Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 150921
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
421 AM CDT Tue Aug 15 2017

.SHORT TERM.../Today through Tonight/
Issued at 421 AM CDT Tue Aug 15 2017

Several concerns today and tonight with the first item focused on
the dense fog across northern portions of the forecast area. Even
with the high clouds streaming into this location, the radiative
cooling effective continues to keep the dense fog and has actually
thickened and expanded further eastward toward the Waterloo
location. Confident enough that the blanket of dense fog will
persist through the mid-morning hours and may consider expanding
the advisory further south and east. Some fog has developed over
the far south as well, but for now has remain above advisory
guidance. Now the latest RAP, HRRR and the ARW/NMM runs suggest
the fog expands over much of the eastern portions of the forecast
area and the dense fog meanders as far south as Interstate 80 but
east of Interstate 35. For now, will handle this development with
SPS if it does materialize.

WRT to convection today and tonight.  Convection has developed along
and north of a stalled cold front extending across Nebraska due to a
ribbon of Theta-E advection pushing northeast. Very slow movement
with this convection and timing to move into western to northwest
portions of the forecast area looks be after 14z this morning and
delayed and cut back pops in the central portions of the CWA. Low
confidence with this convection holding together as these are tied
into the nose of the LLJ and looks to weaken later this morning.
Still, HRRR and ESRL HRRR suggest some of this shower/storm activity
to trickle into the west to northwest through midday. With the
aforementioned boundary laid up across southern Iowa, could see
redevelopment in this location later this afternoon and have low
chance pops going across the central and south.

Late tonight into Wednesday...delayed the timing of the onset of
precip to be confined to just the northwest this evening. The main
forcing looks to finally push into the west past 06z tonight with
still plenty of dry air to contend with across the forecast area.
Condensation pressure deficit along the 300-320K isentropic surfaces
finally become low enough closer to 12z Wednesday. Low confidence
with much of the CWA seeing any rain until past 09z Wednesday and
the best chance looks to be over the west. Thus, may need to
continue to trim back timing of pops further east overnight.

.LONG TERM.../Wednesday through Monday/
Issued at 421 AM CDT Tue Aug 15 2017

Axis of moisture transports will shift into the state on Wednesday
ahead of approaching system. As this flow veers toward the state,
showers and thunderstorms will edge eastward through western Iowa
on Wednesday morning and across much of the area during the day.
The strength of the convection should be limited by cloud cover
keeping instability values relatively low, however any breaks will
allow for some destabilization. In addition, the atmosphere will
be quite moist with PWATS near 2.00" by midday, therefore lapse
rates will likely be near moist adiabatic limited overall severe
potential. The bulk of QG forcing will pass through during the day
and will progress east by Wednesday night with precipitation
ending from west to east during the night. The aforementioned high
PWATS should lead to pockets of heavier rainfall, however the
antecedent dry conditions should preclude much in the way of
widespread significant water issues during this time.

Drier conditions will arrive by Thursday with Canadian airmass
spreading southeast into the state and continuing into early
Friday. Warm advection will begin Friday morning ahead of the next
shortwave dropping southeast into the region. This will bring a
threat of showers and a few storms by Friday evening into early
Saturday across much of central Iowa. Much of the weekend should
be relatively quiet with weak ridging passing through the area.
Main concern will be with the return of moisture/increasing
theta-e advection late Sunday into Monday ahead of the next
system. This will bring an increased chances of storms Sunday
night into early Tuesday from time to time. This may be
problematic for eclipse watching in some locations as cloud cover
may hamper viewing.

&&

.AVIATION.../For the 00Z TAFS through 00Z Tuesday evening/
Issued at 640 PM CDT Mon Aug 14 2017

Frontal boundary to drop southward and stall out across the area
tonight, and may have some fog develop across the northern TAF
sites behind the boundary. Kept MVFR to higher IFR for now.
Otherwise winds to be fairly light and variable overnight, then
shifting around to the east to southeast through the day Tuesday.
Not confident in precip chances at any location, so have left dry
for now.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM CDT this morning for IAZ004>007-
015>017.

&&

$$

SHORT TERM...Podrazik
LONG TERM...Cogil
AVIATION...Beerends


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