Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 230447
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1147 PM CDT Fri Jul 22 2016

...Updated Aviation Discussion...

.SHORT TERM.../Tonight/
Issued at 355 PM CDT Fri Jul 22 2016

Main forecast concern will revolve around the ongoing heat warning
and thunderstorm chances.  Aloft high pressure extending from
Oklahoma through the upper midwest will remain in place through
Saturday.  Northern and Eastern Iowa will be on the periphery of the
upper ridge leaving that area susceptible to lower ceilings from
stratus/stratocu.  That area could also see convection from any weak
wave riding the ridge.

Across Southern Iowa models all point to a stronger shortwave coming
through the ridge later tonight.  There is some credence to this as
Southern Nebraska currently has storms in their area and this may
push into far Southwest Iowa later this evening.  Additionally, a
weak,quasi-stationary boundary currently along/North of the highway
20 corridor will also be a focus for peak heating convection.  This
was pin-pointed by most of the hi-res models though they varied in
coverage and intensity.  Certainly radar has shown storms developing
near this boundary between 2 and 3 PM.  This will likely be the main
issue through the evening hours.  Some of the more robust storms
could produce locally heavy rainfall and gusty wind. A storm this
morning was able to grow upscale enough to produce small hail and
that potential will certainly exist through the evening hours.  It
would not be out of the question to see an isolated report of
damaging wind or nickel hail...possibly quarter hail. As we get past
the peak heating, those storms along the highway 20 corridor should
wane. Then later tonight issues will be more nebulous. The
approaching shortwave across the South could potentially get some
isolated to scattered storms going, the outflow from this evening`s
storms may trigger additional storms and the quasi-stationary
boundary across the North may begin to shift North as a warm front
late tonight. Forcing is better North and East/West boundaries tend
to be problematic so the highest pops tonight will remain up North
with at least isolated/ slight chance pops elsewhere.

As for the heat warning...the across the South today really brought
indices down for a fair part of the day but with afternoon heating
they have recovered.  The additional storms this afternoon/evening
will have the same impact so heat indices will reach warning
criteria in a few places Southwest but be marginal at best
elsewhere.  However, given that Northern locations expire at
00Z...no plans to change anything except to extend the Southern
segment West to line up with neighbors to the West.

.LONG TERM.../Saturday through Friday/
Issued at 355 PM CDT Fri Jul 22 2016

Stout southern plains blocking high that has been responsible for our
stretch of hot days will slowly break down this weekend. This will
allow for the return of more seasonable weather, but also multiple
chances of precipitation. Elevated storms will likely be ongoing at
the start of the period Saturday morning along a lifting warm front
across the north as a weak H500 shortwave progresses through the
flow, though the wide CAM solution variability necessitates lower
confidence POPS. Warm sector should become well established by mid
to late morning with the NAM/GFS/EC pulling the warm front well into
Minnesota. Have upped temps and dewpoints by several degrees in the
north. Despite this increase in T/Td values, the north should remain
below heat advisory criteria. Did expand the Excessive Heat Warning
further west to the border with OAX, though forecast HI values
along the CWA border should only touch Heat Advisory criteria for
a short time tomorrow.

Cold frontal boundary will slide through the state Saturday night
into Sunday morning. Model BUFR soundings show a pronounced cap
present below 700 mb and the best upper level jet support looks to
be relegated further north into Minnesota. This overall northward
shift is reflected in the 12Z POP fields of the EC/GFS/NAM and
only kept chance POPs in the north--which may be generous. The
GFS/EC diverge on the southward progression of the boundary Sunday
with the EC bringing the boundary into Missouri while the GFS is
closer to the IA/MO border. Maintained the going forecast, which
followed closer to the EC. Multiple, poorly timed H500 shortwaves
propagate across the state for the middle to latter part of the
week as northwest flow becomes established over the region, which
will serve as the focus for additional shower and thunderstorm
chances.

&&

.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1145 PM CDT Fri Jul 22 2016

VFR conditions will prevail through the entire TAF period. No
mention of thunder due to the isolated nature and low confidence
in timing and location Saturday afternoon and evening.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning until 7 PM CDT Saturday for Adair-Adams-
Appanoose-Audubon-Boone-Carroll-Cass-Clarke-Crawford-Dallas-
Davis-Decatur-Greene-Guthrie-Jasper-Lucas-Madison-Mahaska-Marion-
Marshall-Monroe-Polk-Poweshiek-Ringgold-Story-Tama-Taylor-Union-
Wapello-Warren-Wayne.

&&

$$

SHORT TERM...FAB
LONG TERM...Skow
AVIATION...Podrazik


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