Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
425 PM CST sun Dec 8 2013
257 PM CST
light snow began earlier today...as a seeder-feeder mechanism
developed across far northeast Illinois. A weak mesoscale-low over the
southern tip of Lake Michigan this morning has shifted north...which
resulted in a burst of snowfall across far southeast Wisconsin and
far Northeast Lake County Illinois. This has since pushed further north.
Temperatures have slowly warmed at the surface into the middle 20s...with
dew points hovering around 20 degree. Guidance continues to indicate
decent lift/Omega along a channel currently lifting north across
central Illinois and stretching west into southeast Iowa...expect this to
Blossom the snowfall across northern Illinois late this afternoon/early
evening before the deeper moisture begins to Peel off to the north
by late tonight. The best Omega does appear to diminish around
04z...however the environment remains rather moist below the
dendritic growth zone. Which raises some concern that some patchy
freezing drizzle may develop in the wake of the low later this
evening. Have trended back from the coverage of the freezing
drizzle...however expect there may be a period of some light
freezing drizzle before all precipitation ends later tonight. The focus
appears to be south of I-80.
Based on Cobb p-type algorithms with this next channel of snow that
is beginning to develop...snowfall rates may approach 0.5"/hr within
the 22-02z window. Many of the points indicating this are along the
far northern row of counties in Illinois...which could allow for snowfall
this afternoon/evening of an additional 1-2". Further south the snow totals
will be much lighter...with generally around 1". The best lift does
appear to remain just below the favored dgz...so dendritic size may
remain smaller and more difficult to get the better accums to occur.
Either way...with the smaller flakes visibilities will fall to around 1/2
mile under the strongest snowfall.
Confidence in snowfall totals...medium/high.
Confidence in freezing drizzle...medium.
Monday through Tuesday...
early Monday the departing surface low will be over northern lower
Michigan...with a 500mb trough axis pivoting east across the Central
Plains. Strong thermal trough will remain overhead...keeping temperatures
aloft in the -8 to -12 degree c range. The combination of slow erosion
in the clouds with westerly winds will help to hold temperatures for much
of the day in the upper teens to low 20s...then possibly warm into
low 20s across the far southeastern County warning forecast area.
High pressure will then build east across the region...setting the
stage for a very cold night Monday night. Arctic air aloft of -20 to
-22 degree c does remain just north of the forecast area Monday
night...however with skies thinning further temperatures should easily
radiate into the single digits above zero. With winds remaining
light from the west/southwest...this should prevent too many areas
from cooling to around 0 to just below. Although wind chill readings
in many areas will be well below zero and close to -10 degree c
readings early Tuesday morning.
High pressure elongates and begins to drift south Tuesday...as a clipper
begins to push southeast into the upper Midwest early Tuesday. This wave
will eventually arrive across the forecast area Tuesday evening. This will
spread light snow across the County warning forecast area Tuesday night...with the possibility
of light accums.
Wednesday through Saturday...
ensembles are beginning to suggest the Arctic airmass that has
pushed southeast across the northern Continental U.S. Will begin to weaken
towards the second half of the week. Guidance is depicting a weak
middle-level ridge...with some moderation to the temperatures. Towards next
weekend the flow does begin to flatten out...however considerable
differences amongst the ensemble members does exist. This indicates
confidence lowering and perhaps the development of yet another ridge
across the pac-NW. This could be setting the stage for a return to
cold air beyond the current extended forecast.
//Ord and mdw concerns...updated 22z...
* steady light snow continuing until 01-02z before decreasing in
* 1-2sm visibility with decrease of 1/2 to 3/4 sm possible in snow and
* MVFR ceilings becoming variable IFR/MVFR next 1-2 hours with IFR also
* Some chance for patchy freezing drizzle as snow ends middle/late
* East to east-southeast winds shifting southeast early this
evening then southwest into early tonight.
Steady snow continues across the terminals with a resurgence in
coverage to the southwest which will move across the terminals
into early evening. Visibility in snow is mainly 1-2sm but it appears
some fog has developed and mixed in bringing visibility to 1/2-3/4sm at
least at times into early evening. Snow intensity/coverage looks
to decrease starting 01-02z allowing visibility to improve as well.
Still see some potential for fzdz at the tail end of the snowfall
but confidence is too low to mention in taf right now but will
Light snow ongoing across all the terminals at this time with
visibility continuing to lower. Despite visible lowering...ceilings
remain VFR for most locations at this time. Expect snow to
continue through this afternoon with snow intensity increasing and
visible lowering to around 1 mile...as ceilings continue to lower to
MVFR. Still anticipate steadier IFR/snow late this afternoon into
the early evening...before this snow diminishes while exiting
around the 2-5z time frame tonight. With MVFR ceilings lowering
this evening...guidance is hinting at the possibility for IFR
ceilings developing tonight. Confidence is low with this
possibility...but definitely cannot rule out. MVFR ceilings will
remain Monday morning...slowly improving through middle day. East
southeast winds prevail...and will remain through the afternoon
with gusts around 16kt still possible. Winds will then turn to the
south and southwest this evening and overnight...becoming more
westerly on Monday.
//Ord and mdw confidence...updated 22z...
* high confidence in steady snow continuing until 01-02z then
decreasing in intensity.
* High confidence in 1-2sm ceilings with periods of 3/4 or even 1/2sm
in snow and fog.
* High confidence in MVFR becoming variable IFR/MVFR. Low
confidence in IFR ceiling potential overnight.
* Low confidence in freezing drizzle potential.
* High confidence in wind trends.
//outlook for Ord/mdw for 00z Tuesday-12z Sunday...updated 12z...
Monday night and Tuesday...generally VFR.
Wednesday...chance of -sn and MVFR ceilings.
337 PM CST
High pressure of over southern Quebec and New England will
continue to move east tonight and be centered over New Brunswick
and Nova Scotia by Monday morning. Meanwhile...low pressure will
move northeast from the middle MS valley to the eastern u.P. Of Michigan.
As the low deepens and continues on across Ontario and to near
southern James Bay during Monday a cold front will move eastward
through the upper Great Lakes shifting winds to out of the west
with speeds increasing to strong breezes from middle morning through
the evening. Outside of a few gusts at or just above gale force
speeds are no longer expected to reach gale force Monday so the gale
watch has been cancelled.
A ridge of high pressure will move east from the upper Midwest to
the eastern Great Lakes with a low moving from South Dakota east to Western
Lake Superior. This low continues eastward Tuesday with a trailing
cold front moving across Lake Michigan. Behind the low and the cold
front west winds will likely increase to near gale or lower end
gales Tuesday. High pressure will be moving from Alberta and
Saskatchewan southeast across the northern plains to the upper MS
valley and the middle MO valley Tuesday night and Wednesday...and then east
over the Ohio Valley Thursday and Thursday night.
Winds and waves are no longer expected to reach Small Craft
Advisory criteria tonight for the Illinois and in nearshore waters.
However...winds are expected to increase to and above criteria
Monday so the Small Craft Advisory stat time has been pushed back
until middle Monday morning.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...9 am Monday to 10 PM Monday.
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