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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
231 am CDT sun Oct 26 2014

Discussion...
237 PM CDT

//short term//
rest of today through Sunday... quiet weather remains on tap for
the remainder of the weekend as upper level ridge translates east
across the area through the period. Surface ridge in place from
the northern plains into the lower Mississippi Valley this
afternoon will also translate east across the area...becoming
centered over north central Illinois by the early morning hours
Sunday. With dewpoints mixing out into the low/middle 30s across
portions of the outlook area today...along with light winds and
mostly clear skies in place tonight...the stage will be set for
strong radiational cooling overnight. Blended overnight lows
heavily towards the gemnm/gemnmbc which appears to capture some of
the typical cool spots west of the Fox River valley well...and
verifies well under this synoptic situation.

Jet stream begins to buckle in the west as a wave moves onshore
over the Pacific northwest and carves out a trough over the Great Basin by
Sunday evening. Meanwhile...at the surface...broad low pressure
will develop over the Front Range of The Rockies with southwest
flow aloft developing Sunday evening. Locally...daytime hours will
be dry Sunday under the influence of the departing surface ridge
but expect an increase in cloud cover from midday on as middle level
warm advection starts to overspread the region.

Sunday night through Tuesday...
surface low remains unorganized Sunday night as broad troughing
moves east into the upper Midwest...however models continue to
advertise a 40-50 knots low level jet developing across the middle
Mississippi Valley with a decent push of moisture into the region
with precipitable waters of around 1.1 inches advecting into the area. On the
nose of the low level jet expect the possibility of some showers
and thunderstorms to develop where there is some modest elevated
instability present. The corridor where the better instability and
forcing line up still favors areas north of the County Warning Area...however
cannot rule out precipitation building down into the northern couple
tiers of counties in Illinois. The surface low begins to deepen
and become better organized Monday into Monday night as the upper
level trough pushes into the upper Midwest. Low level convergence
strengthens along the cold front at this time...and the chances
for showers and thunderstorms increase Monday evening and night as
the cold front sweeps across the area. Both the GFS and NAM now
advertise some prefrontal activity during the day Monday...however
soundings show fairly negligible instability and dry low/middle
levels. Not sure how much precipitation would be possible given the
soundings so will cap any probability of precipitation at 20 before 00z. Taking a step
back and looking a temperatures Monday...guidance continues to advertise
an anonymously warm thermal ridge though has accelerated the
timing of the ridge some from previous runs. Nudged temperatures slightly
towards the latest bias corrected European model (ecmwf)/gemnh but didnt make any
big changes...still calling for middle to upper 70s north of I-80 and
upper 70s with possibly a few 80s to the south. Behind the cold
front on Tuesday...the 850 mb zero degree isotherm will approach the
area. While Tuesday should be dry behind the cold front...breezy
and cooler conditions are expected with afternoon highs only in the
low to middle 60s.

//Long term//
Wednesday through Saturday
models continue to advertise a high amplitude pattern developing
by late in the week with a shot of Arctic air pushing towards
the area...more aggressively now than compared to guidance 24 hours
ago. The low pressure system that impacts our area early this week
will continue to deepen as it lifts across James Bay Wednesday
with cold air starting to wrap around into the northern Great
Lakes. Strong high pressure then builds down over the Canadian
prairies and sinks southeast towards the upper Midwest. The airmass
is quite cold with guidance now showing -10c temperatures at 850 mb sinking
into northern Illinois by Friday. Did not deviate from a general
model consensus blend given it is still a ways out...but these
temperatures might be too warm if current model trends hold.

Deubelbeiss

&&

Aviation...

//Ord and mdw concerns...updated 06z...

* low level wind shear likely overnight with surface winds west-northwesterly at around 5 knots and northwesterly
around 30-35 knots at 1500 feet above ground level.

* Chance for an easterly surface wind shift with a lake breeze
Sunday afternoon.

Krein

//discussion...updated 06z...

West-northwesterly winds will continue to diminish to light/variable overnight as a
strong nocturnal inversion forms under clear skies. Winds aloft at
the inversion layer quickly becm northwesterly 30-35kt...setting up the
likelihood for low level wind shear...so low level wind shear has been added to the forecast.
Anticipate that with clear skies and a dry airmass in place...the
boundary layer will quickly become well mixed and the potential
for low level wind shear should end by 12-13z. As surface high pressure builds across
the region the remainder of the day will be clear and dry with
light winds. The weak pressure gradient should allow for the
development of a lake breeze. There is still some question as to
how far inland the boundary will push and how strong winds will be
behind the boundary. Given that the water-land temperature differential
is not that great...feel that the boundary will be slow to push
inland...likely not pushing through Ord/mdw until 22-23z. Also in
the absence of a strong push behind the boundary...winds should
remain less than 10kt while backing to Ely. Overnight Sunday
night...the high pressure will shift to the east as low pressure
moves across the plains. Winds will trend to southeasterly though the
evening and southerly overnight. Increasing warm advection through the
night should bring increasing cloud cover...but anticipate that
bases will remain at the lower end of VFR through the night.

Krein

//Ord and mdw confidence...updated 06z...

* high confidence in surface winds through early Sunday afternoon.
Medium confidence in timing of the lake breeze and wind speed
behind the lake breeze.

* High confidence in all other forecast elements.

Krein

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

Monday...rain showers likely mainly during the evening with a chance of thunderstorms and rain.
SW winds.
Tuesday...chance of rain showers during the morning with a slight
chance of thunderstorms and rain. West winds.
Wednesday...VFR. West winds.
Thursday...chance of rain showers. West winds turning north.
Friday...slight chance of rain showers. NE winds.

Mtf

&&

Marine...
217 am CDT

A few active periods setting up across the Great Lakes region over
the next few days...as weak ridging centered over the Ohio Valley
shifts east by this evening. This will allow flow to turn
southeasterly to southerly Monday. A developing trough of low
pressure stretching from the Central Plains north into the
northern plains will slowly consolidate into a low across lower
Michigan Tuesday morning. The gradient is poised to increase across
the northern tier and southern tier of Lake Michigan...with gusts
nearing 25kt and possibly a few gusts to 30kt Monday night. It is
possible small craft conds could develop Monday...but with much
warmer/moist air flowing across the slightly cooler lake the wind
gusts could be slightly diminished.

Then the low is expected to deepen as it lifts north across
Eastern Lake Superior Tuesday midday...with a tight gradient
developing once again across the entire lake and producing west to
northwest winds near between 25 to 30 knots Tuesday evening/Wednesday morning.
High pressure then slowly builds back into the region Wednesday/Thursday
with lighter west/northwest winds developing.

Beachler

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...nearshore Gary to Michigan City in until 10 am Sunday.

&&

$$

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