Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 150437
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1137 PM CDT Fri Jul 14 2017

.SHORT TERM.../Tonight through Saturday/
Issued at 311 PM CDT Fri Jul 14 2017

A surface high pressure center currently over Minnesota will slide
southeastward across Iowa tonight and Illinois on Saturday.
Scattered clouds evident in satellite imagery this afternoon will
dissipate with sunset this evening, after which clear skies and
light and variable winds are anticipated overnight. We may see
patchy light fog during the pre-dawn hours Saturday, as we did this
morning, but again it should be quite isolated across the state. The
most likely areas are in our northeastern counties, where the center
of the high and related calm winds will linger longest, and perhaps
in our southwestern counties within the Nishnabotna River valley
where decoupling should allow terrain effects to have more
influence. Have added patchy fog in these areas from about 3-7 am on
Saturday accordingly.

The other forecast question is how high temperatures will rise on
Saturday afternoon. Winds will remain fairly light, around 8-10
knots, but turn through southwesterly ahead of an approaching
frontal boundary, allowing for weak low-level warm air advection.
Forecast soundings also indicate warming temperatures aloft and
modest mixing. Given these factors, have bumped up high temperatures
slightly into the 89-91 range across our north and west and around
the Des Moines metro area. Dewpoints will also rebound into the
upper 60s, and probably lower 70s just ahead of the boundary late in
the day, so heat index values should peak around the mid-90s.

.LONG TERM.../Saturday night through Friday/
Issued at 311 PM CDT Fri Jul 14 2017

Saturday Night through Monday: Confidence: Medium to High

In the near term of the extended mainly minor concerns. Next
upstream system is located in Saskatchewan/Manitoba provinces of
Canada and is expected to drop southeast into the Great Lakes by
Saturday night. The better forcing aloft will remain northeast of
our forecast area...but along the trailing cold front late afternoon
MUCAPE will be approaching 5500 j/kg with a modest H700 cap aloft
along with CIN of about 200 to 250 j/kg. Though afternoon 0-6km
bulk shear values of 30 to 35kts...speed shear profiles remain
rather low with some directional shear in the 0-3 km layer. Any
storms that can develop and punch through the cap will have decent
hail cape profile to work with as well as possible wind threat.
Timing favors a near 00z start over the far north to 12z as the
storms drift south southwest at 15 to 20 mph. Overall...coverage
should remain fairly widely scattered during the evening hours.
The surface boundary is expected to drop south to about the I80
corridor by 12z Sunday. This will keep southern Iowa in the lower
70s for overnight lows with cooler readings in the 60s over the
north. On Sunday the front is anticipated to linger over the east
while lifting/stretching west in response to a Northern Plains
shortwave. With the warm and unstable airmass in place over the
west/south...there may be an isolated thunderstorm or two across
the area. Both the GFS/Euro are hinting at very small chances
mainly during the afternoon hours. Aloft there will remain a
modest H700 thermal cap at 10 to 11C. Other than small threat for
thunderstorms...mid level clouds in response to the return west
southwest flow aloft will temper afternoon highs. For now will
continue with highs around 90 south of the boundary and in the
lower 80s north of the boundary. It is likely that much of the
area along and north of the boundary will have at least 50-60%
cloud cover by mid to late afternoon. Both the GFS/Euro are in
good agreement for Monday...with the warm front hanging over the
eastern sections of Iowa due to a slowly retreating Great Lakes
high parked over Lake Michigan/Michigan. The shift northeastward
shift of the front by late day should allow more of the region to
warm into the mid to upper 80s with lower 90s once again in the
south.

Monday Night through Friday:  Confidence Medium

Models continue on track/agreement into Monday night...but begin to
drift apart Tuesday through Friday. An approaching high over
southern Canada is expected to nudge the front south into Iowa.  The
GFS is faster in bringing the boundary south by about 12 hours...and
is also trending toward convective development nearly a full 12
hours faster even into an unfavorable H700 environment with
temperatures over Iowa hovering around 12C. The Euro seems to be
doing a more honest job in preventing convective initiation too far
into the warm layer aloft...and for this package will try to blend
the lesser convective solution into the forecast...keeping the
better chances for thunderstorms over northern sections of the
forecast area through Tuesday evening as the front buckles south into
the region. This will promote thunderstorms over the area...
especially the north half. Both the Euro/GFS show a renewed surge
of warm air into Iowa Wednesday into Thursday with the front again
cycling south over the next 24 to 36 hours. The GFS continues to
oscillate the boundary north/south much more frequently while the
Euro is keeping the boundary farther north through the period...
in better relation to the stationary H500 ridge just south of the
state in the mid Mississippi River Valley through Friday afternoon.
In either case...periods of thunderstorms will be expected along
and north of the boundary with hotter temperatures south of it.
Given the lack of agreement and uncertainty...details of Wednesday
through Friday are quite muddled...though from the standpoint of
intra-model integrity...the current GFS solution Thursday into
Friday makes little sense.

&&

.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1135 PM CDT Fri Jul 14 2017

VFR conditions likely through the next 24 hours. Some patchy fog
leading to MVFR visibility is possible at KMCW and KALO after
09z, however forecaster confidence is not high enough to include
in the TAF at this time.

&&

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

$$

SHORT TERM...Lee
LONG TERM...REV
AVIATION...Martin



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