Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 180551
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1151 PM CST Fri Feb 17 2017

.SHORT TERM.../Tonight through Saturday/
Issued at 355 PM CST Fri Feb 17 2017

Benign weather continues over Iowa through Saturday.  Aloft a
shortwave will flatten the ridge over the region while a surface
low in Canada drags a weak cold front through the state this
evening. With the absolute lack of moisture in the atmosphere the
only sensible weather with the frontal passage will be some high
clouds and a wind shift from Southerly to Northwest during the
evening.

Temperatures will not be as warm on Saturday as today with highs in
the 55 to 65 degree range but it will still be well above normal for
mid February.  With light wind and ample sunshine it will still feel
very warm.  Of some concern continues to be an elevated fire threat
due to the warm temps and dry condition of the fuels. The primary
threat is for this afternoon.  The threat will be somewhat
alleviated on Saturday with the cooler temps and relative humidity
in the 30 to 40 percent range versus the 20 to 30 percent range.
Also...with the warm up today locations across Northern Iowa were
checked for river ice and reports were that rivers are open with the
only ice being on the banks. This has led to the removal of the ice
jam threat and will be reflected in the weather story and HWO
products.

.LONG TERM.../Saturday night through Friday/
Issued at 355 PM CST Fri Feb 17 2017

The main focus over the extended period was on temperatures
throughout and thunderstorm potential Monday. Leaned closer to the
warmer GFS through Monday as the large thermal ridge remains in
place until a weak cold front and associated upper level trough
move across the region. Good mixing Sunday and even strong mixing
Monday with strong southerly flow to persist over much of central
Iowa before the front drags across the state by Monday evening.
Storms look to develop within the WAA and theta-e advection ahead
of the surface low past 06z Monday and continue through much of
the morning before pressing eastward by the mid-late evening. The
ECMWF is slightly slower than the GFS and thus held onto pops
Monday evening. Weak surface instability present along and ahead
of the front as well as weak forcing for ascent available.
Precipitable water values in the 0.75-1.00" range with decent
moisture transport into the state. Unprecedented warm layer cloud
depths of around 9,000-10,000 feet for this time of year, so
cannot rule out the potential for some heavy rain. Severe weather
threat looks minimal with the lack of shear and forcing present.

GFS and ECMWF continue to show a strong upper level low pressure
system to impact the region by Friday. The GFS is further north
and quicker with the surface low track leaving much of the
forecast area within the warm sector and thus mainly rain. The
ECMWF tracks the low across west-central to northern Iowa with a
tight pressure gradient developing on the backside by Friday
evening. Timing of any storm initiation will lead to the
potential for severe storms. With the slightly more consistent
ECMWF, the severe threat appears to be east of the forecast area
but this is certainly something to continue to monitor. Snow looks
to be likely on the backside of the surface low by Friday evening
but with the continued uncertainty wrt to the low track, any snow
amounts are still highly questionable. Certainly colder
temperatures are expected to filter into the region with 850mb
temperatures as cold as -10C by Friday night.

&&

.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1151 PM CST Fri Feb 17 2017

Confidence is medium to high that VFR conditions will persist
through the period. Low level relative humidity is increasing
leading to some radiation fog potential however drier/cooler air
continues to gradually ooze into Iowa at 06z behind a weak SW-NE
cold front through the state. Thus the forecast remains VFR until
trends suggest otherwise.

&&

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

&&

$$

SHORT TERM...FAB
LONG TERM...Podrazik
AVIATION...Small



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