Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 161740
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1240 PM CDT Sat Sep 16 2017

...Updated for the 18z Aviation Discussion...

&&

.SHORT TERM.../Today through Tonight/
Issued at 413 AM CDT Sat Sep 16 2017

Primary forecast challenges in the short term are: 1)showers and
thunderstorms this morning; 2)possible lull in precip midday;
3)Strong thunderstorm potential late this afternoon into tonight.

Today...Forecast Confidence:  Medium

00Z upper air analysis and water vapor imagery indicated upper
ridging across the Great Lakes and Ohio River Valley with a
seasonable strong trough over the intermountain west.  At the
sfc...MSAS analysis indicated two areas of lower pressure - one over
north-central MN and a second over KS/NE.  09z radar mosaic showed
scattered to numerous showers and tstms over the state.

Early this morning...expect the showers and thunderstorms to
continue through at least mid-morning due to forcing associated with
a lead shortwave trough and a decent surge of moisture in the
low/mid levels. The most widespread activity should be concentrated
across far northern Iowa with a second cluster over west-central IA.
Activity away from these two areas should remain more isolated.  Not
expecting any severe wx this morning...although gusty winds and
small hail will be possible in the strongest cores.

In the wake of this morning activity, a relative lull in precip
should occur as forcing weakens and a capping develops between H8
and H7. However, will maintain at least a slight chance of precip in
many areas as a few rogue storms will be possible per latest CAM
solutions.  By late afternoon and early evening storms are expected
to redevelop along a southeastward moving cold front. High
temperatures in the mid-upper 80s and sfc dewpoints in the 60s
should yield 1000-2000 J/kg of MLCAPE by 00z Sun. Although the
stronger wind fields reside in the post frontal airmass, any storms
developing on or near the boundary could become strong/severe, with
large hail and gusty winds possible. The best potential for storms
prior to 00Z will be across far northwest Iowa.

Tonight...Forecast Confidence:  Medium

Fairly good agreement amongst the various CAM solutions showing a
broken line of storms sliding southeastward across the forecast area
tonight. Diurnal stabilization of the PBL after sunset should lead
to gradually weakening of SBCAPE...although a few stronger storms
with gusty winds will still be possible into the early overnight
hours. Frontal boundary should be remain progressive with most of
the shower/storm activity south and east of the area by Sunday
morning. QPF forecast confidence  is fairly low...but feel many
areas should pickup a quarter to half inch...with some locally
higher amounts possible. Not a drought buster by any means, but
hopefully some beneficial rains nonetheless.  Finally, expect a much
cooler post frontal airmass as winds shift northwesterly.

.LONG TERM.../Sunday through Friday/
Issued at 413 AM CDT Sat Sep 16 2017

./Sunday through Monday Night/

Confidence: Medium

Active weather pattern for the upcoming week with several opportunities
for rain through the extended. Early in the forecast period mainly dry
weather will follow the strong cool front early into Sunday morning
with only a few lingering showers/iso thunder southeast. High pressure
will briefly move across the region with seasonal H850 temperatures
in the low to mid teens by 00z Monday. Light northwest flow aloft should
contribute to some mixing with afternoon highs in the 70s across the
region. Looking at the big picture...a large trough continues over
the western CONUS. Though the stronger wave in the short term will
move east of the region by Sunday morning...water vapor imagery
early this Saturday morning indicates a system near southern
California. This is forecast to move through the flow and arrive
near Iowa by Monday with weak height falls and increasing warm air
advection. Already differences are showing up in the models with
the GFS/NAM rather aggressive with rainfall coverage and amounts.
The Euro is less focused on the higher totals and focuses slightly
more energy south of the state. Highs through the period will
remain mild for this time of the year...considering the west
southwest flow aloft though with the chances for precipitation and
more clouds on Monday...afternoon readings should be several degrees
cooler across the region.

./Tuesday Through Friday/

Confidence:  Low to Medium

With the active pattern continuing...the focus then shifts to the
next system approaching Tuesday night into Thursday.  There remain
larger departures in timing/evolution with the GFS showing another
wave already late Tuesday afternoon/night while the Euro projects a
deepening trough over the western CONUS with little energy working
east until late Wednesday into Thursday. Further model to model
run departures in evolution of the remainder of the week include
the GFS and now Euro dry Thursday with rain/thunder returning
Friday/Saturday. The significant differences between model runs
arise from how the models handle next week`s tropical weather in
the Atlantic. Though both have similar solutions to Jose
initially...the Euro shows another more significant hurricane in
the Caribbean at 168hr which slows the eastward progression of
large scale features over the eastern half of the CONUS with Jose
still over the open Atlantic by 180hr. With the GFS lacking any
significant tropical system approaching until later in the 10 day
forecast...and Jose long gone by 180hr...it forecasts the pattern
over the US to be more progressive leading to a slightly faster
wave train over the central states. Thus confidence moving forward
beyond midweek is lower given the tropical influence and model run
to run differences. The one saving grace is that each model is
similar in H850 temperatures with warming expected from later
Wednesday into Friday. Though the extent differs between the GFS
and the Euro models...generally it looks mild and more humid by
mid to late week with highs in the 70s to 80s and lows in the 50s
and 60s.

&&

.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1237 PM CDT Sat Sep 16 2017

Messy day and evening expected precip-wise. During this afternoon,
not expecting much, if any vsby reduction. With CIGs staying near
to above 5000 ft, expecting VFR through this evening. Once this
evening hits, confidence decreases rapidly. Potential for strong
to severe storms may produce brief gusty winds, periods of heavier
rainfall may reduce vsby to 3 miles or less, and there may be CIG
reductions to IFR criteria. Will likely need semi-frequent TAF
updates from 00z Sun through 06z Sun.

&&

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

$$

UPDATE...Fowle
SHORT TERM...Fowle
LONG TERM...REV
AVIATION...Kotenberg



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