Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250749

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
149 AM CST Sat Nov 25 2017

815 PM CST

For Evening Update...

Minor tweaks based on observed conditions for wind and temp
trends this evening, and to limit sprinkle chances to primarily to
areas north of I-80 (and especially along/north of I-88).

Deep low pressure was passing north of the Great Lakes this
evening, and was trailing a cold front which was pushing east
across northern Illinois as of 8 pm CST. While a sharp wind shift
to the west-northwest will occur with fropa, the air mass behind
the front is of Pacific origin, with temperatures only slowly
falling through the lower 50`s and 40`s in its wake. Winds have
diminished within the weaker gradient of the frontal trough, but
become gusty again 20-30 mph west of the Mississippi where
stronger (5 mb/3 hr) pressure rises were noted. These winds will
likely diminish a bit overnight, but would expect breezy
conditions to redevelop a few hours after fropa overnight.

Upstream soundings depict a relatively dry column, though a band
of mid-level clouds (lower farther north over MN/WI) was evident
behind the surface front, associated with a strong, digging short
wave trough propagating across the upper Mississippi Valley.
Regional radar mosaic indicates precip, in the form of rain,
occurring with these clouds mainly north of the IL/WI border.
However, weaker returns have been developing further south into
far northwest IL and eastern IA. Thus a period of light sprinkles
are expected to affect mainly areas north of the I-80/I-88
corridors from mid-evening through midnight across northern IL.
Clouds should diminish from the northwest with passage of the mid
level trough axis after midnight.



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 06Z TAFs...

A cold front is moving into northwest Indiana late this evening
with winds turning northwest in its wake. Cold advection into
the region should help to maintain steep low level lapse rates
through the overnight hours which will allow winds to gust to
around 20 kt. Cold-air stratocumulus has developed in the wake of
the front over Wisconsin, and models are coming into better
agreement showing upstream MVFR conditions pushing across the
terminals overnight. Have added a tempo for MVFR cigs for a few
hours overnight. VFR conditions should prevail through the
remainder of the TAF period. Winds remain gusty through the day
and should diminish around sunset.



148 AM CST

As low pressure moved into Quebec overnight, an associated cold
front pushed across the lake. This initially provided some
weakening pattern over the lake with a brief lull in the speeds
noted. However, post frontal passage, speeds have quickly picked
back up out of the northwest. Speeds to 30 kt now likely being
observed over much of the lake. Any areas over the far southern
portions of the lake still in a lull, can expect this shift to the
northwest to 30 kt here in the next one to two hours. A period of
northwest winds to 30 kt is then expected for much of today over
the open waters, with these similar speeds reaching the Indiana
nearshore. Along the Illinois side, anticipate these winds to 30
kt to be more occasional. With these winds/speeds, hazardous
conditions for small craft will persist into this afternoon for
the Illinois side but with waves likely remaining elevated, expect
these hazardous conditions to continue through late tonight along
the Indiana side. Speeds briefly diminish later tonight, however,
pick back up Sunday as low pressure passes just north of the
Great Lakes. Speeds across the north half may approach gales
midday Sunday, but think any gales will be limited to a small
window of gusts at this time.



LM...Small Craft Advisory...IN nearshore waters until 4 AM Sunday.

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




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