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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1003 am CST Thursday Jan 29 2015

Short term...
358 am CST

Today through Friday night...

The main forecast concerns this morning involve the area of low
pressure currently over west central Illinois...and include precipitation
timing/type...temperature trends...gusty winds...and eventually the
potential for some lake effect snow showers over Northwest Indiana
tonight into Friday.

The surface low center was just southwest of LaSalle/Peru Illinois
at 3 am CST...with the associated cyclonic wind circulation pulling
relatively mild air north across the eastern 2/3 of the County Warning Area.
Challenging hourly temperature readings may reach 40
degrees in the southeast early this morning before the low passes
and shifts winds to the northwest and pulls in colder air. With the
exception of a band of rain showers late last evening...little
precipitation has been occurring with this system except North/East
of the forecast area overnight. Regional radar mosaic has been
depicting persistent light returns behind the low/cold front however
where some occasional drizzle or "unknown" precipitation has been reported.
High-res guidance trends continue to indicate some very light
precipitation is possible across the area this morning...with
forecast soundings indicating mainly drizzle with saturation mainly
in low levels as colder air moves in...though a brief period of
middle-level moisture with ragged deformation area along west/northwest
side of the low could briefly introduce ice into low levels in a
seeder-feeder situation which could result in a mix of drizzle/light
rain and light snow this morning...though not expecting any real
measurable precipitation or snow with this. Strong drying above the
boundary layer from the northwest by midday should allow any light
precipitation to end from west to east through the late morning and midday

Windy conditions will develop during the morning as cold advection
steepens our low level lapse rates in the tight pressure gradient
west of the low. Forecast soundings indicate gusts around 35 miles per hour
will be possible from middle-morning through the afternoon before winds
gradually diminish tonight. A quick note of temperatures west of the
low this morning indicates readings do not drop off too dramatically
and will likely see steady temperatures for the most part until late today.
(Except in the southeast where temperatures approach 40 this morning...will
likely settle back to the middle 30s there).

As for lake effect tonight...low level winds from the
north-northwest (around 340 deg) support an initial convergence zone
along the southwest Michigan shore this afternoon and early this
evening...though our locally run arw model does veer winds later
tonight and brings convergent band into Porter County late tonight
into early Friday. Larger scale flow backs more westerly Friday
afternoon which should push any lingering snow showers back east of
Porter County by middle/late Friday afternoon. Thermodynamics are not
spectacular...though various model forecast soundings develop
lake-850 hpa Delta-T around 17c early Friday with inversion heights
5000-6000 feet before conditions gradually wane later Friday morning.
Depending upon how long the band backs up into Porter/Eastern Lake
counties will modulate the snowfall accumulation potential...though
a few inches are certainly possible if the band does hang up there
for a while late tonight/early Friday...before expansive surface
high builds across the area through Friday night.



Long term...
358 am CST

Saturday through Wednesday...

The main concern in the extended period continues to be the
potential for some accumulating snow over the weekend...especially
across the southern part of the County Warning Area late Saturday night and Sunday.
Global runs continue to show agreement in first sagging a cold front
south across the area Saturday...then with the amplification of an
upper trough over the region with subsequent surface low development
over the lower Mississippi Valley. While the track of the surface
low has trended south over the past few runs (now from the lower
miss valley to the middle-Atlantic or Carolina coast by Monday
morning)...northern periphery of deformation band snow across
central Illinois/Indiana would still affect southern counties of the
lot County Warning Area at this point. In addition...increasing north-northeast
winds north of the low track combined with Arctic air intrusion
behind Saturday cold front could very well lead to lake
effect/enhanced snow showers into the Chicago metropolitan area. So...
while many details remain to be worked out have maintained decent
chance probability of precipitation for measurable snow across the forecast area for the
weekend. Still looks cold beyond the weekend into next week with
deep trough across the east...though pattern continues to be
progressive in longer range guidance solutions which keeps it from
sticking around too long.




//Ord and mdw concerns...updated 16z...

* northwest winds gusting to 28-33 knots through the
afternoon...gusting in the 20s through at least midnight.

* MVFR ceilings persisting into tonight.

* A brief period of snow flurries with MVFR visibility possible before 11 am

//Discussion...updated 12z...

Low pressure is directly over the Chicago area early this morning.
This feature will continue to move northeast today and as it
does...strong cold advection will spread over the region.
Northwest winds will quickly become gusty this morning with
regular 30 plus knots expected late this morning and afternoon.
The wind direction should be locked in to 320-330 degrees during
that time. The cold advection also is supporting widespread MVFR
clouds to the northwest /all the way back to southern Canada/. An
embedded area of IFR ceilings are over southwest WI...eastern Iowa...and
far northwest Illinois early this morning and do foresee them spreading
into some of the taf sites for at least a brief period this
morning. This could also be accompanied by light
precipitation...including a rain/snow the system
continues to strengthen while pulling away. Any precipitation will
be done by 17z. Due to mild surface temperatures through much of
last night along with the light snow accumulation
is forecast if it were to snow.

//Ord and mdw confidence...updated 16z...

* high in winds.

* High in 1500 feet ceilings through much of today...1500-2000 feet
tonight. Medium on specific scattering some time after 12z.

* High in no snow accumulation. Low on any visibility reduction.

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

Friday night...VFR. Southwest winds.
Saturday...chance of snow and IFR during the night. West winds.
Sunday...chance of snow and IFR. At least MVFR ceilings likely.
Northeast winds.
Monday night-Wednesday...chances of mainly light snow. West winds.



205 am CST

A progressive active pattern will continue across the Great Lakes
region into the beginning of February. Moving eastward early this
morning is an elongated trough with two low pressure centers
composing passing just south of Lake Michigan and another
over Eastern Lake Superior. Together these will consolidate later
today and drive more focused and strengthening cold advection
over Lake Michigan. This within the strong and advecting pressure
gradient will support northwest to north gales...even 45 knots gust
potential across parts of the north. The Illinois and especially
the Indiana near shores should at least experience periodic gale
gusts into this evening. Colder temperatures will be working
southward across the lake later today and tonight. Support for
heavy freezing spray remains over mainly the north and continue
the warning there. Waves are likely to build to 12-15 feet tonight
at downstream fetch locations on the eastern half of the lake.
Winds will gradually diminish overnight into Friday morning and
more quickly afterward as high pressure moves over the lake.

Beyond...there looks like opportunities for at least 30 knots winds
next week. One of these is behind a cold front Saturday afternoon
and more so Saturday night. More 30 knots wind potential would come
possibly Tuesday as return southwest flow spreads back over the



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...nearshore waters
until 10 am Thursday.

Gale Warning...nearshore waters until 3
am Friday.

Gale Warning...lmz080-lmz261-lmz362-lmz364-lmz366-lmz563-lmz565-
lmz870-lmz872-lmz874-lmz876-lmz878 until 3 am Friday.

Heavy freezing spray warning...lmz261-lmz362-lmz364-lmz366-
lmz563-lmz565-lmz567-lmz868...2 PM Thursday to 3 am Friday.



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