Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
416 am CST Friday Dec 13 2013
416 am CST
Synopsis...another round of snow moves into the region today. The
main snow show pushes north this afternoon and expanded the
Winter Weather Advisory to include Cook County. However note that
southern Cook County will meet advisory criteria...3 to 5 inches
of snow...while areas including and north of the city of Chicago
will likely see 2 to 4 inches of snow. Otherwise no major changes
were made for todays system.
Today through Saturday...
an upper level low is over the Desert Southwest. The low will move
over the Central Plains this evening and become an open wave. At
the surface...a low will form over the OK/Texas Panhandle region and
hook NE sliding up the Appalachian Mountains. The main low will
push off the Delaware/New Jersey coast Saturday night while a secondary low
forms over the western Great Lakes Saturday afternoon.
For precipitation...relied heavily on our local 8km arw model. The
majority of today will be snow Free. Lake effect snow is possible
over Lake County Illinois late this afternoon/early evening as NE winds
create a favorable fetch. However...thinking the main snow band
will impact southeast WI more than NE Illinois.
The main system lifts to the north with a quasi warm/stationary
front extending over northern Illinois. The front will provide the forcing
for snowfall. Expecting snow to enter the southern part of the County Warning Area
around 6pm CST and move north. At the same time a upper level
vorticity stream will set up over southern WI allowing a second
band of snow to form. Therefore expecting snow to spread across
the state from both the south and north...so areas along 1-88 will
likely be snow free into the early evening. There is the off
chance that a seeder feeder process could set up over far
northeast Cook County resulting in flurries or light snow showers
developing a little earlier than expected. Expecting snow to be
falling across the County Warning Area around midnight. The heaviest snowfall as
far as rates will likely occur between midnight and 6am Saturday
as Omega is maximized with adequate moisture through the dgz.
The snow tapers off from west to east Saturday afternoon as the low
pulls away. Will probably see some lingering lake effect snow
showers over far northeast Illinois through Saturday evening. As winds
turn north and then northeast behind the low...the lake effect snow
will transition to Northwest Indiana for a brief period late
Saturday evening. Have not changed totals too much from earlier
forecasts...with the highest totals being along and south of I-80.
As mentioned above...will keep the advisory going with the addition
of Cook County. Some locations down south may reach Winter Storm
Warning criteria with 6+ inches...but this should be very local so
will not issue a warning. Areas north of I-80 will see 1 to 4
inches...with the lowest snowfall amounts expected across north
Snow will be more dense than the past 2 events since temperatures
will be just below freezing...especially down south. Capped snow to
liquid ratios at 14:0...but could be as low as 8:0 at times.
The upper level trough axis passes overhead Saturday night with
colder air moving overhead. Cloud cover should be thick enough to
keep temperatures from bottoming out given the fresh snow cover.
Min temperatures Saturday night will be in the teens to 6 degrees in
outlying areas. Winds around 10 miles per hour will result in min wind chills
of +1 over Northwest Indiana to around -10 across north central
Sunday and Monday...
northwest flow sets up aloft Sunday and persists through middle next
week. At the surface high pressure moves over Wisconsin Sunday
night. High temperatures on Sunday remain below normal at 15 to 20
degrees. An upper level shortwave pushes south Sunday night
bringing a chance of light snow Sunday night. Forcing is not very
substantial with this system so thinking an inch at most will fall
at this time.
An upper level trough axis rotates through the region Monday
creating a slight chance of light snow north of a Rockford to Delaware
motte Indiana line. This system does not look significant either.
Monday does look a bit warmer with highs in the upper 20s.
Extended...Tuesday through Thursday...
northwest flow continues through the extended with another shortwave
passing north of the County Warning Area Wednesday. After Tuesday the long term
models differ with when the next system will impact the region with
the European model (ecmwf) being faster than the GFS. To illustrate the
differences...the European model (ecmwf) features 850 temperatures of +6 to +8 Thursday
afternoon while the GFS features -4 to +2. Given the discrepancies
and busy near term...went with the allblend forecast with minor
In general significant precipitation is not expected until late next week
and middle to late next week features more seasonal temperatures...with
a chance of above normal temperatures.
//Ord and mdw concerns...updated 09z...
* SW winds around 10kt through daybreak...then winds backing S to east by
* MVFR ceilings developing midday...steadily lowering to IFR shortly
* Period of light snow early afternoon...possibly a burst of moderate
snow after 06z Sat with a period of LIFR.
Infrared imagery indicates a channel of clouds sliding east/southeast
across Wisconsin into far northeast Illinois. Surface winds have
generally remained southwesterly with high pressure displaced well
to the southeast of the region. A frontal boundary will continue
to sag south across the western Great Lakes region...which will
allow winds to back to south then east by early afternoon hours. Some
lake clouds will likely develop midday...as winds turn easterly.
With high/middle deck clouds arriving...this may help to initiate light
snow developing earlier. At this time have flurries beginning around
00z...then steady coverage/intensity increase by 03-06z Sat. Visibilities
will likely drop to around 1sm with the steadier snow...and ceilings are
prognosticated to fall to around IFR by 03z then it is possible a period of
LIFR ceilings may develop around 06z Sat.
//Ord and mdw confidence...updated 09z...
* high confidence in wind direction and wind direction change.
* High confidence in VFR conds through mid-morning.
* High confidence in snow developing after 21z.
* Medium confidence in precise timing of snow and IFR conds.
//outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...
Saturday...snow and IFR in the morning with a chance of snow in the
Sunday-Monday...slight chance of snow and MVFR.
301 am CST
Broad high pressure has steadily drifted southeast from the Great
Lakes region...which has allowed a frontal boundary to sag south
across Lake Michigan. The gradient was slightly stronger over the
northern portions of the lake with north/northwesterly winds. Along the
southern tip of Lake Michigan the boundary will not completely push
south...resulting in easterly winds. Will maintain the 3 sections
for the open waters forecast as a result. Broad high pressure will
then slide east across central Canada tonight into Ontario and
Quebec by Saturday morning. With high pressure located just north of
lake mcihigan...and a low pressure lifting northeast from the
Southern Plains today...this low should remain just south of the
lake. However this will bring a stronger gradient to the southern
tip of the lake and result in periodic easterly gusts to 25kt
overnight into Saturday morning.
Low pressure will then slide east across the eastern Ohio Valley
into the northeastern United States. High pressure will push into
the Central Plains...resulting in cold air flowing south across the
western Great Lakes and a prolonged period of north winds for the
second half of the weekend. High pressure then shifts southeast
towards the Tennessee Valley Sun night. Winds then back to the
northwest/west. A stronger low pressure develops over central
Canada early Monday...with a frontal boundary approaching the lake
Monday night. A tight gradient returns...with the likelihood that
gales west gales develop Tuesday across the lake.
Illinois...Winter Weather Advisory...ilz019-ilz021-ilz022-ilz023-ilz032-
ilz033-ilz039...6 PM Friday to noon Saturday.
Winter Weather Advisory...ilz014...10 PM Friday to 6 PM Saturday.
In...Winter Weather Advisory...inz001-inz002-inz010-inz011-inz019...6
PM Friday to noon Saturday.
Lm...Small Craft Advisory...lmz740...midnight Saturday to 3 PM
Small Craft Advisory...lmz741-lmz742-lmz743...3 am Saturday to 3
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