Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 252354

Area Forecast Discussion
National Weather Service Des Moines IA
654 PM CDT Mon Sep 25 2017

...Updated Aviation Discussion...

.SHORT TERM.../Tonight through Tuesday/
Issued at 311 PM CDT Mon Sep 25 2017

Shower and thunderstorm chances will be the main challenge through
the short term forecast period. As of 20z a cold front remains
nearly stationary in western Iowa. Temperatures are stuck in the 50s
and 60s behind the front, while those to the east are experiencing
another humid day with temperatures in the 70s and 80s. Weak theta-e
advection over the low-level baroclinic zone in eastern NE/western
IA has supported scattered to widespread shower activity for much of
the day. Clearing skies and heating along and to the east of the
front allowed for sufficient destabilization to break the cap. Hi-
res models have struggled to get a handle on the spatial extent and
progression of precip for much of the day, and thus lowers
confidence in their solutions going forward. Isolated to scattered
convection will continue in the warm sector until instability wanes
later this evening. Favorable shear lags behind the front and poor
mid-level lapse rates limit the threat for severe weather, although
brief heavy rainfall is likely as pwats remain anomalously high.

The western conus trough will finally begin to break up tonight as a
strong shortwave ejects out of the central Rockies. This will give
the sluggish cold front a decent isallobaric shove eastward through
the state. Low level moisture lifted above the frontal axis will
support scattered to widespread precipitation overnight into
Tuesday. Diminishing instability should limit the thunder and heavy
rainfall potential, as most models show little to no MUCAPE left to
work with by 09z Tuesday morning. Model runs today have trended
faster with the progression of the upper level shortwave as it
traverses over the northern Plains tomorrow, and thus faster with
deep-layer subsidence and drier air advecting into the area. POPs
were reduced to only slight/low chance after 18z, and may still need
to be cut back further. Temperatures will be noticeably cooler and
much more in line with seasonal normals.

.LONG TERM.../Tuesday night through Monday/
Issued at 311 PM CDT Mon Sep 25 2017

The cold front and associated showers and thunderstorms discussed
in the short term section above should clear out of our forecast
area to the southeast by Tuesday night. A cool, dry high pressure
area will then dominate the region Wednesday and Thursday, with
mostly clear skies and temperatures nearly 20 degrees cooler than
this past weekend. A strong mid-level trough will swing
southeastward across the Great Lakes late Thursday into Friday,
with a reinforcing surface front progressing across Iowa around
Thursday night. There is some disagreement among the various model
runs as to the strength and southwestward extent of the mid-level
trough, but nearly all solutions keep associated low clouds and
light showers to our northeast across Wisconsin and have
maintained a dry forecast during this period. Behind the
reinforcing front, even cooler and very autumnal weather is in
store for Friday and Saturday, and early Saturday morning lows
may even dip briefly into the upper 30s in a few areas in our
north and east.

By Saturday a broad mid-level trough will have built into the
northwestern U.S., with most of the associated vorticity moving
across the Pacific Northwest while a leading shortwave pivots
northeastward across the Dakotas. The leading wave will probably
have minimal effect on our area, though there is some variability
in the model solutions at that range. Of more consequence, the
larger northwest U.S. trough will slowly progress eastward and
close off/deepen later in the weekend, leading to spotty POPs in
the outer forecast periods. However, the details of this evolution
are still murky at this time.


.AVIATION.../For the 00Z TAFS through 00Z Tuesday evening/
Issued at 644 PM CDT Mon Sep 25 2017

Periods of rain/storms will persist through much of the evening
providing MVFR to IFR visibility and possibly ceilings. Brief
heavy rain threat continues tonight, especially at DSM, MCW and
ALO. OTM is the only TAF site not expected to see rain until after
06z. IFR to possibly LIFR ceilings will set in later tonight into
tomorrow morning...looks like FOD is already in the low stratus
deck. Stratus is expected for much of the day Tuesday, but likely
more in the lower end of MVFR.





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