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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
200 PM CST Thursday Nov 20 2014

Short term...
309 am CST

Today through Saturday...

Quiet weather in the near term...with focus mainly on the effects of
the storm system which approaches from the southwest Friday night
and affects the region through the weekend.

Large-scale upper trough over the eastern half of North America will
become more progressive today...propagating off to the East Coast
tonight and Friday. While another fairly strong middle-level short wave
will dig southeast into the trough across the Great Lakes region
today...upstream soundings from dvn and abr depict very dry air in
middle/upper levels with much less cloud cover than in previous days.
So while it will be blustery and cold...there will be more sunshine
than we've enjoyed of late. Surface high pressure will spread into
the area tonight...with diminishing winds and mainly clear skies
producing one more chilly night with strong radiational cooling
conditions in lingering Arctic air mass. Temperatures in the 20s today will
drop off to lows in the single digits in many areas by Friday
morning...though with light winds by sunrise.

The flow aloft will become more westerly Friday...with surface winds
becoming southerly as the high pressure ridge moves off to the east
and surface pressure falls develop across the plains. Guidance has
been very consistent in bringing a small short wave across the lower
and middle Missouri Valley Friday afternoon and into Illinois Friday night.
Increasing southwesterly winds above the surface...and resulting
warm/moist advection and isentropic ascent...will bring the
potential for light precipitation to develop northward into the
forecast area Friday night. This presents a problem...as
temperatures warm more quickly aloft and sets the stage for a period
of light freezing rain Friday night into early Saturday morning
before surface temperatures rise above freezing. Forecast soundings
indicate elevated warm layer temperatures of +3 to +7 degrees develop above
a 1000-1500 foot layer of sub-freezing air at the surface...making
freezing rain and perhaps a little light sleet the likely initial
precipitation type. Various models in pretty good agreement in depicting
surface temperatures warming to/above freezing generally along/south of the
I-80 corridor overnight...with areas farther north rising above
freezing during the morning hours. While quantitative precipitation forecast amounts are light with
only a few hundredths produced...any ice accumulation will likely
make for some slippery travel conditions as glaze forms on some
roads and many parking lots and sidewalks. Temperatures are expected to
rise into the 40s County Warning Area-wide by afternoon however...with any lingering
light precipitation falling as some occasional light rain during the day.

Ratzer

&&

Long term...
309 am CST

Sunday through Wednesday...

Meanwhile...a fairly vigorous upper trough will have moved ashore
across the southwestern states...and is prognosticated to move east into
Texas through Saturday afternoon before ejecting northeastward as a
strong negative-tilt short wave later Saturday night and Sunday.
Models continue to depict an associated rapidly deepening area of
surface low pressure lifting into northern Illinois by late Sunday
afternoon. Continued low/middle level moist advection coupled with
increasing large scale ascent ahead of the approaching low looks to
produce fairly significant rainfall across the area Sunday...with
models generally indicating amounts approaching 1.00" before rain
tapers off Sunday night as the low lifts north of the area. This
will have to be watched as ground is currently frozen...at least in
the top few inches...which may result in some run-off problems with
some flooding potential and quick river/creek rises.

Models continue to indicate a merging of the short wave with a
digging northern stream trough over the upper Mississippi Valley
Sunday night...with the low continuing to deepen to below 980 mb
across the northern lakes by 12z Monday. Very windy conditions look
to develop from the west in the tight gradient associated with the
deepening low...with 35-40 miles per hour gusts possible Monday. As colder air
gets pulled into the region on these gusty west winds...any
lingering light precipitation will transition from light rain to rain/snow
across the western County Warning Area by early Monday morning...and to light snow
showers across the area Monday night into Tuesday as a series of
short waves rotate about the closed low to our north. 00z runs of
both the GFS and European model (ecmwf) kick the trough off to the east Tuesday night
into Wednesday...with weak surface high pressure bringing dry
weather and decreasing winds by mid-week...though with continued
below-normal temperatures.

Ratzer

&&

Aviation...

//Ord and mdw concerns...updated 20z...

* gusty west winds through this afternoon with maximum gusts 25-30 knots.

Kmd

//discussion...updated 18z...

Some MVFR cumulus trying to form this afternoon but it will have a
hard time getting more coverage than scattered. High pressure will
be moving overhead tonight...thus skies clear tonight with winds
easing from west to east. Wind direction trends to south tomorrow...peaking
around 10 knots or so...possibly with a slight southeast component. Expect
an increase in high clouds through the day.

Kmd

//Ord and mdw confidence...updated 20z...

* high confidence in all elements

Kmd

//outlook for Ord/mdw for 00z Saturday-12z Thursday...updated 12z...

Friday night...schc wintry mix becoming a chance of freezing rain
overnight. IFR ceilings likely. South winds around 10 knots.

Saturday...chance -ra/freezing rain becoming rain and fog overnight.
IFR likely. South-southwest winds 10-15 knots.

Sunday...rain. IFR likely. Gusty southeast winds.

Monday...chance -rasn becoming chance -sn. MVFR ceilings possible. Strong southwest
winds with gusts around 30 knots possible.

Tuesday...schc -sn during the day. MVFR ceilings possible. Gusty west
winds.

Wednesday...VFR. West winds becoming southwest.

Jee

&&

Marine...
200 PM CST

Low pressure over southcentral Quebec will continue to slide east.
An elongated ridge of high pressure stretches from the northern
plains southeast through the Tennessee Valley...and will slowly move
east towards Lake Michigan tonight. High pressure will bring much
lighter winds to Lake Michigan by Friday morning...however high
pressure is expected to continue to slide east towards the middle
Atlantic region by late Friday night. As high pressure departs Lake
Michigan...winds will turn southwest to south Friday night. An area
of low pressure will slide east across Manitoba towards James
Bay...with a tightening gradient expected to develop by late Friday
night across central/Northern Lake Michigan. Have held in place the
gale watch...however this may need to be upgraded to a Gale Warning.
The main concern is that with much warmer air poised to arrive
Friday night...that this may create less mixing to the surface of the
higher wind gusts. Guidance suggests gales may be between 34-37kt so
along the marginal lines.

During the day Saturday the gradient will diminish once again and
turn southeasterly...however this will only be temporary as a more
potent low pressure system is poised to slide southeast from central
Canada Saturday night. A weaker low pressure system will develop
across the Southern Plains early Sunday...then quickly lift north
towards Lake Michigan. Guidance differs on the exact strength and
path of this low...but models indicate the low will deepen to around
975 mb/28.8 inches at the north end of the lake Monday afternoon.
Expecting southeast winds to increase to at least 30 knots through
Sunday evening and then become southwest behind the low. Gales are
expected by late Sunday night and depending on the path and strength
of the low...storm force winds may be included in future forecasts.
Guidance indicated a brief period of high end gales over the
northern end of the lake Monday afternoon. The low moves over
Quebec Tuesday with west winds diminishing Tuesday morning.

Beachler

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...Indiana nearshore waters until 10 am Friday.

Small Craft Advisory...Illinois nearshore waters until 4 am Friday.

Gale watch...lmz261-lmz362-lmz364-lmz366-lmz563-lmz565-lmz567-
lmz669-lmz868-lmz870...6 PM Friday to 9 am Saturday.

&&

$$

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