Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 222339

Area Forecast Discussion
National Weather Service Des Moines IA
638 PM CDT Tue May 22 2018

.SHORT TERM.../Tonight through Wednesday/
Issued at 311 PM CDT Tue May 22 2018

Tonight into Wednesday...The main forecast concern was focused on
heavy rain potential across northern Iowa late tonight into
Wednesday morning.  Utilized a blend of the NAM12/HRRR/RAP for pops
tonight but models are in fairly good agreement with timing and
placement of the heavy rain.

Strong surge of theta-e advection and decent mid-level moisture
transport into northern Iowa this evening into early Wednesday
morning. The strongest forcing for ascent and deep moisture
convergence orients from northwest Iowa through north-central Iowa
slightly southeast toward the Waterloo area. Warm layer cloud depths
around 3500m with precipitable water values around 1.50" (well above
normal this time of year) in northern Iowa. Time frame of heaviest
rainfall looks to develop around 03z to 09z, with some lingering
storms through around 15z. Confident to increase pops and qpf
amounts tonight over the far north and have a sharper gradient of
the higher pops further south. Potential tonight for some localized
flash flooding if the rainfall rates do materialize over far
northern Iowa. Lesser confidence of any storms/rain south of Highway
30 tonight and expecting much of the rain to be out of the CWA by
15z or sooner. Low potential for severe storms with the lack of
significant shear present tonight into Wednesday morning, but
certainly cannot rule out a few elevated hail storms before the
heavy rain develops.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 311 PM CDT Tue May 22 2018

The models and their associated ensembles are in good agreement
with the general pattern progression through the period leading to
greater forecast confidence. The overall theme will be a summer-
like flow regime with persistent warm and humid conditions. NAEFS
and ECMWF ensemble height, temp, precipitable water, and specific
humidity percentiles all remain anomalously high through the
period and beyond. Late Friday looks to be the only appreciable
chances for precipitation. At onset, upper level ridging will have
built through the central CONUS. The best warm/theta-e advection
will be lifting around the ridge in response to the current NV/UT
low opening and through the northern Rockies so nocturnal
convection is not a threat. 2-3K j/kg MLCAPEs with minimal CIN
will be in place during peak heating Thursday, but with little to
no convergence or synoptic support do not feel any PoPs are

However there will be more favorable forcing and CAPE/CIN farther
north and west during peak heating leading to the potential of
Dakotas surface based convection eventually reaching north and
west sections during the overnight hours. The deep shear is quite
weak however so the severe potential is not great, but weak winds
through the column and unseasonally high moisture values may lead
to locally heavy rains. The forcing and convergence will be weaker
by then, but there remains some potential for surface based/peak
heating development over northern IA during the late afternoon or
early evening hours with chances fading overnight due to weak
inflow and unfavorable warm advection.

After this weak short wave starts heading toward the Great Lakes
Saturday heights will build again with insufficient support for
much in the way of PoPs. Although the airmass will remain warm and
humid, deep ridging overhead will result in weak winds aloft and
insufficient convergence for much surface based potential.


.AVIATION.../For the 00Z TAFS through 00Z Wednesday evening/
Issued at 638 PM CDT Tue May 22 2018

Northern TAF locations will become MVFR aft 06Z as thunderstorms
create lowered cigs and vsbys.  Moderate to locally heavy rainfall
is possible with stronger storms.  Lower cigs may linger after the
storms, through the morning hours before becoming VFR later in the
TAF forecast period.





SHORT TERM...Podrazik
AVIATION...FAB is the U.S. government's official web portal to all federal, state and local government web resources and services.