Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 241739
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1239 PM CDT TUE MAY 24 2016

.SHORT TERM.../Today/
Issued at 345 AM CDT Tue May 24 2016

Currently a shortwave shifting across Iowa is responsible for a
broad area of showers and thunderstorms moving across central Iowa.
Radar imagery and lightning data suggest that this area of showers
and storms will be weakening in the next hour or two as it moves out
of the better instability.  Hi-Res models suggest that this precip
will linger into mid morning before dissipating.

Convection for the rest of today is very difficult to predict. We
are unstable and have sufficient moisture available for storms but
shear profiles are weak and pretty unidirectional and lack of a
focusing mechanism suggests that storms today will be isolated...
more hit and miss and the threat of anything severe is pretty
minimal...but still it cannot be ruled out given the instability.
I have noticed that models the last couple of days have been
overdoing dewpoints by several degrees and as a result have been
overdoing the amount of CAPE around.  Looking more at trends versus
overall values of CAPE it looks like the best location for storms
should be the southeast third or so this afternoon. Better
forcing comes in tonight and that period looks much more
interesting.

.LONG TERM.../Tonight through Monday/
Issued at 345 AM CDT Tue May 24 2016

Primary forecast challenge in the longer range remains on shower
and thunderstorm chances and potential for severe weather and
heavy rainfall. Overall, the entire long range forecast features
below average confidence, as the pattern resembles more of a mid-
summer type convective regime -- rather than mid-May.

Tonight: Forecast Confidence = Low

Synoptic pattern will continue to be dominated by a broad upper trough
over the western U.S. with upper ridging over the Ohio River Valley.
Plentiful moisture will remain in place due to persistent s/swly
flow at low to mid-levels. While the 24.00Z models generally agree
on lifting a low amplitude shortwave through the Midwest tonight,
unfortunately significant model differences remain with respect
to convective trends. The GFS/ECMWF continue to indicate
the potential for a nocturnal MCS in the exit region of a 35-50
kts LLJ initiating over northern MO or southern IA. Meanwhile,
the NAM and many of the NAM based CAMS indicate an MCS much
further north over SD/MN with a lower potential further south. At
this time, tried to keep some semblance of continuity regarding
PoPs with high chance to likely PoPs overnight, although if trends
continue farther north adjustments may be required. While
instability /MUCAPE/ will be sufficient for strong storms,
effective shear appears marginal for organized supercellular type
structures, so overall severe threat overnight appears low.

Wednesday Through Thursday: Forecast Confidence = Low

As mentioned above - a conditional threat for strong/severe thunderstorms
will definitely continue on Wed/Thu. Moisture levels /surface
dewpoints in the 60s/ will remain more than sufficient to generate
moderate to strong instability /MLCAPE 1500-2500+ J/kg/ each
afternoon pending sufficient solar insolation/diabatic
heating. While the wind profiles will not be extreme, 25-40kts of
deep layer shear would support multicellular and possibly
supercellular storms. Much of the uncertainty hinges on the large
scale forcing for ascent which remains nebulous at this time
owing to differing mesoscale details. The SPC has highlighted
much of Iowa in a Slight Risk on both Wednesday and Thursday - so
this trend will need to be monitored closely in the future. Heavy
rainfall will also be possible with any storms due to plentiful
moisture. Regardless - conditions will feel summer-like with
highs in the 80s and a fair amount of humidity.

Friday through Monday: Forecast Confidence = Low

Pattern does not change much into the upcoming holiday weekend
with warm and rather humid conditions expected to persist. Many
synoptic and mesoscale details will need to ironed out, however
at least periodic chances for showers and thunderstorms will
continue.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1239 PM CDT Tue May 24 2016

Challenges this period include cigs for now and coverage of
thunder/timing later this evening and overnight. Patch of MVFR
cigs over central Iowa moving east and lifting to VFR. Focus then
shifts to thunder redevelopment today. Possible isolated storms by
21z but more widespread coverage will increase aft 04-06z from
west of I35 t0 I35 then east northeast through 15z. Two areas of
storms across north as well as south expected to develop and move
east through 15z Wednesday. Cigs/vsby will again drop to MVFR as
thunder increases toward 12z.

&&

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

$$

SHORT TERM...FAB
LONG TERM...Fowle
AVIATION...REV



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