Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 222323

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
623 PM CDT Wed Mar 22 2017

158 PM CDT

Through Thursday...

Warm advection will kick in tonight as cool high pressure exits to
the eastern Great Lakes region. Mid and high clouds from low
pressure off the CA coast will stream east-southeast over the area
tonight over a flattening ridge across the plains. With the
increasing clouds expect temps to hold a tad warmer than last

The developing warm front will shift northeast across the area
later Thursday and Thursday night. Lower level warm advection with
this front will likely kick off some scattered afternoon showers.
Weak upper ridging and southeasterly flow maintaining drier lower
level conditions will preclude more widespread shower activity.
In spite of cloudier conditions, highs will recover closer to
normal, except away from the lake in northeast IL where onshore
flow remains.



330 PM CDT

Thursday night through Wednesday...

Theme of the extended is a jump to much warmer temperatures Friday.
A cold front brings temps back down into the 50s and 60s Saturday
and temps stay relatively steady through the remainder of the
forecast. Periods of rain and storms are expected for much of the

A surface low takes shape over the plains Thursday night, and we`ll
be in the warm sector Friday.  Still looking at above normal highs
around 70, but have questions regarding how well we will warm with
thick cloud cover overhead.  Typical gusty southwest winds are
forecast, and continue to think the majority of precip will stay in
WI, north of the forecast area.

Showers spread across the area from northwest to southeast Friday
evening through Saturday morning as a cold front surges down the
lake and upper level vorticity lobes move over the forecast area. I
have medium-high confidence in embedded thunderstorms Friday night,
but less confidence in storms Saturday morning.  CAPE and the better
forcing look to be tied to low itself, so if the low is closer to
the forecast area Saturday, could see more thunder than forecast.
However, if the low is slower and further from the forecast area,
there could be less thunder.  I have high confidence in measurable
precip Saturday, but there could be brief dry periods.

The low moves over the forecast area Sunday and continues northeast
to the eastern Great Lakes Monday.  Showers associated with the
first low come to an end Sunday night.  Long range models differ on
how the next low will evolve, but they do agree that we may be on
the northern fringe of its precip.  High pressure then builds in mid

Extended temps past Friday will generally be in the 50s or low 60s
away from the lake. Onshore flow will keep the lake front around 10
degrees cooler.



For the 00Z TAFs...

Few aviation concerns through Thursday morning, with the potential
for scattered shra and patchy mvfr cigs increasing by mid-late
Thursday afternoon.

Surface high pressure was centered over the Great Lakes region
early this evening, with dry east-northeast winds across the
terminals. Winds will slowly back more east-southeast through late
this evening, becoming southeast during the day Thursday. Modest
gusts around 20 kts develop during the afternoon as the gradient
tightens ahead of an area of low pressure across the Plains. A
warm front will lift north across the terminals late Thursday

Low level flow will remain quite dry into Thursday, while south-
southwest winds aloft bring moisture and a gradual increase and
lowering of mid level VFR clouds through the day. Stronger
moisture transport develops during the afternoon ahead of the
warm front, with guidance in general agreement in depicting
scattered shower development over portions of the area roughly in
the 21-03Z window. Greater shower coverage would likely result in
patchy mvfr cigs. Stronger forcing remains west/northwest of the
area, thus somewhat low confidence in coverage. Steepening mid-
level lapse rates suggest potential for TS, but again potential is
greater north/northwest of terminals and not high enough to
warrant mention. Some model guidance ramps up low-level jet
Thursday evening to 50-55 kts, which may present a non- convective
LLWS threat overnight. May see surface winds remain a bit gustier
than current guidance south of the warm front Thursday night, so
will hold off on mention at this point and monitor for later
inclusion if more model runs come around to this solution.



330 PM CDT

High pressure over the lake will continue to shift southeast
reaching the mid Atlantic coast Thursday.  30 kt south winds set up
on the back side of the high Thursday.  Do not have the confidence
to issue a small craft advisory at this time, but winds and waves
may be hazardous to small craft Thursday. A cold front sinks south
across the lake Friday and Friday night, and winds become north
behind it. Meanwhile, the next surface low forms over the plains
Thursday night into Friday. The low weakens as it moves over
Illinois Sunday morning and then dissipates over the eastern Great
Lakes Monday. Guidance then differs on how the pattern will evolve
early next week.  The ECMWF features a baggy pattern with light
winds while the GFS has another low move up the Ohio valley with
strong northerly winds over the lake.






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