Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 132000
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
300 PM CDT Sun Aug 13 2017

.SHORT TERM.../Tonight through Monday/
Issued at 300 PM CDT Sun Aug 13 2017

Scattered showers have been moving through central Iowa today in
response to a weak area of 700mb warm air advection, but these are
currently pushing off into eastern Iowa. Farther upstream, early
afternoon water vapor imagery shows a shortwave trough moving
through the Dakotas. At the surface, low pressure is over eastern
South Dakota with a warm front lifting into Iowa and a cold front
lingering back over south central South Dakota and Nebraska.
Clouds have been clearing in western Iowa, but overall instability
is rather weak at this time. As the low pressure and cold front
moves slowly eastward, will see increasing chances for showers and
thunderstorms, mainly over northwestern part of central Iowa this
evening and then spreading southeastward toward sunrise.
Instability will be marginal so thunderstorms should be isolated.
Otherwise, placed patchy fog over parts of the western forecast
area with the most likely area being the Nishnabotna Valley.
Forecast soundings show saturated boundary layer beneath an
inversion with cross sections showing abundant low level relative
humidity. Challenge to the fog development will be cloud cover
and any rain, which may limit the fog extent.

As the shortwave trough and associated forcing moves away and weak
mid-level ridging begins to impinge on the area, this will stretch
the boundary across the area. This will lessen the area of
scattered showers and thunderstorms later in the morning and
afternoon. CAMs and short term models show this with
precipitation becoming more scattered and light. Overall, shear
remains rather marginal and with the forcing mechanism moving
away, severe threat seems quite low. With some breaks in the
clouds expected over southeastern and south central Iowa,
temperatures are poised for the middle if not upper 80s.

.LONG TERM.../Monday night through Sunday/
Issued at 300 PM CDT Sun Aug 13 2017

At 500 mb, a weak ridge will approach from the west late Monday,
while a broad trough digs slowly down across the Rockies and
Pacific coast. Small impulses ejecting out of this trough will
flatten the ridge as it crosses Iowa Monday night and Tuesday. At
the surface this will be reflected by the development of a weak
effective warm front somewhere around northern Iowa on Tuesday,
and allow for a return of some relative heat and humidity during
the day. Isolated to scattered showers and thunderstorms will also
be possible, but nothing organized or severe is expected. Of
greatest note will be dewpoints increasing toward 70 degrees,
marking a return of muggy weather we have not seen so far in this
uncharacteristically cool early August.

Between Tuesday night and Friday night the aforementioned western
U.S. trough will gradually shift east northeastward, with its
associated vorticity coming in a series of shortwave troughs, the
leading and most pronounced shortwave crossing Iowa on Wednesday. A
swath of relatively deep atmospheric moisture will precede and
accompany this feature, contributing to moderate instability which
will combine with the modest forcing of the shortwave to produce
fairly widespread showers and thunderstorms, progressing slowly
across our forecast area from late Tuesday night into late
Wednesday. This could potentially result in our most widespread
rainfall in a while. Severe weather potential will be limited by
thick clouds and a resulting lack of low-level instability, as well
as the broad and unfocused nature of the forcing. However, given the
slowly progressing nature of the parent system and expected
convection, some areas may experience fairly heavy rainfall. We will
be monitoring this in the early part of the week, however, given
antecedent conditions it would take a lot of rain in the wrong place
to result in any significant flooding concerns.

From Wednesday night through Friday rain chances will linger, but
with more subtle shortwaves aloft and less moisture and instability
available, any precipitation would be spotty and light. Looking
ahead to next weekend, it appears we will enter a more zonal flow
regime aloft. This should allow for a slow moderation of
temperatures and sporadic rain chances.

&&

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1228 PM CDT Sun Aug 13 2017

VFR conditions have prevailed at the TAF sites with scattered
showers moving across central Iowa this morning and early this
afternoon. These showers may brush KALO and have mentioned it for
the afternoon hours. Also cannot rule out a rogue shower
affecting KMCW or KFOD, but have not mentioned at this time due to
lower confidence. Have continued VCTS later tonight at the
northern terminals and this is also where lower ceilings will
arrive toward sunrise Monday with MVFR restrictions. It`s possible
these lower ceilings will reach farther south and east, but
confidence is lower given SREF guidance maintains the best chance
for lower ceilings over northwestern and north central Iowa.

&&

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

$$

SHORT TERM...Ansorge
LONG TERM...Lee
AVIATION...Ansorge



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