Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 282050

National Weather Service Des Moines IA
350 PM CDT Wed Sep 28 2016

.SHORT TERM.../This evening through Thursday/
Issued at 347 PM CDT Wed Sep 28 2016

The main forecast concern tonight was focused on the
pesky stratus deck if and when it will depart/dissipate. Large upper
level cutoff low pressure located over eastern Illinois/northern
Indiana looks to meander south-southeast into the lower portion of
Ohio Valley tonight. Stratus deck showing signs of thinning
striations via visible satellite over portions of WI/MN. The hires
models (i.e. the ARW-NMM WRF, HRRR, NAM12, RAP, and HopWRF) keep the
low level RH (AOA 70%) over much of the forecast area out through
Thursday morning. Even the GFS and ECMWF keep the low level RH
across Iowa tonight. A few of the aforementioned hires models
dissipate the cloud deck in their respective ceiling height
parameters, but show signs of redeveloping the stratus/fog past 09z
Thursday.  Question whether an inversion can develop and trapped at
least some of this moisture/cloud deck over Iowa. The CAA remains
fairly strong just off the surface, or at least strong enough to
keep the stratus from dissipating completely and develop an
inversion. Hence, higher confidence in leaning toward holding onto
the cloud cover through much of the night. Still anticipating a
chilly night even with the cloud cover sticking around across the
forecast area.

.LONG TERM.../Thursday night through Wednesday/
Issued at 347 PM CDT Wed Sep 28 2016

Little change is expected through much of the period with no
appreciable precip chances until late in the extended forecast and
temperatures staying around seasonal normals. The only concern at
onset will be stratus trends. There is some thinning upstream
toward WI/IL and Lake Michigan at the moment and nearly all
guidance sources suggest this trend will continue for tomorrow
with relative humidity diminishing east to west and then
stratus eventually dissipating altogether. The upper low will
then move to the OH Valley with strong subsidence across IA and
move little through the end of the work week, even drifting back
northward toward where it is currently. This will bring just
enough moisture back westward to warrant some token light shower
PoPs far east by Saturday.

By the weekend the pattern and western CONUS trough will finally be
progressive enough to kick out the upper low and get the surface
high past us by early next week. 12Z models continue to suggest
slower trends so have backed off on PoPs temporally and west to east
aggressiveness. The GFS and ECMWF are in fairly good agreement for
this range as relatively deep forcing starts advancing across the
Plains Monday Night before reaching IA in earnest Tuesday Night and
Wednesday. Instability will be low during this window but both
shallow and deep shear look to be high due to the strong low level
return flow and upper jet surging through the MO valley. This is
still a long way out and quite dependent on favorable timing, but
bears monitoring for a subtle fall QLCS wind/tornado event.


.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1225 PM CDT Wed Sep 28 2016

MVFR stratus looks to stick around for the foreseeable future as
the large upper level cutoff low churns over the Great Lakes.
Leaned toward extended the MVFR stratus deck overnight with
925-850mb RH remaining AOA 70% through 12z Thursday. Winds look to
continue to gust through the afternoon before decoupling tonight.





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