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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
313 am CST Friday Jan 30 2015

Short term...
313 am CST

Through tonight...

Weather none in the short term time frame as high pressure moves
across the region through tonight with southerly winds and warm air
advection kicking in late tonight as the high moves east of the
region.

Izzi

&&

Long term...
313 am CST

Saturday through Thursday...

All eyes in the long term are on the complex weekend storm system
which looks likely to bring shovel-able snowfall to the County Warning Area Saturday
night and Sunday. Main shortwave energy responsible for the
developing storm system this weekend is relatively compact shortwave
about 700 miles west of the British Columbia coast. This shortwave
is forecast to move onshore this evening and then dive southeast
into the northern rockies and High Plains by Saturday
evening...while while phasing with shortwave energy currently over
interior Alaska.

The track of these shortwaves will likely be affected by a vigorous
shortwave pivoting around the Hudson Bay low and a large closed low
developing over the southwest Continental U.S. And dropping south into northern
old Mexico. The complexity of this pattern lends itself to higher
than average uncertainty with respect to the eventual evolution of
the weekend system...its intensity...quantitative precipitation forecast amounts...and placement of
the heaviest quantitative precipitation forecast and snowfall. Given the number of players involved
and the fact the primary shortwave is still offshore it is quite
plausible that there could be additional large swings in the
forecast models' predicted track/intensity of this system...much
like what was seen in the past few days with the main story 24 hours
ago being the big southward trends in the solutions...followed by a
forecast track shift of nearly 200 miles farther north in the 00z
European model (ecmwf) tonight vs its run 24 hours ago.

OK...enough about the uncertainties and on to the current
expectations of the system. As shortwave drops southeast into the
northern rockies look for a Colorado low to develop Saturday...then
track eastward across southern Illinois Sunday afternoon and
eventually east to the middle Atlantic region by Monday morning. The
strong shortwave pivoting around the Hudson Bay vortex and its
associated surface low are still prognosticated to drive a cold front south
across the County Warning Area Saturday with the front eventually stalling out to
out south Saturday night and setting the stage for what looks to be
a long duration warm air advection snow event. Looking at the 280-295k isentropic
surfaces models have upglide starting Sat evening and continuing
through Sunday morning with weak/broad deformation band likely to
keep snows going through much of the day Sunday.

Low/middle level parcels over the area Sat night/Sunday morning are
prognosticated to have originated from the western Gulf...so this system
will have access to much more moisture than most of our systems have
had this winter...though it still will be feeding off a recycled
Continental polar airmass following today's frontal passage in the Gulf.
Strongest ascent/heaviest snowfall in this evening's runs looks to
be centered on Sat night and over our southern County Warning Area. Current run's
thermodynamics forecast moisture/Omega really point toward
snow:liquid ratios near climatology (10-12:1) for the majority of the
event which given latest model consensus/wpc quantitative precipitation forecast forecast would
suggest widespread 3-6" amounts across the County Warning Area with locally heavier
amounts to near 8 inches in our southern County Warning Area. Given the long
duration of the event...warning criteria is 8 inches in 24 hours
which is possible but given all the uncertainties confidence is not
high in if and where this would occur so no watch is planned at this
time.

As Arctic air spreads southward on the back side of the system
Sunday afternoon and Sunday night there could be some lake enhanced
snow that could transition to pure lake effect snow showers Sunday
night. Forecast soundings do show moderately strong low level
instability below the frontal inversion...but inversion heights are
forecast to only be around 5-6kft so lake effect convection would be
shallow. Current forecast thermodynamics and forecast of very dry
air atop a solid inversion...it really doesnt look to favor lake
effect being a big player in snowfall totals. Can't rule out a bit
of enhancement to the snowfall over northeast Illinois Sun afternoon
and night...but given the current model runs the contribution to
totals from the lake look to be trivial compared to the synoptic
snows.

Pattern looks to remain very progressive in the wake of the
system...so Arctic air will not have time to get well entrenched
before warm air advection develops later Monday in advance of the
next clipper which this evening's run have tracking across Wisconsin
keeping US in the relatively dry warm sector Tuesday. Another surge
of Arctic is prognosticated to arrive toward midweek with some snow
possibly accompanying it. While it looks to be very cold
midweek...GFS and European model (ecmwf) beyond day 7 suggest the pattern will remain
progressive with Arctic air not sticking around with long wave
trough migrating east out of the area.

Izzi

&&

Aviation...

//Ord and mdw concerns...updated 09z...

* gusty northwest winds through early this morning backing to
southwest this afternoon.

Krein

//discussion...updated 06z...

Cold air advection and a weak upper disturbance will work in
concert to produce some flurries and brief light snow showers
overnight...likely ending prior to sunrise. Have seen a few brief
visibility reductions in the snow showers in southeast Wisconsin and certainly
possible the same could occur into our terminals...though any
visibility restrictions will likely be short lived. Lower end VFR ceilings
should scatter out at some point Friday morning and not
anticipating any MVFR ceilings through the period. Northwest winds
will gradually ease through the morning hours before gradually
back to southwest Friday afternoon into early evening but with
very light speeds.

Izzi

//Ord and mdw confidence...updated 09z...

* medium to high confidence wind trends.

* Medium confidence in cloud cover trends.

* High confidence in remaining forecast elements

Krein

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

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.

Mdb

&&

Marine...
230 am CST

A progressive...active pattern will continue for Lake Michigan for
the next several days. Winds are slowly diminishing over the lake
and will let the gale and heavy freezing spray warnings expire at
300 am. However...with Lake Michigan still under a relatively
strong pressure gradient...a Small Craft Advisory will be issued
for both the Illinois nearshore waters as winds are still around 30
knots early this morning...but should drop off to 10-15kt by middle day.
Residual wave action from the persistent northerly winds should
keep waves above 4 feet through this afternoon. As the axis of high
pressure moves across the lake this evening a brief period of
relatively mild conditons will set up...but the high will quickly
settle into the Ohio Valley tonight as low pressure tracks across
the upper Great Lakes region. This system will drag a cold front
across the lake Saturday. Following the passage of the cold
front...strong high pressure will build across the Central Plains
and upper Great Lakes satuday night and Sunday while low pressure
develops over the lower Ohio Valley Sunday afternoon. An extended
period of northeasterly winds of 25-30 knots will begin following the frontal
passage and persist into early next week.

Krein

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...nearshore waters
until 4 PM Friday.

&&

$$

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