Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 112325

Area Forecast Discussion
National Weather Service Des Moines IA
620 PM CDT Wed Oct 11 2017

.SHORT TERM.../Tonight through Thursday/
Issued at 325 PM CDT Wed Oct 11 2017

The main forecast concern was focused on stratus deck lingering
into tomorrow and with the drizzle/fog potential early Thursday
morning. Leaned toward the latest runs of the RAP, HRRR and ESRL
HRRR for cloud cover location as they have a good handle on the
current trends across the state.

Strong inversion develops this evening and looks to trap the low
level moisture/stratus deck into the morning Thursday, particularly
across the eastern half of the CWA where the stratus currently
resides. Confident that the eastern portions of the CWA will remain
under the low stratus through at least 12z Thursday, with it
lingering and trying to dissipate b/t 12-15z when the WAA begins to
develop. The real tricky part is the central sections of the
forecast area, from north to south from around Lamoni through Des
Moines northward through Ames to Kossuth County. This is currently
the western edge of the stratus and the question remains if and how
far the stratus retrogrades back west-northwest this evening.  In
this location, have patchy fog mentioned as a few the hires models
keep the stratus as is or erodes it slightly eastward allowing for
radiational cooling to setup and thus fog to develop. Concerned that
the western third of the CWA will see fog develop overnight with the
radiational cooling setup in this location. Dew point depressions do
become less than 5F to even 0F by around 09z Thursday and the latest
runs of the HRRR/RAP and even the ARW/NMM suggest fog developing by
around 09z.  Timing of the WAA the west prior to 12z Thursday will
determine if this will hold off the radiational fog to development.
Across the east, there is very weak lift in the saturated column per
MCW to ALO soundings and kept mention of drizzle overnight.  Low
confidence with any measurable precipitation at this time and
currently left out any pops.

.LONG TERM.../Thursday night through Wednesday/
Issued at 325 PM CDT Wed Oct 11 2017

A cold front will enter northwest Iowa Thursday night then will
continue to move slowly southeast on Friday. The initial airmass
will begin dry as the boundary arrives though mid-level moisture
advection will arrive during the day on Friday which will help
enhance precipitation chances over mainly the northern Iowa.
Precipitation chances would settle south Friday night as the
boundary moves into northern Missouri. Moisture depths though
would support light rain or even drizzle in some areas.

A strong short wave will move into the Nevada/Utah region Friday
night and move east into the Western Plains on Saturday. This
will create increasing southwest flow aloft and will help lift the
surface boundary back north. In addition, this flow will bring
strong low and mid-level moisture advection to the southeast half
of Iowa. PWAT values will increase to near 1.75 inches by Saturday
afternoon or near 250% of normal. Warm cloud depths will be near
12.5 kft and the setup is good for an efficient rain producing
system. The southeast portion of the forecast area is currently
outlooked for severe weather by SPC on Day 4. As of now, the
threat for severe weather looks conditional on if the clouds can
break and temperatures do recover. Any severe weather threat
should be relegated to the far southeast including the Ottumwa and
Bloomfield areas.

The precipitation will quickly end Saturday night. The wind will
increase behind the boundary as cold advection arrives and the
pressure gradient increases as low pressure intensifies to the
north. Wind gusts over 35 mph are possible and may reach wind
advisory criteria for a brief period. High pressure will arrive
during the day Sunday and will diminish the wind gusts by the
afternoon. Currently have low temperatures in the upper 30s for
Sunday night. May not be low enough at this point and could have
frost potential by Monday morning. Current forecast low dew points
are in the lower 30s over northwest Iowa and lows may approach the
dew point values. Generally dry and warmer conditions Monday and
Tuesday as high pressure moves southeast and southerly winds


.AVIATION.../For the 00Z TAFS through 00Z Thursday evening/
Issued at 620 PM CDT Wed Oct 11 2017

MVFR/IFR conditions over the east may expand back to the west
overnight.  This will need to be monitored.  If the stratus does not
move back in, then some patchy fog may develop and bring local MVFR
visibilities to areas mainly west of I-35.





SHORT TERM...Podrazik
LONG TERM...Donavon
AVIATION...FAB is the U.S. government's official web portal to all federal, state and local government web resources and services.