Area Forecast Discussion
Issued by NWS Des Moines, IA

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496
FXUS63 KDMX 142340
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
640 PM CDT Mon Aug 14 2017

.SHORT TERM.../Tonight through Tuesday/
Issued at 320 PM CDT Mon Aug 14 2017

Warm front has swung into northeast Iowa with a cold front entering
the northwest part of the state this afternoon. The cold front can
be identified in the enhanced clouds over north central Iowa at mid-
afternoon. CAMs have shown scattered precipitation over this area,
but this is too aggressive given the amount of dry air aloft as
viewed in GOES-16 low and middle level water vapor imagery and that
the forcing via the shortwave trough is exiting the area. Best
chance for any showers or thunderstorms will be over far southern
Iowa through tonight as the cold front dips southward and stalls,
but have held PoPs back from Superblend given mid-level ridging
arriving overhead and the aforementioned dry air. Lows tonight will
be within a few degrees of normal.

Challenge overnight will be fog potential. Certainly a good setup
for radiational fog over northern Iowa with surface high pressure
dropping through Minnesota into the western Great Lakes. The
question is whether the clouds clear out tonight. Cross sections at
KFOD and KALO show the abundant near surface moisture, but also have
high relative humidity above this. Forecast soundings at the same
sites show quite a bit of dry air aloft above the surface inversion.
Have leaned more towards fog across northern part of the forecast
area given the amount of drier air aloft.

As is expected to be the case tonight, think overall the drier air
aloft will win the battle and much of the day tomorrow will be dry.
There are still some low end PoPs, mainly over the southern and
western parts of central Iowa as the stalled front comes back
northeastward. However, main forcing is currently over the western
US and won`t arrive until Tuesday night into Wednesday. Have kept
PoPs out of the northeastern part of the forecast area as high
pressure over the western Great Lakes will keep the drier air in
place.

.LONG TERM.../Tuesday night through Monday/
Issued at 320 PM CDT Mon Aug 14 2017

Summary...Some much needed rainfall continues to be on track to move
across Iowa beginning scattered Tuesday and more widespread Tuesday
night through Wednesday night. Ahead of the rainfall, temperatures
will rise a few degrees and dew points in the 70s will return. From
Thursday onward, temperatures to gradually rise into mid/upper 80s
with an opportunity for a few storms Fri and Mon.

Tuesday Night through Thursday Morning...Certainly the main window
within the long term forecast, at least from a precipitation
perspective.  Synoptic models have come into fairly good agreement
on tracking of surface low and the 500 mb trough on its heels. Set
up continues to look like the first good widespread efficient
rainfall in a while. A number of variables working in favor of
widespread relatively long duration rainfall such as low tracking
northeast across parts of NE/IA/MN, deep moisture/PWats around 2
inches, warm cloud depths around 13 kft, strong theta-e advection
Tue night/Wed, and some additional 500 mb trough forcing behind
theta-e advection. Threat for any severe is low to nil, with broad
forcing, limited CAPE from cloud cover/preceding rainfall, and weak
shear profiles.

If antecedent conditions were not what they are, D1-D2 drought in
many locations, there might be discussion of localized flooding
potential in areas where multiple storms traverse. It would not be
overly surprising to see a location or two receive in excess of 2
inches. While there is a marginal risk for excessive rainfall,
hard pressed to see any of it result in flooding. Even with some
crops already harvested for silage in the most drought stricken
areas of southern Iowa, it would take a substantial amount of
rainfall to result in flooding. Keep in mind too the more drawn
out nature of this event versus training storms over just a few
hours.

Thursday through Sunday...Euro/GFS/Canadian show a fair amount of
agreement through the weekend. High pressure will slide in Thursday
and linger, only interrupted by a shortwave and weak boundary Fri.
Tendency has been to strengthen the wave, though the GFS is more
than likely over zealous. Otherwise, expect temperatures to remain
seasonable and humid at times with southerly flow often prevailing.

Monday/Eclipse Day...It is a week out, but indications are that
there could be some bad news for eclipse goers. Currently
indications are that a 500 mb shortwave will move through the
Dakotas/Nebraska Mon morning, possibly resulting in cloud cover and
scattered showers/storms through parts of the total eclipse path in
Nebraska/Kansas/NW Missouri. Again, a week out, so details are very
hazy and prone to change. Stay tuned as the event approaches.

&&

.AVIATION.../For the 00Z TAFS through 00Z Tuesday evening/
Issued at 640 PM CDT Mon Aug 14 2017

Frontal boundary to drop southward and stall out across the area
tonight, and may have some fog develop across the northern TAF
sites behind the boundary. Kept MVFR to higher IFR for now.
Otherwise winds to be fairly light and variable overnight, then
shifting around to the east to southeast through the day Tuesday.
Not confident in precip chances at any location, so have left dry
for now.

&&

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

&&

$$

SHORT TERM...Ansorge
LONG TERM...Curtis
AVIATION...Beerends



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