Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 271755

Area Forecast Discussion
National Weather Service Des Moines IA
1255 PM CDT Wed Jul 27 2016

.SHORT TERM.../Today/
Issued at 305 AM CDT Wed Jul 27 2016

Aloft there is a warm front at H850 that crosses western Nebraska
and snakes east to North Platte then due south to near Oklahoma
City. The area of thunderstorms has been tracking both near a
ribbon of moisture along that boundary with the lessening
expansion to the east. Over Minnesota the shortwave forecast to
drop south later today will eventually enhance additional
thunderstorm development along the leading edge of the upper level
forcing. Along with a weak trough at the surface...this area of
thunderstorms should reach northern Iowa in the early to mid
afternoon then slowly drift south into the evening hours reaching
the I80 corridor by about 00z.

Between these two areas of precipitation...sunshine will mix with
cloud cover today bringing a mix of partly cloudy skies and
filtered sunshine across the south and southeast.

A lower confidence forecast is in store today. Somewhat
challenging due to multiple factors. Though the diminishing MCV
over eastern Nebraska should weaken further as it encounters a
more stable air and limited moisture over western and Central Iowa
early this morning...the cloud deck may survive long enough to
create a few sprinkles or light showers to briefly require slight
PoP over the west. For now will monitor and update if the area
holds together prior to 7 am. Though none of the convective
allowing models nor the synoptic models are handling the current
convection well early today...the newest ESRL HRRR now suggests
the MCV may strengthen by early afternoon over central Iowa in
response to increasing instability over central/southern Iowa. If
this is correct... afternoon highs from central to southern areas
would need a slight downward modification by late morning and PoP
would need increasing. For now will go with climatology which
would suggest a weakening complex of storms this morning and let
the day crew reanalyze the trends that develop by late morning.
Will maintain highs in the lower 80s north and mid to upper 80s
across the south. Winds will be light due to the lack of
forcing/synoptic systems...which is also the reason the mesoscale
will be driving todays forecast.

.LONG TERM.../Tonight through Tuesday/
Issued at 305 AM CDT Wed Jul 27 2016

Main forecast concerns include POPs through much of the
period...especially with the system early in the period. In
addition...return to summer-like heat next week.

At the beginning of the long term period...convection is expected
to be underway. Surface front will slide southeast across the CWA.
0-6km shear parameters are not favorable for organized
convection...with the best chance for any stronger storms being in
the vicinity of the front where convergence will be greatest. Main
threats would include winds and moderate to heavy rain. Having
said that...models have backed off considerably on POPs
tonight...and have followed this trend. QPF values are lower as
well...reflecting the lower probability of widespread organized

POPs increase on Thursday from secondary shortwave
drops southeast through the upper-level flow to help intensify the
surface closed low tracking northeast along the surface front from
northeast MO into northeast IL.

Despite the surface low and frontal system to continue tracking
away the CWA on Friday...have kept POPs going. Additional
shortwave will provide for unsettled weather conditions. Thursday
or Friday will see the lowest max temperatures of the long term
period for the CWA...with the west seeing it on Thursday and the
east on Friday...due to combination of cover and
precip. Additional shortwave on Saturday will bring more lower-end
SHRA/TSRA chances. Another relatively cool day is
expected...although a couple degrees higher than on Friday.

Upper-level heights begin rising on Sunday and continue into
Monday as the western U.S. ridge migrates toward the CWA. Have
continued lower-end POPs both days due to presence of additional
weak shortwaves. Having said concerned that POPs on both
of these days especially may be a little on the high side.

From Tuesday afternoon into Wednesday...the surface warm front
will lift northeast across the CWA. Aside from this evening and
tonight...Tuesday night will provide the best chances for
organized convection. Another shortwave will drop southeast along
the southern fringe of the northward-retreating westerlies.
Theta-e advection along with strengthening LLJ will provide
enough moisture for TSRA potential. At this time the actual severe
weather potential is uncertain but will bear watching.

With the ridge closer to the CWA on Tuesday and
Wednesday...summer-like heat will return. Highs behind the warm
front are progged to be in the lower to middle 90s. Humidity
levels will return to levels typical of early August...with
dewpoints generally in the upper 60s to middle 70s. Those
conditions would yield heat index values as warm as the middle to
upper 90s...possibly lower 100s.


.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1253 PM CDT Wed Jul 27 2016

VFR conditions are expected through this evening across central
Iowa although scattered thunderstorms are expected. Will see
widespread status move into the state from the north overnight and
into most of the TAF locations with widespread MVFR to IFR ceiling
heights. These low clouds will linger into Thursday morning with
restrictions continuing. Surface winds will be light and variable
for much of the forecast period.





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