Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201935

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
235 PM CDT Thu Oct 20 2016

235 PM CDT

Through Friday...

Concerns for tonight are potential frost in north Central Illinois and
lake effect showers/possible storms in northwest Indiana.

Quite a different day compared to yesterday with grey skies, brisk
north winds, and temperatures holding in the 50s area wide. A mid
level trough and elevated frontal zone continue to drive a few
light rain showers but these should diminish this evening as the
trough axis shifts east. Lake effect rain showers already made an
appearance across northwest Indiana this afternoon though they
have been shallow and fairly short lived.

High pressure across the plains will inch eastward tonight, and should
provide for some clearing skies from the west as the night
progresses. Cold advection on northwest flow will allow
temperatures to again dip to the mid 30s across north central
Illinois. We do maintain a bit of a wind component tonight as the
high remains to our west making it not an ideal setup for frost,
but forecast soundings do show some decoupling late tonight and
therefore have matched with the neighboring offfices and issued a
Frost advisory for these areas. More cloud cover east and
continued winds hold temps in the upper 30s to mid 40s.

Meanwhile, as this colder air filters south, lake effect already
ongoing across the U.P. of Michigan should become more pronounced
across the lake and northwest Indiana as the evening continues. We
may start to see some development this evening, with better
thermodyanamic parameters and lake convergence setting up
overnight into early Friday. With lake induced equilibrium levels
up at 25000 feet some of these showers could be somewhat robust
and produce thunder, and the lake convergence signal complements
these parameters bringing a potential waterspout concern.

Model guidance is in fairly good agreement that the high will
continue east Friday and allow the convergence axis to shift into
Porter and eventually east of the area sometime Friday afternoon.
Areas away from NW Indy will have largely clear or clearing skies
with modest NW winds. Highs again top out in the lower to mid 50s
but it may look and feel a little nicer with a slightly weaker
wind and some sunshine.




Friday night through Thursday...

216 pm...Forecast concerns include frost potential Friday night/
Saturday morning and an unsettled pattern for the middle of next

A ridge of high pressure will be moving across the area Friday
night into Saturday morning. This will allow for light/calm winds
with temps likely to tank during Friday evening. However...there
will be some weak warm air advection aloft overnight as well as
the potential for some cloud cover. How much these affect frost
formation is uncertain but as long as clouds do not become
overcast...expect at least patchy frost over much of the area with
temps in the mid/upper 30s much of the area. Possible another frost
advisory may be needed.

As the high departs Saturday...winds will turn to the southwest
and temps will begin to moderate reaching into the lower 60s
Saturday. Temps could potentially warm into the upper 60s to lower
70s on Sunday on breezy west/southwest winds. Low pressure will
move across the northern Great Lakes Sunday afternoon into Sunday
evening. Still some timing differences regarding when a trailing
cold front will arrive which may affect high temps. This front
pushes southeast of the area Sunday night with high temps back
into the lower 60s Monday and Tuesday.

Another low will move across the region Wednesday/Wednesday night
next week with the gfs/ecmwf in better agreement today. Possible
precip may develop along a warm front Tuesday afternoon into
Tuesday night with the location of that warm front likely
nearby...possibly southern WI. As the low approaches Wednesday...
winds may become breezy/gusty with a period of showers/
thunderstorms Wednesday into Wednesday night. cms


For the 18Z TAFs...


- MVFR Cigs
- Occasionally gusty NE to NNW winds
- Lake effect showers and possibly t-storms across the lake/NW IN.

MVFR cigs blanket much of the area but some clearing is arriving
from the northwest. Expect a slow upward climb and then at least
some partial scattering of the clouds as the afternoon continues.
The main near term concern is that winds will be occasionally
gusty to the upper teens/low 20s and varying between NNE and NNW,
with a trend toward the NNW direction.

Convergence will set up over the lake tonight which should then
maintain a NW wind tonight into Friday for the NE Illinois
terminals. The pattern is favorable for lake effect rain showers
and even isolated thunderstorms across the lake and into NW
Indiana, depending on where the convergence axis sets up. Think
some of this will impact GYY tonight. The axis of lake effect
rain showers will shift east of the area Saturday with some
continued breezy NW winds but more clearing expected.



232 AM CDT

A cold front is moving down the lake this morning, with brisk
northerly winds behind it, and these will continue through Friday
night. Gusts/speeds will be up to 25 kt on the north half and up
to 30 kt today through this evening on the south half and near
shore waters, followed by up to 25 kt for the entire lake Friday
night before easing late. Much colder air moving over the still
mild lake waters will result in sufficient instability for the
potential of waterspouts and thunderstorms tonight through Friday

West and southwest winds will return Saturday into Saturday
evening, with continued uncertainty in wind direction Saturday
night into Sunday, especially on the north half. A weak low
pressure area will then cross the lake Sunday afternoon or
evening, setting the stage for another period of brisk northerly
winds into Monday, though likely of shorter duration. Strong high
pressure will spread into the region on Monday afternoon, causing
winds to diminish quickly.

A Small Craft Advisory is in effect this morning through Friday
morning for the Illinois near shore waters and mid this morning
through Friday afternoon for the Indiana waters. The advisory may
need to be extended for a portion of the Indiana near shore into
Friday night to account for waves slowly subsiding.



IL...Frost Advisory...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-
     ILZ012...2 AM Friday TO 8 AM Friday.

LM...Small Craft Advisory...ILLINOIS NEARSHORE WATERS UNTIL 4 PM Friday.

     Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 4 AM Saturday.




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