Area Forecast Discussion
Issued by NWS Des Moines, IA

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906
FXUS63 KDMX 240504
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1204 AM CDT Tue Oct 24 2017

...Updated for 06z Aviation Discussion...

.SHORT TERM.../Tonight through Tuesday/
Issued at 330 PM CDT Mon Oct 23 2017

The primary concern into Tuesday remains strong winds. Water vapor
imagery and objective model guidance nicely shows a short wave/PV
anomaly along the western IA/MO border at 19z. Any precip produced
by this feature looks to be driven by high based DPVA and
frontogenetical forcing and should shortly exit the forecast area.
Winds may continue to surge a bit into the early evening hours, even
beyond sunset, with the objective analysis still showing the surface
isallobaric max upstream from SD into MN. The sustained winds may
diminish slightly, but RAP, NAM and GFS soundings with projected
mixing all suggest gusts may reach 40+kts across northern Iowa with
lesser gusts farther south.

After a minor lull into the late evening hours, forecast soundings
again show strong mixing and gust potential into the lower 40s north
06z-15z, which is the same time the MSLP gradient strengthens.
Consensus MOS guidance depicts sustained speeds of 25 to 35 mph
through the early afternoon hours, which is also supported by SREF
mean winds and 25+kt probabilities. NAM and GFS .5-1km layer winds
also depict gust potential in at least the lower 40s. These factors
have all led to medium to high confidence in advisory criteria wind
conditions. The soundings and winds aloft suggest the advisory may
be a tad long, but peak wind potential likely in the 09z-18 window.

The mixing may be tempered slightly however as deep forcing
increases lift on the back side of the Great Lakes cyclone
tomorrow, which may brush eastern sections with light rain early
Tuesday. Tuesday will likely be at or near the coolest daytime
temperatures of the season to date with highs only anticipated to
reach the middle 40s to lower 50s.

.LONG TERM.../Tuesday night through Monday/
Issued at 320 PM CDT Mon Oct 23 2017

Long range forecast begins Tuesday night with steadily relaxing
winds as a deep surface low pushes northeast from the Great Lakes
Region. Quick round of WAA Wednesday as a thermal ridge swiftly
moves east across the central Plains. This will lead to at least
one day of near to above normal highs in the low to mid 60s. A
robust PV anomaly dives southeast out of Canada late Wednesday and
digs into the northern Plains, pushing a cold front through the
CWA Thursday. Strong CAA and a tight pressure gradient behind the
front will create blustery conditions. Bufkit soundings indicate
35-40+ kt winds at the top of the mixed layer, so it`s reasonable
to assume that we will approach wind advisory criteria. Non-
diurnal temp trends are also possible, but that will be highly
dependent on frontal passage timing. GFS/NAM bring the front
through Thursday morning with falling temps through the afternoon.
Euro/Canadian are ~12 hours slower with mild afternoon temps.
This has the temp forecast for Thursday in flux and will need to
be refined once timing issues are resolved. Not expecting any
precip associated with the frontal passage as fronts earlier in
the week scour out most available low level moisture.

Little change to the Friday/Saturday forecast. The trough digs south
and becomes highly amplified over the central CONUS. Highs only in
the 30s/40s remain on track for Friday and blustery northwest winds
will make temperatures feel even cooler. Still a chance we could see
some light rain/snow in Iowa in this time frame. The best moisture
and lift will likely stay north of the surface low tracking across
southern MN. 12z model runs advertise light snow as far south as
DSM Saturday morning, but no mention was added to the forecast due
questions regarding dry air and run-to-run inconsistencies. Below
normal temperatures continue through the weekend as the upper
level low slowly pushes east. A hard freeze and end to the growing
season may occur Fri/Sat nights as temps drop into the 20s.

&&

.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1203 AM CDT Tue Oct 24 2017

Strong winds remain the theme for this TAF package. High
confidence in widespread 25 to 30 kt sustained winds, with gusts
of 35 to 45kts. Slight chance of MVFR remains at KMCW and KALO.
Winds should die down after sunset tonight.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM CDT this evening for IAZ004>007-015>017-
023>028-033>039-044>050-057>061-070>073-082-083.

&&

$$

SHORT TERM...Small
LONG TERM...Martin
AVIATION...Kotenberg



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