Area Forecast Discussion

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 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
forecast period. 
SHORT TERM...Beerends
LONG TERM...Kotenberg