Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 170412
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1012 PM CST Mon Jan 16 2017

.UPDATE...
904 PM CST

Evening Update:

Little change to going forecast this evening, with the exception
of the earlier expansion of the dense fog advisory and extension
in time through midnight. While visibility has improved with rain
in some spots, will likely have to consider extending dense fog
advisory farther north and past midnight.

Current round of moderate rain/showers and embedded thunder has
helped to mix out fog in spots and raise visibilities, though with
trailing edge of more widespread rain moving out over Lake
Michigan and off into Indiana over the next couple of hours would
expect fog to redevelop and thicken. Some evidence of this is
noted across western Illinois where visibility has lowered back
into the 1/4-3/4SM range in spots. Based on this thinking am
planning on letting the current advisory continue, and will
monitor fog/vis trends for possible extension overnight and
northward expansion across remainder of northern IL. Dense fog
will probably become especially likely toward morning as gradient
weakens with approach of surface low.

Another area of rain moving into western IL (which will also
likely modulate fog/low vis trends) is associated with the mid-
level trough and vorticity center, and this is expected to lift
across the area after midnight after a lull in organized precip.
Would expect light rain/drizzle to persist within this lull region
so trying not to get too detailed with hourly pop/wx grids.
Lightning detection network has been indicating isolated thunder
continues to occur across the I-55 and I-57 corridors with
initial band of elevated convection, where RAP mesoanalysis
initializes 100-300 J/kg of MUCAPE available roughly within 850 mb
thete-E axis. Guidance has been consistent in redeveloping this
axis back northward again in advance of the main vort, so have
kept a slight chance of thunder in overnight.

Overall, going forecast has this pretty well in hand and only
further changes will be to consider expansion of fog headlines
during the late evening hours.

Ratzer

&&

.SHORT TERM...
235 PM CST

Through Tonight...

The low over the Kansas/Missouri border will move over northern IL
late tonight/early Tuesday morning. Soggy conditions will continue
through that time. The good news is temperatures will remain above
freezing through tonight so no additional freezing rain or ice is
expected.  Rain continues through the night with periods of moderate
rain possible. An additional 0.5 to 1.0 inch of rain is expected
through tonight with higher totals north of I-80. Forecast soundings
also feature a little elevated instability so a few isolated,
embedded thunderstorms are possible this evening. While an isolated
storm may occur anywhere across the forecast area, the best chance
is along and south of I-80. Convection/thunderstorms may result in
locally higher rainfall amounts.

Dense fog will likely continue south of the current rainfall
(Livingston and Kankakee counties south and all of northwest
Indiana) through this afternoon when steadier rain arrives. I have
medium confidence in how dense fog will be tonight.  The abundance
of moisture will likely result in around 1 to 2 mile visibility, but
periods of drizzle may bring visibilities down to less than a mile.
Decided to go with widespread fog, but no mention of dense fog
tonight due to low confidence.

JEE

&&

.LONG TERM...
235 PM CST

Tuesday through Monday...

High impact weather chances are presently low in the extended
with a pattern atypical for January. Precipitation type looks to
remain mainly if not entirely rain during the latter part of the
week and weekend. While certainly an above normal temperature
pattern for January will strengthen, do have concerns on how far
above normal, especially near the lake front where they could be a
stiff onshore component at times, or much of, later this week and
through next weekend.

The surface low by Tuesday morning is expected to be in southwest
Michigan with gradually increasing northwest surface flow. Early
morning fog, some of it possibly dense, should ease from west to
east across the area. Scattered early morning showers also should
depart east with this system, but clouds will stick around with
high confidence. Temperatures will likely be well into the 40s
across northwest Indiana at daybreak, but as the system warm
sector moves east, temperatures there will cool, and elsewhere
likely hold steady.

