Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 292348

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

237 PM CST

Through Wednesday...

Several disturbances will rotate around the deep upper level low
across the Upper Midwest. The first of these is tracking 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 were more ambitious earlier with some elevated
showers or even a storm given strong mid level lapse rates on
morning forecast soundings. With the dry low levels in place,
expect low coverage, and we do have some returns on radar from the
mid level cloud deck evident of the modest forcing from the wave
and decent mid level lapse rates. If anything were to occur, the
21z- 1z time frame appears the most favorable for some showers or
more likely some sprinkles reaching the ground. Have not mentioned
thunder as the charge separation is not looking as favorable in
this cloud layer, though an isolated strike cannot be ruled out.

A surface trough/cold front will swing through from the west-
southwest tonight, and a surface ridge will bring some brief
clearing behind these waves.

Focus will shift to the core of the deep upper low where a mix of
rain and snow showers have been occurring today. 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 and more so in the evening, but low level profiles
are still fairly mild initially. Wednesday will feature
significantly colder temperatures, closer to normal with even some
cooling possible in the afternoon as cold advection continues and
wrap around cloud cover fills in.



320 PM CST

Wednesday night through Tuesday...

Wednesday night-Saturday...
Primary forecast concern is snow shower activity Wednesday
evening/night into Thursday, with minimal if any impacts due to
above freezing air temperatures.

On Wednesday night, strong positive vorticity maxima/shortwaves
wrapping around mid-upper level low pressure sliding eastward
will provide broad ascent. This combined with fairly steep low
level lapse rates will wring out waves of shower activity through
the night and possibly into Thursday morning. Surface temperatures
will be marginal to start the evening, though accounting for wet-
bulbing, much of the activity should quickly go over to snow
showers, except perhaps a longer duration of rain or mix over the
urban heat island. Given low temperatures remaining above
freezing, not expecting any accumulation on road surfaces, which
will greatly limit any impacts. As mentioned in previous
discussion, some of the snow showers could come down at a fairly
healthy clip, reducing visibility some and possibly whitening up
grassy areas. Went with coverage wording in the grids to convey
higher confidence in occurrence.

Subsidence behind the trailing shortwave should quickly reduce
snow shower/flurry coverage Thursday morning, but stratus will
linger. Progged 925 mb temps on Thursday support highs around
40/low 40s, though cloud cover could result in more than isolated
upper 30s in current forecast. Conditions should be fairly steady
state Thursday night through Friday, with cyclonic flow aloft
keeping overcast in place. High temperatures Friday should step
down a few degrees from Thursday due to additional cooling aloft
plus continued clouds. Expansive nearly 1030 mb high will approach
Friday night and then pass just to our south on Saturday, which
*may* yield some breaks in the clouds, allowing slightly colder
lows early Saturday followed by seasonable highs in mid-upper 30s.

Saturday Night-Tuesday...
A northern stream wave/trough will swing across the area on
Sunday, ramping up warm advection. Latest ECMWF backed off on
phasing with southern stream, but it and 12z Canadian are stronger
and dig farther south with northern wave verus operational GFS.
ECMWF/GEM would suggest a period of light precipitation at least
starting as light snow, while GFS suggests we stay dry. Maintained
low PoPs from blended guidance Sunday-Sunday evening due to
uncertainty and resulting lower confidence. After a dry and slightly
above seasonable Monday, a lead southern wave will amplify mid-
upper ridging over the Midwest, with decent model agreement in
idea of lift and robust WAA causing rain to break out Monday night
into Tuesday as associated weak surface low lifts north. Even at
this vantage point, synoptic pattern would have to significantly
change to present any p-type issues during this period. Beyond day
7, a strong system may take shape over the Central CONUS as a deep
trough ejects into the Plains.



For the 00Z TAFs...

Generally quiet conditions expected across the terminals this
evening/overnight despite weak surface boundary pushing through
the area. Some lower stratus around 3-4KFT is trying to develop
ahead of this boundary, however, this should remain isolated
before shifting to the east. Winds had turned more south
southeast late this afternoon and although the direction is
holding at 170 right now, this should be short lived and become
more south southwest here in the near term. VFR and then MVFR
ceilings return on Wednesday in advance of a large upper level
system. Scattered showers are expected to develop as well, with
all rain the initial precip type. However, conditions Wednesday
evening will become more favorable for a rain snow mix and then
eventually all snow. At this time, intensity is appearing to
remain light with no sig impacts expected.



358 PM CST

Low pressure over the northern Lakes will slowly drift east
through Thursday. As the low lifts northeast late tomorrow and
tomorrow night, west-southwest winds will increase, favoring
speeds/gusts up to 30 kt on the south half Wednesday night through
mid day Thursday. Moderate west-northwest winds will continue
through Saturday and then shift to south-southwest Sunday as high
pressure moves southeast of the lake and weak low pressure moves
along the Minnesota-Canada border. There is uncertainty on wind speeds
Sunday with fairly significant differences in the model guidance.
Not currently anticipating any periods of prevailing gales in the
current forecast period that covers through Sunday. Will likely
need a Small Craft Advisory for the IL and IN nearshore waters
late Wednesday through Thursday. Held off on issuance for now, as
there is uncertainty on the start time of hazardous conditions
Wednesday afternoon, particularly along the Illinois shore.






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