Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191931

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

129 PM CST

Through Saturday...

Southwest winds are occasionally gusty this afternoon as a deepening
low pressure system passes through southern Ontario. Sunshine and
warmer temperatures continue to eat away at the snowpack with
little snow left across southwestern portions of Livingston and
Lasalle counties. Deeper snowpack across northwest Indiana is
holding temperatures down a bit more in these areas.

The low will strengthen further this evening while it will move
into Quebec. Expect winds to remain gusty and even increase this
evening area wide. Gusts to 35 mph are not out of the question.
Otherwise, expect continued higher clouds and generally mild
temperatures in the mid 20s to lower 30s.

Concern will shift to the arrival of an area of stratus currently
across the Arklatex region. Initial trajectories and forecast low
level steering flow initially keeps the stratus south of I-80
through at least a portion of the morning. We do expect an
increase in coverage by noon, and then overspreading most of the
area by tomorrow evening. In spite of the clouds, temperatures
will rebound close to today`s readings in the upper 30s to low
40s. Forecast soundings suggest fog/drizzle will hold off until
the evening as well.



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 18Z TAFs...

- Gusty southwest winds lingering and possibly increasing this
- Possible LLWS this evening
- Stratus arrival times and ceilings on Saturday

Southwest winds are occasionally gusty this afternoon, but expect
sustained speeds and gusts to increase by at least a few knots as
a weather system passes through southern Ontario. 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 22Z-04Z. If this is not the case, then
there will likely be a short period of LLWS centered around the
01z- 04z window. Either way, it will be quite windy out of the
southwest late this afternoon/evening in the lowest 2k feet.

Concern will shift to the arrival of an area of stratus currently
across the Arklatex region. Initial trajectories and forecast low
level steering flow initially keeps the stratus southeast of the
terminals through mid morning, but then expect an increase in
coverage by noon, and then overspreading most of the area by
tomorrow evening. Confidence on specific arrival time is only
medium. Cigs should start MVFR though initially, with again only
medium confidence that IFR will hold off until the evening.



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 until 3 AM Saturday.

     Small Craft Advisory...nearshore waters
     until 3 AM Saturday.




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