Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 242335

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
535 PM CST Fri Nov 24 2017

259 PM CST

Through Saturday...

After a breezy, mild afternoon, a cold front will bring cooler air
and gusty northwest winds tonight.

Gusty south winds may increase by a couple of knots this afternoon
and then will diminish this evening. We are warming a lot more
efficiently than expected so I raised high temps into the mid 60s.
However, a cold front currently stretching from north central WI
through central IA will pass overhead tonight.

I have lower confidence regarding low temperatures tonight and
isolated sprinkles.  We have mixed very well today, allowing dew
points to drop into the mid 30s to around 40. Forecasted cloud
heights are also on the high side for sprinkles to reach the
surface, so I backed off on sprinkle coverage tonight to be more in
line with convective allowing models.  Areas north of I-80 may see
an isolated sprinkle late this evening as the upper level wave moves
through. I am not expecting measurable precip.

It looks like cloud cover will shift southeast with the front, but
gusty northwest winds behind the front will likely limit cooling. I
went a couple degrees higher with low temperatures, mid 30s to
around 40.

Saturday will breezy, but not as breezy as today. Gusts should be 20-
25 kt, but if we maximize mixing, we could see gusts to 30 kt again.
 925 mb temps will be 0 to +5C resulting in high temps in the mid
40s to around 50.  The windchill will make it feel like it`s in the
low 40s to upper 30s.



259 PM CST

Saturday night through Friday...

Looking at a quiet, mainly dry extended period. Milder air returns
Monday as do gusty winds. Next decent chance of precipitation
arrives Wednesday night.

Light winds and relatively clear skies should allow for efficient
cooling Saturday night. Lows should dip below freezing into the
upper 20s.

High pressure dominates the pattern over the Great Lakes through the
weekend. Southwest winds and warm air advection return under the
ridge leading to high temps in the upper 40s Sunday.

Warm air advection continues through Monday night, and Monday could
be as mild as today. I raised high temps slightly, but local
guidance suggests high temps in the 60s are very possible. My
uncertainty is due to inconsistencies in how quickly the thermal
ridge builds over the region. Monday night looks breezy and mild
because the pressure gradient tightens and the temperature ridge
will be  overhead.

A large surface low reaches James Bay Tuesday, and it`s cold front
swings through. Guidance differs on if rain showers will form along
the front, but I kept a slight chance of rain Tuesday night along I-
55.  Widespread rain is not expected. Tuesday could also be a couple
degrees warmer than currently forecast, but not nearly as warm as

A shortwave and associated surface low may bring rain to much of the
region Wednesday evening. The wave weakens as it moves overhead, so
forcing for rain may also weaken.

A stronger upper level wave and associated cold front move through
Thursday and Thursday night.  Colder air associated with the upper
level trough quickly passes overhead with the trough.



For the 00Z TAFs...

A cold front is moving across the Mississippi Valley this evening.
Strong south winds are in place out ahead of the front and will
veer to the northwest mid to late this evening as the front
moves across the terminals. Expect an increase in mid and high
clouds for a few hours but should remain VFR and dry with frontal
passage. Northwest winds will likely remain breezy overnight with
continued gusts into the low to mid 20 kt range, though could be
a short duration of higher gusts back to around 30 kt immediately
behind the front. Northwest winds and VFR conditions will continue
through the day Saturday.



259 PM CST

Did not make any adjustments to the going headlines.

Low pressure over Ontario will continue to slowly weaken as it moves
into eastern Quebec. The pressure gradient overhead will weaken, and
the low`s cold front will swing through this evening. Northwest
winds will gust to 30 kt behind the front, but gales are not
expected.  A Small Craft Advisory will be issued for the NW IN
waters after the gale due to hazardous winds and waves.

High pressure spreads over the Great Lakes Saturday night, and a
weak surface low passes near or just north of the northern Great
Lakes Sunday.  Winds become southwest and gusts to 30 kt are
forecast over the northern end of the lake as that low goes by.

Another high pressure ridge brings light winds to the lake Sunday
night. Low pressure deepens as it crosses southern Canada early next
week, and winds become south 15 to 25 kt by late Monday afternoon.
South gales are possible Monday night primarily over the northern
half of the lake.  Winds shift to the west behind the low`s cold
front. A ridge moves over the lake Wednesday followed by another low
Wednesday night.



LM...Gale Warning...LMZ777-LMZ779 until 9 PM Friday.

     Gale Warning...IN nearshore waters until 6 PM Friday.

     Small Craft Advisory...IL nearshore waters until 3 PM Saturday.




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