Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 200507

Area Forecast Discussion
National Weather Service Des Moines IA
1207 AM CDT Fri Oct 20 2017

.SHORT TERM.../Tonight through Friday/
Issued at 327 PM CDT Thu Oct 19 2017

Another relatively quiet short term, but still a couple of nuggets
to discuss. The weak frontal boundary overnight was quickly replaced
by returning southerly flow this morning. Remnants of the boundary,
namely the 30s dew points, remains over central Iowa at the moment
but will slowly recede northward as low-mid 50s dew points return.
Overnight lows will be much more mild in the low-mid 50s versus last
night/this morning in part because of the rising dew points and also
increased overnight winds from the tightening pressure gradient as a
surface low builds off the Rockies and into the Plains.  Fri,
especially over western areas, will be windy and gusty at times with
previously mentioned surface low building off the Rockies. Would not
be surprised to see sustained 20 mph and gusts over 30 mph at times,
with lesser values further east. Increased winds over guidance by a
couple of kts, but may need to be bumped up more in subsequent
shift. While 850mb temps are a bit cooler Fri than today, highs
likely to reach mid, even upper, 70s with aide of strong WAA.

.LONG TERM.../Friday night through Thursday/
Issued at 327 PM CDT Thu Oct 19 2017

Summary...Quiet, dry conditions will come to an end Friday overnight
and Saturday with showers and storms followed by colder, below
normal, temperatures during the week. The first freeze for some will
be possible during the week as well.

Friday Night through Sunday Morning...Hopefully everyone enjoyed the
pleasant weather this week as it will be replaced by breezy and
stormy conditions this weekend. A strong surge of theta-e
advection/moisture transport ahead of a trough exiting the SW US
will enter the region Friday afternoon and evening, acting to begin
saturating the currently extremely dry column. Considering the
amount of dry air currently in place and a nice 800-850mb cap in
place, it will take quite some time for any precipitation to
develop. Likely not until 06z Sat for any sizable areal coverage.
Euro/GFS/Canadian all suggest a small window of nearly uncapped to
uncapped 1000-1500 J/kg CAPE near the front Sat afternoon, which
would provide the best opportunity for any strong to severe
potential. Even so, deep layer shear profiles are unimpressive, with
most of it concentrated in the lowest 1km. Given progressive frontal
boundary and strong parallel surface, linear convection along the
front most likely with threat for damaging winds the greatest.
Potential for any flooding/water issues, with well above normal
PWats around 1-1.5 in, will be limited by both dry antecedent
conditions and progressive frontal boundary. The front should be
through the CWA by 12z Sunday, leaving in its wake cooler conditions
in the 60s for highs.

Sunday through Thursday...Nothing substantial with regards to
precipitation within this window. The main story will be a troughs
surging south out of Canada both early and mid week that will bring
about the coldest temperatures of the season. Over the last couple
of days, there have been significant differences within the
Euro/GFS/Canadian solutions, with the Canadian the biggest outlier.
Most recent runs (12z) the Canadian continues to be an outlier, but
is coming closer in line with Euro/GFS solutions. The main
discrepancies being the handling of interactions/phasing of a closed
low moving north out of the southern US and the trough surging
southward out of Canada early in the week and precipitation
potential later in the week. 850mb temps may dip into the 0 to -4
deg C range Tue night/Wed morning and -2 to -6 deg C range Thur
night/Fri morning, resulting in coldest lows so far this season.
Currently have low 30s going across the north in both instances,
though Fri morning lows will likely need to be lowered should
consistency be shown in subsequent runs. For the time being, have
made minimal changes to both windows.


.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1206 AM CDT Fri Oct 20 2017

Widespread VFR conditions are expected for the entire period
although ceilings will thicken by the end of forecast timeframe.
Visibilities will be unrestricted with strong south to southwest
winds, especially during the day.





LONG TERM...Curtis
AVIATION...Cogil is the U.S. government's official web portal to all federal, state and local government web resources and services.