Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 280940

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
440 AM CDT Fri Oct 28 2016

354 AM CDT

Today through Sunday night...

Main short term forecast concerns are breezy and unseasonably mild
conditions today into Saturday, before shifting to a cold frontal
passage Saturday afternoon signaling a shift to more typical
seasonable temperatures. Rain is expected to develop post-
frontally Saturday evening, with the highest potential overnight
Saturday night into early Sunday.

Early morning surface analysis depicts high pressure centered
across the Midwest, just off to the east of the forecast area. Low
pressure was developing to our northwest across the Dakotas in
response to a 115 kt upper jet streak and associated mid-level
short wave propagating eastward from the northern Rockies. The
surface low is progged to develop quickly eastward across the
northern Great Lakes through tonight, with surface pressure falls
and a tightening surface pressure gradient inducing strengthening
south winds across the area. Given the relative lack of low
cloudiness upstream this morning, suspect that model soundings mix
the boundary layer a little too shallowly this afternoon this
evening/overnight. Thus have continued to lean 2-3 degrees warmer
than mos guidance for temps today, closer to what 925 mb
climatology would support, with afternoon highs from the low 70`s
west/south to the upper 60`s in the northeast. With models likely
decoupling boundary layer too strongly tonight, expect gusty
conditions to continue with strongest winds likely occurring this
evening in the 35 mph range. Temps will likely dip a little with
sunset, but should then remain fairly steady in the low 60`s
overnight in the persistent south-southwest winds.

As the surface low continues to track away from the northern Great
Lakes Saturday, a trailing cold front will sag across the region.
Guidance has wavered a little with the timing of the front into
the cwa over the past couple of days, though there is now pretty
good agreement that the boundary will settle into northern IL and
to the south end of Lake Michigan during the afternoon. Southwest
winds and dry thermodynamic profiles in the warm sector ahead of
the front will once again support unseasonably mild temperatures
in the mid-upper 70`s in many area, though cold fropa and a shift
to northeast winds will temper things a bit across far
northern/northeast IL. By evening, another short-wave is progged
to be approaching from the west, with increasing forcing for deep
ascent developing and inducing frontogenesis within the baroclinic
zone north of the surface cold front, leading to saturation and
precipitation development across the region. Most of this appears
to be initially off to the north or across the far northern
portion of the cwa near the IL/WI border area into early evening,
eventually spreading south across the area overnight. Passage of
the short wave trough axis Sunday morning, and development of
subsidence should bring an end to precip by midday, with high
pressure spreading in Sunday afternoon and night. While cooler air
spreads in behind the front Saturday night and Sunday,
temperatures are expected to be near normal for late October, in
the mid-upper 50`s north and low-mid 60`s south.



405 AM CDT

Monday through Friday...

Above normal warmth returns early next week as upper pattern
remains a progressive and largely zonal configuration. Broad
ridging across the Midwest Monday, ahead of an upper trough
exiting from the Rockies, will set the stage for another
unseasonably warm day for Halloween as surface low pressure
deepening across the northern plains again produces gusty south
winds across the region. A weak cold front trailing this low is
progged across the area late Monday night, though a dry fropa is
expected with the return of moisture blocked by high pressure
across the Gulf. Guidance is in generally good agreement with
another upper trough and surface low pressure combination
approaching mid-week, with a little better potential for precip.
Rebuilding of upper ridge is depicted by the end of the week, with
no real intrusions of cold air noted. Thus after temperatures in
the 70`s with Monday`s gusty south winds, the remainder of the
week appears to remain slightly above average in temperature with
highs in the upper 50`s and 60`s.



For the 12Z TAFs...

- Patchy fog and brief MVFR cigs this morning
- Increasingly gusty south winds this afternoon with strong
southwest winds this evening
- Transition to low level wind shear late evening/overnight

MVFR/IFR cigs remain sparse this morning, and Latest short term
guidance suggests that the length of time and coverage for low
cigs may be short/low this morning. High clouds continue to move
through the area which will keep sustained visibility reductions
to 3-5sm in most cases.

Concern shifts to increasing south and eventually southwest winds
this afternoon and this evening. Low pressure will quickly move
east across the upper midwest today and north of Lake Michigan
tonight. South winds will largely be the rule today, maybe
acquiring a slight sw component late. At this point critical
crosswind thresholds do not appear to be met today with S gusts
remaining generally under 25 kt. Winds will then veer to
southwesterly as the low shifts east. This does appear to be the
period of strongest winds in the evening, with mixing continuing
during this time, before transitioning to a continued low level
wind shear concern later in the evening and overnight hours. Winds
will be more of a SW than S during the peak winds. Have 30 kt
gusts mentioned in the TAFs. Depending on mixing, these could
certainly get higher, but confidence on specifics is low at this
time, as there is an inversion but very strong winds aloft. Gusts
will likely diminish some overnight as the low pulls away, but low
level wind shear will continue.



409 AM CDT

Active and changing conditions will be the rule across the lake
the next few days with a few periods of gales possible.

South winds will ramp up considerably today as low pressure will
quickly move from the upper Midwest to north of Lake Superior
tonight. A modestly strong pressure gradient will exist ahead
of the low as high pressure remains anchored over the
Appalachians. The strongest winds will be from mid afternoon
through evening as winds shift more southwesterly. Gale force
winds to 40 kt, possibly higher across the north, are expected.

The low will continue east tonight into Saturday with a weaker
pressure gradient behind it. High pressure will move into the
Canadian prairies and the upper Midwest Saturday, and this will
help steer a cold front across the lake from north to south
Saturday and Saturday evening. A weak low will pass just to the
south of the lake Saturday night into Sunday which will maintain a
modestly breezy northeast wind, which if a bit stronger could
result in small craft advisory winds along the nearshore for a
brief time early Sunday. The high will transit the lake later
Sunday and Sunday night.

This will be quickly followed by another quick moving low
pressure system that will move from the upper Midwest to across
Lake Superior Monday night. This will likely result in another
period of southwest gales ahead of the low. High pressure will
bring quieter conditions Tuesday.



LM...Gale Warning...LMZ777-LMZ779...3 PM Friday TO 3 AM Saturday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...3 PM Friday TO 7 AM Saturday.




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