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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
555 am CST Thu Dec 12 2019

Short term...
300 am CST

Through Friday...

Today will find temperatures jump 15 to 20 degrees from yesterday
thanks to gusty southerly winds expected to sporadically peak
near 35 mph. Apart from a chance of sprinkles/spotty light rain in
the far north, through the day Friday looks dry for most if not
all of northern Illinois and Northwest Indiana.

Early morning satellite water vapor imagery reveals a mid-level
circulation across the Dakotas headed quickly east-southeast. This
will pivot more due east in southern Minnesota into Wisconsin as
it rounds the larger scale long wave trough across the eastern
half of Canada. With this feature impinging quickly on the
modified Arctic air mass, this has limited time for any moistening
in its warm sector, and that's why any afternoon/early evening
brief light rain or sprinkles should be confined to near the
Wisconsin state line associated with a 60+ kt low-level jet. It's
this return flow that will be the story for the area, with
sustained southerly winds near 20 mph thanks to a large surface
isallobaric component. Given the unlikely scenario of any lower
base clouds, would expect the boundary layer while shallow to
still be able to mix gusts occasionally to the top of the mixing
channel of 35 mph shown by the rap and GFS.

By advection alone, temperatures will climb into the 40s with high
confidence for most of the cwa, apart from toward Rockford where
readings may stay just shy of that due to the thickest mid-level
clouds. The question is just how far into the 40s. Some limited
snow cover in the Illinois and Vermilion river valleys should
sublimate fairly quickly today and there is some guidance
indication that the southeast County Warning Area sees some thinning of high-mid
cloud cover offering some sun. For now have gone primarily mid
40s, trying to balance the fact that the day after a heart of an
Arctic mass often can run a little cooler than guidance, though
those often have widespread snow cover.

The clipper's associated low-level trough/cold front will move
into the County Warning Area late tonight and gradually weaken through Friday.
Clouds should start to thicken and gradually lower Friday in a
slowly moistening mid to low levels, but forcing for
precipitation looks limited. In the absence of any advection and
with clouds, high temperatures look to peak upper 30s to 40.

Mtf

&&

Long term...
325 am CST

Friday night through Wednesday...

An unsettled pattern will be in place Friday night through Monday,
with Monday having some signal for accumulating snow maybe more
so along/south of I-80, but that is with low confidence. For the
beginning of this time (fri night into sat), the precipitation
intensity looks light and coverage potentially limited, so impacts
look to be small. The one main caveat to that is that there is a
limited chance of freezing drizzle later Friday night and early
Saturday morning.

A pseudo-split flow regime Friday night into Saturday will see
multiple slow-moving short waves in the region, with guidance
favoring the southern stream jet to drive some phasing of this
activity east of the region into Saturday night. This presents
limited deeper upper jet support in the region, as well as
minimized areas of focus/convergence/baroclinicity in the lower to
mid levels. 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. Profiles
show some marginal saturation of the heterogeneous ice nucleation
layer, but also have doubts there will be precipitation actually
occurring in such a profile. 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.

With the strengthening low expected to the east into Saturday
p.M., Cold advection will gradually strengthen over the area with
still some impulses rotating through. So some potential for light
snow Saturday and depending on how temperatures fall out, could
see some very light accumulation. Anything higher than that is a
diminishing forecast outcome at this point. The shot of cold
advection will bring wind chills into the single digits during
Saturday evening persisting into Sunday morning.

A continued quasi-split flow pattern rides into the start of next
week. Guidance is in sound agreement on a southern stream system
anchored along the subtropical jet and amplifying somewhere in
the eastern half of the country on Monday. The European model (ecmwf) has been
consistently south with this while the GFS has been frequently
further north over a larger part of the County Warning Area on its solutions. The
GFS ensemble does have some more members that have trended north
this past 24 hours, and a majority of them are producing some snow
in the southeast forecast area as the system matures. For now
have only inched up blend-provided chances in the southeast cwa,
but tough to stray much from the consistent European model (ecmwf) solution at this
medium range distance. Will be a period to watch as a swath of
accumulating snow is certainly possible somewhere in the Ohio
Valley to southern Great Lakes region.

Beyond looks somewhat quiet as northwest flow aloft takes back
over for at least a multi-day period. Temperatures should be
slightly below normal, though if any snow cover materializes that
would likely tilt the forecast colder.

Mtf

&&

Aviation...
for the 12z tafs...

Main concern for today will be generally southerly winds gusting
to arnd 30 kt and low level wind shear above shallow mixed layer.

As surface high pressure moves to the mid-Atlantic region and low
pressure continues to develop over the upper Missouri Valley,
winds this morning will start out sely under 10 kt. However, as
the pressure gradient will strengthen as the low lifts newd.
Southeast winds will strengthen and become gusty this morning,
before shifting south-southwest as a warm front lifts north of the
terminals later this morning. Slightly deeper mixing as the warm
front lifts north of the terminals will support wind gusts
increasing to 30 kt, or occasionally higher, during the day.
Even with mechanical mixing with the gusty surface winds, winds
above a shallow inversion aloft will increase to 50-60 kts,
producing a period of low level wind shear conditions through the afternoon. The
gradient will eventually weaken this evening and overnight, as a
weak cold front approaches from the northwest and then stalls out
along the Illinois/WI border. This will allow south-southwest winds to
slowly diminish under 10 kt.

Dry low level air mass will result in mainly mid and high level
cloud cover through much of the day Thursday. Forecast soundings
eventually depict lowering VFR clouds late in the day/Thursday
night. The best chances for precipitation should generally be
north of the terminals into WI, but some guidance indicates some
potential for MVFR ceilings and light rain late this afternoon for
krfd. This guidance also suggests that the MVFR cigs may spread
east and south across the area overnight, reaching the Chicago
area terminals after midnight. While confidence in how low ceiling
will lower is relatively low, have gone with MVFR cigs for the
late night hours since forecast soundings indicate that the
inversion will intensify again overnight limiting any mixing of
the drier air from above the inversion.

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...in nearshore waters until midnight
Friday.

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

&&

$$

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