Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 152316
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
616 PM CDT Sat Jul 15 2017

.SHORT TERM.../Tonight through Sunday/
Issued at 329 PM CDT Sat Jul 15 2017

Confidence: Medium

Fairly straightforward short term except for a few possible wrinkles
with the mesoscale. 18z subjective surface analysis shows a weak low
over Upper Michigan with a trailing cold front extending southwest
through Minneapolis back toward Aberdeen South Dakota. A trough
precedes the boundary over south central Minnesota back to about
Sioux Falls. Near and south of the boundary dewpoints have been
pooling to the lower to mid 70s this afternoon...along with
temperatures rising to about 90F by 2 pm. A large area of uncapped
sfc based cape over central/western Iowa now ranges from 4000 to
4500 j/kg...with mlcape a more modest 3000-3500 j/kg. Effective bulk
shear over most of the state ranges from 25-30kts presently and is
not expected to increase much from now through tonight. So
far...weak surface flow/convergence has kept convection from being
realized though a field of cumulus is developing just ahead of the
prefrontal trough. Aloft H700 temperatures remain just below 10C
with little cin also reflected on SPC mesoscale analysis sector.
Hires models continue to focus convective initiation along upper
level boundary/shortwave now in northern Wisconsin/Minnesota between
21-00z then quickly forcing the scattered storms south over the next
12 hours. The trailing edge of the storms may clip the
northeast/east prior to 06z then move east of our forecast area by
12z. Given the timing of the storms moving in after 02z...our area
will most likely lose enough instability to keep the severe threat
in check as the current marginal suggests. The front should reach
the I80 corridor by 12z with overnight lows still in the upper
60s/lower 70s south and mid 60s over the north.  Some patchy fog is
possible...especially nearer the boundary in the south toward
sunrise though widespread fog is not anticipated. Models are now
more aggressive with the boundary and are stalling the front just
south/west of our area by 00z Monday. This should keep most of the
area dry and cooler northeast. Over the south closer to the
boundary...highs will top off near 90 and along the border some
isolated thunderstorms will be possible by late afternoon along the
IA/MO border between 20-00z.

.LONG TERM.../Sunday night through Saturday/
Issued at 329 PM CDT Sat Jul 15 2017

Synopsis...
Extremely Broad area of anticyclonic flow at 500mb has established
itself from the Mississippi River southwestward towards the Baja of
California. By Tuesday, core of anticyclonic flow moves towards the
Ozarks/Oklahoma... serving to push the DMX CWA into return flow.
Long-range model guidance trying to suggest core of high pressure to
puss into the Deep South by next weekend...marking the end of this
upcoming week`s heat wave. However, I am initially skeptical as long-
range models tend to be a little overly fast in terms of moving a
large-scale feature such this out.

Above Normal Temperatures/Excessive Heat...
Confidence: High

With 850mb temperatures around +18C to +22C under scattered clouds,
much of the DMX CWA reached into the low 90s for Saturday afternoon.
850mb temps should remain in this range for Monday and
Tuesday...initially suggesting comparable temps to Saturday pending
cloud cover and precip.

Wednesday and Thursday... 850mb temps jump in a range of +22C to
+26C. The source region of this air was over central to Nebraska on
Saturday...where temperatures were already in the mid 90s by 2 pm
local time. Tracking dewpoints, no reason to see much departure
from persistence forecast from Saturday pm... meaning sfc dwpts in
the upper 60s to mid 70s seems plausible. Combining temperature
and dewpoints, apparent temperatures should be well into the
triple digits both Wednesday and Thursday. With minimum
temperatures bottoming out in the low to mid 70s near and south of
Highway 20, Heat Advisory seems very plausible at this time.
Cannot rule out Excessive Heat Watch/Warning either. If anything,
max temps for Wednesday are conservatively cool and could be
bumped up several degrees in subsequent afternoon forecast package
updates.

Strong to Severe Thunderstorm Potential...
Confidence: Low

Somewhat similar to last week, instability/CAPE off the charts each
afternoon, with sfc dwpts into the 70s. For sustained convection to
generate, location of trigger/focusing mechanism key.

At this time, it appears that each day, with the
exception of Wednesday, will feature a weak/nebulous shortwave
racing eastward through and near to northern IA/southern MN, as
that portion of the region will be socked in W/NW flow. Given the
small size of these shortwaves, expect the models to resolve them
in a highly variable manner in terms of timing and location...thus
impacting where/when boundaries set up across Iowa during peak
heating. Subsequently, see no point in getting into minutia of
detail at this point and have kept with broadbrushed POPs focused
generally across northern Iowa for this time period. POPs for
Friday and beyond may need to be adjusted northward to account for
slow- down of ridge departure. In terms of svr potential, similar
to last week, large hail and damaging winds would be primary
threats. PWATs somewhere between 1.5 and 2 inches... certainly on
the higher side of the spectrum...would suggest at least a
moderate rainfall potential in storms that are able to be
organized. As is typical this time of year, CIN looks to win out
during nightfall...meaning storms will likely weaken each
overnight.

&&

.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 616 PM CDT Sat Jul 15 2017

VFR conditions expected through the forecast period. Light and
variable winds becoming more northerly to northeasterly into
Sunday. A chance for a thunderstorm with development expected
around KALO by late evening. Otherwise majority of thunderstorms
should stay to the east of the terminal sites.

&&

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

&&

$$

SHORT TERM...REV
LONG TERM...Kotenberg
AVIATION...Beerends



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