Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 291738

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1138 AM CST Tue Nov 29 2016

253 AM CST

Through tonight...

Forecast concerns in the short term include temps today and precip
chances this evening. Powerful cyclone continues to dominate the
weather across the central half of the CONUS. Pronounced dry
intrusion is has scoured out cloudiness leaving skies mostly clear
early this morning. The clearing may not last, while low
cloudiness isn`t expected to be a concern today, there is a large
slug of high cloudiness associated with the 145kt+ jet slicing
across the OH Valley. As this jet rounds the base of the trough
expect this slug of high cloudiness to spread north today and
probably put a damper on our chances of reaching 60F today. Even
with the high clouds, highs should have no problem climbing well
into the 50s.

Another shortwave pinwheeling around the huge cyclone will swing
across the region this evening along with surface cold front.
Model forecast sounding do show a stout cap with pretty meager low
level moisture, however guidance does show a plume of 700-500mb
lapse rates near or greater than 8C/km spreading into the area
this evening. The synoptic forcing with the shortwave and steep
lapse rates will probably compensate for limited moisture and
instability to kick of scattered showers and probably some
thunderstorms as well. Best chances look to be this evening over
about the SE half of the CWA and that`s where I focused the higher



253 AM CST

Wednesday through Monday...

The colder air will gradually spread into the area, with highs
likely earlier in the day Wednesday with steady then slowly
falling afternoon temps, especially NW CWA. Center of the deep
vertically stacked cyclone is forecast to wobble SE into WI
Wednesday afternoon. Steepening low/mid level lapse rates within
the cyclonic flow around the cyclone will likely allow for showers
to spread into northern IL late Wednesday afternoon continuing
into Wed evening. Precip could begin as some rain or graupel, but
should transition over to snow showers & flurries Wednesday
evening as 850mg temps plunge below -5C. Surface temps will likely
be too warm for any meaningful accumulation, but given the steep
lapse rates, wouldn`t be surprised to see some brief but rather
healthy snow showers Wed evening that might even whiten a few
grassy surfaces.

Upper low is progged to move east Thursday, while we remain in
cyclonic flow and likely socked in with low cloudiness the forcing
for snow showers/flurries becomes a bit more nebulous, so have
backed off on flurry mention some for Thursday. While some models
show some clearing as early as Thursday night, with no strong
subsidence or push of dry air, suspect it will be a weak dirty
high that should allow generally overcast skies to continue
through Saturday. The clouds will act as a blanket limiting
diurnal temp swings with day to day highs probably in the upper
30-ish range and lows in the lower 30-ish range Wed night through

Big forecast uncertainties with the large scale pattern this
weekend into early next week. One of the big questions is what
will become of strong trough digging into the southwest CONUS. The
ECMWF has been pretty gung-ho kicking this feature out with some
phasing with the northern stream leading to a healthy storm in our
neck of the woods Sunday/Monday time frame. The 00z ECMWF scrapped
the idea of the big snowstorm that its prior run had shown. The
operational 00z GFS keeps the shortwave cut off of the SW and pops
shortwave ridging over our area Sunday/Monday as long wave trough
digs into the Pac NW. This solution would mean dry wx and temps
moderating back to above average levels. Considerable spread in
the ensembles, so really all bets are off and just about anything
is possible in this time range. Until models start to come into
some semblance of agreement, saw no reason to make any changes to
the blended model guidance which has low pops thanks to the ECMWF.



For the 18Z TAFs...

Concerns center on small shower/isolated storm chances late this
afternoon and early evening, and possibly brief MVFR cigs.

Breezy southwest winds remain in place after the passage of a
surface trough earlier today and with strong low pressure across
Lake Superior. The main pressure rises have passed but expect
gusts to the low to mid 20s today to continue.

Several disturbances will rotate around the deep upper level low
across the Upper Midwest. The first of these will track northeast
towards east central IL and northwest IN, and the second one
closer to the low will pivot northeast into north central IL.
As these cross during the late afternoon/early evening, several
high res sources suggest that some elevated showers or even a
storm are possible given strong mid level lapse rates on morning
forecast soundings. With the dry low levels in place, expect low
coverage such that we have continued with dry TAFs except at KGYY.
If anything were to occur, the 22z-1z time frame appears the most
favorable. During this time expect some possible VFR cigs and
maybe even some brief MVFR. Low level moisture advection may even
bring some IFR up along and ahead of the surface trough in NW
Indiana though this is low confidence. A secondary surface
trough/cold front will accompany this wave, so expect a slight
wind shift to more WSW briefly before settling back to breezy

Focus will shift to the core of the deep upper low where snow is
falling currently. This will rotate across our area Wednesday
afternoon and early evening. Wet bulb thermal profiles suggest
some snow could mix in with rain late in the afternoon in north
central Illinois and more so area wide in the evening, but low
level profiles are still fairly mild during this TAF cycle. With
southwest flow in place have largely gone with some VCSH in the
tail end of the 30 hr ORD TAF.



302 AM CST

Headlines...Expired the Gale warning for the nearshore waters at
09Z/3AM CST. Issued a small craft advisory for gusty winds through
18Z/noon CST. Gales may be marginal over the southern half of the
lake, but will keep the warning going until 12Z/7AM CST.
Gales are much more likely to redevelop over the northern half of
the lake this morning. Therefore, did not make any changes to the
northern half gale warning either.

The low over Minnesota rotates in place and slowly fills today.
Winds diminish this afternoon as the low and resulting pressure
gradient weaken, and then become southwest behind the low Wednesday.
Could see gusts to 30 kt over the southern half Wednesday night, and
a small craft advisory may be needed for the IN NSH waters for both
wind gusts and high waves.  Have less confidence in seeing hazardous
winds over the IL waters.  Northwest to west winds 15-25 kt persist
through Saturday as a high pressure ridge builds over the plains.
The ridge moves across the lake over the weekend.




     Small Craft Advisory...ILLINOIS NEARSHORE WATERS UNTIL 3 PM Tuesday.




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