Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1144 PM CST sun Dec 8 2013
630 PM CST
Temperatures and sky are on track...but adjusted precipitation trends for
this evening into tonight. Light to moderate snow continues to
fall across the region as the system continues off to the
northeast. Reports of freezing rain and freezing drizzle have mainly
remained south of the County Warning Area over ilx/S County Warning Area...but thinking snow will
mix and then transition to freezing drizzle over the next few
hours south of a Streator Illinois to Delaware motte Indiana line. The
freezing drizzle will produce a thin glaze at best...but enough to
create slick conditions. As such issued a Freezing Rain Advisory
for the southern counties through midnight.
Snow will mix with freezing drizzle across the area as snow comes
to an end late this evening...but not expecting much of an impact
for areas not included in the advisory. All in all precipitation should
come to an end by 9z as previously thought. In fact wondering if
precipitation will end sooner than currently forecast.
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 06z...
* potential for light freezing drizzle or a few flurries this
* Front pushing across the area with gusty winds increasing
* Ceiling trends.
Low pressure is lifting across far southeast Wisconsin this evening
with a trailing cold front pushing across north central Illinois
this hour. Radar shows increasing returns immediately ahead of the
front which appears to be an area of very light drizzle and mist.
Expect this area to clear the Chicago terminals in a few hours as
the front sweeps across. Farther west...snow is persisting behind
the front across portions of Iowa. Models indicate this area will dry
out as it lifts northeast...but still has a decent chance of
clipping rfd this late evening. At the moment...do not anticipate
this area to impact the Chicago terminals.
Behind the front...westerly winds will increase with gusts into the
high teens overnight...and further increase into the low 20 knots
range during the day Monday. Cold advection should keep the lowest
levels of the column mixed preventing IFR ceilings from forming
overnight. Ceilings should remain MVFR though with a slow diurnal upward
trend...then may scatter out around or shortly after sunset Monday
//Ord and mdw confidence...updated 06z...
* medium confidence in precipitation trends early in the period.
* Medium confidence in ceilings staying at or above 010 through the
* High confidence in wind direction...medium-high confidence in
//outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...
Tuesday night and Wednesday...chance for -sn and MVFR cigs/vsby.
Thursday and Friday...VFR.
Friday night and Saturday...chance for -sn and MVFR.
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.
Illinois...Freezing Rain Advisory...ilz023-ilz032-ilz033-ilz039 until
In...Freezing Rain Advisory...inz010-inz011-inz019 until midnight
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...9 am Monday to 10 PM Monday.
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