Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 180027

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

...Updated Aviation Discussion...

.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

Monday and Tuesday...

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 00Z TAFS through 00Z Sunday evening/
Issued at 622 PM CST Sat Feb 17 2018

Fog potential remains across northern Iowa overnight tonight into
Sunday and introduced IFR mention for visibility at FOD, MCW and
ALO by around 08z SUnday. Strong WAA develops by 12z Sunday or
slightly sooner and this should help dissipate the fog. Kept
mention of LLWS on Sunday with winds aloft increasing to 50 to 65
knots around 2000 feet. Winds look to decouple prior to 00z
Monday, but gradient winds remain strong enough to have mention of
AOA 12 knots.





LONG TERM...Kotenberg
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