Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 212123

Area Forecast Discussion
National Weather Service Des Moines IA
423 PM CDT Thu Jul 21 2016

.SHORT TERM.../Tonight/
Issued at 348 PM CDT Thu Jul 21 2016

Tonight`s forecast is rather difficult in that there are a couple
different scenario`s that can play out and each are equally as
viable and my confidence is low that we will see precip.

The upper ridge remains in place and heat indices over much of the
warning area are indeed above 110 and other places as of 3 PM are
very close.  A weak boundary is forecast to drift into Iowa and
remain in place overnight.  This is coupled with a shortwave
expected to top the ridge axis and roll into Eastern Iowa.  An MCS
is expected to roll into Eastern Iowa tonight possibly affecting the
Eastern portions of our forecast area...mainly Waterloo to North of
Ottumwa.  While this seems plausible based on the last couple of
nights, the better forcing is off to the East and actually across
the West/Northwest.  Looking to the West...ACCAS and weak isolated
storms have been festering across Eastern NE all day and are
dissipating as they shift to the East into Western Iowa.  My
confidence is a little higher that the activity over NE
some point...make it into Iowa.  Whether that`s this evening or
overnight is questionable but given the tendency for stronger
forcing across the NW late I believe that it will happen...and
likely more in the overnight period.

Given the low chances for precip in any given area and the low
overall confidence in the precip forecast, I blended slight chance
pops over the areas of concern mainly after 06z through the end of
the tonight period.  Overall shear is weak but we are very unstable
and with the boundary around I cannot go completely dry though I do
not expect severe storms or heavy rainfall.  An isolated strong wind
gust however wouldn`t be out of the question.

.LONG TERM.../Friday through Thursday/
Issued at 348 PM CDT Thu Jul 21 2016

Primary forecast issues in the longer term will temperature and
heat indices...along with thunderstorm potential.

Friday and Friday Night...Forecast Confidence = Low to Moderate

Expansive upper ridge will remain entrenched over the much of the
central CONUS on Friday. A series of weak impulses/shortwaves will
ridge along the northern periphery of the ridge which could
provide enough forcing/ascent to trigger areas of convection.

Starting with temperatures...850mb temps on Friday will be similar
to today...perhaps even a degree or two warmer in the south.
However...a weak frontal boundary is expected to drop into the
state from the north...with an easterly flow on the north side of
the boundary with a southerly flow to the south. Cooler easterly
flow north of the boundary...combined with a higher potential for
thicker cloud cover will likely result in temperatures trending a
few degrees cooler than today...especially across northern Iowa.
With the boundary lining up right along I-80...Des Moines will be
a challenging forecast depending on the exactly location of the
front. With dewpoints remaining in the 70s to near 80F...heat
indices will continue to range from 100-110F.

Regarding convective potential...could see a scenario similar to
what portions of eastern Neb experienced today...with high based
showers/storms early in the day. Showers/storms could fester into
midday. A more likely scenario is a lull in activity /due to
capping/ with a possible uptick during peak heating as weak
convergence continues along the aforementioned boundary. Latest
CAM guidance continues to suggest isolated/scattered storms across
eastern or western Iowa...but overall confidence in convective
evolution is low.

Saturday through Thursday...Forecast Confidence = Low to Moderate

Boundary is expected to lift north as a warm front on Saturday.
This combined with weak shortwave energy is expected to lead to an
uptick in convection especially across western and northern Iowa.
A more pronounced temperature gradient is possible on
Saturday...with cooler readings north. Will keep the Excessive
Heat Warning going for now...pending convective trends.

Slightly cooler readings with occasional chances for showers and
thunderstorms will prevail through the middle of next week.


.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1256 PM CDT Thu Jul 21 2016

VFR conditions forecast with a potential for brief MVFR cigs through
00Z across the Eastern TAF`s as they lie on the periphery of a high
pressure system extending from OK.  There is also a potential for
fog or haze with the Eastern TAF`s later tonight for the same
reason.  After 12Z a weak frontal boundary will sink across central
Iowa with weak shortwaves dropping across the state coincident with
the surface feature.  I expect to see a few more clouds...perhaps a
mid level cloud deck and possibly some isolated TSRA.  This would be
similar to what is over central/Eastern Nebraska at 18Z Friday.


Excessive Heat Warning until 7 PM CDT Saturday for Adair-Adams-

Excessive Heat Warning until 7 PM CDT Friday for Audubon-Black
Hawk-Bremer-Butler-Calhoun-Carroll-Cass-Cerro Gordo-Crawford-
Kossuth-Palo Alto-Pocahontas-Sac-Webster-Winnebago-Worth-Wright.



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