Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 211729

Area Forecast Discussion
National Weather Service Des Moines IA
1129 AM CST Tue Feb 21 2017

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

.SHORT TERM.../Today through Tonight/
Issued at 348 AM CST Tue Feb 21 2017

Confidence: Medium to High. Main concerns today will be the
extent of morning fog over the northeast/west and then recovery in
temperature for the afternoon hours. Trough continues to pull
east of the area early today with lingering clouds across the east
in the boundary zone. Boundary layer moisture remains high with
light winds and clear skies over much of the north/northwest/west.
Temperatures have been hovering near the dewpoints and expect
that near sunrise patchy fog now will become more
widespread...especially over the north and west. A weak ridge of
high pressure will move over the area early today...resulting in
light winds and little mixing during the morning hours. HIRES
models break out fog across the northeast and west to a lesser
extent. Some uncertainty as to how widespread it will be during
morning commute...will most likely first apply an SPS to the areas
of concern and then headlines if needed. Models in good agreement
to have much warmer H850 temperatures pushing northeast into
western/southern areas by 00z tonight. Though afternoon mixing is
by no means significant...mixing should increase boundary layer
gusts to 15 to 20 mph at times across central and western areas
from 18-22z this afternoon...enough to warm the area back to near
record highs this afternoon. This afternoon highs should reach
about 60 in the far north to the lower 70s over the
southwest/south during the afternoon. By tonight another trough
will approach from the northwest...but overnight H850 temperatures
will actually increase over the region ahead of the trough. This
warming aloft should keep fog from developing to any appreciable
extent and also keep overnight lows mild...though the far north
may dip to just below 40 after slightly cooler highs this
afternoon. The south once again should remain in the mid 40s
overnight. There may be some high clouds moving over the region
tonight but otherwise skies should remain generally clear and
winds light over the region.

.LONG TERM.../Wednesday through Monday/
Issued at 348 AM CST Tue Feb 21 2017

The unseasonably warm weather will continue on Wednesday as 850mb
temperatures climb into the 11 to 15C range. At the surface,
winds from the southwest will prevail ahead of a cold front
associated with low pressure moving across southern Minnesota. The
warm low-level temperatures and warm breezes from the southwest
should provide another day of near record to record warmth.
Unfortunately, this spring-like warmth in the middle of February
will begin to come to an end as the aforementioned cold front
passes through the state by late Wednesday. High temperatures on
Thursday will still be warm by typical February standards
averaging 10 to 15 degrees above normal.

Attention then turns to our late week weather system as a long wave
trough digs into the Western US on Thursday and the jet energy
ejects out onto the Plains and cyclogenesis begins. Ahead of the
developing low pressure, isentropic lift will aid in areas of
showers forming across the state. While models have tracked the
low a bit farther north from previous runs, still fairly
consistent with a low moving from western Kansas late Thursday
into southeast Iowa by early Friday. This will put southern Iowa
in the warm sector and rumbles of thunder are possible as there
will be marginal MUCAPE Thursday evening.

The big story though is the potential for a significant winter
storm with a plowable snow over most of the northwest half of
Iowa. Rain will begin to transition to snow over northwest/north
central Iowa Thursday night. As the low continues to move to the
northeast on Friday, the rain to snow transition will continue to
move to the southeast as winds will quicken and blow from the
north and northwest pulling colder air into the state. The
transition is expected to be rain to snow as the soundings show a
quick cooling of the thermal profile. Winds are also a concern.
As with the previous forecast shift, have increased winds above
initial guidance and this yields sustained winds of 15 to 25 mph
with gusts to around 40 mph. This of course will cause areas of
blowing snow and have incorporated that into the forecast for
Friday and Friday night for areas primarily north of Highway 20
where sustained winds and gusts are expected to be highest. Winds
will gradually abate Friday night into early Saturday as the snow

Zonal flow takes over for the weekend into early next week. A
rather weak system is posed to pass on Sunday with precipitation
chances. A push of milder air early next week will also bring
another chance of precipitation.


.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1126 AM CST Tue Feb 21 2017

Medium confidence in VFR conditions throughout TAF period. High
confidence in LLWS primarily at KFOD, KMCW, and KALO. KDSM
included in LLWS as well. May need to add fog at KCMW and KALO as
IFR possible with 1sm vsby. Confidence not high enough in fog at
this time... 00z Wed TAF update may need to include fog.
Confidence high in clear to mostly clear skies.




LONG TERM...Ansorge
AVIATION...Kotenberg is the U.S. government's official web portal to all federal, state and local government web resources and services.