Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 230914
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
414 AM CDT MON MAY 23 2016

.SHORT TERM.../Today/
Issued at 328 AM CDT Mon May 23 2016

Primary forecast challenge today will be timing and coverage of
showers and thunderstorms...and a low potential for severe storms.
overall forecast confidence is medium.

00z upper air analysis and water vapor imagery indicated a large
trough over much of the western CONUS with upper ridging remaining
over the Midwest and Great Lakes. Several low amplitude shortwaves
were also noted on water vapor imagery...with the most distinctive
wave currently over AZ/UT. fairly impressive moisture plume observed
from OK/TX northward to SD with 850 mb dewpoints exceeding 12C, sfc
dewpoints in the low/mid 60s, and PWAT values exceeding 1 inch.
However, areas to the east remain much drier with the 00z KDVN
sounding observing a PWAT of only 0.47"...with sfc dewpoints near
50F.

Large MCS over SD/NE continues to push slowly e/sewd this morning
aided by moisture convergence/theta-E advection along the periphery
of the LLJ. Unfortunately, short range model and CAM guidance
continues to struggle with convective trends associated with the
eastward propogation of this system with some members maintaining a
rather healthy line of showers and storms, while other solutions
have it outrunning the instability and showing a rapid weakening by
mid-morning. At this point, feel a least scattered convection will
reach central Iowa, with less confidence east of I-35. Depending on
cloud cover and diabatic heating, models suggest the possibility of
thunderstorm re-development this afternoon across portions of Iowa.
Best potential for strong to severe storms is across northwestern
Iowa where the most favorable CAPE/shear parameter space exists. The
SPC has included this area in the Slight Risk...with a Marginal Risk
elsewhere.  Temps today should be warmest in the east /low 80s/
which will likely see the most sunshine, with readings in the 70s
west.

.LONG TERM.../Tonight through Sunday/
Issued at 328 AM CDT Mon May 23 2016

There is little change to the going forecast as we enter a very
active pattern. Southwest flow will already be established as an
upper low digs into the Southwest United States and sharpens an
upper ridge just to the East of the region and into the great
lakes. With plenty of moisture and instability available the key
will be where the forcing develops and where boundaries will lie.

Beginning with tonight...we have have ongoing convection from the
afternoon to contend with during the evening. The best forcing
appears to be across the Northwest which is where the strongest
storms should occur. Overnight there will be strong warm advection
developing across Southern Iowa and as such we could see an MCS
develop to the West and come rolling across mainly Southern Iowa
overnight into Tuesday morning.

Tuesday afternoon a warm front becomes established across Eastern
Nebraska into Northern Missouri...possibly as far North as
Southern Iowa and this difference will play an important role as
to where the strongest storms develop and the heaviest rain will
fall.

Tuesday night through Wednesday the front lifts across Iowa and we
will likely see storms going all night with severe storms during
peak heating along the frontal boundary...somewhere across central
Iowa. The front stalls through Wednesday night so there could be a
prolonged period where several rounds of storms impacts central
Iowa.

Thursday another shortwave lifts across the state with another
surface low and boundary developing late day and overnight. That
will then lift across Iowa Thursday night into Friday keeping
storm chances going. The severe threat tonight Through Tuesday
night appears to be more a hail and wind threat but Wednesday
through Wednesday night with the boundary across the state...all
modes of severe weather is possible along with heavy rainfall.
Locations that receive multiple rounds of storms will likely run
the risk of flooding as well.

Beyond Friday the models still differ in their solutions but both
bring a shortwave or closed low across the region with Southwest
flow continuing. We will need to see where any boundaries exist as
things progress and how much we can destabilize but Saturday still
looks wet with the threat shifting Northeast on Sunday.

&&

.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1141 PM CDT Sun May 22 2016

Showers and possibly thunderstorms to move through the area during
the day Monday. May have some redevelopment later in the
afternoon/evening, however uncertainty is high. Still have the
stronger low level winds aloft, therefore kept the mention of LLWS
in through sunrise Monday.

&&

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

$$

SHORT TERM...Fowle
LONG TERM...FAB
AVIATION...Beerends


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