Area Forecast Discussion
Issued by NWS Des Moines, IA

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397
FXUS63 KDMX 240453
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1153 PM CDT MON MAY 23 2016

...Updated Aviation Discussion...

.SHORT TERM.../Tonight/
Issued at 344 PM CDT Mon May 23 2016

A moist and unstable airmass has spread across Iowa and much of
the region today beneath southwesterly flow aloft. A shortwave
trough moving slowly through this flow has generated persistent
showers and thunderstorms across about the western half of Iowa
and portions of adjacent states, mainly focused on a couple of
weak boundaries. Modest instability magnitude and an absence of
vertical shear have precluded severe weather with just some small
hail reported at times, but a few storms have produced locally
heavy rainfall across narrow strips of western and northwestern
Iowa.

While the aforementioned shortwave is progressing slowly
northeastward across the region, even in its wake the instability
associated with daytime heating has allowed more thunderstorms to
fester along the boundaries across eastern Nebraska, and it is
expected the redevelopment will continue into the evening,
resulting in scattered showers and storms across much of our
forecast area. Have carried generally 40 to 60 POPs to account for
this bubbly mess and any details of categorical POPs will be very
transient in forecast updates this evening. Later tonight the
shortwave will have moved away and a low-level jet will generate
more thunderstorms roughly over northern Kansas, but it remains to
be seen whether these will develop far enough north to affect
southern Iowa much, so will hold chance POPs in that area and time
frame. Again severe weather will be unlikely, but locally heavy
rainfall is expected.

.LONG TERM.../Tuesday through Monday/
Issued at 344 PM CDT Mon May 23 2016

Tuesday...
Trending to be a dry afternoon. Though CAPE values to be more
than sufficient for thunderstorms, overall meso environment
largely disorganized. 850mb winds struggle to reach 15kts, which
is not going to bode well for organizing shear/moisture transport.
For the second consecutive day, relatively dry air encroaches upon
the DMX CWA from the NW, dropping PWATs below 1 inch.

Though 850mb temps remain the same Tuesday as they were on Monday,
low level cloud cover will increase. This will drop temps around 5
degrees for much of the CWA versus Monday`s temperatures.

Wednesday...
Models in good agreement with depicting an MCS trying to sweep
across the DMX CWA from SW to NE during the morning period. There
is a lot of disagreement with the specific timing and location of
this feature, so have kept POPs somewhat broadbrushed for now,
with expectation of locally heavy rain with the MCS.

By 12z Wed, consensus with another reinforcing shot of moisture
aided by a 25kt LLJ to push across southern/SW Iowa. Decent
isentropic lift/theta-e boundary in good alignment with the LLJ
should help organize and support precip. With +1 to +2 std dev PWATs
over 1.5 inches, QPF upwards of 1 inch seems plausible.

Wednesday afternoon severe threat challenging to pin down. Southern
Iowa may be "gunked" over from morning precip. KDSM fcst soundings
also keep DSM area capped. Northern Iowa looks like the best shot to
have over 2000 j/kg uncapped CAPE. DCAPE values jump to over 1000
j/kg, introducing damaging winds, along with large hail as main
threats. Tor parameters not the best...So while not completely
ruling out a tornado or two, focus will be on hail and winds.

Thursday into the weekend...
Medium-long range models continue with pushing a sfc low into
KS/NE. An attendant boundary NE of this low will help continue to
push warm air and moisture into the DMX CWA. Sfc dewpoints from
the source region are in the low to mid 70s, so have bumped up
dewpoints vs guidance. Max temps are conservative in the low to
mid 80s and may be increased as well. Ultimately, Thursday looks
to be the peak of the heat/humidity.

Confidence low, but it looks like the aforementioned boundary may
not make it far into Iowa until later Thursday. Thus, Thursday
morning and afternoon may be storm-free, and the evening into
overnight period may be active thunderstorm-wise. Given overall
ambiguity with this setup, this all could change in future updates.
It appears plausible this pattern could continue into Saturday,
after when, gulf moisture flow becomes pinched off from Iowa.

&&

.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1148 PM CDT Mon May 23 2016

Low confidence with any TSRA affecting an TAF site overnight into
Tuesday morning. May see some patchy fog develop across northern
Iowa and have mvfr visibility mentioned at FOD and MCW. At ALO,
MVFR ceilings are possible towards the morning. Otherwise, have
VCTS mentioned at DSM with weak area of convection trickling into
western Iowa and may hold together long enough to come close to
the airport. Otherwise, VFR conditions are expected.

&&

.DMX Watches/Warnings/Advisories...None.

&&

$$

SHORT TERM...Lee
LONG TERM...Kotenberg
AVIATION...Podrazik



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