Thursday, March 23, 2017, 12:14 AM EDT
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Area Forecast Discussion
Valid At
Wed, Dec 31, 7:00pm EST
FXUS63 KLOT 222031

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
331 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 18Z TAFs...

Lake effect stratocumulus clouds in the 3000-3500 ft range will
dissipate soon Low level winds will shift to a bit more of an 
easterly direction, so the clouds will hang on the longest across 
northern Illinois. 

High pressure over Lake Michigan will shift to the eastern Great
Lakes region tonight while low pressure will organize in lee of
the Rockies. This will allow winds to shift southeasterly tonight
and strengthen on Thursday. No operationally significant clouds
will enter the picture until Thursday afternoon ahead of a warm
front, though clouds will generally continue to lower through the
period. This feature will also lead to a few scattered rain 
showers in the afternoon. 



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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