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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
410 PM CST Thursday Jan 29 2015

Short term...
234 PM CST

Through Saturday...

The main focus during this period will be with the potential for
some accumulating lake effect snow across portions of northwestern
Indiana tonight into Friday morning.

Low pressure now across the eastern Great Lakes region...drove a
cold front across the area this morning...and has since continued to
producing gusty northwest winds across the region through the
afternoon. Water vapor imagery this afternoon indicates the next
short wave disturbance now digging southward over Lake Superior.
This disturbance is expected to race southward across Southern Lake
Michigan this evening...and as it does could produce enough
lift to squeeze out a few I have added the mention to
the forecast through much of the evening. appears that lake effect snow showers will continue
to develop over Southern Lake Michigan tonight...and as the boundary
layer flow over Lake Michigan begins to back appears
that some bands of lake effect snow will begin to impact portions of
northwestern Indiana overnight tonight through Friday morning. Lake
induced equilibrium levels are expected to reside around 5,000
feet...and due to the cold lower level atmosphere...this should be
high enough to reside well into the dendrite growth zone.
Therefore...some efficient snowfall looks to result in some
accumulating potential...especially across portions of Porter County
Indiana in my area...where some isolated areas could get a couple
inches. These lake effect snow showers will come to an end by early
afternoon as the flow begins to shift westerly in response to a an
area of surface high shifting southeastward towards the Ohio

A brief warming trend looks to be on tap late Friday night and
Saturday as this high shifts over the Ohio Valley...setting up a
west southwesterly flow of warmer air over the area. This should
allow temperatures to warm well into the 30s area-wide.



Long term...
328 PM CST

Saturday night through Thursday...

The main focus during this period will be with the increasing
potential of a decent accumulating snowfall event across at least
portions of my area Saturday evening through Sunday.

A complex weather pattern will be in place across the Continental U.S. During
this period. Overall...the main disturbance of interest is now
ejecting east-northeastward across the northeastern Pacific along
the eastern periphery of an upper low south of the Aleutians. This
feature will moving onshore across the British Columbia coast by
Friday morning...then is expected to dig southeastward along the
northeastern periphery of an upper level ridge in place across the
western Continental U.S.. the main complexities with this if and
how much of the southern stream energy...emanating from the
southwestern Continental U.S. Trough...gets tapped into by this digging system.

Model guidance has flipped flopped a bit over the past couple of
runs on how far north the accompanying precipitation shield will get
Saturday night into Sunday. It appears one of the main Keys that
will play a role in how far north the snowfall ends up getting
across Illinois will be the timing of colder airmass/cold frontal
passage expected on Saturday night. A much colder airmass is
expected to spill in across the northern Continental U.S. Ahead of a Stout 1044
mb high building southward out of southern Manitoba Saturday
night...and this should set up a good baroclinic zone potentially
right across The Heart of my County Warning Area Saturday night into Sunday. At the
same time...isentropic ascent and moisture transport will ramp up to
our southwest in advance of the approaching disturbance. This should
result in a large area of snowfall development in the cold air across
Missouri and west central Illinois by Saturday afternoon. It appears
this snowfall will gradually develop northward across Illinois by
Saturday evening...and may end up impacting most of my area to some
degree overnight Saturday and into Sunday. It appears that a decent
band of frontogenesis could set up somewhere near the I-80 corridor
into Sunday morning as a deformation axis tries to develop with the
925-850 mb layer ahead of the approaching cold airmass. This could
result in some areas of heavier snow. However...confidence on where
exactly this enhanced mesoscale forcing sets up is low at this
time...given this system is 60+ hours away. It does appear that most
areas may see at least some light snow accumulations...but the
potential of a few to several inches of snow accumulation is
certainly a possibility.

The other factor to consider with this event is the potential for
lake enhanced snow showers later Sunday and into Sunday night across
northeastern Illinois. As the colder airmass begins to spill
southward over the area on Sunday...winds will become northeasterly
off of Lake Michigan. The colder airmass...featuring 850 mb
temperatures in the teens below zero celsius...should also produce
favorable thermodynamics in the lowest levels of the atmosphere for
enhanced snow showers later Sunday and Sunday night. Model soundings
indicate inversion heights reaching up around 5-6,000 feet...which
should extend well into the dendritic growth zone. Therefore...we cant
rule out the potential for some heavier snow totals across portions
of northeastern Illinois. At this time...I have remained a bit
conservative on snow amounts across northern Illinois...mainly due
to the high uncertainties with this evolving system.

