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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1200 PM CST Monday Mar 2 2015

1146 am CST

Have echoed the previous shifts concerns for mixed precipitation
during the Tuesday morning commute...and have issued a Winter
Weather Advisory for the entire area late tonight through Tuesday
morning. Models are still showing precipitation beginning as snow
north of Interstate 80...and some form of snow...sleet or possibly
even freezing rain in the far south. It could be a pretty good
burst of wet snow initially before the transition begins.

A warm front will lift through the area during the morning hours.
This is the period where models a showing a more significant push
of precipitation. We are coming out of a pretty cold air
mass..and dewpoints are still very low across the area...with
dewpoints above freezing not very close. Temperatures look to
remain below freezing through at least middle morning area wide.
Modest precipitable water values in excess of 0.75 inches coupled
with a 35-40 knots low level jet feeding into the low level frontal
zone would suggest modest precipitation rates during this time when
the low level (925-850 mb) temperatures are heading above
freezing. Some of the recent NAM/GFS was slightly cooler in the
thermal profile for this period suggesting that sleet may
dominate and if precipitation rates continue a bit heavier this
may slow the low level warming...but the signal is still there
for freezing rain accumulation. Snow amounts may be higher near
the Wisconsin border where the low level warming may be a touch

More details to follow this afternoon.



Short term...
250 am CST

Through Tuesday...

High pressure centered over the middle Missouri Valley this morning
will translate east across northern Illinois this afternoon and then
off the Atlantic coast by early Tuesday. Meanwhile...closed upper
low within a deep trough is in place across the West Coast and lead
wave is prognosticated to eject northeast from the Desert Southwest this
evening with a surface low developing over the Colorado Front Range
which will lift into the upper Midwest Tuesday. This will be a
challenging system with the potential for modest snow accumulations
on the front addition to light ice accumulations...before
precipitation changes over to all rain. These details of course will
hinge on the track and strength of the low which models continue to
struggle with. The 00z European model fits well withing the best
clustering of the gefs and has shifted north from previous runs...a
move the GFS made with earlier guidance...though the latest run is
north of the gefs mean. Sref is even an farther northern outlier.
Continue to lean heavily on the European model (ecmwf) guidance but with a northern
stream system pushing east across the Canadian prairies and high
pressure along the East Coast...wouldnt be surprise to see farther
shifts north which would likely lessen the impacts from frozen
precipitation type.

Current expectations are that precipitation will begin as snow with
light accumulations possible before middle levels warm enough for a
transition to sleet and freezing rain during the morning. As
surface/boundary layer temperatures continue to warm...precipitation should change
over to rain for most if not all of the County Warning Area by afternoon. Given the
rapid warmup...any snow/ice accumulations would likely quickly melt
off. Unfortunately the best chances for frozen precipitation will be right
around the morning commute. Middle level dry slot punches into the
region middle afternoon into the early evening and precipitation will
likely transition to a light rain or drizzle...then most of the
remaining precipitation should start to push east of the forecast
area through the evening.



Long term...
250 am CST

Tuesday night through Sunday...

High pressure will gradually build from the northern plains Tuesday
night to the Midwest Thursday then to the middle-Atlantic coast Friday
into Saturday. Ridging will be in place across northern Illinois
much of the period bringing an extended period of drier weather. The
high will be accompanied by colder air...with 850 mb temperatures pushing -20c
moving across northern Illinois Wednesday night and Thursday
morning. This will be the coldest morning in the forecast with near
or below zero lows expected. A low pressure system will move across
northern Ontario Friday with a thermal ridge overspreading northern
Illinois resulting in warmer conditions by the end of the week into
the weekend.




//Ord and mdw concerns...updated 18z...

* light/variable winds this afternoon before flopping southeast by
early evening. South-southeast winds becoming gusty 20-25 knots by
Tuesday morning.

* Snow...mixing with sleet then changing to freezing rain/rain
early Tuesday morning. VFR conditions to quickly deteriorate to
IFR/LIFR in precipitation.


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

It will continue to be a quiet weather day across the area today. A
surface ridge of high pressure pushing across the area this
afternoon will result in a period of light and variable
winds...though the winds should settle into a southeasterly direction
by early this evening. Otherwise VFR conditions expected.

The weather will become much more problematic by daybreak Tuesday.
Winds will increase out of the southeast and become gusty during the
morning. Snow will also develop over the area just prior to
daybreak...before mixing with sleet and changing over to a period of
freezing rain by middle morning Tuesday. It currently appears that
about a 1 to 3 hour period of snow and sleet will occur from roughly
11 UTC through 13 to 15 UTC. During this period IFR visible in snow and
lowering ceilings are likely. Ceilings will continue to deteriorate into the
IFR and LIFR categories during the morning as warmer air aloft
begins to change the snow and sleet over to some freezing rain.
Surface temperatures should also be on a warming trend during the
morning...and are expected to warm above freezing by any
lingering precipitation after this time will likely no longer produce a
freezing threat.


//Ord and mdw confidence...updated 18z...

* medium-high in precipitation timing/trend early Tuesday. Medium in
ceiling/visible impacts.

* High all other elements.


//outlook for Ord/mdw for 00z Wednesday-12z Monday...updated 12z...

Tuesday night...MVFR ceilings. Gusty west winds.

Wednesday...MVFR ceilings.

Thursday and Friday...VFR.

Saturday and Sunday...MVFR ceilings possible.



217 am CST

High pressure spreading east from the plains eastward to the Midwest
and Great Lakes today will allow westerly winds to diminish quickly
this the gradient weakens with the approach of the
surface ridge axis. Winds will back to the south-southeast by
tonight as the high pulls away to the east...and will increase into
the 25 knots range by Tuesday morning as an area of low pressure lifts
northeast from the plains to the middle-Missouri Valley. The low is
expected to track northeast across Wisconsin and Northern Lake
Michigan Tuesday...with winds further increasing into the 30-35 knots
range on the southern 2/3 or so of the lake during the day. Current
model runs are not as bullish in deepening the low as it approaches
and it appears that a brief period of marginal gale force winds
midday-afternoon Tuesday. Will continue to mention gale force gusts
but will hold short of a Gale Warning at this time. Winds will then
ease just a bit by Tuesday evening as the trailing cold front moves
across the lake...but are expected to remain near 30 knots as the shift
to the west-northwest Tuesday night as cold air spreads across the
lake and another large high pressure system builds from Canada
southeast across the plains. Winds look to hold in the 25-30 knots
range Wednesday as the center of the high slowly tracks
southeast...eventually moving into the lower Ohio Valley by late
Thursday. This will allow winds to gradually diminish and back to
the southwest Thursday night. Gusty south-southwest winds return
Friday as the high moves off to the southeast and low pressure
tracks north of the lakes.

While most of the Illinois/in near shore waters appear ice covered per
satellite and webcam imagery...will carry a Small Craft Advisory
Tuesday/Wednesday with 25+ knots winds during that period.



Lot watches/warnings/advisories...
Illinois...Winter Weather Advisory...ilz003-ilz004-ilz005-ilz006-ilz008-
ilz022-ilz023-ilz032-ilz033-ilz039...3 am Tuesday to noon

In...Winter Weather Advisory...inz001-inz002-inz010-inz011-inz019...3
am Tuesday to noon Tuesday.

Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...6 am Tuesday to 6 PM Wednesday.



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