Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 110454
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1152 PM CDT Tue Oct 10 2017

.SHORT TERM.../Tonight through Wednesday/
Issued at 354 PM CDT Tue Oct 10 2017

Upper low currently moving into northern Missouri with broad swath
of warm advection precipitation across north and east Iowa. Also,
some convective cells continue in the far southeast just ahead of
the upper low. These cells will have little impact overall in the
forecast area as the bulk will push quickly east of the area by
late afternoon. Meanwhile, the strong forcing producing the broad
shield of precipitation will continue to lift into Illinois and
Wisconsin with rain ending across the northeast by early evening.
A few isolated to scattered showers will persist into the evening
with the passage of the upper low and associated instability.
However, the overall threat for precipitation should end toward
midnight with relatively dry conditions from overnight into
Wednesday as forcing subsides. Otherwise, low clouds will linger
overnight into much of Wednesday as low level moisture will be in
place across the state. The exception will be in the far western
forecast area where the moist layer will be thin enough for
partial clearing. This will allow temperatures overnight to drop
into the mid to upper 30s with warmer readings to the east where
clouds persist. Highs on Wednesday will remain below normal given
the persistent clouds for much of the area.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 354 PM CDT Tue Oct 10 2017

The main concerns during the extended period were focused on
drizzle potential Thursday morning, precip chances over the
weekend, and windy weather on Sunday. Models are coming into much
better agreement with the stalled front, strong low pressure
system, and a pair of 500mb shortwaves to impact the state over
the weekend. There remains some question on the timing of the
secondary shortwave to roll through the state, with the GFS much
quicker than the ECMWF which would make a more significant change
in the winds Monday. Regardless, highly confident in precip
chances Saturday into Saturday night and even into Sunday morning.

Wednesday night through Friday...surface high shifts east
Wednesday night into Thursday allowing for the return flow (albeit
weak) over the CWA. Forecast soundings via NAM/GFS and even the
SREF suggest saturation below 850mb to the surface with some very
weak mid-level theta-e advection moving across the state Thursday
morning. Have low confidence with any measurable precip and thus
just have mention of drizzle from 06-12z Thursday over the eastern
half of the forecast area. The drizzle soundings linger through
the late morning hours Thursday and may have to extend the timing
of the drizzle mention through 15-18z if solution continues. WAA
ahead of the frontal boundary on Friday will make it the warmest
day in the extended period. Weak frontogenetical forcing looks to
provide enough forcing for shower activity across northern Iowa
Friday and as the front continues to sag further southeast, storm
chances increase Friday night over central and southern Iowa. Low
confidence with any severe threat with the very minimal
instability and weak shear available.

Saturday into Tuesday...An area of low pressure develops over the
central Rockies and transitions across the Plains into the Corn
Belt on Saturday. The low will push the aforementioned front back
north before it passes across the CWA by late Saturday night. A
strong cold front associated with this low pushes across the state
by early Sunday. Both the ECMWF and GFS have two upper level vort
maxes to push across the state with the ECMWF continually being
the stronger, slower, and further south solution wrt to the first
vort max Sunday. The southeast half of the forecast area looks to
remain in the warm sector on Saturday providing some concern for
severe weather potential. MUCAPE values increases to 500 J/kg to
near 1000 J/kg by 00z Sunday with LCLs over 1000 to 1200 meters,
thus mainly looking at elevated storm potential. Low confidence
with severe weather potential but certainly something to continue
to monitor if models shift the surface low track further north
placing more of the CWA within the warm sector.

High confidence though winds will be strong on Sunday and
increased sustained/gusts a few knots across the CWA. Strong PV
anomaly punches across the state on Sunday morning with the first
vort max. Winds atop the mixed layer range in around 40 knots per
ECMWF soundings later in the afternoon and with the GFS a bit
weaker but still strong enough to warrant the increased winds on
Day 6.

&&

.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1152 PM CDT Tue Oct 10 2017

high pressure undercutting the low level moisture far nw creating
clearing and potential frost mainly near EST.  Elsewhere VFR to MVFR
cigs expected over central Iowa with IFR over eastern Iowa becoming
MVFR around daybreak.  Sfc wind becomes SE aft 12/00Z which should
produce some fog and/or lower clouds again.

&&

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

&&

$$

SHORT TERM...Cogil
LONG TERM...Podrazik
AVIATION...FAB



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