Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220507

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1107 PM CST Sat Jan 21 2017


957 pm...Evening update...issued a dense fog advisory for the
northern two thirds of the cwa and this may need to be expanded
south for the rest of the area late this evening or overnight.
Cloud cover late this afternoon started to move east after sunset
and a ribbon of dense fog developed along the leading edge of this
cloud cover and extends across portions of the northwest cwa. It
appears this has slowed its eastward movement...but now dense fog
is developing in other areas along the I-80 corridor...likely in
part from radiational cooling. There still remains uncertainty
regarding how widespread the dense fog will become...especially in
the next few hours. But winds are expected to become light and
variable then shift light northeasterly by morning and this should
support at least areas of dense fog. If dense fog does become
widespread...its possible it may persist longer than the current
expiration time of 15z.

Further south...showers and scattered thunderstorms have developed
from dnv to laf in the past hour or so and short term/hi res
guidance has been struggling with this activity. While it appears
the bulk of this will remain just south of the cwa...opted to bump
pops up into chance and also maintained slight chance thunder.
Overall confidence with this activity is low and some showers may
continue into the cwa...generally south of the Kankakee river trends will need to be monitored. cms


308 PM CST

Through Sunday...

After a sunny and unseasonably warm day by January standards,
forecast concerns shift quickly back to the redevelopment of fog
and low stratus across the forecast area tonight.

Afternoon surface analysis depicts low pressure lifting northeast
across southern Minnesota, with a warm front extending east across
southern Wisconsin and southern Lake Michigan. South of the warm
front, skies were generally sunny with temperatures in the mid
50`s to lower 60`s across the cwa. The surface low is progged to
fill quickly tonight across the upper Midwest as the parent mid
level short wave lifts north, with the pressure gradient weakening
and becoming baggy across the area later this evening. Given
generally clear skies and quickly diminishing winds, radiative
cooling and surface dew points in the 40`s are expected to allow
for relatively quick fog/low stratus development in the hours
after sunset. Low clouds and light fog were already trying to
creep east across northwest IL this afternoon, slowed by better
mixing in the sunny/breezy warm sector across the area. Would
expect this stratus fog to expand east and southeastward this
evening as we lose the gradient and mixing, along with new
development in cooling with loss of diurnal mixing. Will very well
need fog headlines sometime later this evening, though confidence
in speed of development/expansion is low at this point. Temps
to remain above freezing through the night, so at least no
freezing issues expected.

Next upper level wave across the southern Plains is progged to
lift out across the lower Ohio and lower Mississippi river valleys
through Sunday, with surface low pressure tracking well south of
the forecast area. Guidance is in good agreement in depicting an
inverted surface trough developing northeast into northern
Indiana later tonight, which becomes the focus for some light
precipitation across our far southeastern tier of counties late
tonight into Sunday. Best chances remain south of the cwa, though
have included some low pops across that area. Forecast soundings
depict some weak elevated cape as well from late evening into the
overnight hours, thus an isolated thunderstorm can`t be ruled.
out. Low level winds which go light/variable tonight become
northeasterly Sunday, pulling cooler air into the area and likely
slowing any erosion of fog/stratus. Even so, temps appear to
remain well above climo in the 45-50 degree range through the day
Sunday, a little cooler along the Lake Michigan shore.



231 PM CST

Sunday night through Saturday...

The main challenges during the period will be with the timing, and
associated local impacts, of a storm system expected to impact
the Lower Great Lakes region by midweek.

Prior to the midweek storm system, a potent storm system will
move over the Ozarks on Sunday, than to the Central Atlantic Coast
on Monday. While this storm system will be too far to our south to
produce much precipitation over the area, a few rain showers will
still be possible on Monday over far eastern Illinois and into
northwestern Indiana. Otherwise, this system will result in a
period of breezy cooler northerly winds Sunday night and Monday.
Expect high temperatures for Monday and Tuesday to be several
degrees cooler (low to mid 40s) than this weekend temperatures.

