Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181920

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
120 PM CST Thu Jan 18 2018

120 PM CST

Through Friday...

The clipper system to our north will quickly exit the region
tonight. Southwest winds have ushered in some more seasonal
temperatures getting close to the freezing mark. High pressure
aloft will shift across the Great Lakes tonight. At the surface
high pressure will to the southeast USA, while a second low will
dig into south central Canada. This will maintain breezy southwest
winds through tonight and Friday. Expect lows to hold near or even
above yesterday`s highs, with lows tonight mostly in the 20s.
Continued warm advection on Friday will steer high temperatures up
near 40. We will have some higher clouds due to the low passing by
to the north and from a system well out in the west.



336 AM CST

Saturday through Wednesday...

The main forecast focus in the long term period is on a deep low
pressure system which is expected to emerge from the Plains this
weekend, and track into the western Great Lakes region through
Monday. Medium range model guidance remains in general agreement
in tracking the surface low just northwest of the forecast area,
keeping the WFO LOT cwa in the warm sector, and resulting in
primarily snow melt, drizzle/fog and rain event until colder air
wraps in late Monday behind the departing low.

A deep upper trough is evident off the West Coast of the CONUS
early this morning in GOES vapor imagery. Several aircraft obs
depict a strong jet streak, of greater than 140 kts, translating
into the base of this trough over the Pacific. Model guidance
amplify this trough as it moves ashore over California and Baja
early Saturday, with the trough further deepening and forming a
closed upper low as it emerges from the south-central Rockies
Sunday. Global models are in pretty good agreement with the
evolution, though some spread in speed and placement develop as
the system lifts into the western Great Lakes region during the
Monday-Monday night time frame. Given the strength of the upper
trough, have some preference toward the slower ECMWF and GEM
depictions, which are clustered closer together than the faster
GFS. Surface low development is indicated in the lee of the
Rockies Saturday night, with the low then deepening on a general
track across Iowa by early Monday, and passing near or just north
of northwest IL later in the day/evening. This is generally along
the location of a low level baroclinic zone which lays out along
that axis in association with a cold front which settles south
from a northern stream disturbance Friday night.

Based on the slightly slower model representations, and the
aforementioned surface low track, low level flow looks to remain
primarily southwest across the forecast area Saturday, with a
gradual increase in low level moisture as the western Gulf gets
tapped. A surge of above freezing dew point air is progged to
spread into the cwa Saturday night, with forecast soundings
suggesting the development of low clouds, fog and drizzle as the
milder, moist air spreads over our lingering snow cover. Surface
temps should remain above freezing Saturday night, though it may
be close along the IL/WI border for a time which could support
some brief patchy freezing drizzle there. The warm front lifts to
the IL/WI border Sunday, with the entire area expected to warm
into the 40`s, before more widespread rain develops Sunday night
into early Monday as large scale forcing increases ahead of the
approaching low. Forecast soundings even depict some weak elevated
instability associated with the low-level theta-E axis ahead of
the cold front Monday, which could even support an isolated rumble
or two of thunder. Temps, which hold in the 40`s Sunday night,
could reach/exceed 50 degrees Monday ahead of the front. With the
slower ECMWF/GEM progression, colder air slowly wraps in behind
the cold front late Monday, which would support a transition from
rain to snow or snow showers, which would persist in some fashion
into early Tuesday before the system finally exits the area. No
real punch of real cold air is depicted however, with temps around
average through mid-week.

Will have to monitor the potential for flood issues as this
system unfolds as melting of our snow pack (which may have up to
0.50 inches of water equivalent in places), and model qpf of
0.50-1.00 inches, could cause some issues with frozen ground.



For the 18Z TAFs...

Only significant aviation concern is a window for LLWS this

Southwest winds with gusts in the 20s will continue this afternoon
with high pressure parked over the Tennessee Valley and low
pressure dropping southeast through the upper Great Lakes. This
evening the gusts may drop off for a time while winds around
2000-2500 feet hold around 40 kt or so. It is sort of a marginal
setup but certainly possible for some LLWS periods depending on if
gusts continue this evening. Will leave it out of the TAF for now.
The high will shift off to the southeast on Friday while another
low will drop into south central Manitoba. This will maintain
breezy southwest winds on Friday with continued high cloudiness.



300 AM CST

A broad ridge will remain in place over the Deep South through the
remainder of the week and into the weekend while a series of lows
move east across Canada. Expect a west to southwest gradient to
prevail over Lake Michigan through at least Saturday. Low pressure
will move from Alberta this evening to near James Bay Friday
evening. Winds will pick back up to gale force across the Lake
Friday afternoon through Friday night, and should ease back below
gale force during the predawn hours Saturday. Will hoist a gale
watch this morning for the open waters to cover this time frame.

Another low will take shape over the central Great Plains over the
weekend then lift over or near Lake Michigan on Monday. Models
continue to have modest differences with timing, strength, and
the exact track of the low so the details are still a bit unclear.
In general though, winds are expected to turn south to southeast
on Sunday ahead of the low. A more northerly low track will also
favor high dew point air lifting into the region resulting in
stable conditions over the lake and likely fog. Gales or near
gales are possible Sunday and Sunday night. As the low shifts east
of the lake on Monday, gales then appear likely and could
potentially be strong as cold air spreads back over the region
behind the deepening low.



LM...Small Craft Advisory...nearshore waters
     until 4 PM Thursday.

     Gale Watch...LMZ777-LMZ779...noon Friday to 3 AM Saturday.




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