Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 131748
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1245 PM CDT Wed Sep 13 2017

...Updated for 18z Aviation Discussion...

.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.

Tonight...
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 18Z TAFS through 18Z Thursday afternoon/
Issued at 1245 PM CDT Wed Sep 13 2017

VFR conditions and light wind forecast for all TAF sites through
12Z.  There is some concern with the light flow more southeasterly
over the far eastern TAF locations that a brief period of patchy fog
or ground fog would restrict vsbys but with as much dry air that is
in place, confidence was too low to mention fog.  KALO and KOTM
would be the locations that might see a brief period of 5SM BR.
Otherwise winds become more southerly aft 12Z and increase to
15g22kts by the end of the TAF period as warmer air moves into
place.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Kotenberg
LONG TERM...Beerends
AVIATION...FAB



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