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FXUS63 KLOT 130457

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1057 PM CST Thu Dec 12 2019

214 PM CST

Through Friday night...

Northern IL and northwest IN are solidly in the warm sector of a 
clipper system across the upper Midwest with widespread 40s this
afternoon, though it may not feel quite that warm with the gusty
south winds that are in the 30-35 mph range. With the warm front 
across southern WI, the precipitation shield will remain confined 
across WI and north of the local area other than a few sprinkles 
along the border. The surface low will weaken some while it shifts
across Lake Michigan, therefore winds will slacken off this 
evening and overnight. 

The clipper's associated low-level trough/cold front will move into 
the area late tonight and gradually weaken through Friday. Clouds 
should start to thicken in the mid levels, with some periods of 
lower level cloudiness. In spite of the cloud cover the forcing 
for precipitation looks limited as the next upper level wave again
will track to our north. In the absence of any advection and with
clouds, high temperatures look to peak upper 30s to lower 40s.

A split flow regime Friday night into Saturday will see several 
weak short waves in the region, though it appears that much of 
the lower Great Lakes will largely remain in between these two 
systems and in a region of more subtle forcing. On Friday night, 
any precipitation looks light and associated with either very weak
isentropic upglide or pockets of ascent ahead of/within any of 
the mid-level impulses. Forecast soundings do depict a cloud layer
supportive of ice nucleation above a lower cloud layer, with the 
depth between these two layers close enough where some seeder- 
feeder process could support rain/snow if any precipitation were 
even to fall, which again is in question. So while 
drizzle/freezing drizzle is possible it is again low confidence. 
Equally if not slightly more likely if any precipitation occurs is
that it may be patchy very light snow/flurries. Lows on Friday 
night/Saturday morning have consistently looked like they would be
right around freezing. 



337 PM CST

Saturday through Thursday...

The primary concerns through the long term remain the potential 
light mixed precip event on Saturday and a potentially more 
impactful winter system somewhere in the Great Lakes/Ohio Valley
region on Monday. 

On Saturday, a series of multiple weaker shortwaves appear to phase 
as they track across the area. Models continue to have 
variability in the placement of any resulting precip. The latest 
run of the GFS is a bit more aggressive with precip across the 
southern half of the forecast area, whereas the NAM/NAMNEST tend 
to favor lighter precip/drizzle scenario. Area soundings suggest 
that there is a window for some light freezing drizzle at times 
during the late morning period mixed in with light snow/drizzle. 
Surface temps will be generally be in the low 30s across the area.
If precip does occur, any accumulation should be light.

Our next system approaches the area by Monday. Models have been 
consistent in a developing low moving across the Great Lakes/Ohio
Valley region, but the exact track this low will take still 
remains uncertain. Model ensembles have been trending closer to 
our area and have adjusted precip chances up slightly across our 
southern counties to account for this. Due to the amount of 
uncertainty in whether we will even see snow, it is too early to 
discuss snowfall amounts. Currently the best chances for 
accumulating snow remain to our south, but we will continue to 
monitor this storm closely over the coming days.

Looking past the Monday system, relatively quiet conditions are 
expected across the area. Winds will be generally out of the 
northwest and temps will be chilly with highs in the 20s Tuesday 
and Wednesday and 30s on Thursday.



For the 06Z TAFs...

Weak cold front will sag slowly south into northern IL overnight
into Friday, eventually stalling out and washing out. The should
allow the winds at the terminals to weaken and eventually go
light/variable to calm later Friday afternoon into Friday night.
CIGS generally expected to remain VFR, though area of lower CIGS
lurking to the north could make a run at RFD Friday morning.
Confidence is low, so just including a TEMPO for MVFR CIGS for
now, but if confidence grows in lower stratus making it into the
area then later TAFS could need to introduce a period of even IFR
CIGS during the morning. Northerly low level flow expected to
develop later Friday night which could pull some of the lower CIGS
in the colder air to the north southward into the terminals. 

- Izzi


LM...Small Craft Advisory...nearshore waters 
     until midnight Friday.




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