will arrive with the wave and
will cut off precipitation from southwest to northeast during the
day, except near the front where deeper forcing where deeper
forcing on warm side of dry slot will allow precipitation to
linger in the southeast.

A second shortwave will begin to influence the weather by Monday
night with renewed warm advection/moisture transport north of the
boundary into central Iowa. This event likely shaping up to be the
more problematic of the two as heavier QPF is expected in areas
behind the front. Impressive broad expanse of isentropic lift
will be in place across much of the upper Midwest during this
time, supporting widespread precipitation. Unfortunately for the
forecast area, thermal profiles indicate a broad area of freezing
rain from southwest through central into northeast Iowa Monday
night into Tuesday with ice accumulations that could be upwards of
a few tenths of an inch possibly approaching or exceeding ice
warning criteria, therefore a Winter Storm Watch has been issued
for these areas. Meanwhile, in the far southeast CWA, temperatures
will remain relatively warm with rain and even thunder during
this time. Rainfall amounts of an inch to inch and half are
certainly not out of the question given the moisture transport
into the state and this may lead to some decent rises on streams
over the southeast.

The system will finally drop southeast of the state on Tuesday
night with large Canadian high pressure approaching. However, both
GFS and Euro indicate that the western trof will remain in place
through the week with various waves traversing through the base of
the mean trof an into the central United States. Therefore, there
will continue to be additional threats of precipitation through
the week with timing and placement problematic as GFS and Euro
disagree with the specifics of the individual shortwaves.


.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1154 AM CST Sun Feb 18 2018

Many concerns this period. Strong gradient winds with gusts
nearing 60 mph reported near KCIN moments ago...though many areas
this afternoon in the 40 to 50+ range at times...should diminish
over the next several hours by 21z. Winds aloft remain strong so
LLWS again appears near/after 00z most areas through 03-04z. With
system on the way MVFR/IFR cigs expanding across area between
03-09z and will remain through period. Also expecting potential
-FZDZ increasing to-FZRA at KFOD/KMCW aft 09z continuing through
 18z...but lessening to -FZDZ by 15z west areas. South sites will
 remain liquid this package. Winds not problematic Monday. /rev


Wind Advisory until 2 PM CST this afternoon for IAZ004>007-

Winter Storm Watch from Monday evening through Tuesday morning
for IAZ027-028-037>039-048>050-059>062-072>074-082>084-093-094.

Winter Weather Advisory from midnight tonight to 3 PM CST Monday
for IAZ004>007-015>017-023>026-033>036-044>047-057-058.



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