Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 260504

Area Forecast Discussion
National Weather Service Des Moines IA
1204 AM CDT Wed Jul 26 2017

.SHORT TERM.../Tonight through Wednesday/
Issued at 406 PM CDT Tue Jul 25 2017

Confidence: Medium to High

Main challenge this period will be temperatures and extent of severe
weather/heavy rain. Overnight tonight will be continuing with
southwest flow and increasing low level and mid level moisture. 13z
surface analysis shows ribbon of 65-70+ dewpoints moving north ahead
of the trough into eastern Nebraska and western Iowa. Weak low in
South Dakota with a warm front stretching east toward Minneapolis
and southeast toward LaCrosse.  Overall...forecasts have remained
consistent the past 24 hours with overnight lows tonight into the
lower to mid 70s across the region. Models continue to advertise
slowly increasing thetae advection along the warm front late this
evening into Wednesday morning. By 12z Wednesday the approaching
trough will be entering the far west/northwest then slowly edging
east throughout the day. Once again today the GFS/Euro forecasts
suggest that bulk shear will be minimal with 30 to up to a max 40kts
overall expected for Wednesday suggesting that the overall severe
weather threat should be limited in scope. The Euro also remains
less robust than the GFS forecast...only increasing the areal
coverage of higher bulk shear toward the end of the day over eastern
Iowa.  Soundings also continue to support a lesser risk of severe
storms as well with tall/skinny CAPE profiles with more emphasis on
efficient rainfall.  As the front sinks south during the day...
precipitable water forecasts continue to show 2 to 2.5 inches over
the area which will be focused by the boundary as it moves
southeast. Warm cloud depths are forecast to reach 14 to 16kft by
the afternoon to early evening hours. Along the boundary and into
the evening hours as the front drifts southeast storm motion vectors
suggest storm motion will be slow...with a more pronounced slowing
as the low level jet increases into the latter evening hours either
in far southern Iowa or northern Missouri...helping storms backbuild
along the boundary. This should increase rainfall rates/amounts
especially after dark in the southeast. For the daytime hours...
given the pwat available and the expected slow movement along the
front...localized rainfall totals of 2 to 4 inches with locally
higher amounts are very possible.  Hires models show banding amounts
greater than 4 inches in some areas which may lead to some isolated
hydro problems tomorrow. Given the extremely dry conditions over the
south...most of the region there could handle any totals of that
magnitude...though there may be some runoff issues if rainfall rates
become too high over an given area.  For now will hold off on
headlines to see if next round of data support possible flooding
potential. Overall... the threat for any widespread flash flooding
is rather limited... though local runoff issues could develop in
urban areas and areas that are prone to poor drainage. The only
known area of concern for localized heavy rainfall would be the
northeast corner where Bremer County and surrounding areas are just
slowly recovering from this past week`s rainfall...but the better
forcing and likely heavier rainfall appears more focused south and
west of that area Wednesday afternoon...then moving into southern
and eastern Iowa Wednesday evening. Should a signal for heavy
rainfall over northeast Iowa become more pronounced in the next 12
hours...then flood watch headlines may be needed there. Highs
Wednesday will be cooler than today with a few areas in the
southeast nearing 90 F by afternoon prior to the arrival of the

.LONG TERM.../Wednesday night through Tuesday/
Issued at 406 PM CDT Tue Jul 25 2017

Main focus for the extended term is really on Wednesday night and
then temperatures throughout. Sfc trough/frontal boundary should
be stretching across the central/southern portion of the state by
early Wednesday evening with convection ongoing across the state.
Areas near this boundary and to the south will have the best
chance of lingering strong to severe storms into the evening hours
as the frontal boundary continues to sink southward into northern
Missouri by late Wednesday night. PWAT value still to be very high
in the south/southeastern portions of the area pushing above 2
inches with very high freezing levels on the order of 17kft should
suggest very efficient rainfall production. Some training also
possible with moisture advection into the boundary and storms
forming along the boundary and then pushing east southeast.
However the heavier rainfall is expected to occur across an area
that has been very dry and could take some rain, therefore any
flash flood threat is somewhat limited with mature crops in the
fields as well. If storms train through an urban area, the
potential will increase for flash flooding. Otherwise storms to
diminish late Wednesday night into early Thursday morning as the
boundary moves south of the area.

Upper ridging to then build into the central US with northwest to
northerly flow across Iowa to end the week, with high pressure
dominating at the surface and cooler air moving in aloft. A weak
boundary to move through Sunday may bring an isolated chance for
storms otherwise the extended remains dry for much of the period
and cooler for much of the period. H85 temps mainly in the mid
teens Celsius for Thursday into the weekend and early next week.
This should yield below normal temperatures for this time of the
year with highs in the upper 70s to mid 80s and lows in the upper
50s and 60s.


.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1202 AM CDT Wed Jul 26 2017

Thunderstorm chances will increase overnight across northwest Iowa
then spread across Iowa through the day Wednesday. Potential for
heavy rain with these storms that could greatly reduce vsbys. Cigs
will also be decreasing through the day Wednesday with likely
widespread MVFR cigs by late in the day and into the evening.




LONG TERM...Beerends
AVIATION...Donavon is the U.S. government's official web portal to all federal, state and local government web resources and services.