Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 261145
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
645 AM CDT Wed Jul 26 2017

.SHORT TERM.../Today through Tonight/
Issued at 409 AM CDT Wed Jul 26 2017

Today into tonight will be active across the forecast area as a weak
surface low and front pass through the state.  Currently, the front
was located from western Nebraska into southern Minnesota where most
of the active convection can be found. Main axis of the low level
jet is feeding the convection and is expected to veer toward the
state this morning.  Therefore, the storms in Nebraska are expected
to push into western Iowa by daybreak and to near the I-35
corridor by late morning. This convection is not expected to be
severe but will likely contain some heavier downpours with PWATS
climbing above 1.5"

The main show however is expected later this afternoon into
tonight.  Instability will increase across the southeast by
afternoon where there is the least chance of contamination by
ongoing convection and clouds.  This will fuel redevelopment by
afternoon near the boundary.  The initial storms will have the
highest likelihood of being severe with large hail but then main
concerns will be with wind and heavy rainfall as the system matures.
Please see the Hydrology discussion below for heavy rainfall issues.
The convection will drop southeast during the afternoon into tonight
as the boundary gradually passes to the south with subsidence
increasing on the backside overnight.  This will end the
precipitation from north to south during the night as the drier air
arrives and lift subsides.

.LONG TERM.../Thursday through Tuesday/
Issued at 409 AM CDT Wed Jul 26 2017

Bottom Line up Front...

One of the more quiet long-term fcst periods in quite a while. Near-
normal temperatures and no appreciable weather hazards to speak of
beyond the lightning produced via a slight possibility of a few very
isolated/"hit-or-miss" non-severe thunderstorms Sunday, Monday, and
Tuesday PM.

Thursday through Sunday Morning...

At 12z Thu, precip from the system discussed in the short term will
be exiting the forecast area to the southeast. Meanwhile, the
extensive ridge that will effectively encompass the western CONUS
will begin to settle in near the Four Corners region. The Ohio
Valley and eastward will be influenced by a deep longwave trough.
Through the next several days, a 1020 mb sfc high will propagate
through Minnesota and into the Northwoods of Wisconsin. This
synoptic setup will serve to keep Iowa into NW to NE flow above 850
mb through the weekend. Resultingly, 850mb temps look to dip down to
the +12C to +15C range... versus the +20C to +22C range they were at
yesterday. The forecast 850mb temp range is near-normal for this
time of year...supporting temps in the 80s through the weekend.

Sunday PM and Beyond...

Depending on model preference, the sfc high discussed above will be
located near northern Illinois by peak heating Sunday. The GFS is
slightly faster than the ECMWF in progressing the high...but since
this is Day 5, will not get into too much detail. The GFS solution
has a shortwave pushing across northern Iowa during peak heating
Sunday. This shortwave can be traced back to California and is
clearly picked up on the 08z Wed water vapor imagery. The sfc high
more or less stations itself over Illinois, potentially setting us
up for isolated late afternoon thunderstorms during this period. The
Gulf of Mexico is effectively closed off during this time, so any
isolated storms that do develop will struggle to find sufficient
moisture. With the high so close to Iowa, shear/a solid focusing
mechanism will be very hard to come by. 850mb temps slowly creep up

&&

.AVIATION.../For the 12Z TAFS through 12Z Thursday morning/
Issued at 641 AM CDT Wed Jul 26 2017

Very active period aviation wise with ample convection expected
across Iowa into tonight. Initially, VFR conditions will be
widespread this morning but quickly give way to MVFR to IFR
conditions as thunderstorms spread east across the forecast area.
By tonight, lower ceilings are expected to develop across the area
with widespread MVFR to even IFR ceilings late tonight as cooler
air arrives. Surface winds will be relatively light today from the
south and southwest but variable near thunderstorms. Winds will
become north to northeast overnight.

&&

.HYDROLOGY...
Issued at 409 AM CDT Wed Jul 26 2017

Moisture transport will continue into the state today on decent
southerly flow.  PWATS increase to an impressive 2.00-2.50" by late
this afternoon across the southern half of the state.  Convection is
expected to develop ahead of the boundary this afternoon and
gradually sink south through the state this evening with decent LLJ
feeding the convection.  Given the good inflow, deep warm cloud
layer of greater than 13.5kft and the high PWATS, heavy rainfall
with high rates will be quite possible across the southern half of
the state by this afternoon into tonight.  While this area is quite
dry, the intense rates should be sufficient to produce some flash
flooding, especially in urban settings and other low lying favored
areas.  Therefore, have gone with a flash flood watch along and
south of the I80 corridor for the afternoon into the overnight
period.

Will also continue to monitor the Bremer county area for heavy
rainfall as well given the recent deluge and incipient moist
conditions.  While the heaviest rainfall is expected south of this
area, any northward shift of precipitation could affect this area
which would quickly produce issues.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 1 PM CDT this afternoon through late
tonight for IAZ058>062-070>075-081>086-092>097.

&&

$$

SHORT TERM...Cogil
LONG TERM...Kotenberg
AVIATION...Cogil
HYDROLOGY...Cogil



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