Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 241917
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
117 PM CST Sat Feb 24 2018

.UPDATE...
Issued at 115 AM CST Sat Feb 24 2018

Have ended the east central counties advisory a bit earlier than
expected as temperatures have risen above freezing. The remainder
of the advisory in central Iowa/Des Moines metro may end up being
marginal although with the precipitation early this evening, some
snow/mix along with freezing afterwards will lead to icy
conditions on area roads.

&&

.SHORT TERM.../Today through Tonight/
Issued at 351 AM CST Sat Feb 24 2018

Precipitation is beginning to push into southern Iowa with dense
fog in place across northern Iowa. Main upper trough spinning
across the four corners region currently and will lift
northeastward through the region today into tonight. The
associated surface low situated across the southwestern high
plains, with a lobe into AR/TX/OK region. This sfc low will lift
through MO into IL toward Lake Michigan by this evening with an
inverted piece of the trough to track across central Iowa this
afternoon into evening. Strong moisture advection ahead of this
system and WAA aloft today, with the def zone across the area
tonight, along with the passage of the sfc trough axis. This will
result in precipitation spreading northward across the state this
morning, with a warm nose aloft and sub-freezing temps at the
surface. Therefore still on track for some light icing potential
across the southern third to half of the forecast area this
morning. As precipitation area spreads into the northern portions
of the forecast area by late morning/early afternoon, thermal
profile cold enough for snow. Biggest changes to the ongoing
scenario is that latest round of models showing temperatures
trending cooler today than previously expected with the track of
the sfc low a bit further south/east than in previous days.
Therefore have lowered highs a few degrees across the entire
forecast area and this will allow the threat for freezing
precipitation to linger longer in portions of the
western/central/northeastern forecast area today with colder air
moving in tonight and any lingering precip to changeover to snow
this evening. Therefore have made several changes to the headlines
regarding extensions in time through this afternoon into this
evening, and expansions to the east for today and northeast for
today into tonight. Also moved up the start time in the northwest
with precip to spread there by late morning. Also have a
tremendous amount of conditional stability aloft this afternoon
across the entire forecast area so did add in a mention of
isolated thunder. This could enhance some rates of the
precipitation and cause some periods of intense snowfall across
the north/northwest. Therefore visibilities at times in the snow
may drop to around one half mile or less, which could cause
hazardous travel with slick, snow-covered roads expected. Snow
amounts remain similar in the north/northwest with a total of
around 2 to 6 inches expected with the highest amounts toward the
northwestern portions of the forecast area and toward the MN
border. Ice accumulations across the southwest to west central to
central IA around one tenth of an inch. Still expecting a
changeover to all rain in the far south/southeast/east central.
Precip should then quickly move out of the forecast area by
mid/late evening as the system lifts quickly north of the area.
Therefore have also ended the advisory at 06z.

Have an area of dense fog across the north, and expect this area
of fog to persist until precip starts to move into the area, thus
extended the advisory through 15z at this time. Temps well below
freezing so fog deposits may freeze on roadways, bridges and
walking surfaces may become slick this morning.

.LONG TERM.../Sunday through Friday/
Issued at 351 AM CST Sat Feb 24 2018

Upper-level southwest flow will dominate the weather during much
of the period. Main concern is a storm system to affect the region
from Wednesday into Thursday. The 00Z GFS/ECMWF have significant
differences in handling the upper trough. The GFS is stronger with
this system. Meanwhile, the ECMWF is weaker. Expecting rain in
some locations, mainly the southern CWA, snow in some locations,
mainly the northern CWA, with a mix of rain and snow in between.
The mix line will migrate north and south with daytime heating and
cooling.

Look for dry weather early in the week with a warming trend.
Temperatures should be well above normal with highs approaching 60
in southern Iowa on Tuesday. Fairly high POPs with the storm, but
with no significant cold air, the temperatures should only
retreat back to near normal for late February.

&&

.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 115 PM CST Sat Feb 24 2018

Poor general aviation conditions this afternoon into the early
evening with widespread IFR/LIFR ceilings and visibilities. Areas
of fog, light rain, light freezing drizzle, sleet and snow are
expected across central into northern Iowa ahead of a front which
will pass through this evening. Strong west winds on the backside
of the this system will allow for rapid clearing with VFR
conditions becoming widespread after midnight across much of the
area. The west winds and VFR ceilings and visibilities will
persist into Sunday morning.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 9 PM CST this evening for IAZ033-
034-044>049-057>060-070>072-081-092.

Winter Weather Advisory until midnight CST tonight for
IAZ004>007-015>017-023>028-035>039.

&&

$$

UPDATE...Cogil
SHORT TERM...Beerends
LONG TERM...Zogg
AVIATION...Cogil



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