Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 210451
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
1151 PM CDT Tue Sep 20 2016

.SHORT TERM.../This evening through Wednesday/
Issued at 326 PM CDT Tue Sep 20 2016

Through the day today high pressure continued to slide eastward as a
surface low over southern SD moved towards the area. With the
tightening surface gradient, winds were gusty across the
west/northwest, boosting highs beyond original guidance into the
upper 80s. Additionally, showers pushed across southern MN/northern
IA during the early afternoon associated with a mid-level theta-e
push.

Main concern through rest of period will be whether or not storms
that develop tonight affect northern/northeast CWA. Guidance
continues to peg significant push of theta-e advection through the
low and mid levels as moisture returns in earnest. While a
stationary surface boundary/wind shift exists over southern MN, it
will not be the impetus for initiation. Main warm front/moisture
discontinuity further south across central IA will continue its push
northward, though has been slower than originally anticipated. With
boost from expected ~40kt LLJ, widespread initiation north of Iowa
likely to occur after 03z this evening. Anticipate convection to
eventually turn SE and ride instability gradient, possibly bringing
northern/northeast CWA into play. Potential for severe weather
greatest to the north, however with MUCAPE around 2000-3000 J/kg and
0-6km shear around 30-40 kts, cannot rule out isolated elevated
hailer early if convection initiates further south and damaging wind
potential as storms conglomerate. Heavy rain will be primary threat
with PWats approaching 1.75 inches or more and great low level
moisture transport into region through the overnight hours. However,
do expect a more progressive nature to storms as evening progresses,
reducing flooding concerns.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 326 PM CDT Tue Sep 20 2016

Heavy rainfall potential the main concern through the period...though
heat and humidity likely to be a short concern for the southern counties
tomorrow. Models continue to advertise potential for repeated rainfall
across the north possibly late tonight...and more likely tomorrow
and continuing into the overnight hours Wednesday night...lingering
into Thursday. Though front passed south of the region yesterday...wave
already developing over south Central South Dakota with very moist
airmass south of a stationary front in northern Missouri. Aloft...several
factors will combine for a quick return of moisture to the Midwest and
Iowa. A ridge of high pressure is parked over the lower Mississippi
River Valley with a pronounced fetch of southwest flow from Texas
northeast to Minnesota. Also...remnants of Tropical Storm Paine
is expected to be caught up in the flow over the next 1 to 2 days
and help to fuel the heavy rainfall potential...especially for
tomorrow evening through Friday. There will also be a semi-persistent
warm frontal boundary that sets up over northern Iowa to southern
Minnesota.

Some uncertainty still exists as to where the boundary will set
up...with convection possibly farther south into Iowa through the
period.  None the less the better potential for heavy rainfall and
flash flooding will remain over the northern half of our forecast
area through the period. Scattered thunderstorms will likely continue
into the morning hours tomorrow north...then redevelop in the
afternoon across the north closer to the boundary and just north
of the 10C cap at H700. By evening another stronger push of Thetae
advection and increasing low level jet to 40kts by 06z as moisture
from old TS Paine gets ingested into the persistent southwest flow
aloft. The threat for heavy rainfall should be maximized during
the 00-12z period Thursday with continued scattered thunderstorms
during the day on Thursday. Temperatures will continue warm Wednesday
with highs in the upper 70s north and upper 80s south...cooling slightly
Thursday. The nearly stationary boundary will again lift north Thursday
night into Friday with lesser chances for thunderstorms Friday while
temperatures again heat up. With the mean trough west of the region
until Saturday night...another period of wet weather is anticipated
Saturday night into Sunday with the potential for additional longer
duration rainfall due to another tropical connection. Finally by Monday
into Tuesday dry weather briefly returns.

&&

.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1151 PM CDT Tue Sep 20 2016

Storms have fired across southern MN and may trickle southeastward
toward KMCW and KALO. Current trends expect the storms to slide
just east of the CWA, but will have to monitor. Still left the
VCTS for the possibility. Otherwise southerly winds picking up
again during the day Tuesday. Additional storm activity expected
late Wednesday. May end up being beyond the current forecast
period, so have not included in the TAF currently.

&&

.HYDROLOGY...
Issued at 326 PM CDT Tue Sep 20 2016

Heavy rainfall is expected over portions of southern Minnesota and
northern Iowa through Thursday afternoon...with additional showers
and thunderstorms possible Friday...and again Saturday night
through Sunday. Current river outlooks suggest that portions of
the Winnebago River...Cedar River and its tributaries may
experience some flooding by Friday and remain in flood into the
weekend. With the potential for repeated rainfall from late
tonight across the north and from Wednesday evening through
Thursday afternoon for most of northern Iowa...and rainfall
amounts of several inches possible over far northern Iowa...flash
flooding and areal flooding will be possible for areas generally
north of Highway 20 tonight and along and north of portions of
Highway 30 Wednesday evening through Thursday evening.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Thursday evening for IAZ004>007-
015>017.

Flash Flood Watch from Wednesday evening through Thursday
evening for IAZ023>028-033>039-044>048.

&&

$$

SHORT TERM...Curtis
LONG TERM...REV
AVIATION...Beerends
HYDROLOGY...REV



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