Area Forecast Discussion
Issued by NWS Des Moines, IA

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358
FXUS63 KDMX 230853
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
353 AM CDT Sun Apr 23 2017

.SHORT TERM.../Today through Tonight/
Issued at 351 AM CDT Sun Apr 23 2017

Surface ridge currently sliding across central Iowa with light and
variable winds across the southern half of the state.  Farther
north, winds have increased from the southwest as warm advection
commences.  The high will continue its retreat to the southeast
today with a tightening pressure gradient over the state with low
pressure developing in western South Dakota.  This will increase the
southwest winds through midday with gusty conditions in the
northwest corner of the forecast area by this afternoon.  The
increasing winds will help mixing today and should push temperatures
into the 70s across the entire area.  The tighter gradient will
persist tonight with surface winds remaining stronger overnight,
especially in the north and west.  This, in turn, will keep near
surface mixing ongoing overnight helping temperatures to remain
relatively warm in the northwest forecast area.  The lighter winds
in the south and east will allow for better cooling and slightly
lower temperatures overnight.  Otherwise, little threat of
precipitation today or tonight with a dry atmosphere in place.


.LONG TERM.../Monday through Saturday/
Issued at 351 AM CDT Sun Apr 23 2017

Monday into Tuesday...
Confidence: High

08z Sun water vapor imagery picking up on a subtle upper low
spinning over northern California. This low/longwave trough will
continue to propagate eastward... with its sfc reflection nearing NW
Iowa by 00z Tue. There continues to be very little model spread with
the track of this low, leading to high confidence forecast. With
high pressure over the Gulf of Mexico, gulf moisture access will
be closed off ahead of this system. An attendant cold front will
be extending southward through Iowa will serve as a sufficient
focusing mechanism for convection. BUFKIT analysis of KFOD, KSUX,
and KFSD shows a lot of low-level dry air...keeping any convection
elevated and relegating precip, if any, to mainly virga.

More significantly in terms of a hazard, Monday looks to feature
very strong winds. Sounding analysis shows the mixed layer above
800mb. Slight veering with the wind profile and upper clouds may
inhibit complete mixing of 45kt winds at top the mixed layer. Sfc
winds should be out of the south, meaning high profile vehicles
traveling along East-West roadways (such as Highways 20 and 30)
may be vulnerable to jackknifing.


Tuesday into Wednesday...
Confidence: Medium

Confidence has increased greatly in this time period, as models
beginning to show run-to-run consistency with clustering around a
solution. Have favored a general consensus model blend for track
and timing. 08z Sun picking up on a more well-defined upper low
spinning just off the Washington coastline...right on the heels of
the Monday system. This upper low looks to be centered over the
Dakotas by 12z Wed, with the associated trough digging down
through Texas. A band of strong frontogenetical forcing,
especially evident at 950mb, will be a signature of this system.
The frontogenetical forcing will be oriented from SW to NE and cut
right through central Iowa Tuesday into Wednesday morning. By 18z
Wed, models place a 995mb sfc low just east of Iowa...taking
along the best forcing for precip with it. During the day Tuesday,
PWATs of 1-1.25 inches will be advected into Iowa. Sounding
analysis does not come close to saturating the profile completely,
so lack of precipitation efficiency will likely keep QPF in the
0.25 to 0.5 inch range during this prolonged and widespread period
of rain.


Thursday and beyond...
Confidence: Low-Medium

Much uncertainty has existed in the Thursday through Saturday
timeframe. As has been the case time, after time, after time in
the long range, the ECMWF has been the first to show run-to-run
consistency in a plausible solution, and the GFS has been playing
catch-up. In this particular instance, the ECMWF has kept the DMX
CWA cool, with the aforementioned 995mb sfc low over the Great
Lakes by 12z Thu, and a weak, ill-defined sfc high quickly passing
through the upper Midwest. 00z Sun ECMWF 850mb temps drop to 0 to
-5C across Iowa Thursday morning. Frost/freeze headlines almost a
likelihood across northern Iowa.

Both the 00z Sun ECMWF and GFS show stunning agreement with a
potentially high-impact system Saturday into Sunday. By 00z Sun,
both models showing an upper low coming off the Rockies near
Nebraska. This upper low is beginning to come into range in the
Pacific, so the next few runs should greatly improve upon capturing
this low, especially its amplitude/strength. 990mb sfc reflection
coming across the Oklahoma panhandle and tapping into gulf moisture
big time. Won`t get into minutia at this point, other than to say
that the GFS is pushing PWATs of 1.5 to 1.75 inches into Iowa.
Obviously a lot of time for things to change between now and then,
but PWATs of this magnitude this time of year are effectively off
the charts per SPC sounding climatology at KOAX and KDVN.


&&

.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1153 PM CDT Sat Apr 22 2017

Little has changed since 00z. There is high confidence in VFR
conditions through the period with mostly clear skies. Winds will
be light and variable early Sun morning and staying below 10 kts
through 06z.


&&

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

$$

SHORT TERM...Cogil
LONG TERM...Kotenberg
AVIATION...Small



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