Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 222110

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
310 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 18Z TAFs...

Visibilities continue to improve across the area late this
morning. The only exception is areas along the lakeshore in
Northwestern Indiana, including KGYY. Light flow off the lake
will likely keep the visibility lower at KGYY this afternoon as a
result. Otherwise, CIGS should gradually improve into the 1,000 to
1,500 FT AGL range this afternoon. A rather potent area of low
pressure will track well south of the area this afternoon. The
main effect this system will have over the area is to produce a
continued north-northeasterly wind through the forecast period.

CIGs will likely not improve much tonight into Monday. In fact,
CIGs could drop back into the IFR category tonight. There could
also be some additional areas of fog that develop again tonight.
However, with more of a wind component tonight, it appears the
threat of dense fog at the terminals will be low.



324 AM CST

Fog, much of it likely dense, continues over the lake this
morning. Northerly flow will steadily increase from late morning
into the afternoon, and should at least push/dissipate some of the
fog from north to south into the afternoon. The northerly flow
will further increase tonight into Monday, and a Small Craft
Advisory is likely for the Indiana shore for Monday and Monday

After a weak ridge passes over the lake on Tuesday during the day,
strong low pressure passing over or just south of Lake Michigan
will bring increasing northeast winds shifting northwest during
midweek. While this event presently does not have a gale
magnitude with it, it does look it will have 30 kt winds at times
and Small Craft Advisory criteria in the Illinois and Indiana



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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