Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 300447
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1147 PM CDT Mon Aug 29 2016

...Aviation Update...

.SHORT TERM.../Tonight/
Issued at 326 PM CDT Mon Aug 29 2016

The main concern into tonight will be convective trends. Water vapor
imagery shows a noted short wave/PV anomaly over NM/CO with an
associated stream of deep moisture into the mid Missouri River
Valley. Relatively unorganized thunderstorms have developed in this
deep moisture fetch and instability. MLCapes are generally uncapped
at 1500-2500 j/kg with weak shear. Soundings show you have to go to
around 6.5km just to get 20kts of flow so storms will be slow moving
and disorganized with heavy rain and potentially flash flooding a
concern, and an outside chance of a microburst with stronger storms
if strong cells and updrafts collapse. Heavy rains and potentially
flash flooding are a concern, but have held off on a headline at the
moment as the disorganization and lack of a particular focus would
warrant issuing for the entire area, which seems like overkill
considering the situation.

The synoptic situation and near storm environment will change little
into the night with only gradual weakening of instability and loss
of heating the only change. The low level flow and moisture
transport will certainly not be strong, but the trajectory
sufficient to sustain gradually weakening storms into the night. The
best 0-2km moisture convergence should remain north of Iowa near the
frontal boundary/baroclinic zone, but most models do show a weaker
secondary max into Iowa associated with a weak KS surface trough
ahead of the NM/CO short wave. Thus will keep chance PoPs through
the first period. The environment is currently conducive for funnel
clouds as well, with weak flow and high values of 0-3km Capes, but
this should wane toward sunset.

.LONG TERM.../Tuesday through Monday/
Issued at 326 PM CDT Mon Aug 29 2016

Tuesday...

Main Impact: Heavy rainfall potential, strong to severe
thunderstorms
Confidence: Med-High

Many similarities between today`s wx overall and the setup for
Tuesday. Throughout the day time a cold front will be passing
through Iowa from north to south. A lot of upper level cloud cover
expected throughout the morning hours. Decently high 0-1km lapse
rates, low-level CAPE near 200-500 j/kg, MLCAPE approaching 2000
j/kg and increasing sfc relative vorticity priming the
environment for landspouts/funnel clouds once again.

Beyond landspouts/funnel clouds, heavy rainfall potential will need
to be monitored. Moisture surging across SW Iowa will keep PWATs
near 2.0 inches and warm cloud depths near 12kft south of Interstate
80. Low to mid level winds remain weak and suggest storm motion will
be slow once again for pockets of storms that generate.
As of now, threat appears to be highest over our southwestern to
south-central counties. Flood potential will need to be monitored as
some locations could receive over one inch of rain per hour
rainfall rates.

Wednesday through Friday...

Main Impact: No hazardous wx expected
Confidence: Very High

There may be a few lingering showers and storms near the IA/MO
border into Wednesday. Otherwise, a period of dry and cooler
conditions is expected from Wednesday into at least Friday as high
pressure will propagate eastward from North Dakota across northern
Minnesota. Sfc dewpoints will go from the upper 60s to mid 70s
experienced today and tomorrow, down do the 50s and low 60s by
Wednesday afternoon. Concurrently, 850mb temps will fall rather
appreciably. The 12z Mon solutions are cooler and drier than than
the 00z Mon solutions, which is notable because the source region
for the progged air has only moved into the CONUS, and thus is
becoming more accurately sampled.

Saturday through Monday...

Main Impact: Returning showers and thunderstorms
Confidence: Low

Long-range models trying to zero in on a solution which brings
return flow into Iowa by the weekend as the aforementioned area of
high pressure slides into the Great Lakes Region. The big negative
for re-introducing precipitation into the area too soon is the a
large area of anti-cyclonic rotation slated to be near the Gulf
Coast. Given the size of this area, models may be pushing it away
too fast, meaning precip may struggle to return to Iowa until at
least Monday. Have begun to slightly scale back timing of POPs vs
guidance.

&&

.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1141 PM CDT Mon Aug 29 2016

Main concern overnight will be details in fog development.
Currently best conditions for development around KMCW/KALO. Continuing
shower/storm possibilities remain at KFOD/KDSM/KOTM and will limit
fog at times, though development should be seen during any
extended precip free times. That said, push of energy/showers
moving into SW Iowa currently will spread NE over the next few
hours. Daytime concerns will be with scattered convection once
again prior to frontal passage and high pressure moving in.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...Small
LONG TERM...Kotenberg
AVIATION...Curtis



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