Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
526 am CST Monday Dec 9 2013
327 am CST
In the slightly modified words of Frank sinatra...the worst is yet
to come and it won't be that fine. After wading around in the
shallow end of the Arctic pool the past few days it looks like our
area is poised to meander into the deep end of the pool for bit this
week. And for good measure it is looking increasingly likely that
portions of the area could freshen up our new snow pack with a
couple more poorly timed inches around rush hour Wednesday morning.
The one Silver lining...it does look like the Arctic air will loosen
its grip on the area later this week into the weekend though it
should remain cold...just a more typical brand of cold.
System responsible for sunday's snow is pulling out with any
lingering spits of freezing snizzle or flurries likely to exit this
area by sunrise...with clearing line arriving shortly thereafter.
Despite clearing skies...blustery west winds and cold air advection
should result in temperatures falling a few degrees this morning followed
by a possible slight early afternoon recovery before falling off
quickly toward early this evening. Will probably see some clearing
tonight and lessening of the winds this evening as weak ridge axis
passes by...which could result in temperatures really tanking this evening
with the fresh snow cover. Some remaining wind followed by warm air
advection starting up after midnight will hopefully keep temperatures from
getting too out of hand...but potential exists for some of The Holes
to dip below zero. Those heading to the bears game this evening will
really need to bundle up with wind chills near or just below
zero...which is one heck of a contrast from the last home game which
featured severe thunderstorms and temperatures not far from 70 back
on Nov 17th!
Clipper passing to our north late tonight and Tuesday could allow
temperatures to begin to rise late tonight with that rise continuing into
Tuesday with a brief shot of "warm" air advection. Secondary cold
front arrives in the afternoon and after a quick rebound could see
temperatures begin to fall again in the afternoon with the next shot of
Arctic air. Tuesday looks to be very windy...so wind chills not
expected to get much above zero all day as west winds gust to over
30 miles per hour. The gusty west winds could result in some blowing snow in
open/rural areas but not expecting it to be a huge deal.
Next clipper should result in warm air advection starting up again
Tuesday night...with yet another period with an atypical diurnal
temperature trend as we see temperatures begin to rather quickly rise after
evening lows. Tuesday night clipper is prognosticated to track farther south
and better forcing is farther south placing our area in the
potential swath of snow. Moisture is limited and forcing not
tremendous...but pretty good model agreement in this system laying
out a swath of about a tenth of an inch of liquid over northern
Illinois. Being such a subtle shortwave...models may well change
their tune some in the next 12-24 hours...but given current forecasts it
would seem reasonable that a swath of 1-3 inches of fluffy snow
could fall with this system. Current forecast path would favor areas
north of I-80 again with timing looking to be after midnight Tuesday
night through rush hour Wednesday morning.
Yet another re-enforcing blast of Arctic air arrives Wednesday
afternoon in the wake of this clipper. European model (ecmwf)/GFS/WRF-NAM are all in
good agreement on bringing what appears to be remarkably cold air
for so early in the season with 850mb and 925mb temperatures both prognosticated
to drop to around -19c...which looking back to 1979 we have only
seen cold of this magnitude a few times during the first 15 days of
December. One of the events was 12/15/1989 when Ord got to -8 and
rfd -16...another was 12/14-15/1985 when Ord once again hit -8 and
rfd -11. It doesnt look like it will get that cold this time around
thanks to the progressive nature of the pattern...however that does
give somewhat of a baseline for what an air mass like this one can
bring. Thinking now that temperatures will follow another atypical diurnal
trend Wednesday with steadily falling temperatures Wednesday afternoon...probably
into the single digits by sunset over most of the area with wind
chills well below zero.
