Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 161747

Area Forecast Discussion
National Weather Service Des Moines IA
1243 PM CDT Tue May 16 2017

.SHORT TERM.../Today through Tonight/
Issued at 326 AM CDT Tue May 16 2017

08z Satellite and sfc obs analysis depicts a somewhat
disorganized, fast-moving shortwave passing north- northeastward
through Nebraska this morning...making it into north-central
Minnesota by this afternoon. Pressure gradient tightens to around
3mb/110 km. With no additional localized wind- boosting mechanisms
of note, winds should stay below advisory criteria. Strong
southerly winds to boost WAA once again. Overall, have kept max
temps for today close to where they were yesterday.

Severe threat mainly confined to western/northwestern Iowa...with
an arrival time in the 00z Wed - 03z Wed timeframe. Models in
good agreement with attendant boundary to the aforementioned
shortwave draping through this area to serve as focusing
mechanism. MLCape values reach 2500 J/kg. Low-level lapse rates
reach 8 C/km. DCAPE values exceed 1000 j/kg...with the highest
values seeming placed well to our west. In all, tornado parameters
not too primary svr mode will be large hail, with
damaging winds as a distant secondary threat.
Additionally...PWATs look to exceed 1.65 inches per the GFS.
Interpolating between KOAX and KMSP on the SPC climatology page,
this value would be near the top of the charts. In short, heavy
rainfall is possible and flash flooding cannot be ruled
out...though NW Iowa can typically handle higher rainfall amounts
better than most other portions of the DMX CWA.

.LONG TERM.../Wednesday through Monday/
Issued at 326 AM CDT Tue May 16 2017

Wednesday PM-Thursday PM...

Conditional svr threat, but if storms become severe, mini-outbreak
cannot be ruled out. Models coming into good agreement with
pushing parent upper low located near the Four Corners region of
the CONUS at 09z Tue, to the upper Midwest. By 00z Thu, models
place this upper low over the IA/SD/MN/NE area. with sfc low
slightly lagging to the south. As of now, central to SE Iowa looks
to be the focus for the potentially worst weather. 0-3km Bulk
shear values approach 50 kts, which is off the charts. 0-6km
MUCape is only 1000 J/KG which is not great, but sufficient.
Tornado setup is what could be interesting. LCL heights below
1000m, KDSM and KOTM hodographs very much "sickle-shaped", 0-1km
helicity approaching 200 m2/s2. Top 15 CIPS analog events tend to
show a few tornadoes, damaging wind reports, and isolated large
hail reports basically along and east of I-35, along and south of
I-80... which seems reasonable. Much uncertainty, however, as to
how morning/early afternoon will pan out with cloud cover +
ongoing rain possible. Next forecast discussion should pin down a
little better... if timing lines up, SPC upgrade to enhanced-
moderate risk cannot be ruled out in our southeast.

Friday into the weekend...

Another slow-moving upper low/broad longwave trough will dig
through the Plains states. 00z Tue GFS and ECMWF in good agreement
with reinforcing moisture making it into Iowa...raising PWATs into
the 1.25 to 1.5 inch range through Saturday. 850mb temps drop to
around +10C by with cloud cover and rain potential,
highs will struggle to make it into the 60s...a stark contrast
from 90 degree wx yesterday and today. Thunder potential there,
though svr threat not impressive at this time.


.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1243 PM CDT Tue May 16 2017

VFR conditions expected through 00Z.  After 00Z a low pressure
system will lift out of the southern Plains with a frontal boundary
extending across western into north central Iowa.  This boundary
will lift across the state overnight bringing MVFR conditions with
strong storms in aft 03Z.  There may be a lull in activity in the
morning but a second system will move in late morning and storms
will regenerate lifting across the TAF location even beyond the
forecast valid period.





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