Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 130824

Area Forecast Discussion
National Weather Service Des Moines IA
324 AM CDT Wed Sep 13 2017

.SHORT TERM.../Today through Tonight/
Issued at 324 AM CDT Wed Sep 13 2017

Bottom Line Up Front...

Only potential impact in the short term is from areas to potentially
widespread fog this morning. Another day of sunshine and well above
normal temperatures into the mid 80s.

This Morning and Today...

At 08z Wed, a broad area of high pressure/anticyclonic flow was
centered over Wisconsin. 08z sfc obs show dewpoint depressions of
zero, with light and variable winds... leading to radiational fog
development. As this potential was mentioned in yesterday morning`s
AFD, confidence is high that fog will continue and become more
widespread through sunrise this morning. A Dense Fog Advisory may be
needed in the central to northeastern portion of the DMX CWA from
Fort Dodge to Waterloo and northward/northeastward. IR imagery shows
a band of mid to high-level clouds moving into Iowa from the west.
These clouds may arrive in time to preclude dense fog development in
our NW in locations such as Emmet to Sac Counties.

Yesterday`s 850mb temps were around the +15C to +18C range... with
WAA slowly infiltrating from the west. Today`s 850mb temps are
slated to be very similar, around +15C to 18C, with WAA making into
far western IA and raising 850mb temps to +20C. Looking at hi-res
forecast soundings, it seems overall mixing for today should be
comparable to yesterday...perhaps a tad higher, but not much.
Therefore, have generally gone persistence and boosted max temps a
few. Strangely, the 00z Wed GFS is placing low to mid 90s in our
western CWA today. Yesterday, these portion of Iowa was in the low
80s. Do not realistically see how the slightly warmer temps aloft
can attribute to a 10 degree temperature increase, so have
discarded 00z Wed GFS.


A shortwave, fairly vertically stacked with a sfc low, is slated to
come off the rockies and propagate eastward, making it into South
Dakota by 12z Thu. This approaching system will continue to bring
WAA into Iowa, and will also help push away the broad anticyclonic
flow from Wisconsin into the Ohio Valley Region. Ultimately, this
interaction will keep sfc dwpt depressions in the 2 to 5 degree
range, and winds around 3 to 5 kts. Factoring in the dry conditions
across the DMX CWA, fog appears to be unlikely to develop tonight.

.LONG TERM.../Thursday through Tuesday/
Issued at 324 AM CDT Wed Sep 13 2017

Still anticipating a fairly active period setting up for the end of
the week into the upcoming weekend as a large upper level trough
digs into the western US through Friday. An elongated area of
surface low pressure will develop across the central US associated
with the digging trough aloft. This will set up southerly flow in
the low levels off of the Gulf with moisture streaming northward.
WAA also in place through the end of the week as a thermal ridge
builds north northeastward into the state by early Saturday. Some
storms may form across the north/northwestern portion of the
forecast area Thursday night as the LLJ picks up and theta e
advection increases. A similar setup will then be in place again
Friday night into early Saturday with continued chances for storms.
The sfc trough axis becomes oriented from nrn MN southward into NW
IA and then into central KS by early Saturday. As the western US
upper level trough begins to lift northeastward  into the Northern
Plains by Saturday, the associated cold front will drop
southeastward through the state Saturday night. This will bring
additional chances for showers/storms through the state. High
pressure will build briefly into the state early next week with
upper ridging aloft. Another upper trough to dig into the western US
which will again setup southerly flow off the Gulf by
Tuesday/Wednesday. Extended range models showing energy ejecting
from the western US trough through the central US with additional
chances for precipitation coming mid/late next week. Decent
instability in place Friday night into Saturday night across the
state, but overall deep layer shear to be fairly marginal around 20
kts or so. Therefore not anticipating much of a severe threat, but
an isolated strong to severe storm cannot be ruled out.

Temperatures are expected to be quite warm for this time of year
through the end of the week with 80s to around 90 expected as H85
temps climb into the lower 20s Celsius ahead of the sfc trough.
Temps to fall back into the 70s behind the cold front for Sunday,
then gradually warm back up through next week as warmer air
continues to build in aloft.


.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1140 PM CDT Tue Sep 12 2017

Main concern overnight will be fog development. Current trends
highlight the HIRES models fog development from near KMCW back to
KFOD south through KALO/KOTM with lesser problems at KDSM. Though
some MVFR BR likely already at 09z...peak development should be
between 11-13z then improving by 13-14z. Have lowered vsby at
KALO/KMCW to 1sm and 3-5sm elsewhere. Some potential for IFR FG at
KALO and potential for brief period of shallow 1/4SM FG. For now
have limited confidence to go any lower than 1SM. Remainder of
period will be VFR with light winds to light south/southwest wind.





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