Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 222326

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
526 PM CST Sun Jan 22 2017

239 PM CST

Through Monday...

The main concerns again tonight will with fog trends, especially
over northwestern Indiana and near Lake Michigan.

A powerhouse storm system over the Lower Mississippi Valley, will
shift towards the Central Atlantic Coast by late Monday. This
system will remain far enough south of the area to prevent much
more than a passing shower tonight into Monday. However, this
system will continue to support a northerly wind across the area
through Monday. It appears that with a bit more of a wind
component tonight that the fog may not end up being nearly as
widespread as it was this morning. However, fog will still be a
possibility. It appears the areas that would be best favored for
dense fog will be portions of northwestern Indiana, especially near
the lake. Area weather webcams show quite a bit of dense fog over
the lake. As as result, with the continued northerly wind tonight,
this fog will likely be pushed onshore over northwestern Indiana.
With this in mind, we have opted to reissue the dense fog advisory
for Lake and Porter Indiana through early Monday morning. I don`t
have a good feel for areas farther south, so I have opted to hold
off issuing, but if trends support it, an extension to the
advisory may be needed this evening. Additional areas of fog will
again be possible over portions of north central Illinois as well,
but the extent of this fog is still somewhat uncertain.

Otherwise, expect mainly cloudy skies through the day Monday. The
morning fog should again abate during the morning. Highs are
expected into the lower 40s.



310 PM CST

Monday night through Sunday...

Above average temperatures will linger into mid-week, before a
pattern shift brings a return to more typical late January weather
conditions for the second half of the week, next weekend and

Surface low pressure currently across the southeastern states will
be moving off of the mid-Atlantic coast by Monday evening, with
weak surface high pressure moving across the Mississippi Valley.
Lack of strong dry advection and the persistence of an inversion
around 900 mb (2500 ft agl) will likely prevent low clouds from
scouring out despite rising surface pressures, which should in
turn help keep temperatures in the 30`s overnight, a little warmer
than our normal daily highs. Some potential for fog will exist,
though in the absence of any substantial cloud breaks it would
likely remain patchy/light.

Attention then turns to the next upper trough and associated
surface low which emerges from the Plains Tuesday, and tracks
northeast across the western Great Lakes region through Wednesday.
Some model differences remain with the details and speed with this
system, though overall there are more similarities amongst the
various guidance than large differences. Surface winds will turn
east-southeasterly as the low approaches, while south-southwest
flow increases above the boundary layer by Tuesday evening.
Forecast soundings depict a gradual mid-level and eventual top-
down saturation which spreads into northwest IL Tuesday
afternoon, and across the remainder of the forecast area Tuesday
evening. Mild low-levels (low-mid 40`s Tuesday afternoon) and
persistent warm advection aloft should support all-liquid precip
from Tuesday afternoon through the night, with mild surface temps
and wet-bulbs and 1000-850 mb thicknesses around 1315-1320 meters.
Period of greatest organized/measurable precip appears to be
Tuesday evening, when deepest saturation is present. Some mid-
level drying is evident later Tuesday night into Wednesday, which
along with veering 850 mb flow and the transition of better
moisture transport east/southeast of the area suggests precip
would become lighter by that time. Upper trough then propagates
across the area Wednesday, with a morning passage of the occluding
cold front resulting in a shift to westerly low level flow and a
gradual cooling of the column. Lingering lighter precip would
likely mix/change to some wet snow from west to east across the
area through Wednesday afternoon/evening, though with minimal qpf
and no snow accumulation is expected.

The upper pattern undergoes a substantial change from Wednesday
onward, as additional short wave energy drops into the developing
positive-tilt long wave trough which develops across much of the
northern CONUS. This sets up a period of blustery west-northwest
cyclonic flow across the forecast area, which will maintain mostly
cloudy skies and draw colder air into the region. While little in
the way of organized precipitation is indicated, some periods of
flurries or light snow showers may occur into the weekend,
especially with a couple of reinforcing pushes of colder air
Friday and Saturday. Flow looks to remain westerly enough to keep
lake effect snows east of the cwa, though northeast Porter county
could be briefly clipped at some point during the weekend as low
level winds veer a little more northwest. While a colder pattern,
nothing brutal is anticipated with expected temps actually fairly
close to our normals for late January, with highs around 30 and
lows in the teens and 20`s. It does appear that this pattern will
continue next week, beyond the end of the current forecast period.



For the 00Z TAFs...

Low CIGS and fog potential remain the primary aviation concerns.
Fairly widespread MVFR deck blankets the area, though there have
been and continue to be a few holes where low clouds scatter out
and conditions go VFR for a time. Not going to try and capture
what should be brief periods of VFR in the TAFs. IFR CIGS over
southwest WI are expected to spread SE into RFD and probably DPA
tonight. Can`t rule out IFR CIGS spreading into ORD/MDW or
building down, but guidance not very gung-ho on the idea, so opted
to keep things MVFR in TAF until observational trends suggest
otherwise. Mainly expecting light fog tonight, while some dense
fog cannot be totally ruled out tonight, set up appears far less
favorable compared to last night. CIGS and VSBYS should both
improve some diurnally by mid-late morning Monday.

For GYY, dense fog off the lake has kept conditions on the deck
all day. While some drier air could eventually advect in and
improve conditions, limited observations over the lake makes it
pretty hard to say when. Given lack of data to suggest otherwise,
persistence of dense fog seems to be the most reasonable forecast
for GYY with potential for improvement Monday morning.



311 PM CST

Main concerns for Lake Michigan are the persistent dense fog in
the near term, then a period of stronger west-northwest winds
beginning mid-week and persisting through next weekend.

Dense fog continues to plague much of the lake this afternoon,
and is expected to persist into Monday as weak high pressure and
light winds combine with mild/moist air across the area. Surface
low pressure passing south of the region tonight will allow winds
to shift to the north and increase a bit Monday, which should help
pull drier low level air across the lake and improve visibility. A
dense fog advisory is in effect for the entire lake through mid-
morning Monday.

Another low pressure system is expected to develop across the
Plains by Tuesday, and will track northeast across Lake Michigan
through Wednesday. The main impact with this system will be a
period of blustery west-northwest winds behind the low Wednesday,
which will persist and become more true northwesterly with time
through next weekend. Model guidance continues to show some
differences with the strength and timing of the low center
passage, but it appears that winds will increase into the 20-25 kt
range Wednesday afternoon and will remain blustery for several
days as a few reinforcing pushes of colder air move across the
region. Lake effect snow showers will be common on the down-wind
eastern side of the lake.



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

LM...Dense Fog Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 UNTIL 9 AM Monday.




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