Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211128

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
528 AM CST Thu Feb 21 2019

234 AM CST

Through Friday...

In spite of southwest flow aloft and a sharp upper jet overhead, we
will get a bit of breather in the weather department in the short
term. At the surface, northern Illinois and northeast Indiana are
in the gradient zone between low pressure across Lake Superior
and high pressure the lower Mississippi and Ohio valley. The low
will will quickly exit to the east coast today which will slacken
winds today as the high inches closer. With the southwest flow/jet
aloft, cirrus clouds will continue to stream overhead at times
today and tonight. Seasonal high temperatures will reach the 30s
northwest, where cooler air is filtering in behind our departing
wave, to near 40 south.

The high will spread overhead tonight resulting in a bit more
clearing overnight. This will make for a chilly night with readings
in the lower teens toward the Rockford area to the lower 20s
across northwest Indiana. The high will shift to the eastern
Great Lakes and much of the northeast US on Friday. We will still
feel its influence with dry easterly winds, and therefore just
expect increasing clouds ahead of the next upper low across the
desert southwest.



255 AM CST

Friday Night through Wednesday...

A strong low will impact the region over the weekend bringing
primarily rain, but also some windows of wintry precip along with
strong winds.

A deep longwave trough will be in place over the Great Basin to
close out the week as a strong shortwave rounds the base of this
trough Friday evening. The shortwave will take on a negative tilt
as it lifts across the Great Plains Saturday with strong
cyclogenesis occurring at the surface. A 990mb is progged to be
near the middle Missouri Valley by Saturday evening, further
deepening to below 980mb as it lifts into the Great Lakes Saturday
night. There remains some uncertainty with respect to the exact
track and strength of the surface low, but generally expect it to
track over or clip northern Illinois Saturday evening lifting to
northern Lake Michigan Sunday morning. The GEM and NAM are
colder/southerly solutions while the GFS and ECMWF are
warmer/northerly low track solutions. Run-to-run trends have been
inconsistent so far making hard to pin down details which will
impact precip type. There is high confidence in warm temperatures
aloft at the onset of precipitation Friday night with the initial
warm advection wing which will drive 850mb temps several degrees
above freezing. Have lower confidence in surface temperatures,
though models continue to suggest northern portions of the CWA,
particularly north of I-88, will be below freezing at the onset of
precip. Will continue to mention freezing rain potential north of
the low track of the southern model solutions, but wouldn`t at
all be surprised if the freezing rain threat stays northwest of
the CWA as models will often struggle with the magnitude of the
warm advection. Precipitation arriving late Friday night (after
09Z) and non-diurnal trends overnight may be enough to mitigate
the threat for much of the CWA.

The remainder of Saturday should be wet and breezy, but otherwise
mild with temps warming into the 40s across most of the CWA with
a few spots south of I-80 tagging the 50 mark. Expect periods of
rain and later drizzle as the dry slot punches across the area
late Saturday afternoon into the evening.

A cold front will sweep across the region Saturday night. Strong
cold advection will promote efficient mixing of stronger winds
aloft, especially late Saturday night. In addition, strong surface
height rises will overspread the CWA. Both the NAM and GFS show a
corridor of 10mb/3hr pressure rises pushing across the CWA behind
the front Sunday morning. Given these factors, seems probable
winds will be near or in excess of wind advisory criteria (gusts
>=45 mph). GFS advertises winds at the top of the mixed layer of
around 50kt, and if we do see shallow convection behind the front,
it`s possible we could mix down some of these higher magnitude
winds nearing or exceeding wind warning criteria. Looks to be very
windy all day Sunday. These strong winds will help drive down
temperatures through the day. Expect calendar day highs at
midnight Sunday AM with temps falling into the 20s Sunday evening
and single digit/teens Sunday night.

Upper levels trend to a more zonal pattern/broad flat troughing
early through the middle of next week. Temperatures will undergo a
gradual warming trend Monday into Tuesday, moderating closer to
but still below seasonal norms Tuesday. A lower amplitude Pacific
wave is still being advertised by the models Tuesday/Tuesday night
but there remain differences in the track. Expect accumulating
snow if not over then near the local area should this wave stay
the course. Temperatures fall again behind the wave and it looks
like we remain in a colder than normal pattern the remainder of
the week.



For the 12Z TAFs...

Southwest winds will shift westerly this morning. Winds will continue
around 10-15 kts today as low pressure will quickly shift across
the eastern Great Lakes today and high pressure will also head
toward the area from the southeast. The high will shift overhead
of the terminals tonight, which will mitigate some of the mostly
passing higher cloud cover through the period.

Wind speeds will diminish under 10kts this evening and become
northwest...eventually shifting light northeast early Friday
morning, and eventually more easterly with the approach of the
high pressure center. Some patchy fog may develop away from the
main Chicago terminals overnight. Additionally, some clouds
VFR/MVFR clouds over the lake may sneak into ORD/MDW once the
flow turns off the lake late tonight into early Friday.



LM...Small Craft Advisory...nearshore waters
     until 9 AM Thursday.




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