Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 151212

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
612 AM CST Wed Nov 15 2017

319 AM CST

Through Thursday...

Main forecast focus in the near term is precip today with fairly
vigorous upper trough digging into the Upper Great Lakes region.
Southerly low level flow has been advecting a plume of moisture
northward into the region with large scale ascent with the upper
trough and warm air advection leading to blossoming area of
showers and a couple of thunderstorms. This trend should continue
this morning with likely an increase in coverage of the rain as
the upper trough nears. Cold front will steadily move across the
CWA reaching western CWA mid-late morning and clearing NW IN by
late afternoon. FROPA should result in an end in the precip and
probably a period of at least partial clearing today. Push of
cooler air lags behind the front, so today will be rather mild
behind the front.

Forecast challenge for tonight is potential for stratus. Area post
front stratus upstream over the Dakotas could pivot into the area
tonight. Confidence is not particularly high, should we end up
under stratus then forecast low temps will likely end up being too
cold. May trend a bit in that direction, but would like the day
shift to get another look and make the final call. An concerned
that is the stratus deck does lock in that a portion of the day
Thursday could be socked in as well, which would mean temps cooler
than forecast.

- Izzi


319 AM CST

Thursday night through Tuesday...

By Thursday night attention will already be focused upstream at
the next system taking aim on the region. Winds will become
southerly and gradually increase Thursday night which should keep
temps in check and probably allow for a rebound in temps prior to
sunrise. The GFS has finally come on board with the ECMWF and is
slower to phase the northern and southern stream shortwave
troughs, which should result in a more progressive system as it
moves across the area Friday night into early Saturday. Have
maintained a mention of thunder Friday night as strong warm air
advection taps into seasonably moist air mass resulting in some
weak destabilization.

Cloud cover will probably temper the warm up during the day
Friday, but as strong southerly winds drag the 50F+ dewpoints
north into the CWA Friday night look for rising temps after dark
and daily highs likely being reached at night. Cold frontal
passage Saturday morning will bring an end to the rain, followed
by another cool down.

While the system is forecast to phase and deepen over southeast
Canada over the weekend, it now looks to be fairly progressive
with the next northern stream trough given resulting in a quick
return to warm air advection by early next week. The active
progression of strong Alberta Clippers tracking to our north
look to result in a temperature roller coaster with alternating
shots of cold and mild air and likely below average precip next
week given the storm track to our north.

- Izzi


For the 12Z TAFs...

Forecast concerns remain pcpn coverage and duration this morning
along with associated cig/vis restrictions, chances for thunder
and then wind trends.

A widespread area of rain with some embedded thunder is moving
across the area this morning. The general area of pcpn is fairly
quickly moving to the east. While there is some isold, embedded
thunder, have not included and ts mention in the tafs as coverage
is sparse and confidence in impacting the terminals is low. The
greater impacts will be cig/vis trends in the pcpn and the wind
shift with the frontal passage. At issuance time, cigs have been
trending gradually lower through mvfr levels. Vis impacts are a
bit more spotty, but still expect periods of mvfr vis until the
fropa. Latest guidance still suggests that cigs should lower to
ifr levels, and at 12Z, much of the ifr cigs are still arnd the
Mississippi River, but have decent confidence that cigs will lower
to ifr with top-down saturation.

Latest radar imagery indicates a very sharp back edge to the more
convective-appearing pcpn in the warm sector ahead of the cold
front, with pcpn quickly ending behind the front. Also, the abrupt
wind shift line with the front is still over ern Iowa, and have
changed the forecast very little with respect to the wind trends.
The generally southerly winds to 15kt will continue for a couple
more hours before veering more swly just in advance of the front.
Winds with shift to westerly to northwesterly as the cold front
moves across the area. Speeds will increase with gusts into the
mid 20kt range expected in the cold advection and pressure rises
following the frontal passage. Winds should remain gusty into the
evening while veering to northwest. The only other significant
change to the forecast is to bring back mvfr cigs for the evening
and overnight hours. RH profiles suggest an extended period of
mvfr stratus in the backwash around the swrn periphery of the main
upper low.


319 AM CST

A very active pattern will continue for Lake Michigan for the next
several days.

Main changes to the current forecast were to extend the SC.Y for
the nearshore waters for an additional six hours. Otherwise, no
changes to the current Gale Warnings.

Series of systems will impact the Great Lakes, with periods of
Gale Force winds expected. South winds will shift to nwly
following the passage of a cold front today. South Gales over the
north half of the lake will diminish slightly with the passage of
the frontal trough, but in strong cold advection behind the front,
expect a period of generally northwest Gales for this evening and
into Thursday morning. A brief period of diminishing winds is
expected for Thursday as high pressure quickly builds across the
lake, but by Thursday night, another period of south gales is
likely for late Thursday night into Friday night as the high
moves off to the east and deepening low pressure tracks across the
plains. This low is expected to track across the southern portion
of the lake Saturday morning. South gales will become northwest
gales Saturday through Saturday night as cold advection and a
strengthening gradient set up over the lake between the exiting
low and the next area of high pressure building into the plains.


LM...Gale Warning...LMZ777-LMZ779...6 PM Wednesday to 6 AM Thursday.

     Small Craft Advisory...IN nearshore waters until 11 PM Thursday.

     Small Craft Advisory...IL nearshore waters until 4 PM Thursday.




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