Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191123

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
523 AM CST Fri Jan 19 2018

235 AM CST

Through tonight...

Benign weather in the short term with the only real forecast
challenge being temps as warm air continues to attempt to surge
into the area, but being partially thwarted by the snow pack.

GOES-16 imagery from Thursday afternoon nicely depicted the
rapidly eroding snow cover over much of MO, IA, east into portions
of northwest and central IL. The NOHRSC`s (National Operational
Hydrologic Remote Sensing Center) snow analysis indicates the 2"
snow cover line skirts our southwestern CWA, with a fairly good
likelihood that these areas and points west and south will lose
their snow cover today. Have attempted to adjust high temps today
to go warmer in the lighter snow cover areas of western LaSalle,
Lee, and Livingston Counties. In addition, also went a bit warmer
over the highly urbanized areas of Chicago where snow cover was
also analyzed largely less than 2" and where more buildings
provide for a lower albedo and a better chance of warming up.
Elsewhere, a fairly stout snow pack should help keep temps in the
mid to upper 30s today, but without the arrival of higher
dewpoints until later in the weekend, the snow melt should be a
bit more gradual.

- Izzi


235 AM CST

Saturday through Thursday...

No big changes in the pattern heading into Saturday, though
guidance is in good agreement in beginning the northward transport
of higher theta-e air mass, likely resulting in a northward moving
stratus deck. A good consensus exists that stratus will arrive by
Saturday evening at the latest with overcast conditions likely to
persist until the cold front moves through Monday. Both the GFS
and NAM have a pretty solid looking drizzle sounding Saturday
night with guidance even attempting to squeeze out a hundredth of
an inch here and there. Have maintained a light rain/drizzle
mention Sat night into Sunday. Fog chances look to increase later
Saturday night and particularly into Sunday and continued carrying
fog in the grids through this time frame.

Sat night into Sunday, cyclogenesis will take place over the
central/southern high plains, with warm frontogenesis occurring
east from this low into central IL. The result will be southerly
winds backing to easterly over much of the CWA, which combined
with the cloud cover should limit the temp rise during the day
Sunday. This warm front is expected to lift north as the sfc low
track northeast toward the Lower Great Lakes region, though
guidance has wavered a bit with timing/track/intensity of this
system. The latest ECMWF is the slower, weaker, and farther south
and would keep much of our central/northern CWA in the fog,
drizzle and chilly air Sunday night before the warm front lifts
north briefly early Monday. The NAM and GFS are a bit faster with
warm FROPA and for now generally followed a blend of guidance and
do show markedly rising temps Sunday night into Monday morning. As
the higher dewpoints arrive Sunday night, look for what is left of
the snow cover to quickly melt.

The intermittent drizzle/very light rain will likely build into
steadier and heavier rain Sunday night into Monday as the main
system lifts northeast into the region. Cold front will move
across the area Monday afternoon/evening time frame with colder
air rushing into the area. ECMWF is farther south with the track
of the system and would keep the mid level dry intrusion farther
south and would provide for more of a chance of some snow showers
in the area Monday night vs the GFS and GEM which would tend to
favor snow on the back side of the system staying farther north in
WI. Still plenty of time to watch things, but the southward trend
in the ECMWF is noteworthy and often times the ECMWF leads the
path toward changes in guidance like this one.

Dry weather with fairly seasonable temps expected in the wake of
this system Tuesday into Wednesday. By the later part of next
week, medium range models dig another trough into the western
CONUS which would support downstream ridging and another mild
spell late in the week possibly into the first part of next
weekend. Assuming no significant snow cover is left behind Monday
night/Tues morning with this first system, then the late week warm
up would likely not be impeded by snow cover and could be more
pronounced than this current warm up.

- Izzi


For the 12Z TAFs...

The aviation forecast concerns for today and tonight are gusty
south-southwest winds, with late this afternoon and evening being
the period with the strongest.

South-southwest winds will show more common gusts by mid-morning,
with direction expected to remain between 200 and 220 degrees
through today. By mid-late afternoon, sustained speeds and gusts
are likely to increase by at least a few knots as a weather
system passes well to our north. While confidence in that increase
is high and that the peak should last through the evening,
confidence is somewhat low on just how high surface gusts will be.
There is a chance of gusts occasionally reaching 30-33 kt between

Even with the higher surface winds and gusts this evening, there
is also the possibility for low-level wind shear (LLWS), helped
by a strong temperature inversion aloft between 1000 and 2000 ft.
Southwest wind speeds above this inversion are forecast to be 50+
kt, possibly even close to 60 kt. Given the gust magnitude
currently in the TAF, feel that true LLWS criteria may be
difficult to reach, but it is worth noting the strong channel of
winds aloft.



1258 AM CST

An active pattern continues over the lake into next week. A
quick-moving but fairly strong Alberta Clipper system will cross
Ontario today and this evening. A tightening pressure gradient
will result and enough so for Small Craft Advisory winds over the
Illinois and Indiana nearshores by late morning and for gales over
the open water by mid-afternoon. These conditions will persist
through this evening before easing late tonight.

The associated cold front from this low will dip southward into
the northern part of the lake and weaken on early Saturday. A
stronger Plains low will develop Sunday afternoon and move
northeast, lifting a warm front over the lake on Monday.
Increasing temperatures and dew points over the cooler waters, as
well as added moisture from snow melt to the south of the lake,
may increase the likelihood of fog across the southern part of the
lake late Sunday night into early Monday. Confidence on this is
low at this time. Winds ahead of this system may touch near gales
across the north and central late Sunday night into Monday, but
the better widespread potential for gales it on the system
backside later Monday night into Tuesday. With quite a bit of
variance in model guidance on where this low cross the lake, and
just how strong it will be (though it should be gradually
deepening), confidence in gales at this time is low.



LM...Gale Warning...LMZ777-LMZ779...2 PM Friday to 3 AM Saturday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...9 AM Friday to 3 AM Saturday.




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