Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 130509
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1209 AM CDT Fri Oct 13 2017

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

.SHORT TERM.../Tonight through Friday/
Issued at 319 PM CDT Thu Oct 12 2017

Low clouds that have hung around much of the day breaking up and
dissipating and should continue to dissipate across the far east
into the early evening hours. An elongated area of sfc low
pressure stretches from the plains of Canada southward through the
eastern Dakotas into central NE and the high plains of CO/KS.
This sfc trough will shift south and east through Iowa late
tonight into Friday. A weak shortwave trough will eject out of the
western US upper trough through the central US interacting with
the sfc boundary resulting in a chance for some showers/drizzle
across portions of central IA Friday. Moisture looking more
limited in the morning with front pushing a bit further south than
previously anticipated, this have trimmed some pops across the
far north/northwest and decreased pops overall through the morning
hours. An overrunning type situation setting up through the day,
with deeper saturation expected toward the afternoon hours with
increased moisture advection/waa across much of central IA. This
should result in more widespread shower chances through the
afternoon hours, and maybe a an isolated thunderstorm across the
far south/southeast where some weak instability is present toward
late afternoon in vicinity of the frontal boundary.

As for temperatures will have CAA aloft this evening, but then waa
increase ahead of the shortwave late tonight as the front stalls
across the forecast area. Also expect low clouds to fill back in
tonight as moisture advection increases. Therefore had temps drop
off fairly decent this evening, then hold steadier across the
central/south late tonight. Warmest temps expected Friday across
the southeast with boundary getting hung up across the central
portion of the forecast area. Therefore expect highs only around
60 in the far northwest with highs in the mid 70s far southeast.

.LONG TERM.../Friday night through Thursday/
Issued at 319 PM CDT Thu Oct 12 2017

The main focus on the extended was on the severe weather potential
Saturday and strong winds Saturday night into Sunday. Models are
good agreement with the overall synoptic setup, timing and
location of the frontal boundary on Saturday into Saturday night.

Friday night into Sunday...WAA wing of a fairly potent low pressure
system strengthening over the Central Plains will cause some shower
and thunderstorm activity across southern Iowa Friday evening and
into the overnight hours towards Saturday morning.  The front is
expected to push back northward Saturday morning as the main surface
low pushes northeast across Iowa throughout the day and continues to
deepen as it moves northeast. The warm sector, generally along and
south of Interstate 80 to the Missouri border, will be the focus of
strong to potentially severe thunderstorms in this location late
Saturday morning into the afternoon. Certainly a very conditional
setup if some instability has a chance to build into the south after
the overnight convection diminishes/pushes northward.  MUCAPE per
NAM/ECMWF/GFS suggestion that it increases by 21z to 2000-2500 J/kg
over the south to southeast sections of the forecast area. Lapse
rates through the column range around 6 to near 7 C/km per OTM
forecast sounding. Shear is backing somewhat in this location and
the 0-1km and 0-3km bulk shear range around 30 knots and 45 knots
respectively. Deep layer shear looks strong to enough to allow
storms to be long-lived but do have low confidence with supercells
during the entire period per the lack of significant directional
shear. More likely scenario is initial development in the afternoon
to be supercellular but quickly transition into linear QLCS type
across the south. Wind damage looks to be the primary severe threat.

Heavy rain is another concern along and south of the front. PWATs
over 1.5 inches which is well above normal for mid-October. WLCD
around 3500m with strong moisture transport into southern Iowa
should provide for efficient rain producers.  Storm movement looks
to be fairly fast and not perfectly oriented with Corfidi vectors,
so training is possible but not as clear-cut.

By 00z Sunday, the surface low deepens to around 1000mb over
southern Wisconsin producing a very sharp gradient across the state
Saturday evening. Strong PV anomaly punches into the state Saturday
evening providing some initially  strong winds/gusts across the
forecast area. Winds atop the mixed layer range near 40 knots or
even higher closer to the surface low. Increased winds several knots
Saturday night into Sunday morning.

Monday through Wednesday...CAA continues throughout the day on
Sunday as a surface high pressure builds into the region by
Sunday night into Monday. No significant storms systems
anticipated over the work week next week with above normal
temperatures forecast as large ridge over the southeast remains
in place.

&&

.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1152 PM CDT Thu Oct 12 2017

Slight tweaks to the 00z aviation package. Have bumped up category
at KALO. At this moment, it appears the best threat of IFR will be
just east of KALO, impacting KCCY. However, will monitor closely
for fog potential near sunrise. Added light precip to the TAFs
during the afternoon. Precip should be light enough to generally
keep vsbys above 6 SM, but will monitor this as well and adjust
later this morning if it appears heavier precip will affect TAF
sites. high confidence in winds under 10 kts.


&&

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

$$

SHORT TERM...Beerends
LONG TERM...Podrazik
AVIATION...Kotenberg



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