Area Forecast Discussion
Issued by NWS Des Moines, IA

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861
FXUS63 KDMX 220848
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
348 AM CDT Sat Jul 22 2017

.SHORT TERM.../Today through Tonight/
Issued at 348 AM CDT Sat Jul 22 2017

Boundary has drifted south and is oriented west-to-east along the
Interstate 80 corridor resulting in some cooler temperatures
north of the boundary. Ongoing convection in the northeast looks
to move east by around 12z or slightly sooner but there is still a
heavy rain threat until then and thus left the Flash Flood Watch
going for now.

The other concern was focused on the heat headlines.  Confident
enough to shave off the northern tier of counties of the advisory
and transition a few of the western counties from a heat warning to
an advisory. Kept the heat warning going for the central to
southeast, including the Des Moines Metro, based on the boundary not
anticipated to drift much further south until later today and wrt to
heat warning criteria (3 or more days of 105 heat index or higher)
where this location should see at least heat index values of 105 or
slightly greater by late this morning into the early afternoon
hours. There is the potential to downgrade/cancel early this
afternoon once the boundary does begin to drift further south into
Missouri. Plus, dew points seem to pool slightly higher along the
front and even if temperatures do not reach the lower 90s, the
stress of the previous several days of the humidity is enough to
warrant the heat warning for today.

GFS suggests a couple weak vort maxes to cut across the state
tonight providing enough forcing to mention slight pops in the
northeast. May need to consider adding some pops across far southern
Iowa in the vicinity of the front in northern Missouri with another
weak shortwave moving through this location. Left dry for now as
boundary looks to be far enough south in Missouri.

.LONG TERM.../Sunday through Friday/
Issued at 348 AM CDT Sat Jul 22 2017

Deepening trof across the eastern United States will allow for
cooler air associated with Canadian high to spread across Iowa
into Sunday. Conditions will be much more comfortable with lower
dewpoints and a modest northwest winds. Monday will be quite nice
with thermal trof grazing the northeast early before warm
advection starts by late in the day. The warm advection will
intensify into Tuesday with moisture returning and some threat of
convection in the west and north where best theta-e advection
occurs. A surface boundary is forecast to drop into the state on
Wednesday and slowly drop south as a shortwave passes north of the
state. Convection is likely to develop on Wednesday afternoon in
the vicinity of the boundary and continue into early Thursday with
low level jet/moisture axis feeding the convection across the
state. Models indicate the front is expected to slide south of
Iowa by Thursday with ridging building across the upper Midwest.
Subsidence and drier air associated with this ridge should bring
end the precipitation threat with cooler temperatures toward the
end of the work week.

&&

.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1154 PM CDT Fri Jul 21 2017

Thunderstorms have developed near KMCW and north of KALO between
3z and 4z. Expect these storms to very slowly sink southward this
evening as they pull off to the east. Therefore, have a several
hour period of SHRA/VCTS at KMCW with just VCTS at KALO. Other
challenge is low clouds and fog that have developed. Confidence
has grown since earlier in the evening that visibilities/ceilings
will drop into IFR at KMCW and may even drop in and out of LIFR
toward sunrise. MVFR conditions are expected at KFOD and KALO,
though these sites could possibly slip into IFR as well. Farther
south, low clouds and fog will not form as these sites are south
of the boundary with saturation in the low levels not forecast.
After the low clouds lift and fog dissipates Saturday morning, VFR
conditions will prevail through the TAF period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning until 8 PM CDT this evening for
IAZ057>062-070>075-081>086-092>097.

Flash Flood Watch until 9 AM CDT this morning for IAZ027-028-038-
039-050.

&&

$$

SHORT TERM...Podrazik
LONG TERM...Cogil
AVIATION...Ansorge



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