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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
918 PM CST sun Dec 21 2014

914 PM CST

While thermal profiles are still a bit tricky in our western County Warning Area
overnight...feeling is that if some precipitation sneaks in overnight...surface
temperatures would be around freezing or possibly slightly warmer which
would mitigate impacts significantly. Mli is reporting unknown
precipitation this evening but with a surface temperature in the middle 30s. When
precipitation arrives in the morning in the west and south...temperatures should
largely still be above freezing. While the chance is not zero for
some light fzdz or sleeet...confidence is low.



Short term...
246 PM CST

Through Tuesday...

Tonight through Tuesday...
broad surface ridge has become centered over the northeast Continental U.S....and
continues to slowly drift east/northeast. The gradient across the
Great Lakes has remained light this afternoon...with a mainly southerly
flow. Clouds remain locked in across the forecast area...with some
clearing just southeast of the County warning forecast area in central/northern in. Surface
temperatures have been able to warm to the middle/upper 30s...with dew points
hovering around 30-33 degree. Most the evening will remain dry...with
the weak middle-level wave sliding northeast across Iowa/wisc with some
light snow...otherwise most of the forecast area should remain dry through
the overnight period.

500mb vorticity begins to lift northeast from the Central Plains before
daybreak Monday...pushing the moist channel further east across the
forecast area. With soundings suggesting a warm wedge developing
aloft...p-type could be a challenge around daybreak. Guidance would
suggest temperatures could be warm enough to limit precipitation to light
rain...with near surface temperatures holding around the freezing have
continued the mention of freezing rain mainly south of Rockford. Closer to
Rockford the thermal profile is a little more tricky and could allow
some light snow/sleet at the onset changing to light rain/freezing rain before
midday. Throughout the afternoon temperatures will steadily warm...nearing 40
degree in most locations as warm/moist air continues to advect north
across the County warning forecast area. Most guidance indicates light rain will persist through
the afternoon/evening hours...with the dry wedge not arriving until closer to
daybreak Tuesday. Local solution suggests slightly earlier with bringing
a temporary end to the precipitation...just after midnight Monday night.

Temperatures overnight will remain mild in the upper 30s/near 40. With the
trough taking on a negative tilt midday Tuesday...the surface low will be
wrapped up across central wisc. Guidance has bounced around possibly
keeping either dry/low chance probability of precipitation over northern Illinois/northwest in
Tuesday/Tuesday afternoon...but have opted to hold onto a slt chance of rain. With
warm air continuing to advect north...surface temperatures could push into the
middle/upper 40s with a few locations in the southeast reaching 50 degree.
By Tuesday evening the trough axis is prognosticated to eject another surface low
across the Gulf states...then quickly lift north across the tenn
valley late Tuesday night. Moisture will fill back in across the forecast
area Tuesday night...with rain becoming mixed with snow overnight. Temperatures
will slowly the period of rain/snow mix could linger



Long term...
246 PM CST

Wednesday through Sunday...

Middle-week ensembles continue to forecast a slight strengthening to the
downstream ridge over the Canadian maritime...which will limit how
far east the secondary low will progress. Operational solutions
continue to lift the strengthening low north across the Ohio Valley
Wednesday afternoon...and quickly north of Lake Huron by late Wednesday night. Wrap
around moisture and cooler air will bring some light snow to northern
Illinois Wednesday.

For Christmas day the downstream ridge will begin to
weaken...allowing the middle-level trough to become progressive and push
east. This should bring an end to the light snow/flurries by daybreak.
Temperatures should return to the middle/upper 30s...along with perhaps some
breaks in the cloud cover by the afternoon. Middle-level heights should start to
increase Thursday ngt/Fri...however this looks to be short-lived as yet
another trough is prognosticated to dig south from the Canadian rockies
into the northern plains. The stronger push of colder air looks to
arrive Sat...although this may only be a short duration with the
semi-zonal pattern setting up for the second half of the weekend.




//Ord and mdw concerns...updated 03z...

* primarily VFR or MVFR ceilings through Monday morning...likely
lowering to IFR Monday afternoon and possibly LIFR Monday
* rain develops around midday Monday and continues through Monday


//discussion...updated 00z...

Another day of stratus across the region. Southern flank of
stratus had been lifting north toward I-80 but appears to be
recently redeveloping. Confidence in ceilings tonight is low with many
models suggesting improvement to VFR...though in light of recent
trends have opted to run with MVFR with very low confidence. Rain
will likely overspread the terminals midday Monday and continue
fairly steadily into the evening hours. Ceilings/visibility should fall with
the onset of rain with IFR conditions likely...probably lowering
to LIFR Monday evening.


//Ord and mdw confidence...updated 03z...

* low confidence in ceilings tonight/Monday morning increasing to low-
medium confidence later Monday afternoon and evening
* medium-high confidence in general precipitation timing
* high confidence in remaining elements


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

Tuesday...MVFR ceilings likely. IFR ceilings possible...mainly early.

Wednesday...MVFR ceilings likely. Chance of ra/sn.

Thursday...chance of MVFR ceilings...mainly early.

Friday...chance of MVFR ceilings. Chance of -ra/-sn

Saturday...MVFR ceilings likely. Chance of flurries.



204 PM CST

Small Craft Advisory...kept the current small craft as is and
expanded it two zones south through Calumet Harbor. Waves will
become hazardous overnight north of Wilmette Harbor and then winds
and waves will become hazardous tomorrow afternoon. Marginal small
craft conditions are possible along the Indiana nearshore waters and
do not have enough confidence to issue a Small Craft Advisory at
this time.

High pressure is over the East Coast while the next low is over the
northern plains. South winds have increased to 15-25 knots across the
lake and will continue to increase to 30 knots this afternoon. Winds
become southeast tomorrow as the low moves over the eastern plains
and weakens. The low then weakens over Wisconsin Tuesday afternoon
through Tuesday night. Winds weaken across the lake Monday night and
become west to northwest 10-15 knots Tuesday night. A second low forms
over the southern Mississippi Valley Tuesday night and quickly
deepens as it moves north. Guidance differs on how far to our east
the low will pass...but thinking it will be over the eastern Great
Lakes. Guidance has also backed off on wind speeds behind the low
and will only carry occasional gales in the Gulf at this time. So
have west to northwest winds increasing to 30 knots Wednesday night.
The best window for gales would be late Wednesday night. The low
then continues northeast into Quebec Thursday and have southwest
winds decreasing Thursday. Another low slides across the plains
Thursday afternoon and will impact the lake Friday. Guidance
differs on how strong the low will be when it moves over the have low confidence in the forecast late next week.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...nearshore Winthrop to Wilmette harbors until 3 am Tuesday.

Small Craft Advisory...lmz741-lmz742...noon Monday to 3 am



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