Area Forecast Discussion
Issued by NWS Des Moines, IA

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351
FXUS63 KDMX 211729
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1129 AM CST Tue Nov 21 2017

.SHORT TERM.../Today through Tonight/
Issued at 344 AM CST Tue Nov 21 2017

Bottom Line up Front...
Northwesterly wind gusts upwards of 35 to 45 mph will lead to
potentially hazardous travel conditions across Iowa, especially
for high-profile vehicles traveling on SW-NE oriented roads.
Temperatures will more or less stay steady throughout today before
dropping down into the teens overnight tonight.

At 09z Tue, 500mb vorticity/1.5 PVU analysis clearly picks up on a
shortwave racing across Ontario from west to east. The shortwave
will phase in with a deeper upper low located over the Hudson Bay,
with the associated longwave trough pulling down into the Ozarks
through today. 09z Tue SFC analysis shows an attendant cold front
oriented roughly from NE to SW across northeastern to western-
central Iowa. This cold front will be the curtain call for
anticipated strong winds plus strong CAA to infiltrate Iowa today as
the boundary progresses southeastward.

Winds...
Winds will be the most noticeable and potentially hazardous
impact today. As they have since yesterday, effectively all BUFKIT
models show winds becoming mixed shortly after sunrise across
northern Iowa. Wind profile looks to stay well-mixed throughout
the day, and even towards the midnight hour. As done yesterday
morning, have bumped up winds after sunset to account for this.
Otherwise, strong wind indicators mentioned in yesterday morning`s
long-term AFD remain exactly in place... sfc pressure gradient of
3mb/125km, coupled with 3-hr pressure changes of around +4mb and
strong, widespread low-level subsidence. Transport layer winds
range between 35kts and 45kts, with strongest winds at KMCW.
Strong winds extend to KEST, and down to KALO... which could very
plausibly experience wind criteria as well for a brief period this
morning. With 09z obs at KMCW already coming in at 20G27 kts, obs
seem to initialize well with hi-res profiles. Again, with going
obs/initialization, forecast sounding analysis and other
scientific reasoning, think Butler, Bremer, Black Hawk counties
should experience winds close to Mason City, and these locations
could be justified to be in the Wind Advisory as well, as strong
winds should cover Floyd county as well.

Temperatures...
06z Tue 850mb temps start from around +6C in NW Iowa to +10C in
southern Iowa. By 18z, 850mb temps are around -10C in northern
Iowa and 0C in southern iowa. Though blue skies look to be present
across much of Iowa today, given this strong CAA, hard to justify
temps warming up much, if at all today. Thus, have gone with non-
diurnal temperatures for today...which may need lowering this
afternoon.

A 1033mb sfc high will make its way south-southeastward through the
Great Plains this afternoon, arriving in eastern Nebraska by 03z
Wed. This high will quickly push the strong winds east of Iowa as it
nears the state. With skies likely clear, this will make for a
strong radiational cooling night. Only tricky part here is if the
arrival of the high is delayed, stronger winds may linger, which
would slightly dampen radiative transfer processes. If high is
faster, min temps for Wed am may need to be lowered as much as 5
degrees... which would bring single digits near to western Iowa.

.LONG TERM.../Wednesday through Monday/
Issued at 344 AM CST Tue Nov 21 2017

Very little of note in the extended range with mainly northwesterly
to westerly upper level flow. Biggest issues will be wind on Friday
and temperatures throughout. Forecast area may get sideswiped with
some precip chances Friday afternoon, but otherwise dry conditions
expected through early next week. A series of weak waves to drop
through the region during the forecast period, with the next one
coming through late Wednesday into Wednesday night. Warmer air to
build in ahead of it with highs a bit warmer into Wednesday compared
to expected temps today. Little moisture with only an increase in
clouds expected as the wave moves through, then even warmer air to
push into the state for Thursday with westerly flow in the lower
levels. This should push temps up into the 40s/50s across the
forecast area as the upper ridge nudges NE into the region.

Thermal ridge continues to build in ahead of a stronger cold front
late Thursday into Friday, with H85 temps climbing into the teens
Celsius. Therefore expect temps into the 50s/60s for Friday, with
the GFS/EC differing in timing with the passage of the cold front.
GFS is a bit faster coming through mid/late day, with the EC more
toward late afternoon/early evening. GFS tries to clip the area with
some precip chances as well, so have some slight chance/low chance
pops across the far north for Friday afternoon. Better chances
situated closer to the sfc low across WRN Ontario and into far NE
MN. Have low confidence any precip will actually push into Iowa at
this time. Otherwise bigger issue behind the front is the winds.
GFS/EC advertising very strong winds behind the front with 40-50 kts
at 850 mb. Timing will depend on how strong winds will get given
daytime mixing, etc. However with the strong CAA and a very tight
sfc pres gradient, should still see gusty winds into the evening.
Therefore went ahead and bumped up winds some as could potentially
see another advisory criteria event by late Friday into Friday
night. Did not go quite as high as advisory criteria at the moment
given the timing differences, but will have to monitor over the next
few days for adjustments. Gusty winds look to continue into
Saturday. Soundings showing decent winds at the top of the mixed
layer with gusts to 30 mph possible through the day Saturday.
Another warming trend with dry conditions then expected for Sunday
into Monday as upper ridging moves over the region. H85 temps climbs
back to around +10C by Monday, so could see temps back into the 40s
to around 50 and possible warmer by early next week.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1127 AM CST Tue Nov 21 2017

Gusty northwest winds will persist through the afternoon before
diminishing then becoming light overnight. A brief period of MVFR
cigs still possible vicinity of KMCW/KALO though clouds continue
to dissipate. Otherwise VFR for remainder of the period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 2 PM CST this afternoon for IAZ004>007-
015>017.

&&

$$

SHORT TERM...Kotenberg
LONG TERM...Beerends
AVIATION...Donavon



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