Attention then turns to what appears to be a quick larger scale
pattern shift towards a +PNA pattern (Upper ridge in the west,
Upper trough in the east) over North America by mid to late week.
As this pattern shift occurs, the eastern Pacific storm system
will be driven eastward over the central CONUS and into the Lower
Great Lakes Region sometime around midweek. While model and
ensemble forecast guidance is in fair agreement with the
evolution of the larger scale pattern supportive of this storm
system, there are still some rather large differences in the
timing and strengthen of an associated surface low. Most notable
has been the faster trend in the GFS solutions over the past 24 to
36 hours. The 12Z GFS now appears to be on the fast side of all
the guidance, with the forecast surface low already shifting
towards Lake Huron by 12z Wednesday, which even outpaces the GEFS
mean. Meanwhile, on the other end of the envelop is the 12Z
Canadian, which has the surface low all the way back over
Northwest Missouri at 12z Wednesday. Finally there`s the 12Z
ECMWF, which seems to fall right in the middle of the pack, with
the low over Northwest Illinois during this time.

All in all these large differences in timing still result in low
confidence with the specifics of this system, especially in terms
of timing a change from rain to snow over the area. Because of
this, we have not made any major changes to the forecast during
this period. If the track of the low does go over far northern
Illinois or southern Wisconsin, the P-type would likely be mainly
rain Tuesday night. The question is than when and how much of the
precipitation could change to snow on the back side of the storm
system late Wednesday or Wednesday night. Also, in addition to
the precipitation, if the surface low ends up on the stronger
side, and tracks just north of the CWA, it could support a period
of warm temperatures followed by strong westerly winds over the

Colder weather and some lingering periods of snow showers or
flurries look to continue during the later portion of the week.



For the 06Z TAFs...

A near wall of dense fog oriented NE to SW has been steadily
moving east across northern Illinois. By 0430Z the leading edge
had passed ORD and based on webcams and sfc obs appears to extend
to just west of KJOT. While the pseudo wall of fog moves slowly
east, the western extent has been re-expanding back west with fog
developing again in KRPJ and recently in KRFD. In addition, ahead
of the wall, shallow patches of dense ground fog have also
developed. Hard to find any meteorological reason that the wall
won`t continue east and reach MDW between 0530Z and 0630Z.
Guidance has been struggling terribly with this fog development
and none have had a good handle on trends this evening, which
lowers confidence in forecast overnight into Sunday morning.

The LAV guidance has been oblivious and of no help in forecast the
development of the fog, but as it initializes a location with the
dense fog, its forecast instantly changes to keep vsby on the
deck until late Sunday morning. Given the meteorological set-up,
this seems like a very plausible and the best forecast, even
though confidence in the duration of the on the deck forecast is
only medium. Have opted to depict improvement to prevailing vsby
at MDW at 12Z, but this is only due to medium and reluctance to
shut down the airport this far out. Meteorologically, there is no
sound reasoning for conditions to improve at 12Z, but fog often
has a mind of its own.

Only looking for gradual improvement Sunday, first to IFR, then
likely climbing to MVFR cigs and VFR vsby during the afternoon.
Some guidance would suggest skies clearing out and going VFR
Sunday evening, which doesn`t seem terribly likely given the
upstream extent of stratus/fog. Some guidance does bring cigs/vsby
back down, which is more probable, but at this point opted to
split the difference and just maintain MVFR cigs through Sunday



323 PM CST

The main weather concerns over the lake continue to be the dense
fog over the open waters of Lake Michigan. Given this fog will
likely not be going anywhere anytime soon, we have extended the
fog advisory for most of the open waters through 3 PM Sunday.
Northerly winds are expected to set up over the lake by Sunday
night into Monday as a strong area of low pressure tracks well
south of the lakes region. While the low will be well south of the
area, northerly winds up to around 25 KT will still be possible
over southern Lake Michigan Sunday night and Monday. These winds
will likely result in small craft advisory level waves of 4 to 7
FT over the southern Lake Michigan near shore waters during this

Another storm system is expect to develop over the Plains by
Tuesday morning, then track east-northward over the southern
lakes region by midweek. This system could result in a period of
enhanced winds over the lake around midweek.



IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     UNTIL 9 AM Sunday.

IN...Dense Fog Advisory...INZ001-INZ002 UNTIL 9 AM Sunday.

LM...Dense Fog Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 UNTIL 3 PM Sunday.




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