Following this appears the region will remain in and
active west/northwesterly flow pattern. This could support a few
additional clipper type systems with snow chances and reinforcing
shots of cold air through the extended period. appears
that we will remain below to well below average for temperatures
next week. A period of very cold conditions could also be in the
works for the latter part of next week...with the possibilities of a
system digging southward over the area from the high latitudes of




//Ord and mdw concerns...updated 22z...

* northwest winds with erratic speeds between 12 and 17 knots gusting
to around 25 knots or so through at least midnight...tapering some

* MVFR ceilings lifting to VFR late evening or early tonight.

* Flurries this evening possibly reducing visibility to MVFR.


//discussion...updated 22z...

Northwest winds continue to be quite variable in speed. One upper
wave is crossing the area and am thinking that winds will pick up
a bit closer to 00z behind this wave. Another wave will then cross
middle evening and may result in another brief decrease in speeds
before increasing again late this evening. Ceilings have improved
upstream to the north so have gone with slightly higher ceilings in
the latest taf but may need to speed the improvement to VFR by a
few hours.


From 18z...

Main aviation concern is with gusty northwest winds this afternoon and
tonight. We have had a hard time realizing deeper mixing with the
solid MVFR cloud deck in place. We have seen occasional 30 knots
gusts on 1-minute ASOS observations...but in general sustained
winds have been in the middle to upper teens with gusts into the middle
20s. Expect these winds to remain in place this afternoon and
evening as a second wave of cold air advection ensues tonight.

Next concern is around MVFR ceilings and possible flurries tonight.
Ceilings should generally remain above 1500 feet with the gusty winds
and mixed very low levels. A weaker trough axis will move through
the terminals this afternoon...followed by a sharper vorticity maximum this
evening. Upstream observations in northern Minnesota ahead of this
wave have been reporting some flurries and possibly even some
freezing drizzle. As this wave moves through this evening...lower
level temperatures cool to that point to support some flurries
and possible visibility reduction if this activity. Confidence on
this is low on specifically impacting any particular
it has not been mentioned in the tafs at the moment. But
confidence is pretty high that the MVFR ceilings linger through much
of the night. High pressure builds in tomorrow which results in
lighter winds and likely some scattering out of the low cloud
deck...but confidence timing the scattering is not very high.


//Ord and mdw confidence...updated 22z...

* high in sustained speeds being erratic and medium-high in gust

* Medium confidence in timing of improvement to VFR ceilings and time
of scattering. May need to speed up improvement to VFR.

* Low on flurries and any associated visibility reduction this evening.


//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.



153 PM CST

Active pattern looks to persist through the weekend with much
colder air arriving early next week. With the slow and steady
departure of two low pressures over the western Great Lakes this
morning...the two lows are now consolidating over Northeast Lake
Huron. Along the back edge of these low pressures...the pressure
gradient has tighten and was producing gale force winds to 35 knots
from the northwest. Most of the afternoon winds have been held to
around low end gales or just under gale force...however as the
afternoon progressed mixing has been able to bring to the surface
much stronger winds. As a result will maintain the current Gale
Warning through this evening. With the strongest gradient over the
northern half of the lake...producing gales to 40kt at times...this will
likely produce heavy freezing spray. Further south the gales will
likely remain around 35kt and the freezing spray should only be
along the moderate level and not require a headline.

Waves will continue to build early this evening...with the highest
waves between 12-15 feet tonight. High pressure then quickly arrives
and brings lighter winds to the lake by midday Friday. Then high
pressure quickly slides east/southeast...with winds turning
southerly ahead of yet another low pressure system over the
weekend. A frontal boundary will slide south from Canada
Saturday...and could develop a tight gradient over the lake for a
period Saturday night into Sunday. At this time it looks like
north/northeast winds will top out around 30kt...but could see a
few gale force gusts for a few hours early Sunday. Then high
pressure pushes over the western Great Lakes Sunday night into
Monday. Unfortunately another active pattern looks favorable for
early next week.



Lot watches/warnings/advisories...
Lm...heavy freezing spray warning...lmz261-lmz362-lmz364-lmz366-
lmz563-lmz565-lmz567-lmz868 until 3 am Friday.

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



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