Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 131052

Area Forecast Discussion
National Weather Service Des Moines IA
552 AM CDT Tue Apr 13 2021

...Updated for 12z Aviation Discussion...

.DISCUSSION.../Today through Monday/
Issued at 307 AM CDT Tue Apr 13 2021

Forecast Highlights:
-- Frost potential remains this morning and Wednesday morning,
   mainly over southern Iowa
-- Below normal temperatures through Saturday
-- Little in the way of widespread rain chances until late Friday
   at the earliest

Details: GOES-East upper level water vapor imagery shows an upper
low departing the Middle Atlantic States and another over Minnesota
with easterly mid-level flow along and north of the US-Canadian
border. A trough is digging over the Intermountain West with an
objectively analyzed upper low beginning to form over northern
Idaho. These two upper lows be the main drivers of our weather this
week providing cooler than normal conditions for mid-April. The
upper low over Minnesota will remain put today before slowly
beginning to depart to the east on Wednesday. With steep lapse rates
and the upper low, should be able to get clouds to develop at least
over the northern half of Iowa with the highest coverage over far
northern Iowa. This is also where there could be some showers that
rotate around the upper low late this morning and more so this
afternoon. It will also be another windy spring day today. Already
this morning, there have been a few sites sporadically around Wind
Advisory criteria. The winds should decouple over the next few hours
for a short period before daytime mixing ensues. Forecast soundings
show the boundary layer growing to around 850mb/5300 feet, which is
a bit less than the past few days where it grew to around 750mb/7000
feet. Winds at the top of the mixed layer will be around 30 knots
and momentum transfer to the surface is expected. While it will be
windy today, relative humidity should be a bit higher. The 00z HREF
ensemble minimum has no less than 30 percent with the current
forecast near 30 percent as dewpoints were trended toward the 25th
percentile of available guidance. So a somewhat elevated fire
weather day is in store, largely due to the windy conditions, which
puts the GFDI in the high category at worst. Temperatures were
nudged down a bit today as National Blend of Models is on the hotter
end of guidance, perhaps a symptom of our earlier warm weather and
its bias correction. Temperatures were lowered a bit in the
Nishnabotna Valley tonight as well and expect another round of frost
possible in some areas south of Highway 30. This may necessitate a
headline for tonight (Wednesday morning) across parts of our
southern counties where there may be susceptible vegetation and

As the upper low begins to move over the Great Lakes, this should
keep the rain chances northeast of Iowa. It will be breezy as well,
but forecast soundings do not show as strong of winds at the top of
the mixed layer as compared to today. That said, sustained winds of
15 to 20 mph will be common across central Iowa. Attention then
turns to the upper low that is over northern Idaho early this
morning. That will dig and meander over the western US until the
upper low over Minnesota begins to move east. It will then start to
move into our region on Friday with the upper low passing south of
Iowa on Saturday. Shower potential will increase toward Friday night
as QG convergence ahead of the upper low increases over the state.
The ECMWF has generally weaker QG convergence compared to the GFS,
which has strong QG convergence just south of the state bleeding
into southern Iowa. That forcing lingers into early Saturday, but
wanes much faster in the ECMWF compared to the GFS, which maintains
forcing through Saturday night. Looking at ensembles, the ECMWF and
CMC are split pretty evenly on whether or not there will be rain on
Saturday. The GEFS, likely owning to the forcing displayed in its
deterministic run, has a majority of its members with rain chances
somewhere in the state. At this point, if rain does occur, the
highest chances will likely be over western Iowa transitioning into
southern Iowa. Following closely behind this upper low, a shortwave
will dive in from central Canada late in the weekend, which may
foster additional rain chances. All in all, what does amount from
all these rain chances will be on the light side. The next notable
feature otherwise will be a cold front dropping through the northern
Plains early next week. This could bring additional precipitation
chances followed by perhaps breezy winds.


.AVIATION.../For the 12Z TAFS through 12Z Wednesday morning/
Issued at 552 AM CDT Tue Apr 13 2021

Clouds are moving over northern portions of the state with bases
on the lower end of VFR. Some ceilings below FL030 are possible at
MCW this morning, but bases should largely remain in VFR. There
could also be a passing shower over far northern Iowa, but
confidence is not high enough to include at MCW at this time.
It will be windy today with gusts to around 30 knots over northern
Iowa later this morning into this afternoon.




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