Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 221745
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1245 PM CDT Sun Oct 22 2017

.SHORT TERM.../Today through Tonight/
Issued at 330 AM CDT Sun Oct 22 2017

The boundary that moved across the state through the overnight is
now located along the Mississippi River corridor. A region of post
frontal showers and thunderstorms still exist east of Interstate 35
and south across a large portion of Missouri. The precipitation will
linger over the far eastern forecast area through 9-10 am before
shifting to the east. Drier air has been moving into central and
western Iowa and cloud heights are a reflection of the dry air
arrival with ceilings below 2000 ft southeast to above 25 kft
northwest. The dry air will continue to arrive through the day today
as PWAT values drop from 1.5 inches overnight to around 0.3 inches
by this afternoon. Much of the forecast area should have an
abundance of sunshine this afternoon with the exception over the
southeast where cirrus may remain.  Well mixed profiles to 850 mb or
above and the dry air will allow temperatures to warm back into the
60s.

South to southwest winds overnight combined with increasing mid to
high level cloudiness ahead of an approaching short wave, will keep
temperatures from falling into the 30s at most areas. Not expecting
any precipitation through 7 am Monday as an abundance of dry air
will remain below 12 kft and any forcing arriving during this time
is weak.

.LONG TERM.../Monday through Saturday/
Issued at 330 AM CDT Sun Oct 22 2017

The big story for the long term will be the colder temps in store of
Iowa.  On Monday a deep trough drops down through the Upper Midwest.
Models are not handling precip with this wave very well but a closer
analysis shows about a 4 to 6 hour window where some saturation
occurs along with some pretty good forcing.  High-res models are
beginning to pick up on this as well, though showers for northern
Iowa are still being underdone.  For now I have expanded the slight
chance PoPs across the northwest Monday afternoon and the northeast
Monday evening.  It will also be pretty breezy on Monday and
something of interest if the showers develop, would be the potential
for the showers to tap that wind and bring some of it down to the
surface.  Soundings show winds off the surface at 35-45kts which
could potentially mix down.  At this point we are borderline
advisory on the winds but if this scenario becomes increasingly more
likely, a headline may be considered.

Colder air pushes in for Monday night and Tuesday giving us our
first taste of fall in awhile. After lows in the 30s Monday night,
highs on Tuesday will only be in the 40s and 50s.  For Tuesday night
and Wednesday weak ridging moves over the state and temps return to
more seasonal readings in the mid 50s to mid 60s and it will be dry.

Beyond Thursday, models are somewhat divergent on the smaller scale
details but they all swing a deep trough through the state through
the weekend and this trough taps some very cold air and brings it
down into Iowa.  Highs for Friday and Saturday will likely not make
it out of the 40s with only minor modification for Sunday in the mid
40s to mid 50s. We will likely see a hard freeze Friday night across
most if not all of the forecast area.  Freezing temps will also be
seen northwest Thursday night.   At this point the models cannot
agree on the extent of precip ahead of this trough or the wrap
around precip as the associated low pulls out.  The GFS is most
bullish here with the Euro and Canadian keeping the wrap around
precip to our north. At this point the feeling is that the precip is
overdone but with little to argue on because of model differences, I
just lowered the PoPs that the model blends gave me.  One thing of
note...if we do get precip in the evening and overnight hours, there
will likely be some snow mixed in with the rain and perhaps all snow
Friday night/Saturday morning.

&&

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1244 PM CDT Sun Oct 22 2017

There is high confidence in VFR conditions through the period.
Mid cloud ceilings currently over eastern IA will exit later this
afternoon leading to nothing beyond patchy mid-high cloudiness
later tonight into early Sunday. There could be some light precip
with these clouds as well, but confidence was not great enough to
mention. NW winds will increase and become gusty Monday, but this
will mainly be west of TAF sites and beyond the valid period.

&&

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

&&

$$

SHORT TERM...Donavon
LONG TERM...FAB
AVIATION...Small


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