Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 191145
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
643 AM CDT Mon Sep 19 2016

.SHORT TERM.../Today/
Issued at 412 AM CDT Mon Sep 19 2016

Currently a shortwave is exiting to the East and the associated
forcing with that is East so the threat for early morning precip is
quickly becoming a non-concern.  A cold front is poised to push into
far Northwest Iowa by 12Z and likely pass to the East of Mason City
by 15Z.  By 18Z the front should lie somewhere along a Waterloo to
South of Fort Dodge to Omaha line.  The front should be through the
metro area by 21Z and into far Southeast Iowa by 00Z.  South of the
boundary moisture and heating will be pretty impressive. The front
will be bisecting the state during peak heating and instability will
peak out at between 3500 and 5000 J/kg.  That is quite impressive
for mid-September.  Adding to the plus column for storms today will
be deep shear...especially over East and Southeast Iowa from 21Z to
00Z.  On the negative side of things...we will be pretty well capped
except for the Southeast and the best forcing between 18Z and 00Z
will be over East/Northeast Iowa eventually working into Southeast
Iowa so the best area to see the cap break and have explosive storm
development.  So even though we have a narrow window and fairly
narrow zone of where storms are most likely to fire...the dynamics
are very good for robust development and strong/severe storms.
SPC SWODY1 outlook puts a 5 percent tornado probability into Eastern
Iowa this afternoon an while that`s a small percentage...I cannot
argue against the possibility.

Adding to the scenario will be a surface low progged to develop
along the boundary which could bring the storm chances a bit further
to the Southwest during the late afternoon but for now and for the
reasons discussed above...my confidence is too low to bring pops
further Southwest.  As for temps...they were underdone with the warm
advection yesterday and South of the cold front temps should again
shoot up today.  I went a couple degrees above the warmest guidance
across the Southeast forecast area for this reason.

.LONG TERM.../Tonight through Sunday/
Issued at 412 AM CDT Mon Sep 19 2016

The threat for thunderstorms will linger across the southeast this
evening as the NAM/RAP and CAMs are more aggressive eroding the
elevated mixed layer. A few severe storms will remain possible.
While an isolated tornado is possible the low level helicity and
shear is marginal the greatest threat will be damaging wind gusts
with the 0-3 km theta-e differences exceeding 30K. Large hail will
be possible with intense updrafts with melting levels being deep.
Storms will be progressive and should be out of the area by late
evening.

Little change in the forecast Tuesday through Friday except
increasing confidence in storms over northern Iowa and the
increasing probability for heavy rainfall. As mentioned in
previous discussions the large upper low will begin digging into
the Pacific Northwest and begin to transition the upper flow from
zonal to southwest. This will allow the low level jet to veer into
Iowa Tuesday night with theta-e advection spreading across the
state and will bring a return of good instability and moisture.
Thunderstorms should develop over southern Minnesota into northern
Iowa overnight. If storms can become rooted storm motions will
slow and begin to become slow to the south. Heavy rainfall will be
a concern if this occurs. In addition given the amount of
instability and increasing deep layer shear a few elevated
supercells may occur thus severe weather will also be possible.
Thunderstorm chances on Wednesday will linger north but a favorable
environment for cold pool development could generate outflow from
these storms and drive south under the instability bringing chances
into central Iowa.

Wednesday night may bring a more significant threat for heavy
rainfall. Moistures and fragments from tropical storm Paine over the
eastern Pacific will be ingested into the steepening southwest flow
as the upper low continues to settle south. The moisture associated
with Paine will arrive late Wednesday and push PWATS back to near 2
inches. Storm motions will again be quite slow and will have the
potential for several rounds of storms over the northwest half of
the state. Pending on how the Tuesday night event evolves with
rainfall across this region, potential is there for a significant
hydrologic event to occur with a significant amount of rainfall
occurring. Threat would be two fold with an initial threat for flash
flooding followed by main stream flooding potential. Will continue
to monitor with finer details in upcoming forecasts. Thunderstorm
chances may linger Thursday into Friday with a surface boundary hung
up over the state. A warm front will lift north through Iowa as the
upper low to the west begins to eject northeast and a surface
reflection develops. This will bring a break in the precipitation
chances late Friday into a portion of Saturday and perhaps beyond
until a slow moving cold front arrives with more chances.
Temperatures through this week will be quite warm for mid September.

&&

.AVIATION.../For the 12Z TAFS through 12Z Monday night/
Issued at 643 AM CDT Mon Sep 19 2016

A cold front will push Southeast across TAF locations today and lie
between KALO and KDSM between 18Z and 22Z then to KOTM around 00Z.
VFR conditions expected over TAF locations except for a brief period
of potential MVFR cigs and vsbys in TSRA associated with the FROPA.
TAF sites expected to be impacted by potential MVFR conditions are
KALO...KDSM and KOTM.

&&

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

$$

SHORT TERM...FAB LONG TERM...Donavon AVIATION...FAB



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