Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
543 am CST Thursday Dec 12 2013
345 am CST
First forecast concern is the Wind Chill Advisory expiring early
this morning. Wind chills that fell below -20f in portions of the
area overnight should slowly be on the increase as a bit more cloud
cover moves overhead and winds diminish slightly. Most spots fell
only to the -10 to -15 range so plan to let the advisory expire on
schedule unless something changes over the next couple of hours.
Either way it is still going to be a very cold morning.
After a very slight warm up today and Friday...the focus then turns
to snowfall expectations Friday night into Saturday. At this point
it appears we will be looking at headline criteria snow amounts for
some portion of the County Warning Area but most likely advisory rather than
warning...and mainly south of the Chicago metropolitan area. Easterly flow
off the Lake May also enhance totals in the Illinois counties of
lake and Cook...and areas north...but again only to advisory amounts
or possibly even less.
Models do not seem to be showing pronounced mesoscale effects that
would enhance banding...so would expect more of a synoptic scale
broad isentropic ascent ahead of the inverted trough associated with
a low sliding from Texas into the Tennessee Valley. Then
again...mesoscale effects are tough to pin down at this time
range...so it is possible some locations may see higher amounts.
What seems more likely is for snow lovers to be somewhat
disappointed. The temperature profile is not favorable for overly
efficient snow growth...with most of the column between 0c and -10c
with a very shallow dendritic growth zone. Will let the day shift
take another look and decide where headlines might be needed...but
as for the timing of this system...it appears that far southern
portions of the area would start to see snow by late afternoon
Friday. It would then spread across the entire County Warning Area by Friday
evening and continue to fall off and on for most of the night and
into Saturday morning.
By Saturday afternoon the snow moves out quickly and high pressure
takes over for the remainder of the weekend. Next week does not
look quite as cold as this one...and aside from slight chances of
snow Monday and Monday night...the storm track appears to remain
north of the area for a few days. It may be Worth mentioning that
late next week...Friday into Saturday...both the GFS and European model (ecmwf) show
a strong system lifting through the Ohio Valley on a course that
would be favorable for significant weather locally...but we know a
lot can change before that time.
//Ord and mdw concerns...updated 12z...
* possible period of MVFR ceilings through this morning.
* SW winds increasing with speeds generally around 10-12kt and gusts
up to 20kt.
* Middle/high level clouds midday through afternoon.
High pressure centered over the southern Ohio Valley stretches
continues to drift east/southeast. A weak middle-level wave was sliding
southeast through the upper Midwest into the western Great Lakes
this morning...producing an area of patchy MVFR ceilings around 3kft
above ground level. This wave will remain just north of the taf sites...however
based on infrared imagery the clouds will be sliding overhead this
morning. As a result have introduced a period of ceilings at
3000-3500ft above ground level through late morning. Then ceilings should improve back to
VFR conds. Winds will generally be southwesterly. With better
mixing this morning through the afternoon speeds should come up to around
10-12kt with gusts up to 20kt.
Then quiet weather moves back in for the overnight hours...and
focus turns towards the developing storm system in the Southern
Plains that is expected to approach the region Friday night. This
system may bring a period of MVFR to IFR ceilings with light snow Friday
night into Sat.
//Ord and mdw confidence...updated 12z...
* high confidence in wind dir/speed/gusts.
* Medium/high confidence in ceilings and tempo forecast.
//outlook for Ord/mdw for 00z Saturday-12z Thursday...updated 12z...
Friday night...possible MVFR ceilings by afternoon. Snow likely in
MVFR/IFR visibility and ceilings.
Saturday...snow and IFR likely during the day. Accumulation
Sunday- Tuesday...slight chance of snow and MVFR.
245 am CST
High pressure centered south of Lake Michigan has maintained a west
flow across the lake...with a gradient producing gusts between
20-25kt at times for the northern half. Further south the gradient
is slightly less. This will persist through the day into tonight.
Then high pressure will slide southeast with a cold front sliding
south across the upper Midwest...arriving over the northern portion
of Lake Michigan late tonight. This front will steadily slide south
across the lake Friday...with winds turning north/northeasterly. Then a
developing low pressure across the Southern Plains...continues to be
the focus for Friday night into Sat...and slides northeast through the
Guidance continues to indicate this low pressure will slowly weaken
as it lifts northeast across the Ohio Valley...meanwhile a strong
ridge of high pressure over central Canada is expected to begin
sliding southeast across the upper Midwest Saturday. This will help
to initially bring winds to the northeast and produce a tight
pressure gradient for the central/southern portions of the lake
through Sat evening. Then a strong ridge of high pressure will build
in from the west for the second half of the weekend...with winds
turning northerly and increasing to 30 knots Sat night. High pressure
then slides overhead sun...and departs to the east for Monday with
winds turning west/southwest early next week.
Lm...Small Craft Advisory...nearshore Gary to Michigan City in until 3 PM Thursday.
Visit US at http://weather.Gov/Chicago (all lowercase)
Follow US on facebook...twitter...and youtube at: