Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 241738

Area Forecast Discussion
National Weather Service Des Moines IA
1238 PM CDT Sun Sep 24 2017

.SHORT TERM.../Today through Tonight/
Issued at 341 AM CDT Sun Sep 24 2017

There will finally be some changes in the weather this period as the
current Rockies long wave trough begins to slowly advance eastward.
Short waves will continue to eject into the SSW upper flow producing
periods of thermodynamic and kinematic forcing along the baroclinic
zone. This will mainly be along and behind the surface front so the
eastward progression will be slow with the front only reaching Fort
Dodge by 12z, if that. Examination of the 300-310K isentropic
surfaces suggest the lift will only be moderate at best here with
better baroclinicity farther south and west. Instability will be
fairly weak with effective shear only high enough to support
organization along and just ahead of the front so the severe
potential is low.  Although the better potential will be just out of
our forecast area, anomalously high specific humidities,
precipitation waters and integrated moisture transport are all noted
in the 00z NAEFS so moderate rains may be at or on the doorstep of
the far western forecast area toward daybreak.

PoPs will not enter the picture until early this evening far west
however so until that time another day of persistence can be
expected. Forecast soundings with projected mixing suggest highs
should be approaching or just exceeding records again today reaching
the upper 80s to lower 90s. Dewpoints should be similar to yesterday
as well, in the low/mid 60s, so heat indices are expected to be
tempered somewhat and only at or just above air temps.

.LONG TERM.../Monday through Saturday/
Issued at 341 AM CDT Sun Sep 24 2017

Monday the upper trough that has been parked over the western U.S.
and Rockies will begin to inch eastward. Meanwhile a cold front
will enter the far northwest and slowly push across the state
through Tuesday. Overall I`m still not impressed with the dynamics
of this system. Instability is in the 500-100 J/Kg range except
for a period Monday afternoon. Shear is weak along the front and
better post frontal, which is not conducive to strong or severe

Having said that, there are a couple things going for this system
that make it somewhat interesting. First, there is at least
pockets of better forcing which will yield periods of moderate
rainfall and some stronger storms. Next, despite marginal to weak
lapse rates, this system is moving very slowly and while
widespread heavy rainfall is unlikely, localize areas of heavy
rainfall is possible, especially west into central Iowa. There are
several periods where a quarter to a half inch will fall which
could lead to localized amounts well above three quarters of an
inch where stronger showers/storms are.

Beyond Tuesday, we will be in a cool and dry fall pattern with a
secondary shot of cool next weekend. Highs Tuesday through Friday
will be in the 60`s and 70`s with 60`s area wide over the weekend.
Both the Euro and the GFS drop a trough from a Great Lakes low
through northeast Iowa but they diverge on precip with the GFS dry
and the Euro dropping light precip through the northeast Thursday
night/Friday. Confidence is too low at this point to include.


.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1237 PM CDT Sun Sep 24 2017

Main challenge is timing of precipitation. Mixing this afternoon
will relax by 00-01z. Blocking pattern continues to delay the
progression of the cold front over eastern South Dakota/Nebraska.
The front is finally expected to reach western Iowa by 12z
Wednesday. With most of the rain/iso thunder behind the boundary...
will see cigs gradually lower to near MVFR by the end of the
period near KFOD/KMCW along with increase in shower chances to
VCSH/-SHRA at those two locations. Elsewhere VFR conditions will
remain with southwest winds. /rev




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