A northern stream upper short wave disturbance will move east into
Wisconsin tomorrow afternoon and likely tap some of the moisture
left in the wake of the preceding, deeper system. Convective
allowing guidance with solutions that far do indicate isolated to
scattered showers across Wisconsin and northern Illinois during
the late afternoon shifting southeast into the early evening. Have
nudged up pops a tad during that time frame. The column will be
cooling, with a small chance for some brief wet snow before
showers end.

With fairly quick height rises late Tuesday night into early
Wednesday, a scattering of the persistent clouds is expected.
Temperatures will gradually moderate on southwest to south winds
Wednesday and then Thursday, with 40s for highs likely areawide.

A large blocking upper ridge across eastern NOAM, with heights
near record across eastern Canada, will keep systems from an
active Pacific jet slowing as they move into the middle of the
country. Not only will they slow, they look likely to occlude,
keeping precipitation influence potentially minimal over our area,
or at least slowing chances. Do have the chances for rain lifting
north late Thursday night and Friday with a first system. Have
higher chances with the next system predicted by 12Z global models
to move into the Ohio River Valley one week from now. Confidence
in this system and timing is very low.

The forecast 925mb temperatures are well above normal, especially
by Friday into the weekend. Given the multiple systems over the
plains, at the surface an east wind is presently forecast, and at
times fairly stiffly. Given water temperatures in the lower 30s,
lake adjacent counties including downtown Chicago would remain
quite a bit cooler than inland locations that have a good
potential for low 50s one or both days this weekend.

MTF

&&

.AVIATION...
For the 06Z TAFs...

IFR/LIFR conditions in ra/dz will be the primary concern for the
rest of the night and into tomorrow morning.

Low pressure has moved over nern MO and will continue to track
northeast through the night with a warm front lifting north to
near a KPNT to KRZL line. In the cold sector north of the front,
cigs/vis has been a mixed bag ranging from lifr to mvfr, with
somewhat improving conditions in areas of more  convective pcpn
with shra and isold tsra stirring up sfc vis. Cigs have generally
remained lower ifr to lifr and will continue to remain in the
200-500ft range.

No sgfnt improvement is expected through the night. At 04Z, a
relative dry slot is moving over nern IL with one area of more
convective pcpn lifting across Lame Michigan. Under this dry slot,
expect cigs/vis to drop to lift for a short period, with pcpn more
dz than ra/shra. The upper low center will move over the terminal
areas btwn 07-12z. As the cold core of the upper low moves
overhear, there will be a very small chance for some isold ts, but
generally, expect a period of shra with slightly higher vis, but
still ifr.

Following the passage of the low, winds will gradually veer from
sely to swly durg the morning hours and then nwly by late
afternoon. The pressure gradient will be weak and wind speeds
should generally be under 10 kt through the period. As winds veer
from slwy to nwly, increasingly drier air will move into the area,
allowing prevailing vis to very gradually improve, but expect that
cigs will remain arnd 300ft through late afternoon.

&&

.MARINE...
235 PM CST

Fog will continue to be a problem into Tuesday morning across the
southern part of the lake, especially the far south. Webcam
visibility and reports indicate near 1/2SM visibility, and have a
marine fog advisory now issued. There will be times when rain
showers abate some of the fog, especially this evening. However
there is a good possibility of the visibility dropping again
overnight.

Otherwise, this evening will see an increase in easterly winds
across the southern part of the lake. There could be a period
north of Chicago where winds and more so waves briefly reach Small
Craft Advisory criteria, but it should be short lasting. As the
low center moves over and past the lake, winds will turn northerly
across the entire lake, dissipating the fog. Winds turn southwest
on Wednesday and last into Thursday. Confidence in the wind
forecast Friday into the weekend is low, as trends are now
indicating more of an east to northwest wind given multiple slow-
moving low pressures to the southwest of the area.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory...ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-ILZ032-
     ILZ033-ILZ039 UNTIL MIDNIGHT Tuesday.

IN...Dense Fog Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 UNTIL
     MIDNIGHT Tuesday.

LM...Dense Fog Advisory...NEARSHORE WATERS
     UNTIL 9 AM Tuesday.

&&

$$

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