Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KLOT 212005

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
Issued by National Weather Service Milwaukee/Sullivan WI
305 PM CDT Tue Mar 21 2017

205 PM CDT

Through Wednesday...

Area of very light showers or sprinkles is clipping the far southern
portions of the forecast area, associated with some 700 mb
frontogenesis response. Added small PoPs for light showers in those
areas for late this afternoon.

Cold front has already pushed southward along the lakeshore early
this afternoon, with winds shifting to the northeast and becoming
gusty. Winds are expected to shift northeast this evening and linger
later tonight. Gusty winds will linger near the lake. Cold air
advection develops over the area tonight, bringing a colder airmass
for tonight into Wednesday.

Added small PoPs for light lake effect snow showers near the lake
for this evening, as combination of delta T values, moisture below
the inversion, and favorable fetch develops. Did not add any
accumulations due to some uncertainty with how long favorable
conditions will remain, but will need to be watched later on.

Brought PoPs down later tonight, as drier air works into that area.
Still kept lake effect clouds later tonight into Wednesday morning,
which should push into northern portions of the forecast area.

Quiet weather with cold temperatures are expected Wednesday, with
high pressure passing by to the north.



205 PM CDT

Wednesday night through Tuesday...

A big upper level ridge will slide across the western Great Lakes
and the Mississippi River Valley through Thursday, with some
short wave energy already riding over the top of the ridge by
Thursday afternoon. The low level high pressure ridge will be over
the central and eastern Great Lakes by 12z Thursday, allowing
return flow and a decent shot of warm air advection with a warm
front to surge north into the area. This, coupled with the upper
level energy should result in a band of showers and possibly a
thunderstorm Thursday afternoon into Thursday evening lifting
north across the area.

The sfc front will likely stall somewhere near/along the IL/WI
border with the h8 frontal boundary much farther north. This
should put the focus for precip later Thursday night through
Friday north into Wisconsin.

Friday is looking quite nice, but it is providing one of the
biggest challenges for this forecast period. The stalled frontal
boundary will be hovering very close to the state line, extending
east to the Lake Michigan shore. The range in high temps will be
quite extreme. Far northeast IL, near Waukegan and Zion will have
high temps ranging from the lower/mid 50s (possibly lower if
northeast winds kick in), to the lower 70s about 20-30 miles
inland. A classic spring challenge with that very cold lake water.

Eventually, a large closed upper level low will lift out of the
4-corners region of the southwest and head northeast into the
southern Great Lakes by Saturday. The associated surface low will
track across central Illinois toward northern Indiana through
Sunday with a secondary low right behind it moving a tad more
south through Monday night. This means we transition into a
cooler, damp period for the weekend into early next week.



For the 18Z TAFs...

N/NW winds of 10-15kt will gradually become N/NE this afternoon as
high pressure builds into the Upper Midwest.  Will continue to
indicate wind direction change to N/NE at KORD by 19z, although some
high-res models such as the HRRR are now suggesting it may be
delayed until 20-21z.  Mid/high clouds will prevail through the
afternoon before MVFR clouds develop/advect off Lake Michigan this
evening into the overnight hours.  Forecast soundings show ceilings
lowering to around 1500ft at KORD after 03z.  The lower ceilings
will persist through the night before clouds scatter by mid to late
Wednesday morning.



310 PM CDT

Small Craft Advisory continues for the nearshore waters of Lake
Michigan into early Wednesday afternoon. Gusty north to northeast
winds are expected to develop by early this evening, lingering
into later tonight. Gusts up to 25 knots are expected at times
tonight. The increasing winds will build waves of 4 to 8 feet
tonight, remaining in the 3 to 5 foot range on Wednesday morning,
before subsiding early Wednesday afternoon.

North to northeasterly gusts up to around 30 knots are expected to
continue over the open waters of Lake Michigan into later tonight,
before weakening Wednesday morning. The highest waves of 5 to 8 feet
will be over the open waters toward mid lake tonight, then shift
toward the southern end of the lake into Wednesday morning. Waves
will quickly subside from north to south by early Wednesday



LM...Small Craft Advisory...NEARSHORE WATERS
     UNTIL 4 PM Wednesday.




WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.