Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/13/19

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 545 PM CST Thu Dec 12 2019

.SHORT TERM...
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.

KMD

&&

.LONG TERM...
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.

Petr

&&

.AVIATION...
For the 00Z TAFs...

Recent aircraft observations and radar data suggest that winds of
40-45kt continue just off the deck, so will maintain LLWS in the
TAFs for the first couple hours before winds ease. Otherwise, some
MVFR CIGS could creep into northern IL late tonight and Friday,
with greatest chances at RFD. Can`t rule out MVFR CIGS into the
other terminals, but for now maintaining just low end VFR in TAFs.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...IN nearshore waters until midnight
     Friday.

     Small Craft Advisory...IL nearshore waters until 9 PM Thursday.

&&

$$

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