Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 132102
AFDDMX

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

.SHORT TERM.../Tonight through Thursday/
Issued at 349 PM CDT Wed Sep 13 2017

A short wave will ride across the upper ridge across the region
and flatten it somewhat as it pushes a surface low over the
central Plains towards eastern South Dakota and Nebraska. This
will increase the warm advection across Iowa bringing even warmer
temperatures to central Iowa for tonight and Thursday. For
tonight, the wind across western and central Iowa will remain
south to southwest and light with the better warm advection
occurring west. Soundings show a pretty dry airmass in place so
fog is not a concern tonight with one exception...the wind across
the eastern part of the forecast area will have a more
southeasterly component and may advect a little more low level
moisture in. There is a possibility that areas from Waterloo
through Ottumwa could see some ground fog towards daybreak but
confidence is low of this occurring and it would be only for a
brief period so I am not going to put it in the forecast for now.

Thursday a south wind will increase bringing in even warmer temps
and more moisture by late in the afternoon. Eastern portions of
the forecast area will not be as warm as central and west where
lower 90`s are expected but those locations will still see mid to
upper 80`s. There is some hint that a secondary shortwave further
south across Kansas/Nebraska in the afternoon could spark some
showers/storms across the southwest and south central portions of
the forecast area, however we are dealing with some pretty dry air
despite the increasing moisture which will come in late. So even
though we will destabilize I believe the lack of moisture will
limit the precip threat. The weather becomes more active after
Thursday which will be discussed in the long term.

.LONG TERM.../Thursday night through Wednesday/
Issued at 349 PM CDT Wed Sep 13 2017

Certainly a bit more active weather pattern in store over the
next several days that Iowa has had in quite some time. A few
concerns; winds and temperatures on Friday and then periodic
storm chances Friday into early next week. Leaned toward a blend
through Saturday then closer to the ECMWF the rest of the period.

Thursday night into Friday...the upper level flow begins to shift
more zonal to southwest flow during this period.  An increasing
surface pressure gradient on Friday combined with deep mixing,
completely mixing dry adiabatically to around 800mb, should yield
a fairly breezy afternoon Friday with some gusts over 30-35 mph
possible. Confident to raise sustained winds/gusts and even
maximum temperatures. Utilized a blend of the ECMWF/NAM for
temperatures on Friday as they have much of central Iowa in the
lower 90s. The ECMWF seems slightly too warm, but then again with
the dry air and deep mixing, certainly cannot completely rule out
a few areas topping the middle 90s. Also on Friday is fire
weather concerns with the gusty winds and low relative humidity.
Lowered dew points slightly due to the deep mixing and thus the RH
values get as low as 30-35 percent by Friday afternoon.

Low confidence with any precip chances Thursday night into Friday
with the abundance of dry air in the lower levels and very weak
upper level support forcing with the shortwave embedded within the
500mb southwest flow. May see a few stray showers and storms
across the west to northwest late tomorrow night into Friday.

Saturday through Tuesday...a deepening upper level trough over the
central to northern Rockies will dig into the central CONUS
Saturday afternoon and evening. There maybe some elevated storms
in the WAA regime Saturday afternoon, but a strong cap remains in
place across the forecast area and have low confidence with any
storms developing until the cold front/trough advance across
central Iowa late Saturday night into Sunday. The surface low
pressure associated with the trough will skirt across central
Minnesota and drag the cold front through the state b/t 03-09z
Sunday. This seems to be central Iowa`s best potential for any
measurable precipitation, but not enough to alleviate the drought
stricken areas.

GFS has been fairly consistent on bringing the remnants of a
tropical system, originating off the coast of western Mexico/Baja
Peninsula, into the state by Tuesday into Wednesday of next week.
If this holds true, certainly some much welcomed rain across the
state. The ECMWF shears apart the system as it comes into Texas
and keeps any remnants to the south of the state.

&&

.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...FAB
LONG TERM...Podrazik
AVIATION...FAB



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