Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182333

Area Forecast Discussion
National Weather Service Des Moines IA
633 PM CDT Mon Jul 18 2016

...00z Aviation Update...

.SHORT TERM.../Tonight through Tuesday Night/
Issued at 349 PM CDT Mon Jul 18 2016

The upper level ridge will continue to build north through the
Western Plains tonight then expand east through Iowa Tuesday and
Tuesday night. As this occurs the focus for thunderstorms will shift
northeast as a capping layer overruns the region. The band of
showers and isolated thunderstorms over far Southern Iowa are
gradually diminishing with that area beginning to stabilize a bit.
Broad area of shallow cumulus has developed over West Central Iowa
and into Nebraska. Proximity soundings show this area remains a bit
capped aloft. The lack of strong forcing and mid level stability
should keep the area dry through much of the evening. The best
chance for thunderstorm initiation should be across Nebraska
overnight as a short wave lifts into the area in addition to an
increasing low level jet. This activity should begin to approach
Western Iowa late tonight and into Tuesday. Possible this activity
lingers much of the day while the area of focus gradually shifts
northeast. Heavy rainfall will remain a threat with deep warm cloud
depths and PWAT values of 1.75 to 2 inches. Not planning Flash Flood
headlines at this time due to overall uncertainty of evolution of
potential MCS during this period. There will also be an attendant
large hail and damaging wind threat.

The warmer air will lift into Iowa Tuesday night with thunderstorms
possibly lingering over Northeast Iowa. High temperatures Tuesday
will be conditional on how long clouds and precipitation remain.
Increasing dewpoints Tuesday night will help hold lows in the low to
mid 70s and will be the transition to the much warmer and more humid
conditions for mid to late week.

.LONG TERM.../Wednesday through Monday/
Issued at 349 PM CDT Mon Jul 18 2016

Main Impact:  Excessive Heat
Confidence:  High to Very High

Main focus for Wednesday into the weekend will be the high potential
for excessive heat. Models have consistently shown a large high
directly over the heart of the country beginning on Wednesday and
lasting into the weekend (at least). CIPS analog are comparing this
event to 13-14 July 1995 and 18-19 July 2011, where we had record
dewpoint values in SD/MN/ND.

As of now dewpoint values are progged to be well into the mid 70s.
It will be notable to see how much moisture/rain the Tuesday system
brings to our area.  If rainfall amounts start exceeding the half an
inch range, this added moisture would likely act to nudge
dewpoint values up slightly.

The models have been depicting the synoptic setup for this heat
event the past couple of days. 850 mb temperatures look to be well
stuck into the +25C to +29C range, which is a solid +2 to +3 std
dev. With the high to our southeast, moisture will remain locked in
place through the weekend. This will also serve to keep min temps up
overnight, which will also have an adverse public impact.

For precipitation and this weekend, models are trying to bring some
sort of a boundary/shortwave through on Saturday into Sunday, but
confidence is not terribly high at this time. Thus, have bumped up
temperatures vs guidance as a nod to persistence. Future shifts will
need to entertain the possibility of a heat advisory Saturday into
at least Sunday for our area.


.AVIATION.../For the 00Z TAFS through 00Z Tuesday evening/
Issued at 629 PM CDT Mon Jul 18 2016

Much of remaining cumulus field will dissipate through the
evening, eliminating local MVFR conditions. Main concern overnight
will be development and timing of storms in Nebraska moving
eastward into Iowa. MVFR conditions may develop as storms
progress, but given uncertainty of extent and timing at the moment,
have only introduced VCTS and kept VFR cigs at TAF sites.


Excessive Heat Watch from Wednesday morning through Friday
evening for Adair-Adams-Appanoose-Audubon-Black Hawk-Boone-
Bremer-Butler-Calhoun-Carroll-Cass-Cerro Gordo-Clarke-Crawford-
Mahaska-Marion-Marshall-Monroe-Palo Alto-Pocahontas-Polk-



SHORT TERM...Donavon
LONG TERM...Kotenberg
AVIATION...Curtis is the U.S. government's official web portal to all federal, state and local government web resources and services.