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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
235 PM CST Monday Dec 22 2014

Short term...
1227 PM CST

Tonight through Christmas day...
main challenges for tonight will be on placement of precipitation along
with how much rain will fall across the forecast area. The
northern stream surface low has become wrapped up across the
Dakotas...with the middle-level trough axis stretching south through
the Southern Plains. Low level flow across the forecast area has become
more easterly...with a feed of some slightly drier air at the surface
into northern Illinois/northwest in. This has been able to delay the
onset of steady precipitation until later this afternoon. Surface temperatures have been
slow to warm...and latest guidance indicates that maximum temperatures may
not occur until just after 00z as a steady conveyor of warmer/moist
air advects north.

Local solutions bring the better slug of precipitation into the forecast
area this evening...then slowly lift the steadier rain north
overnight. Quantitative precipitation forecast totals have decreased from earlier solutions...but
have only made minor adjustments to the going forecast. The surface low
over the Dakotas will be sliding southeast towards southeast Iowa
overnight...with a dry wedge becoming entrained over Missouri.
Guidance has been leaning towards possibly an earlier arrival of the
dry slot into the forecast area just before daybreak Tuesday. Given the
continued uncertainties on track/placement of the low...have held
onto some light rain at daybreak then lift the precipitation shield north of
into wisc by middle-morning Tuesday. Temperatures overnight Monday should be nearly
steady...then after midnight with a 850mb thermal ridge of 3-6 degree c
lifting north expect temperatures to be slowly rising into the low 40s.

Tuesday expect the surface low to be slowly weakening across southwest
wisc...with the middle-level trough axis ejecting the secondary surface low
along the western Gulf. This secondary low is the main concern for
Wednesday afternoon-Wednesday night across our region. The dry wedge should keep precipitation
west/north of the forecast area through Tuesday morning...then wrap around
moisture will bring light rain back into the area Tuesday aftn/eve. Temperatures
should warm into the middle/upper 40s...possibly touching 50 degrees in the
southeast County warning forecast area. The southern stream low will be rapidly deepening
and lifting north across the tenn valley Wednesday morning...then it
becomes a challenge on the placement of the low beyond Wednesday morning.
Guidance continues to show corrections...with a few solutions
bringing the low north into northeast in and a few further east.
This will be a key player on the placement of the potential
deformation zone which could bring accumulate snow to the forecast area
Wednesday aftn/eve. Temperatures will be falling during the day Wednesday...and with
the increased forcing...this could br strong enough to overcome the
warmer air at the surface and allow rain to quickly change to snow and
start accumulating. Gefs ensembles are beginning to demonstrate a
weak easterly low level jet at 850mb lifting northeast over the forecast
area Wednesday afternoon/early evening...which would support the idea of a
deformation snow band for the central/eastern County warning forecast area. Considerable
uncertainty remains with this system...but either way it will impact
a portion of the Great Lakes region on one of the busiest travel
days of the year.

For Christmas day...the downstream ridge over the Canadian maritime
will begin to weaken and allow for the 500mb trough to push east of
the Great Lakes...lifting the deeper moisture/precipitation northeast with
it. Most guidance comes into slightly better agreement for highs
around the middle/upper 30s...with possibly thinning cloud cover.



Long term...
234 PM CST

Wednesday night through Monday...

Friday through Monday...

Main forecast concerns this period surround a pair of storm systems.
The first to pass to the north of the area Friday-Friday night and
the second to pass to the south and east (should current trends
continue) on Sunday. First surface low develops over the Central
Plains Thursday night in response to the digging upper trough over the
western U.S. Models showing northern Illinois generally dry Friday during the
daytime hours Friday with maximum temperatures running around 10 degree above
average. A cold front then sweeps across the area Friday night
bringing a chance of rain or snow.

Meanwhile a secondary upper level shortwave rotates into the base
of western U.S. Trough then on Saturday amplifying the southern
stream. Models not completely resolving this system yet and have
leaned toward model consensus for a forecast scenario where a
sharper shortwave helps to organize surface low pressure near the
Texas/la Gulf Coast Saturday. This low then moves northeast across
the Tennessee Valley and into the central applications Sunday. Will
confine probability of precipitation with this system to just the southeastern counties of
the forecast area currently with temperatures expected near seasonal for
the time of year.

Ed f



//Ord and mdw concerns...updated 20z...

* ceilings lowering to IFR/LIFR this afternoon/evening.
* Periods of rain through tonight
* reduced visibility at times 1-3 sm.


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

Conditions are steadily deteriorating already with the 18z taf
issuance and expect this trend to continue through the afternoon
hours. A lead upper level disturbance has allowed rain to overspread
the terminals late this morning. A few sites have reported a few ice
pellets but soundings indicate the column will quickly warm and will
no longer support frozen precipitation through the remainder of the
taf period. Ceilings are expected to fairly quickly drop to IFR early to
middle afternoon today and will remain there overnight. Behind the lead
wave...there may be a lull in the precipitation intensity but moist
conditions and some forcing may result in a very light rain or
drizzle continuing overnight. The main upper wave and warm front will
lift across the region early Tuesday resulting in precipitation intensity
increasing once again. Once the warm front lifts north of the
terminals...precipitation should taper and there may be a gradual rise in
ceiling heights back into MVFR.


//Ord and mdw confidence...updated 20z...

* high confidence in general ceiling trends but low-medium confidence
in timing.
* Low confidence in visibility trends this evening and tonight.


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

Tuesday night...dry. Mainly VFR...though MVFR possible late.

Wednesday...chance of rain/snow changing to snow. IFR likely.

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

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

Saturday and Sunday...mainly VFR.



233 am CST

Forecast for Lake Michigan remains complicated this morning...with
two areas of low pressure expected to bring periods of strong winds
across the lake over the next several days.

The first low was moving southeast across the Dakotas early this
morning...with a tightening pressure gradient extending east into
the western lakes. Observation platforms along the west and
northwest shores of the lake were reporting south winds in the 25-30
knots range...with winds to 30 knots expected to back to the east as the
low moves to the Iowa-Missouri border tonight. The low will then
begin to fill as it turns northeast...lifting across Northern Lake
Michigan late Tuesday.

Expectations are that a second low will develop to the south across
the Tennessee and Ohio River valleys by early Wednesday morning...
deepening as it lifts northward into the central Great Lakes region
by Wednesday evening. Models continue to show differences in the
exact track and strength of the low as it passes east of Lake well as north and northwest wind speeds Wednesday
afternoon into Thursday morning. At this appears that
winds of 25-30 knots are likely...and possibly still gales for a time
if the low deepens a little faster. Winds then look to diminish and
back to the west and then southwest Christmas day. Models are in
better agreement in another low approaching and lifting across the
northern lakes Friday and Friday night.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...Illinois nearshore waters until 9 am Tuesday.



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