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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
425 am CDT Friday Apr 18 2014

Discussion...
352 am CDT

Period of quiet/dry weather through Easter Sunday puts focus of
forecast on temperature trends through the weekend...warming into the 70s
after cool northerly winds today. Precipitation potential appears to be
mainly later Sunday night-Monday. Progressive pattern continues next
week with another potential warm-up middle-late in the week.

Isolated showers were occurring across the far southeastern part of
the forecast area early this morning...south of about a
Pontiac-Kankakee-valpo line. These showers were occurring in a
region of low level f-general and weak moisture transport associated
with the elevated baroclinic zone behind a departing cold front...
and just ahead of a sheared middle-level short wave trough propagating
northeast into the central Great Lakes region. Radar trends
supported by short-range guidance which indicates a quick
diminishing of this precipitation as it moves off to the east/northeast
early this morning...and it should not be a factor beyond sunrise if
even that long. Clouds also quickly decreasing from the west per GOES
infrared imagery this morning...setting up mostly sunny conditions for
today. Surface high pressure building southeast from Ontario and the
upper Mississippi Valley in response to subsidence in the wake of
the upper trough will result in cooler north-northeast winds today
especially immediately downwind of Lake Michigan where highs in the
middle 40s are anticipated...while areas well inland to the west and
south of The Heart of the metropolitan area should see upper 50s/low 60s
for highs. Cool/dry weather will continue through tonight as the
surface high continues to drift east across the area as broad upper
ridging develops aloft.

South-southeast winds return Saturday as a fairly strong middle-level
short wave currently over the Pacific northwest tracks along the
Canadian border...and eventually occludes across far northwest
Ontario. Low-level moisture return will be impeded by southern
periphery of our surface ridge which will block flow off the western
Gulf of Mexico...keeping precipitation potential north/west of the forecast
area along cold front which trails southwest across WI-MN-IA through
Saturday night. Dry/breezy south winds will support a nice warm-up
Saturday with low level model thermal forecasts supporting lower 70s
across our western County Warning Area...and middle-60s farther east. It will be cooler
along the lake from the city northward however...due to a southeast
component to the wind off of the Lake. Lake-modified air should
support temperatures in the low-middle 50s near the shore. Modest south winds
continue Saturday night...which along with slowly increasing surface
dew points will make for overnight lows in the 40s/near 50
especially west.

Another upper trough will slowly emerge eastward from the Desert
Southwest on Easter Sunday...with models depicting a series of much
weaker short-wave disturbances rippling northeast into the Midwest.
Guidance remains largely dry through the day Sunday however...with
relatively weak forcing and moisture transport continued to be
focused along cold front to our northwest. Despite potential for
some increased middle-high cloud cover especially north...south winds
and warm advection look to support temperatures in the middle-70s most areas
and even along the lake as flow becomes more southwest. Rain chances
then increase slowly from the northwest Sunday night into Monday as
the front eventually moves into the forecast area and upper trough
axis approaches late Sunday night and passes across the County Warning Area Monday.
Current model forecasts indicate the cold front clears the eastern
counties of the County Warning Area around midnight...with showers ending from the
west behind the front. Model soundings are not very impressive with
lapse rates...with no widespread thunder expected.

Meanwhile...guidance remains fairly consistent in digging a northern
stream short wave across the northern lakes Monday-Monday night
phasing into a deeper eastern Continental U.S. Trough by Tuesday with high
amplitude upper ridge across the west. Surface high pressure once
again builds across the upper Midwest and lakes Tuesday and
Wednesday with dry weather...but attendant north/northeast winds and
cooler temperatures. Extended period continues to feature persistent high
amplitude but still progressive upper pattern...leading to warmer
temperatures again by Thursday...but with a better potential for showers
and thunderstorms as western Gulf opens up.

Ratzer

&&

Aviation...

//Ord and mdw concerns...updated 09z...

* northeast winds increasing to 10-13kt by around 16z.

//Discussion...updated 06z...

A weak cool front will continue to move south through the area
very early this morning. While some patchy 4000 feet clouds are seen
in the wake of this...little in the way of sensible weather is
expected. Winds will turn north to northeast behind this front.
Speeds at taf sites look to remain below 10 knots through most of the
morning hours...before an increase to 10-13kt this afternoon and
early to middle evening as the thermally-induced pressure gradient
increases around the lake periphery. Some middle to high cloud are
possible late in the day into this evening...but otherwise no impacts
expected.

//Ord and mdw confidence...updated 09z...

* high in all forecast elements.

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

Saturday...VFR/wx none.

Sunday...VFR/wx none.

Monday...chance rain with MVFR ceilings/visible likely. IFR ceilings possible.

Tuesday...VFR/wx none.

Wednesday...chance rain with MVFR ceilings/visible likely. IFR ceilings
possible.

Mtf/krein

&&

Marine...
133 am CDT

A generally quiet pattern over the lake in terms of headlines is
expected into the start of next week. High pressure will move over
the lake today allowing for a 10-20 knots push of onshore winds
across Illinois and Indiana nearshore areas this afternoon into
early evening. Low pressure will move northeast into northern Ontario
this weekend and steer a cold front into the northern half of the
lake on Sunday. With the parent low well north of the area and
moving away...this front will stall through early Monday.
Southerly winds across the area on Sunday could yield 20 knots gusts
or slightly higher in nearshore areas. Eventually the front will
clear the lake as a low pressure riding up the boundary brings it
across the lake. This front will turn winds northerly into early
next week. Some long range guidance members are indicating a
potential gusty southerly period of winds late in the forecast
/Thursday/.

Mtf

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

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