Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 171312
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
712 AM CST Sun Dec 17 2017

.UPDATE...
Issued at 712 AM CST Sun Dec 17 2017

Have sent out an update as trends from radar and the latest CAMS
suggest precip potential to push a little further north across the
area through early afternoon. Therefore have expanded pops and
refined precip types somewhat as still expect majority of the area
to see rain. A wintry mix is possible toward the northwest side of
the precipitation area where cooler/drier air is in place aloft.
Also expanded the dense fog advisory a few counties further south
with reports near Clarion and Iowa Falls of visibilities around
1/4 to 1 mile.

&&

.SHORT TERM.../Today/
Issued at 344 AM CST Sun Dec 17 2017

Complex scenario across the area today, fog/low stratus issues to
the north and some mixed precip changing to rain across the south.
Currently the area is sandwiched between two upper level troughs,
one moving eastward through the northern Plains today and another
ejecting northeastward through the central Plains and through IA/MO.
There is weak shortwave ridging over MN/IA currently, with an axis
of surface high pressure nosing westward into MN/nrn IA/SD. Low
level moisture trapped beneath this surface high is resulting in
some low stratus and fog across the north half of the area. Several
sites to the northwest and northeast of the area have dropped to
below 1 mile with a small area of clearing across the north expected
to dissipate with fog spreading across the north. Have decided to
issue a dense fog advisory across the far northwest to match up with
surrounding offices at this time, and will have to monitor to see if
it will need to be expanded over the next few hours as the
stratus/fog is expected to expand and close up the clearing area.

Further to the south the southern system will continue its path
toward IA with an associated area of low pressure centered down
across the panhandle of TX with a warm front extending northeastward
into far northern MO. This front will lift northward into southern
IA as the sfc low lifts NE through nrn MO/se IA by late this
afternoon. A large area of precipitation associated with this system
is lifting through KS into NE Nebraska and western MO and is
expected to continue to lift into southern IA this morning.
Isentropic lift evident on the 290K/295K surfaces matching up well
to the current areas of precip as does radar progs from the HRRR.
Therefore trended pops toward these two trends for the morning into
the afternoon hours. This will bring precip chances across the
southern third of the forecast area this morning, and into the
southeastern portion of the forecast area into the early afternoon
hours before moving east out of the area. Warming aloft should keep
the majority of the precipitation liquid, with sfc temps at the
onset of precipitation on the northern edge below freezing. This
will allow for a short period of light freezing rain across portions
of the southwest to southern forecast area in the vicinity of the
hwy92/hwy 34 corridors, with mainly rain toward the MO/IA border. A
light glazing will be possible especially in areas closer to Creston
and Corning. However as the warm front lifts northward, sfc temps
should quickly rise above freezing with all precipitation changing
over to rain by late morning.

.LONG TERM.../Tonight through Saturday/
Issued at 344 AM CST Sun Dec 17 2017

From tonight through Wednesday the weather will be quiet, dry and
mild with persistent roughly zonal flow aloft. A weak surface
trough will sink across Iowa early Tuesday, but the airmass behind
it will be of similar temperature to that preceding it and
temperatures will remain well above normal for the first half of
the work week. All eyes then turn to the first real winter storm
system of the season set to impact the region around Thursday into
Friday.

By Thursday morning there will be a 500 mb longwave trough
trailing from south central Canada southwestward to a cutoff low
over the desert southwest. On the leading flank of the trough a
broad surface low pressure area will be developing over the
southern High Plains around the Oklahoma panhandle, with an
inverted trough stretching northeastward over the Midwest and Iowa
as an effective stationary front. Southeast of this boundary much
of Iowa will initially remain within a warm region characterized
by south to southeast low-level flow, while northwest of the
boundary a tight pressure gradient will promote brisk north winds
and substantial cold air advection. A stretched zone of broad
vertical forcing associated with the mid-level southwesterly flow
will reside over this low-level cold/northerly flow region,
promoting a wide swath of developing precipitation northwest of
the surface boundary. This is a classic set-up for an early season
synoptic cyclone of this kind.

As the mid-level trough ejects eastward from Thursday into Friday
it will be followed by a robust shortwave impulse moving quickly
into southern Canada and carving out a large cyclonic flow region
over the northern U.S. This will cause the advancing leading
trough to stretch out and weaken somewhat, however the forcing
associated with that feature will still be sufficient in the
presence of decent atmospheric moisture to generate a swath of
accumulating precipitation over or near Iowa as it moves through.
Thermal profiles suggest the precipitation could initially be rain
over parts of Iowa, but should switch quickly to snow behind the
boundary and after sunset Thursday night. This should lead to the
first widespread accumulating snowfall of the season. However,
considerable uncertainty remains in the track and timing of this
system. The weakening nature of the large-scale trough is
promoting forecast QPF supportive of only light to moderate
snowfall amounts, but when and where this may fall is the subject
of considerable conjecture. The long-range models were in relative
agreement for much of the last 24-36 hours with the snow band
affecting about the northwest two thirds of Iowa, primarily on
Thursday afternoon and evening. However, tonight the 00Z ECMWF
has come in later and considerably further northwest with its
solution, which has the effect of not only delaying the
precipitation by 6-12 hours, but also making a nearly dry forecast
for about the southeast half of our forecast area and a higher
probability of wintry mix instead of straight snow with warmer
temperatures lingering through Thursday night. At this time, given
that it is only one run, have maintained a fairly persistent
forecast from previous packages and will stick with the message of
a high likelihood of accumulating snowfall across portions of
Iowa. However, if the latest ECMWF is a harbinger of a new trend
in the model solutions, this may need to be adjusted later today
and tonight.

After the initial system on Thursday-Friday, the aforementioned
broad cyclonic flow being carved out across the northern U.S. will
intensify and persist through next weekend and Christmas Day. This
general scenario will promote a continuation of cold temperatures,
general cloudiness, and perhaps some light precipitation at times.
If we do get accumulating snowfall with the Thursday-Friday
system, it appears there would probably be little melting through
Christmas.

&&

.AVIATION.../For the 12Z TAFS through 12Z Monday morning/
Issued at 542 AM CST Sun Dec 17 2017

Low CIGS and VSBYS across the northern sites expected this AM as
fog expands. Potential for IFR to LIFR CIGS, and similar VSBYS at
KALO and KFOD. Uncertainty with how far south the dense fog will
expand, so tried to go with my best thoughts thus far. Otherwise
precipitation area migrating northeastward across the southern
sites this morning into the early afternoon hours. CIGS likely to
fall again tonight, but uncertainty in how low so again tried to
put the best thoughts forward.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until noon CST today for IAZ004>007-015>017-
023>026-033-036-037.

&&

$$

UPDATE...Beerends
SHORT TERM...Beerends
LONG TERM...Lee
AVIATION...Beerends



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