Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 211800
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
100 PM CDT Wed Sep 21 2016

.UPDATE...
Issued at 1259 PM CDT Wed Sep 21 2016

Outflow from morning convection continues to slowly push into IA,
from the Quad Cities to near Mason City at noon. MLCapes have
reached uncapped 3500 j/kg into central IA with shallow
convergence noted along the outflow boundary. The 12z Nam and HRW
ARW/WRF runs are not recognizing the current SE WI cluster with
the HRRR/RAP much better. They suggest scattered surface based
late afternoon convection will develop per current forecast.
Shallow and deep shear is fairly high into NE IA so all modes of
severe weather would be possible, including some tornado
potential.

&&

.SHORT TERM.../Today/
Issued at 405 AM CDT Wed Sep 21 2016

Thunderstorms have remained north and northeast of the forecast
area early this morning and just north of a warm front that is
currently near the IA/MN border. The lower res solutions such as
the GFS/ECMWF/NAM all tried to drive this convection down the
instability gradient and into central Iowa this morning. The CAMs
are much more in tune with the current situation generally keeping
convection out of the forecast area until mid to late afternoon.
The exception is potential for a few sprinkles/light showers over
the far north where high level theta-e advection has commenced.

A broad cirrus shield is currently moving into western IA and may
play a role in limiting convective development today. Good elevated
mixed layer in place this morning with steep lapse rates and good
instability aloft. Convective temperatures in the upper 80s to low
90s may difficult to reach today though if the cirrus deck is
thicker than expected. In addition the moisture advection and first
wave of energy associated with tropical storm Paine will not arrive
over northern Iowa until late this afternoon. Overall have backed
off PoPs today before increasing across the north late this
afternoon. The EML should hold most of the day and may not have much
convective development until the low level jet strengthens this
evening. Deep layer shear will remain weak into this afternoon
therefore it will be difficult for organized storms to develop
through 7 pm however given the amount of instability any surface
based storm that does develop will have the potential of damaging
wind gusts and localized hail along with locally heavy rainfall. The
flash flood watch remains in effect for the far north though areal
coverage of storms should remain limited. PWAT values by this
afternoon will be nearing 2 inches or over 200 percent of normal.

.LONG TERM.../Tonight through Tuesday/
Issued at 405 AM CDT Wed Sep 21 2016

The main forecast concern was focused on heavy rain potential
tonight into Thursday across western to northern Iowa. Models
remain in fairly good agreement with the location and timing of
convection and leaned toward a blend of the NAM/GFS/ECMWF tonight
into tomorrow and then closer to the GFS through the remainder of
the forecast.

Tonight into Friday...Very impressive moisture plume evident on
Water Vapor Satellite will continue to pump significant tropical
moisture into the state tonight into Thursday. Stationary front
looks to remain planted along the Minnesota/Iowa border tonight and
possibly get shifted northward slightly with the strong WAA
throughout the day into this evening. Iowa remains entrenched within
west to southwest flow aloft with several shortwaves training across
the state tonight into Thursday evening. The strongest push of Theta-
E advection looks to be between 03z and 12z Thursday and have
highest pops going across northern Iowa during this time. Plenty of
moisture transport into the forecast area with PWATs ranging from
1.6 to 1.9 inches, which is roughly 200-250% of normal or +2-3 sigma
for this time of year.  Even surface dew points remain near 70
tonight, but there is some mid-level dry air tonight that will
ward off the heavy rain until after 03z Thursday when the column
becomes deeply saturated over northern Iowa. Warm layer cloud
depths are around 3500 meters with weak Corfidi vectors generally
oriented with the storm motion vectors tonight into Thursday
morning, and thus efficient rain producers and training are
likely over northern Iowa. Therefore, no changes to the ongoing
Flash Flood Watch.

Another shortwave combined with weak Theta-E advection in the
vicinity of the surface boundary draped over western to north-
central Iowa Thursday night will lead to another round of storms
over northern Iowa. Similar PWATs and slightly higher warm layer
cloud depths but weaker moisture transport into the state and a
bit stronger Corfidi vectors, so not the best set up for training
heavy rain producers but still the threat exist. Depending how
tonight into Thursday shapes up, another Flash Flood Watch might
be needed over northern Iowa. Some lingering storms are possible
Friday morning over northern Iowa, but overall Friday looks to be
dry for much of central Iowa with a strong ridge building into the
region.

Saturday through Tuesday...Very deep and strong upper level trough
digs into the western CONUS at the beginning of the weekend and
slowly transitions eastward and pushes a cold front through the
state Saturday night into Sunday. Yet another round of storms and
heavy rain potential for Iowa during this time. Storms linger
across the east Sunday night, but much of the precip looks to
further east by early Monday. However, GFS and even the ECMWF are
suggesting the low to strengthen and become cutoff or remain
stationary across the central CONUS through late next week. Thus,
providing focus for periods of showers, cloudy skies, and cooler
temperatures into the state by Monday and lasting for much of the
week. 850 mb temperatures drop to +2C by mid-week.

&&

.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1259 PM CDT Wed Sep 21 2016

VFR conditions are in place area wide with main feature an outflow
boundary along KMPZ-KFXY line at 17z with a sharp discontinuity
from SW to SE wind. Convection is expected to develop near this
feature into the evening and also along a NE/SD warm front. Thus
expect periods of convection to affect KFOD/KMCW/KALO into the
night. Timing, location and duration of MVFR or less conditions
and thunder is still in question however so have nothing more than
vicinity wording and VFR for now.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Thursday evening for IAZ004>007-
015>017.

Flash Flood Watch from 7 PM CDT this evening through Thursday
evening for IAZ023>028-033>039-044>048.

&&

$$

UPDATE...Small
SHORT TERM...Donavon
LONG TERM...Podrazik
AVIATION...Small



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