Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
329 am CST Tuesday Dec 10 2013
329 am CST
Clicking my heels three times and chanting there's no place like
Chicago in the Summer has yet to produce the desired results.
Forecast challenges this morning include the potentially high impact
light/moderate snow late tonight/Wednesday morning...bitterly cold
temperatures/wind chills Wednesday PM into Wednesday night...then the next potential
snow maker Friday night into the start of the weekend.
Temperatures will moderate pretty quickly from the bitterly cold start this
morning with temperatures rebounding into the teens...but the rise in temperatures
will be offset by the increasingly strong winds which will hold wind
chills to near or below zero much of the day. Water vapor imagery
shows well defined shortwave digging east-southeast across Minnesota this
morning with some snow showers associated with this feature across
Minnesota int northern Iowa. Typically not too difficult to squeeze snow
out of an air mass this cold...so as the shortwave pivots east this
morning could be some snow showers across mainly northern portions
of our County Warning Area. The strong west winds could result in some blowing and
drifting snow this afternoon in open areas.
Next shortwave in The Hopper diving south into the northern rockies
will quickly translate eastward toward the Midwest by late tonight
into tomorrow morning. Strengthening low and middle level frontogenesis
in advance of this shortwave combined with backing winds in the
270-285k pressure surface will result in increasing lower
tropospheric ascent...while 150kt 300mb jet streak is prognosticated to
develop resulting in increasing divergence aloft setting the stage
for a pretty impressive area of snow developing with this wave.
Fairly deep dendritic growth zone (5000ft+) Colorado-located with the
ascent could allow for a pretty fluffy/high ratio snow. Unlike the
last system...depth of the moisture and saturation is greater and
extends well above the -20c level...so suspect that this system will
produce a higher snl ratio...likely at least 15-20:1. Guidance
coming into pretty good agreement on the axis of highest quantitative precipitation forecast setting
up in the I-80 to i88/290 corridor where it's looking pretty likely
that a swath of 2 to locally 4 inches of snow will fall. Snow looks
to peak in the 07-13z time frame western County Warning Area and 09-15z over Chicago
area...coinciding with rush hour. In addition to the horrible
timing...another factor to make this a higher impact event is the
cold air temperatures which will result in untreated salt being less
effective along with very cold pavement temperatures also adding to the
challenge for Road crews. While forecast snowfall amounts fall a
little shy of advisory criteria...the potential for this to be such
a high impact event seems to justify a Winter Weather Advisory. Plan
to issue it for the corridor that looks most likely to get the
highest impacts...it is possible the day shift may need to expand
the areal coverage of the advisory a bit...but wanted to draw early
attention to this potentially very problematic event.
Re-enforcing blast of Arctic air comes rushing into the area
Wednesday afternoon in the wake of the snow. Temperatures will
likely fall into the single digits by evening over most of the area
with temperatures possibly dropping below zero northwest County Warning Area by sunset.
Strong northwest winds will result in wind chills plummeting well
below zero...likely 10-20 below by late Wednesday afternoon...then
possibly as cold as 15-30 below zero Wednesday night. Low temperatures will
be tricky with pressure gradient likely to maintain some wind
through the night. In addition...backing winds late at night and
developing warm air advection (mainly aloft) with the potential for
cloudiness moving in could result in keeping temperatures from getting too
out of control. Wouldn't be hard to envision late evening lows of
10-15 below in the coldest locations assuming clouds don't make an
early arrival...but the bust potential with lows is high in both
directions with current forecast probably the Middle Range of the
possibilities. Temperatures should level off and possibly rise late at
night as cloudiness and warm air advection increase.
Some moderation in temperatures is expected Thursday with cloudiness and
southerly winds likely to result in near steady temperatures Thursday
night. Looks like things could get unsettled with another chance of
snow Friday night into the weekend...but forecast confidence is low
at medium range models are attempting to resolve a very complex
evolution of shortwaves. First...a cut off low meandering off the
Southern California coast is prognosticated to drift eastward into The Four
Corners region before a northern stream shortwave dives southeast
and causes the upper low to open up and begin to lift out into the
Southern Plains. By the weekend the models are attempting to depict
some degree of phasing between these two shortwaves with a polar jet
shortwave digging south into the upper Mississippi Valley region.
Medium range models tend to phase shortwaves too much and the range
of possibilities for this weekend system is still very large so it
is far too soon to sound any alarms. Anytime you have a southwest
upper low lifting out and potentially affecting the area it is a
situation that requires monitoring. Another lobe of Arctic air is
currently prognosticated to spread south into the area behind this system
but models disagree on just how cold...so stay tuned.
//Ord and mdw concerns...updated 09z...
* strong westerly winds during the day Tuesday...with gusts in
excess of 30 knots.
* Potential for light flurries or a few snow showers late
* More significant snow potential late Tuesday night.
Transient surface ridge is pushing across the areas this evening
allowing winds to briefly back to the south-southwest/SW and weaken. Winds will
veer to the west-southwest again tomorrow after daybreak as a clipper system
skirts just north of the area. A tight gradient and strong changes
in the surface pressure will result in a sharp increase in winds.
Winds at the top of the mixed layer are around 35 kts and would
expect to tap into these stronger winds by early afternoon. Main
forcing associated with the upper wave will be focused over
Wisconsin...though some lift is noted south into northern Illinois
where the moisture is shallow. Despite the meager moisture...a few
flurries are possible though would not anticipate any impacts to
visibility nor any accumulation. Winds will settle around sunset as
another ridge translates east across the area...but yet another
clipper system will be right on its heels. This next system looks to
track right over northern Illinois late Tuesday night into early
Wednesday...pivoting across the terminals which could result in a
few inches of snow accumulation and likely periods of IFR visibility.
//Ord and mdw confidence...updated 09z...
* medium-high confidence in wind speeds/direction.
* Low confidence in snow impacting terminals midday
Tuesday...though there is an area of -sn expanding over central
Iowa that will have to monitor.
* Medium-high confidence in accumulating snow late Tuesday night
into early Wednesday.
//outlook for Ord/mdw for 12z Wednesday-12z Monday...updated 00z...
Wednesday...snow ending toward midday. Becoming VFR.
Friday...VFR with snow chances increasing Friday evening. IFR
developing Friday evening.
Saturday...chance for snow. IFR/MVFR.
Sunday...improving to VFR.
324 am CST
Transient ridge is moving across the lake early this morning
allowing winds ot briefly taper...and will allow the first Gale
Warning to expire this morning for the north half of the lake. The
respite will be short lived however as a clipper system will move
across Lake Superior this afternoon with southwest winds quickly
increasing back to gales across the south half of the lake by middle
morning ahead of the system. Winds turn westerly by middle afternoon
and the gales are expected to spread to the northern portions of
Lake Michigan. A Gale Warning remains in effect for the entire lake
for this system. Yet another clipper system will move across Lake
Michigan late tonight and early Wednesday. Expect winds to top out
just below gales though...around 30 knots. Winds will remain breezy
through the remainder of the week and into the weekend.
Lm...Small Craft Advisory...nearshore Gary to Michigan City in until 8 am Tuesday.
lmz870-lmz872-lmz874-lmz876-lmz878...8 am Tuesday to 9 PM
Gale Warning...lmz261-lmz362-lmz364-lmz366-lmz563-lmz565...4 PM
Tuesday to 3 am Wednesday.
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