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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
549 am CST Tuesday Mar 3 2015

Short term...
300 am CST

Through Wednesday...

Main forecast concerns this morning will be dealing with precipitation type
issues with warm front lifting across the region early this morning.
Surface low is in place across eastern Colorado and western Kansas
early with axis of strongest pressure falls sprawled across the middle
Missouri Valley into southeastern Minnesota. Middle level warm
advection is ramping up across the Midwest and will continue to
increase across northern Illinois over the next few hours...then
should start to spread north and northeast of the forecast area from
middle morning on. Widespread precipitation noted on radar already this
morning with reports of freezing rain and sleet upstream. Best
chance for any snow accumulations with this system will be along and
north of I-80...but particularly closer to the WI state line which
will remain marginally cold enough aloft for snow through middle
morning. H9-800 mb layer maximum wet bulb temperatures will rise above freezing
south of the I-80 corridor by around 12z...and across the entire County Warning Area
by 15z at which point sleet or freezing rain can be expected for
areas with surface temperatures still below freezing. Guidance is
initializing too cold with surface temperatures this morning...and some
models such as the NAM seem to hold on to freezing temperatures too
long...so feel the threat for freezing rain may be somewhat
limited...though any ice accumulations will cause issues
particularly with the timing during the morning commute. Areas south
of I-80 should see surface temperatures rise above the freezing mark 13-15z
and through 15-17z the freezing line should lift up to the state
line marking the end of the freezing rain threat. Meanwhile...late
this morning into the early afternoon middle level dry slot will punch
in from the west with precipitation tapering to a light rain or drizzle
before ending altogether this evening.

High pressure will begin to build across the northern plains tonight
with a moderate northwest gradient and cold/dry advection in place
tonight. Steep low level lapse rates develop and there is some
indication there may be a few light flurries that develop this
evening and early overnight before drier air moves in...but for now
chances seem too low to include mention in forecast. Cold advection
will drive temperatures into the single digits in the northwest
County Warning Area...and upper teens to low 20s in the southeast. Temperatures will
not rebound much on Wednesday with highs in the high teens to lower

Deubelbeiss

&&

Long term...
300 am CST

Wednesday night through Monday...

Fairly quiet weather expected from midweek through the weekend. High
pressure will build from the northern plains Wednesday night to
downstate Illinois Thursday. Colder air will accompany the high with
models continuing to advertise 850 mb temperatures around -20c Wednesday
night. High pressure centered to our west will allow western
counties to begin to clear and winds to decouple allowing temperatures to
radiate below zero...though a modest gradient and lingering cloud
cover in eastern counties may prop up temperatures above zero. High
pressure shifts to our east later on Thursday and return flow and
warm advection will result in temperatures leveling off or rising Thursday
night. This is in advance of low pressure over northern Ontario
dragging a cold front across the upper Midwest. Temperatures should
be able to rise above freezing Friday and Saturday before the cold
front passes through northern Illinois Saturday afternoon/evening.
Front is pretty moisture starved so maybe a few flurries if it
doesnt come through dry.

Deubelbeiss

&&

Climate...

A few record low and and record low maximum temperatures are in
jeopardy on Wednesday and Thursday.

Here are the current records for Rockford and Chicago. The ones that
stand the best chance to be reached or broken have a Star (*) next to
them.

Wednesday 3/4 Thursday 3/5
Rockford

Record low-11 (2002)-11 (2002)
record low maximum 14 (2002)* 17 (1960)*

Chicago
record low-12 (1873) 0 (1978)*
record low maximum 17 (1890)* 12 (1901)

Kmd/kjb/bmd

&&

Aviation...

//Ord and mdw concerns...updated 12z...

* snow...sleet and freezing rain this morning eventually turning
to rain/drizzle as temperatures rise above freezing late
morning/midday. Generally IFR/LIFR conditions this morning with
conditions improving to MVFR ceilings and higher visibilities late afternoon and
evening.

* Southeast winds becoming gusty around 20 knots from the south by
late morning and southwest this afternoon. West-northwest winds
tonight remain gusty.

Ratzer

//discussion...updated 12z...

Area of precipitation has overspread the terminals early this
morning...with some moderate snow at times from Ord
north/northwest. Northern terminals...rfd...dpa...Ord will likely
continue to be the focus for heavier snow/sleet this morning with
occasionally lower visible/ceiling conditions resulting. Farther south
some drying in the middle-levels is resulting in lighter precipitation which
will affect mdw/gyy...and may result in more of a freezing rain/fzdz trend
rather than snow for much of the morning at those locations.
In any case...warming aloft will eventually turn precipitation from snow
to sleet/freezing rain or drizzle over the next few hours...and
eventually to rain/drizzle as surface temperatures rise above freezing
later this morning. Precipitation is expected to end later this afternoon
as winds shift west and drier air filters in.

Speaking of winds...winds are currently southeast around 10 knots but
will increase 10-15 knots with gusts around 20 knots by late morning...
backing to the south and southwest into this afternoon as low
pressure moves across Wisconsin. Trailing cold front will shift
winds west this evening...with gusts 20-25 knots lingering into
tonight.

Ratzer



//Ord and mdw confidence...updated 12z...

* medium in timing of precipitation type changes and ceiling/visible impacts.

* High in wind trends...and in MVFR or better conditions by this
evening.

Ratzer

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

Wednesday night through Friday...VFR.

Saturday through Monday...MVFR ceilings possible.

Ratzer

&&

Marine...
214 am CST

Main focus for the Lake Michigan forecast remains in the near
term...as south winds increase quickly this morning in advance of an
area of low pressure moving quickly from the plains to the western
lakes by this evening. South winds to 30 knots...with a few gale force
gusts to 35 knots...are expected on mainly the southern portion of the
lake. The low will trail a cold front across the lake early this
evening which will shift winds to the west initially and more
northwest overnight and Wednesday...as the gradient continues to
support 25-30 knots winds as high pressure slowly builds from the
northern plains to the Ohio Valley. The high will pass southwest of
the lakes Thursday and Thursday night allowing winds to ease and
back southwest by early Friday. The gradient will tighten up once
again Friday however...between the high retreating to the southeast
and an area of low pressure moving north of the lakes across
Ontario. 30 knots southwesterlies likely Friday...before the low trails
a cold frontal trough into the area early Saturday and allows winds
to relax a bit and shift to the west.

Small craft headlines remain up for the Illinois and Indiana
nearshore today through Wednesday evening for the 25-30 knots winds.
Recent polar-orbiting satellite imagery from Monday afternoon shows
a good portion of the Illinois shore to be mainly open water or thin
ice floes...thus have included wave forecast for Illinois waters
while Indiana shore appears to remain largely ice-covered. The same
GOES for western portions of Northern Lake Michigan...which appear
to have a fair amount of open water.

Ratzer

&&

Lot watches/warnings/advisories...
Illinois...Winter Weather Advisory...ilz003-ilz004-ilz005-ilz006-ilz008-
ilz010-ilz011-ilz012-ilz013-ilz014-ilz019-ilz020-ilz021-
ilz022-ilz023-ilz032-ilz033-ilz039 until noon Tuesday.

In...Winter Weather Advisory...inz001-inz002-inz010-inz011-inz019
until noon Tuesday.

Lm...Small Craft Advisory...nearshore waters
until 6 PM Wednesday.

&&

$$

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