Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 060220

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
820 PM CST Mon Dec 5 2016

820 PM CST

No big changes planned to going forecast this evening. Dense
freezing fog certainly remains a potential tonight, but I have
little to no confidence fog development, and if so, where. Unlike
last night, there are conflicting signals, low level flow and been
backing to more easterly and will continue to do so on the
northwest flank of low pressure tracking through the TN valley.
Upstream air mass with easterly flow is drier and for the Chicago
metro area is off the warmer lake which also wouldn`t favor dense
fog. Conversely, there is a still a fairly pristine snow pack and
1) over north central IL light fog is already present with LIFR
stratus and 2) over SE CWA skies have temporarily cleared out
which could allow some radiational cooling and ground fog
development. Other than backing off the fog mention a bit in the
grids in NE IL, not planning any other changes to fog forecast
tonight out of pure lack of confidence.

Other item of note is that increasing short range guidance
suggests precip will miss our far SE CWA late tonight/Tue morning.
Chance isn`t quite zero down there, so not planning to remove the
slight chance pops.

Other than adjusting the sky grids to account for the clearing in
the southern 1/3rd of the CWA, no other changes planned. Updated
grids/text products are forthcoming.



231 PM CST

Through Tuesday...

We remain between a departing high pressure ridge and low pressure
over the plains.  As such, winds drop off tonight. There should be
ample low level moisture with the dense snow pack so expecting
widespread fog tonight. Dense fog is possible especially in outlying
areas.  Low temps will be at or just below freezing. The fog may
freeze on elevated objects, and if roadways cool enough, fog may
create slick conditions on some roads. Confidence is low in
widespread freezing fog and slick conditions.

Another low forms over the southern Mississippi valley tonight
and moves up the Ohio valley. Precip may clip the southeast
portion of the outlook area...mainly southeast of a Valparaiso, IN
to Chatsworth, IL line. Precip will be a wintry mix at the
beginning. Freezing rain is possible given forecast soundings and
forecast low temps. Ice accumulation will mostly be a light glaze
as temps will climb above freezing by mid to late morning. As
such, precip should turn over to all rain before coming to an end
late in the morning.

Fog dissipates in the late morning as the cold front moves through
and west winds pick up.  Otherwise we should see some melting
tomorrow as high temps will be in the mid to upper 30s.



231 PM CST

Tuesday night through Monday...

Well below normal temperatures will prevail throughout the latter
half of the week as a strong Canadian high builds into the Great
Plains and eventually shifts across portions of the Midwest.

Low pressure over the northern Plains today is expected to lift
across Ontario to near James Bay  Wednesday with strong cold air
advection occurring across the northern Plains and upper Midwest in
the wake of this low. Ridging from a 1050mb high over the Canadian
Rockies will stretch southeast into the central and southern Plains
helping to drive 850mb temps into the -10 to -12C range Wednesday
night into Thursday. Highs in the mid 20s north to low 30s south
Wednesday will likely only top out in the low 20s Thursday and
Friday with morning lows in the teens. While the lake effect
machine ramps up for the latter half of the week, outside of any
of this activity creeping west into Porter County, anticipate the
remainder of the CWA to remain dry through Friday.

Upper pattern becomes more zonal over the weekend into early next
week with an upper level disturbance digging into the northern
Plains by early next week. At this distance, we are still dealing
with large track and timing differences, but do anticipate
temperatures to moderate out ahead of the wave this weekend with the
baroclinic zone lifting back towards the local area. Precip chances
will return under the mid level WAA and again with a cold front as
it pushes across the region early next week.



For the 00Z TAFs...

Tricky and fairly low confidence forecast through Tuesday morning.
Synoptically winds have already begun to back to a more easterly
direction and this trend should continue through the night.
Currently thinking the most likely scenario is that the easterly
winds (albeit light) will have a tendency to keep cigs/vsbys from
tanking at Chicago terminals due to some moderating temp influence
from the lake, plus upstream conditions are also better. It is
possible that the mitigating effects of the easterly winds could
be offset by nocturnal cooling and crashing cigs/vsbys. Confidence
is low and will continue to monitor trends and amend as needed.

Farther west at DPA and especially RFD, the potential is greater
for conditions to tank tonight. Not the same set up as last night,
but considering how low conditions already are it wouldn`t take
much for them to drop down to the deck. Signals in guidance are
mixed, leading to low confidence, but will maintain trend of
bringing cigs/vsbys down at DPA and RFD for now.



233 PM CST

Moderate south flow will remain in place across Lake Michigan today
behind a departing high over the Ohio Valley and ahead of a
developing low over the Plains. The low will lift across northern
Minnesota Tuesday to James Bay Wednesday with a trailing cold front
pushing across Lake Michigan Tuesday afternoon and evening turning
winds westerly. Tuesday night into Wednesday, expect westerly winds
to peak around 30 kt with a few gales possible, mainly over northern
Lake Michigan. Expect west to northwest winds to around 30 kt to
persist through the remainder of the week with another possibility
for gales behind a secondary front Thursday night into early






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