Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 202352
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
652 PM CDT Wed Jul 20 2016

.SHORT TERM.../Tonight/
Issued at 256 PM CDT Wed Jul 20 2016

The warm front is currently near an Estherville to Cedar Rapids
line. The boundary will push through the area tonight. A few
clouds and southeast winds have kept temperatures a bit lower thus
far northeast of the boundary. Dew points have responded well due
to the moisture advection in addition to moist ground and crop
evapotranspiration. This will all help keep temperatures very warm
overnight and in the mid to upper 70s. Warm advection continues
across Central Iowa through tonight with 700 mb temperatures of
12C to 14C arriving along with a strong capping inversion below.
This will shift the focus for thunderstorms north of the state
with dry conditions prevailing.

.LONG TERM.../Thursday through Wednesday/
Issued at 256 PM CDT Wed Jul 20 2016

Heat and humidity continue to be the main focus in the forecast
through Saturday. Thursday continues to look to be the hottest
day in this stretch of heat but 850 temps through Saturday
continue to be +25C with upper 20s in some locations. While
confidence is high in oppressive conditions persisting...there are
some caveats to the forecast. For Thursday night into Friday...a
shortwave tops the ridge in place over the upper midwest and an
MCS is progged to move East across the Northern Plains and
Minnesota. The shortwave and potential cloud cover from the MCS
could hamper temps across the North Thursday night into Friday and
if thunderstorms should develop a little further South...across
Northern Iowa then these impacts could spread a little further
South. Then on Friday afternoon into Saturday morning, a cold
front extending from the Great Lakes all the way back into Kansas
will slowly settle South across the forecast area. All the medium
range models depict a shortwave breaking the ridge during this
time with at least some potential for thunderstorms. We have a cap
in place with 700MB temps around +14 or so but if the cap is able
to be broken, then thunderstorms will rapidly develop along the
boundary and cloud cover and rainfall could again squash the heat
potential. At this point confidence in that scenario is low though
I do keep the mention of isolated storms. The cap will likely hold
but if it breaks, the reality is that we are so unstable that
storms would develop along the boundary. Shear is weak so not sure
they would be severe but deep moisture is in place and PWAT`s are
2+ inches so heavy rain would be a concern.

The front sinks South for Sunday into Monday and a strong system
sweeps across the Northern Plains flattening the upper ridge over
the region with a surface high to the East of the state so temps
early next week will be back in the 80s to around 90 and humidity
will be lower as well. The surface high will begin to shift
further East late Monday into Tuesday but models vary on this
movement. The GFS brings warm advection back into the South and
West by Tuesday while the Euro model keeps the warm advection to
our West, thus this would mean a difference between warming back
up with storm chances or not. I did not change the forecast much
out that far but by Wednesday all models push the warm advection
back over Iowa and bring a more active pattern back to the region.

&&

.AVIATION.../For the 00Z TAFS through 00Z Thursday evening/
Issued at 652 PM CDT Wed Jul 20 2016

Fairly high confidence in VFR conditions through the period with
SCT/BKN mid clouds at worst. Precipitation, mainly aloft, may just
brush KMCW early this evening. Little change in wind expected into
Thursday.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning until 7 PM CDT Saturday for Adair-Adams-
Appanoose-Boone-Clarke-Dallas-Davis-Decatur-Guthrie-Jasper-Lucas-
Madison-Mahaska-Marion-Marshall-Monroe-Polk-Poweshiek-Ringgold-
Story-Tama-Taylor-Union-Wapello-Warren-Wayne.

Excessive Heat Warning until 7 PM CDT Friday for Audubon-Black
Hawk-Bremer-Butler-Calhoun-Carroll-Cass-Cerro Gordo-Crawford-
Emmet-Franklin-Greene-Grundy-Hamilton-Hancock-Hardin-Humboldt-
Kossuth-Palo Alto-Pocahontas-Sac-Webster-Winnebago-Worth-Wright.

&&

$$

SHORT TERM...Donavon
LONG TERM...FAB
AVIATION...Small


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