FXUS63 KDMX 151148
Area Forecast Discussion
National Weather Service Des Moines IA
548 AM CST Fri Dec 15 2017
.SHORT TERM.../Today through Tonight/
Issued at 334 AM CST Fri Dec 15 2017
Area of freezing drizzle/drizzle has moved into far southern Iowa
and should be exiting the area soon. Could still see some scattered
slick spots on roads across central IA from the light glaze that
occurred overnight into the early morning hours. Across the
remainder portion of central and northern Iowa, low level saturation
has become deeper allowing for CIGS to rise and for ice introduction
into the column. This means that any additional precipitation across
central and northern Iowa, and soon to be southern Iowa should be in
the form of scattered flurries. An elongated area of waa will move
into the area today ahead of a weak shortwave trough dropping into
the northern plains late tonight. Already seeing a large area of
light snow with this waa band setting up across North Dakota
currently. This area of snow is expected to spread into far
northern/northeastern Iowa this morning into early afternoon.
Therefore have highlighted with some mention of flurries across much
of the northern/northeastern third of the forecast area. Also
included some small pops for light snow during the best chance for
light snow in the far north/northeast. Still not really expecting
any accumulations, but should likely see a period of light snow for
an hour or two as this band spreads southeastward.
With the WAA in place today expect temps to be about 5 to 10 degrees
warmer than readings from Thursday. This should set highs in the mid
30s north to mid 40s south. Continued push of warmer air into the
region with winds shifting around to the southwest for tonight,
should allow for fairly mild overnight temperatures down into the
mid 20s to around 30 tonight. Expect dry conditions tonight as the
WAA band of precip is expected to spread further east into WI as the
associated shortwave trop begins to drop southeastward through the
.LONG TERM.../Saturday through Thursday/
Issued at 334 AM CST Fri Dec 15 2017
Bottom Line Up Front...
Sunday Morning into the early afternoon could bring some close to
nil-impact wintry precipitation primarily south of Interstate 80.
Our first potent, potentially high-impact winter system of the
season could target Iowa next Thursday into Friday.
Models are now coming into very good agreement with tracking a
well- advertised shortwave slated to push northeastward from west
Texas, through the Midwest, and into the Ohio Valley region.
Ultimately, the slightly western ECMWF solution appears to have
won out, with the GFS members converging towards it. The
baroclinic zone will be the main focus of precipitation as it
looks to set up from the SW corner of Iowa through the WI/IA/IL
state line intersection. There is decent low-level frontogenetical
forcing in a favored location and ascent through a loosely
saturated area from 600mb to 800mb that is near the ice
introduction layer. The big knock to this system is the (GFS)
forecast soundings show a notable amount of dry air below 850mb.
Impact-wise, temperatures are slated to be in the low to mid 40s by
the late morning, which will serve to transition any lingering
precip into rain and will certainly melt any wintry precip that fell
during the early morning. With winds under 10 mph expected, this
should be a very little to no impact event.
Monday Through Wednesday...
Though long-range models differ slightly on the location and
magnitude, they are agreeing on a sizable area of high pressure
pushing towards the middle of the country by Tuesday. The ECMWF
has been trending ever so slightly north with the location of this
high, and the GFS seems to be playing "catch up". So, am thinking
slight upward nudge of temps into the upper 40s and low 50s
justified more or less along and south of I-80.
Thursday into Friday...
The 00z and 12z model runs from 12z Wed to the most recent 00z
Fri run have all featured what could be our first high-impact
system of this season for Iowa. The most recent 12z Thu and 00z
Fri runs, in particular, have hinted at narrowing in on a track
that would place this system near northern Missouri by 18z Thu,
and the Great Lakes region by around 09z Fri. Given that this is
still in the 160+ hour range, there is time for a lot to change in
terms of timing, location and magnitude...so certainly not
advisable to go even close to "all in" on a given solution yet, or
to dig too much into details. The ECMWF has been stronger,
slightly slower, and has shown a little less run-to-run
variability, so initial lean is towards it.
Strong winds definitely look plausible as sfc pressure gradient of
around 3mb/75km may be in play. KDSM forecast sounding shows
transport/mixed layer winds in the 25 to 35 kt range by 00z Fri.
Time of arrival will play a big factor... as current forecast has
the warm sector arriving during daytime heating, which would result
in temps in the 40s along and south of I-80...which would keep
precip as rain for the southern half of Iowa and a wintry mix in the
northern portion of the state. If arrival is delayed to Thursday
night, which cannot be ruled out, there would be more of a wintry
mix across central to southern Iowa, and the pure snow region would
drop closer to I-80.
Regardless of arrival timing, signal growing for temperatures to
plummet rapidly behind this system. Models not resolving this well
yet, but there is historical precedence for a sizable area of high
pressure to lock into place over the northern Great Plains...serving
to blast bitterly cold air into Iowa for several days.
.AVIATION.../For the 12Z TAFS through 12Z Saturday morning/
Issued at 548 AM CST Fri Dec 15 2017
Mainly VFR CIGS expected through the period, with a chance for
some MVFR CIGS at KMCW and KALO this morning into the afternoon
hours. Some light snow could move through these two northeastern
sites as well when the lower CIGS are possible. Westerly winds
will shift around to the southwest to more southerly through the