Area Forecast Discussion
Issued by NWS Des Moines, IA

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051
FXUS63 KDMX 172139
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
339 PM CST Sat Feb 17 2018

.SHORT TERM.../Tonight through Sunday/
Issued at 336 PM CST Sat Feb 17 2018

System that affected the area earlier this morning is pulling east
of Iowa at this time. As expected the airmass behind the snow is
warmer and temperatures have responded nicely over most areas that
have received sunshine today.  GOES16 vissat shows the remaining
snowpack across the northwest toward the east central this
afternoon. Subjective sfc analysis this afternoon shows ridge of
high pressure to our west right on schedule for the overnight hours.
With that sliding east...skies will remain clear for at least the
first 6 hours of the forecast tonight. This...combined with todays
snow/melt and lingering snowfield over the north bodes for some fog
formation in the latter evening through overnight.  HRRR/RAP13 and
ARW/NMM suggest coverage greater over the north. Between 06-12z warm
air advection aloft will result in more clouds over the north which
may mitigate the fog to some degree. For now will introduce patchy
fog over the north and let the evening/overnight shift refine the
details. Mins tonight will fall quickly into the teens north and mid
20s south with the briefly clear skies. Any fog/lower clouds that do
develop will likely extent into the 12-14z timeframe Sunday. Sunday
will see a decent warming once again. Models in good agreement
regarding highs tomorrow. Strong southwest winds will help move the
warm airmass north and despite the snow cover north...temperatures
should respond nicely in the afternoon. Have made only minor
adjustments to the highs tomorrow given the good agreement amongst
the guidance.  Winds Sunday will be brisk during the day with
stronger gusts expected. Some concern across the west tomorrow with
soundings suggesting winds may exceed 45 mph over the western
counties on Sunday with some areas seeing sustained winds greater
than 30 mph between 15z and 21z. Will pass along to evening shift
for possible headline issuance later this evening. Possible that
with warmer air the higher winds will remain slightly elevated.
Highs tomorrow will rise into the lower to mid 40s north and into
the mid to upper 50s south.


.LONG TERM.../Sunday night through Saturday/
Issued at 336 PM CST Sat Feb 17 2018

Bottom Line Up Front...
Hazardous weather likely Monday and Tuesday as two rounds of
wintry precipitation likely. Round 1 will begin Monday morning,
bringing freezing rain the northwestern third of IA, Round 2 will
impact central to SE Iowa beginning Tuesday morning. With elevated
winds, adverse impacts from freezing rain may be realized more
quickly.


Monday and Tuesday...

Setup...
20z Sat water vapor imagery picking up on an impulse spinning off
the Baja California. Models now coming into pretty decent
agreement with the track and the timing of the forecast track of
this impulse. Run-to-run consistency also starting to improve,
which ups forecast confidence. Overall, have gone with a model
blend, minus the NAM, which seems like an outlier. Worth noting
the ECMWF has has an ever so slight trend in its past 5 runs of
being slightly further north with the track of the low... which
would bring the warm sector further north/Northwest. The GFS has
been slightly oscillating around its current solution, with the
CMC/GEM not too far behind. So again, a blend seemed most
appropriate here, but worth keeping an eye on the slightly
further north/northwest tendency of the ECMWF.

SFC low should be near northern KS by 06z Mon, central to SE Iowa by
18z Mon, and near southern Wisconsin by 00z Tue... so it will be a
fast-moving low. Moisture advection is one major player with this
system... Leading the way for this low, a huge 850mb 40+ jet streak,
with a 55kt jet core, will extend all the way down to the Gulf of
Mexico, serving to get abundant gulf moisture to the Midwest in a
hurry. The other player is strong/off-the-charts frontogenetical
forcing from 950mb to 900mb. This is all well-phased with strong
low- level moisture flux convergence and widespread theta-e
advection. Interestingly, there will be a somewhat extreme
temperature gradient across Iowa, with SE Iowa likely hitting 60
degrees, and NW Iowa struggling to reach 20 degrees, with a wind
chill near zero. Thunder threat seems likely in warm sector.
Severe threat nearly nil.

Of note, there have been hints at a dry slot oriented from SW to NE
arriving in SW Iowa sometime between 00z Tue and 06z Tue to give
a "break" to the precip across places not SE Iowa. The 12z Sat run
was much less weaker with this dry slot than the 00z Sat runs, so
will see if this trend continues... if it does, precip amounts
will need to be increased.

From around 12z Mon through 06z Tue or so, NAEFS show an anomaly
of +3 to +4 std dev for PWAT across SE Iowa. Climatological
sounding analysis at KDVN shows daily max PWAT for this time of
year around 0.8 to 0.9 inches. 12z Sat GFS forecasting PWAT close
to 1.2 inches. CIPS Analog Guidance pinning around 0.75 inches to
1 inch of QPF in SE Iowa, which is close to what the GEFS plumes
show. Moral of the story, is confidence is high with over 0.75
inches of rain in SE Iowa, and medium-high in over 1 inch.

Mixed Precip...
In terms of mixed precip types, fcst soundings across northern
Iowa show a distinct lack of saturation below -8C. So, ice
introduction may not be there. Warm layer around 3.5kt thick, and
then AOA 3kft, temperature profile quickly crosses below zero and
stays there. Winds atop a very shallow mixed layer in the 20 to 30
mph range, with a pressure gradient near 3mb/85 km. Other
enhancers not present, so winds near 20 mph seem plausible.
Putting it together, sounding analysis suggest sleet/freezing
rain. Strong winds will enhance icing impacts. Winter WX Advisory
will likely be needed and Ice Storm Warning cannot be ruled out at
this point. As time increases, minor oscillations will hopefully
be straightened out.


Wednesday and Beyond...
A 1040mb high propagates across the upper Midwest, keeping the
forecast dry through mid-week. The GFS and GEM have been trying to
hit Iowa with a quick-moving snow producer on Friday. The ECMWF,
however, has been slower and strong with the sfc high... lingering
it around the Great Lakes/Ohio Valley Regions into next weekend.
So, Friday system has much lower confidence with it than current
forecast projects.


&&

.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1213 PM CST Sat Feb 17 2018

Area of snow and lower visby/cigs quickly moving out this
afternoon. Some mixing has followed the clearing and will be
updating winds at KDSM/KOTM through 21z. Overnight some increase
in clouds again...and some uncertainty whether lower stratus/br
deck will reform due to todays snowfall/melt and tonight clearing
skies. For now have not included...but hires models suggesting
some potential. Winds increase aloft aft 12-14z Sunday with
potential llws across the area. Will pass along to evening avn
forecaster to evaluate these two items further. /rev


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

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



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