Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 210541
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1134 PM CST Sat Jan 20 2018

.SHORT TERM.../Tonight through Sunday/
Issued at 357 PM CST Sat Jan 20 2018

The main concern during this period was focused on drizzle onset
tonight and the potential for any freezing drizzle late tonight
into Sunday morning. Models are in good agreement with the timing
of drizzle tonight into Sunday morning and only change was delayed
the onset slightly from previous forecast and have less mention
of freezing drizzle overnight, especially over central to southern
Iowa.

Low level moisture continues to increase tonight into Sunday
morning with a decent push of Theta-E advection and moisture
transport along 290K to 310K isentropic surface. Lift is weak, but
enough to produce areas of drizzle and advection fog late
tonight. There is no ice introduction in the column, so kept with
mention of only dz/fzdz tonight into Sunday. The most problematic
location looks to be from around Highway 30 northward to Highway
20 overnight tonight into Sunday morning. FOD BUFR sounding via
GFS/NAM indicate a long duration period of FZDZ with a decent warm
nose near 850mb. Surface temperatures remain below freezing
through much of the morning Sunday and with the northeast winds in
place, this should help keep the surface temperatures cool enough
for the FZDZ to occur. There is a potential for a light glaze of
ice Sunday morning over the aforementioned location. QPF is very
light, so low confidence with issuing an advisory attm due to the
uncertainty how this FZDZ will impact roads. Most road
temperatures are well into the 40s to around 50, so mainly looking
at elevated surfaces such as trees and powerlines. Certainly
cannot rule out the overpasses/bridges, but it is something to
monitor for potential headline.

.LONG TERM.../Sunday night through Saturday/
Issued at 357 PM CST Sat Jan 20 2018

In the headline location across northern Iowa, ice introduction
remains the key issue here with the precipitation teetering b/t
snow/wintry mix Sunday night into Monday. Higher confidence in the
far northwest corner of the CWA to see more snow with the strong
forcing within the dendritic layer combined with it being in the
vicinity of strong frontogenetical forcing, the snowfall rates
will likely be enough to keep the entire column cool enough to see
snow start to pile up b/t 06-12z and continue through much of the
day Monday. The caveat is the TROWAL and how far west the mid-
level warm nose reaches early Monday morning and may cut down on
snow amounts slightly b/t 06-12z Monday. Soundings are already
near isothermal and any warm nose push will switch the
precipitation over to a mix for a short period early Monday
morning. The warm nose doesn`t appear to be large enough though to
completely melt the snow. Plus, the strong frontogenetical
forcing increases across northwest Iowa and will expect all snow
by 12z Monday at the latest. Winds increase to 40-45 knots atop
the mixed layer by this time and with the falling snow, cannot
rule out the potential for blizzard conditions.

Further east/southeast along a line from MCW to FOD could see a
period of FZRA/wintry precipitation late Sunday night into Monday
morning.  The TROWAL will definitely impact the precipitation type
in this location and looks to be more FZRA (possibly sleet) as
there is a deep warm layer b/t 850-700mb and warms to around +5C
by around 09z Monday. Surface temperatures teeter around the
freezing mark Monday morning before the precipitation transitions
to all snow b/t 13-16z Monday. Added a Winter Storm Watch along
the southeast edges of the heavier snow band in the event of
higher snow amounts and even the ice accumulation. Still plenty of
time to hash out the fine details and hence to upgrade to a
warning/advisory in this location.

Monday night through Friday...not a significantly cold air mass
Monday night into Tuesday but still in the CAA regime of the
departing low pressure.  The ECMWF is slightly slower in departing
the low and there may be some lingering snow showers across the
east Tuesday morning. Upper level ridge moves in over the region
mid to late week and generally west to southerly flow at the
surface keeps temperatures slightly above normal.

&&

.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1134 PM CST Sat Jan 20 2018

Brief clearing/lifting of cigs is allowing br to form and this will
bring MVFR vsbys to TAF`s.  Moisture will deepen and low cigs will
develop overnight.  If we can get drizzle to develop across northern
TAF locations aft 09Z FZDZ would be possible for a few hours this
A.M. but confidence in this is not high enough to include in TAF`s.
After 15Z DZ will begin to develop further reducing cigs/vsbys to
IFR with rain developing late in the period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late Sunday night through Monday evening
for IAZ006-007-016-023-024-033-034-044.

Winter Storm Warning from 9 PM Sunday to midnight CST Monday
night for IAZ004-005-015.

&&

$$

SHORT TERM...Podrazik
LONG TERM...Podrazik
AVIATION...FAB



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