Wednesday night should be coldest night of the season...and possibly
one of the coldest nights in years. While not an ideal radiational
cooling set-up with some gradient persisting through the night and
warm air advection aloft...it does look like skies could be mostly
clear and with a fresh snow pack think that lows will drop below
zero over most of the County Warning Area outside of the urban heat island of
Chicago. Conservatively knocked lows down to 5 to 10 below in the
coldest spots...but if winds decouple and we stay clear then lows
colder than 10 below would not be a stretch at all for places like
arr/rpj. Officially it will be close for Chicago ohare...but if we
drop below zero it would be only the second time in the last 30
years that temperatures have dropped below zero in Chicago this early in
the season. Wind chills look to bottom out in the 10 to 20 below
range...getting close to the -20 Wind Chill Advisory criteria over
north central Illinois for time Wednesday night.
Warm air advection should take hold Thursday with temperatures rebounding
nicely from the bitterly cold morning lows. Have gone a smidge below
guidance out of fear of the snow cover and very cold start to the
day making it hard to rebound more than 20f. Warm air advection
continues Thursday night with temperatures probably not falling much from
day time highs and likely to end up 20f+ warmer than the previous
night in many areas.
Another shortwave could affect the area late Friday into early
Saturday...though models are not in great agreement in their
handling of that system just yet. At this time not looking like a
block-buster...but could be cold enough for mostly snow if it does
impact the County Warning Area. More moderate brand of cold expected to arrive in
the wake of this system...but still below average.
//Ord and mdw concerns...updated 12z...
* flurries ending early this morning.
* Ceiling trends through the day.
* Gusty westerly winds.
A cold front is departing east of the area this morning with a few
lingering flurries and increasingly gusty west winds in its place.
MVFR stratus deck has a clear back edge pushing across north central
Illinois early this morning and should briefly clear out clouds middle
morning before middle level clouds currently over Wisconsin move into
the area. It is uncertain if an MVFR deck will reform later
today...but with snow cover on the ground have decided to back off
on the lower cloud coverage this afternoon. Certainly wouldnt be
surprised though if we had bkn025 ceilings after around 18-19z through
sunset...but leaning against it for now. Winds have already begun
gusting to around 20 knots and should gradually trend up towards the
middle 20 knots range this afternoon. Winds may be slow to come down this
evening...but eventually expect gusts to taper and winds to back to
the southwest as high pressure starts to spread east along the Ohio
//Ord and mdw confidence...updated 12z...
* medium-high confidence in ceiling trends through early
afternoon...low confidence in MVFR ceilings reforming middle afternoon.
* High confidence in wind direction...medium-high confidence in
//outlook for Ord/mdw for 00z Wednesday-12z Monday...updated 12z...
Tuesday night...snow likely late...accumulations possible. Dropping
Wednesday...snow ending. Improving to VFR.
Friday...chance snow. Becoming IFR/MVFR.
Saturday...chance snow. IFR/MVFR.
Sunday...improving to VFR.
249 am CST
With a strong area of high pressure over the Mississippi and Ohio
valleys...and a series of lows quickly moving across the northern
Great Lakes region...anticipate winds to remain elevated with a
couple periods of gales expected. First...low end period of gales
is expected primarily across the northern portions of Lake Michigan
this afternoon and evening as departing low pressure deepens as it
lifts to near James Bay. Have hoisted a short duration 6 hour Gale
Warning for this event. Winds diminish back below gales Monday night
as a transient ridge moves across the lake...but winds are expected
to increase back to gales again Tuesday as a clipper system moves
across the lake. 40 knots gales are expected on the south end of the
lake by middle morning...and should spread north across the lake by
afternoon. Winds will gradually taper Wednesday into Thursday as
high pressure builds into the upper Midwest towards the Ohio
Lm...Gale Warning...lmz261-lmz362-lmz364-lmz366-lmz563-lmz565...3 PM
Monday to 9 PM Monday.
Gale watch...lmz261-lmz362-lmz364-lmz366-lmz563-lmz565...4 PM
Tuesday to 3 am Wednesday.
Small Craft Advisory...nearshore Gary to Michigan City in until 8 am Tuesday.
lmz870-lmz872-lmz874-lmz876-lmz878...8 am Tuesday to 9 PM
Small Craft Advisory...lmz740-lmz741-lmz742-lmz743...9 am Monday
to 10 PM Monday.
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