Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 212333

Area Forecast Discussion
National Weather Service Des Moines IA
633 PM CDT Mon Aug 21 2017

.SHORT TERM.../Tonight through Tuesday/
Issued at 406 PM CDT Mon Aug 21 2017

Bottom Line up Front...
Severe weather and heavy rainfall potential main focus through 7
am tomorrow morning. After this rain moves through, expecting
rain-free conditions and temperatures around 5 degrees below
normal for the rest of the week.

Severe weather and heavy rainfall potential...
At 4 pm, strong thunderstorms present across northwestern Iowa
associated with an MCS that has persisted through much of the
afternoon hours, traversing eastward across southeastern SD. SPC
Mesoanalysis shows pocket of instability ahead of line where there
is semi-decent clearing. Additionally, there is a compact pocket
of moisture pooling oriented from NW to SE ahead of the line that
drapes into the Des Moines metro. EBS ahead of the line is at 40
kts. Soundings show vertical profile slightly warmer than
previously progged at 750mb, which is producing a cap in Iowa that
is hindering these storms from becoming sfc-based, thus limiting
there severe potential attm. Additionally, convection starting to
outrun the boundary, which is not good for intensification. SPC
mesoanalysis also picking up on 925mb frontogenetical forcing
ahead of the line, near the clearing portion where the moisture
conv exists. DCAPE values are over 1000 J/KG with the line.
Putting it all together, conditions not highly favorable for
intensification. Any intensification would likely be confined to
daytime heating hours, after that cap will win out and storms will
become solely rain producers.

For flood potential, system fair progressive, which is a good thing
and the big difference between last night`s system and this present
one. Forward storm motion of around 30 kts. Last night, much of
western Iowa received over 2 to 4 inches of rain, so will
certainly keep an eye on those locations as flash flood guidance
likely much lower relative to the rest of Iowa. General consensus
is for precip to be SE of the DMX CWA near 7 am tonight.

.LONG TERM.../Tuesday night through Monday/
Issued at 406 PM CDT Mon Aug 21 2017

Very well-advertised, broad area of high pressure moving in
behind this rain system. Dry air entrainment plus subsidence
yielding high- confidence forecast of sunshine and CAA. With a
very persistent pattern, will stick with a persistent forecast.
Highs should be in the mid 70s to low 80s most of the week, with
lows in the 50s.


.AVIATION.../For the 00Z TAFS through 00Z Tuesday evening/
Issued at 632 PM CDT Mon Aug 21 2017

Main concern will be the continued convection through 06z...One
band moving into east/central/southeast affecting KMCW/KALO/KDSM
and KOTM with secondary band still holding together over northeast
Nebraska. This may eventually affect KFOD and KDSM...weakening
later prior to affecting any additional sites. Cigs will lower to
MVFR behind the storms briefly with a few sites possibly near IFR
w/i the main band. VFR anticipated aft 12z with winds increasing
from NW gusting to 20-22kt at KFOD/KMCW/KALO during prime mixing
hours of 15z-22z. /rev




SHORT TERM...Kotenberg
LONG TERM...Kotenberg
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