Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182342
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
542 PM CST Sun Feb 18 2018

...Updated Aviation Discussion...

.SHORT TERM.../Tonight through Monday/
Issued at 342 PM CST Sun Feb 18 2018

Confidence: Medium

Minor adjustments made to previous forecast. Overall there is
model to model run consistency within each model...but less
overall consistency between the Euro/GFS after 00z. First push of
moisture modeled well and should arrive between 06-12z tonight.
Have slightly cut back on PoP/qpf for the period with the main
push of moisture arrive between 09 through about 18z Monday. With
the front moving south after 06z...upglide isentropic lift will be
the main focus for light precipitation through tomorrow afternoon
- the first portion of this two part event. There are subtle
differences between how fast the cold air arrives (front
placement) and also the eventual amount of light precipitation
tomorrow. For the most part the north will see roughly 0.10 liquid
from overnight to 00z Tuesday. This will result in roughly .10 to
.20 inch ice accumulation across the north tomorrow. The forcing
lets up in the afternoon enough with a push of drier air aloft
that overall chances for precipitation diminish somewhat but there
may be just enough lift to continue periodic light freezing
drizzle. Our original advisory was to expire by 3 pm...with the
possibility of another being issued for the remainder of the event
across the north. At this time it looks probable that
precipitation will again increase during the evening hours through
midday Tuesday...so the advisory will now continue until that
time mainly to simplify the headlines and express the reality that
the storm is not really over until Tuesday. With the front
arriving tonight...cold air advection will advance south with lows
to the upper teens northwest to the lower 40s south. Tomorrow we
will see a continue slow push of colder air southeast during the
afternoon with readings slowly dropping by late afternoon. Any
remaining light precipitation during the afternoon hours may
result in a light glaze of freezing drizzle/ice. By late afternoon
temperatures will near the freezing mark by 00z Tuesday near the
metro with sub freezing temperatures northwest...and above
freezing southeast. Models still advertising some thunder
southeast up to about Des Moines...and have carried that through
the forecast at 7 pm Monday.

.LONG TERM.../Monday night through Sunday/
Issued at 342 PM CST Sun Feb 18 2018

Bottom Line Up Front...
Forecast confidence only medium, medium-high for Tomorrow night
through Tuesday. If going forecast pans out, impacts could be
serious across the central to SE portion of Iowa, including the
Des Moines metro. Rest of the week quiet until next Friday or
so.

Tomorrow Night and Tuesday...
"Round 1" of wintry precipitation should be east of our area as
"low 1", as captured by the 950mb height field should be into SW
Wisconsin around 00z Tue. A second, less deep core moves into SW
Iowa immediately on the heels of its predecessor. Model dynamics
show more differences than would like to see at this point.
General driver for precip Tuesday is broad pocket of isentropic
lift/theta-e advection, that lines up from the SW corner of IA to
the NE corner of IA. This is phased with a pocket of low-level
moisture flux convergence. Best low-level frontogenetical
forcing, may remain in southern IA/northern MO, however. For model
splitting, the 12z Sun NAM brings the frontogenetical forcing
furthest into central IA, whereas the GFS keeps it furthest south.
This seems to have a huge impact on QPF... the 12z Sun NAM is
cranking out around 0.25 inches from Waterloo through Des Moines
and to Creston, where as the 12z Sun GFS has only a few hundredths
during the Tuesday morning timeframe. This would make a huge
difference in terms of ice storm potential. The 12z Sun ECMWF is
somewhere in the middle. Ultimately, have gone with a model blend
as confidence not high enough to run away from guidance towards a
specific model.

Have extended time of Winter Weather Advisory through Tuesday and
added on Winter Storm Watch in SW. Forecast ice accumulation in
Clarinda is 0.2 inches, with slightly more in Creston... therefore
see no responsible reason to not have this area under a headline.
If the higher-QPF NAM and ECMWF solutions are correct, the need
for an Ice Storm Warning would be very plausible... not only in
our SW counties, but much of the central portions of the state as
well. For the Advisory, though there may be a slight lull in
measurable precip across west-central Iowa tomorrow evening,
confidence is not high enough to pin down the lull timing/duration
and location... So have continued the Advisory, as there still
may be slight impacts through the evening hours. As models
(hopefully) converge on a solution, would hope to be more precise
with headlines.

Winds also worth a special mention, as they will be strong enough
to amplify impacts and "efficiency" of freezing precipitation.
High confidence in winds of 10 to 15 mph tomorrow night,
increasing to the 10 to 20 mph range Tuesday afternoon will help
precipitation freeze on surfaces much more readily. In all,
Tuesday morning commute around the Des Moines metro to Ames will
really have to be watched. There could be well over 0.1 inches of
ice already accumulated before sunrise, making travel potentially
very hazardous.

Wednesday and Beyond...
Not much emphasis placed on the extended forecast at this point.
Confidence very high in 1040mb or so sfc high racing across the
upper Midwest Wednesday into Thursday. By Friday, this high should
be somewhere over the Great Lakes/Ohio Valley region. 12z SUN
ECMWF now coming more in line with the GFS in racing a shortwave
from SW to NE through Iowa sometime near Friday. At this point,
this shortwave looks very disheveled. At this time, 850mb
jet/moisture transport vectors stop short of Iowa, making it only
into central MO and then central IL with gulf moisture. So at this
time, looks like a quick-moving, low-impact system. Will have to
monitor more closely after Tuesday system is out of our area.

&&

.AVIATION.../For the 00Z TAFS through 00Z Monday evening/
Issued at 541 PM CST Sun Feb 18 2018

IFR to LIFR ceilings look to move into the state past 06z tonight
and continue throughout much of the TAF forecast. Confidence is
high wrt to the ceilings, but lower confidence wrt to visibility
restrictions and only have MVFR/IFR by around 12z Monday morning.
Hires models are in very good agreement with cloud deck b/t
400-800 feet during the day Monday.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Monday evening through Tuesday morning
for IAZ027-028-037>039-048>050-059>062-071>074-081>084-092>094.

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

Winter Weather Advisory from 6 PM Monday to noon CST Tuesday for
IAZ070.

&&

$$

SHORT TERM...REV
LONG TERM...Kotenberg
AVIATION...Podrazik



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