Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
917 am CST Sat Nov 22 2014
917 am...morning update...temperatures have warmed into the upper
30s and lower 40s with the threat of freezing rain over and let
advisory expire at 9 am.
Focus now shifts to the potential for dense fog across the far
northern and northwest portions of the County Warning Area today and tonight.
Visibilities currently between 1/2sm and 2sm. Not expecting much
improvement in these areas today so added fog to the forecast...
generally north of I-80 but confidence regarding how soon dense
fog may become prevailing and then how long it will last is still
low. But a dense fog advisory for portions of the northern County Warning Area may
be needed later today or this evening. Cms
315 am CST
The primary concerns with the forecast center on freezing rain
threat early this morning...then rain potential and amounts through
the rest of the weekend.
An area of showers is moving quickly northeast over the immediate
region as of 315 am. This is within strengthening forcing from a
short wave trough...and this greater lift is coincident on the nose
of a 45 knots low-level jet per VAD wind profilers. Overall much of
this forcing and modest shower activity will continue to shift east
through sunrise. Steadily rising temperatures had leveled off some
very early this morning with the heavier precipitation intensity...although
a slow increase should restart prior to daybreak and persist through
today as warm advection wins out. Temperatures of 31 to 33 as of
315 am mainly remained in the northern forecast area...including
Rockford and the North Chicago metropolitan...and parts of Northwest
Indiana including Valparaiso. Pavement temperatures from a handful
of sources varied some...but for the most part were above freezing
in Chicago and southward from there. So will continue advisory as
is for now and make a reassessment just prior to 6 am to see if
any parts need to continue to 9 am.
While much of the modest precipitation should shift eastward early this
morning...broad ascent in the middle-levels will remain with an upper
speed maximum moving overhead. This probably will be enough for
scattered/intermittent showers especially through early afternoon.
In the lower-levels...high dew point air will continue to be
advected northward establishing the low clouds. Isentropic Omega
within these lowest levels looks to support areas of drizzle...and
already seeing indications of this near the dvn rda. So continue
areas of drizzle wording. Temperatures will be the warmest they have
been since late on the night of November 10th...with 40s expected
areawide and even 50 tagged south of Interstate 80 in some
locations. Tonight temperatures should basically hold steady and
maybe even rise some. If temperatures are able to drop a few
degrees...the threat for fog becomes higher...although forecast
soundings generally do not look to support anything dense given
the already present low cloud deck.
Rain should begin to develop within greater moisture advection and
convergence across central Illinois later tonight and spread
northward into the forecast area. Greater intensity looks to be
seen beyond tonight into Sunday.
315 am CST
Sunday the region will remain in a broad warm air advection regime
ahead of a pair of troughs approaching from the west. Broad deep
southerly flow will continue to usher warmer and increasingly more
moist air into the region. Broad isentropic ascent will continue to
support at least some occasional showers or drizzle early before building
into a steadier/heavier rain later in the day as the strong southern
stream shortwave begins lifting northeast into the region. If early
portion of the day isnt too wet/showery then highs could reach well
into the 50s...especially southern County Warning Area as guidance is focusing the
better forcing north early.
The northern and southern streak shortwaves are still prognosticated to
begin phasing over the western Great Lakes region Sunday night into
Monday. Guidance continues to suggest that an impressively moist air
mass will be transported north into the region Sunday into Sunday.
Given the magnitude of forcing and ample moisture with precipitable waters nearing
2 Standard deviations above average some healthy rainfall totals
look likely...with amounts Sunday into Sunday night probably ending
up close to an inch. Long duration of the precipitation should result in
little more than maybe some minor ponding of water in low spots. Rfc
contingency forecasts do suggest some respectable rises on area
rivers...though if current quantitative precipitation forecast verifies rivers will remain below
As northern and southern stream systems phase guidance is in
agreement that the surface low will rapidly...and potentially
explosively if the GFS verifies. There is a fairly large spread in
model guidance with respect to just how deep the low gets Monday afternoon with
the GFS and several of its ensemble members in the between 961 and
966mb...while the European model (ecmwf) is a still impressive but less extreme 979.
Lack of strong high pressure in the wake of this system will
mitigate the magnitude of the winds some...but latest guidance has
trended stronger with winds again Monday. Still possible there will
be some waffling in guidance for the Monday time range as models
often struggle with phasing systems...but have followed the trend
from the day shift of stronger winds Monday with gusts of 35-40 miles per hour
possible. If the more extreme GFS model Camp verifies then gusts
could near advisory criteria Monday...a trend Worth watching the
next couple days.
Warm conveyor belt precipitation should get shunted north of the area later
Sunday night...however models suggest that deformation precipitation could
potentially wrap as far south as our County Warning Area Monday...especially
northern County Warning Area. Colder air will come flooding in on the strong
westerly winds meaning any defo precipitation that does fall would
transition to snow/snow showers Monday morning. Far too soon to have
much confidence in any potential accums...but better threat of snow
accums looks to remain north of the area with accums...if
any...probably on the light side in our area.
Stratus deck likely gets locked in Monday night tempering the low
temperatures a bit but magnitude of the cold air advection should still
allow 20s across the area. Cloud cover may stick around into a good
chunk of the day Tuesday as The Heart of the cold air mass traverses
the area which looks to result in very little temperature recovery from
overnight lows Tuesday morning.
Brief and somewhat muted shot of warm air advection Tuesday night
into Wednesday should occur ahead of a clipper system...potentially
bringing some light snow to the area Wednesday. Medium range
guidance varies with respect to the track of this system so
maintaining probability of precipitation in the lower end of the chance spectrum for now.
Appears likely that another blast of unseasonably cold air will
follow this clipper in time for Thanksgiving and the start of the
long Holiday weekend with highs once again heading back down into
//Ord and mdw concerns...updated 14z...
* IFR ceilings likely through the day with temporary LIFR
possible...namely this morning and tonight.
* IFR visibility in drizzle this morning and potentially lasting
into this afternoon.
* Low-level wind shear tonight.
* Near due south winds Sunday morning.
* Rain Sunday morning and continued IFR conditions.
Temperatures have risen above freezing...with all precipitation
now falling as drizzle to light rain. Abundant warm/moist air
continues to feed north across still cold ground...and will likely
keep visibilities reduced through the day along with lower ceilings. Could
see IFR conds linger through the entire day.
Previous discussion updated 12z...
Widespread IFR ceilings have overspread the region as moist air
continues to stream back northward. Within that are areas of
drizzle and IFR visibility which should continue through early
afternoon and possibly even longer. The threat for freezing
precipitation continues...but will slowly diminish. Rfd is the
most likely area taf site to experience any of this beyond
12z...but should be done by 14z or 15z.
The IFR ceilings may improve this afternoon but are still expected to
remain in the IFR category...and upstream observations are
supporting this. Tonight the pattern favors ceilings to ease their way
back down. Drizzle will be possible tonight with rain likely late
and chances increasing into Sunday morning.
Winds will be from the south-southwest through this evening and
then turn more toward the due south overnight into Sunday morning.
Once again tonight a low-level jet around 2000 feet will support
low-level wind shear near the inversion height.
//Ord and mdw confidence...updated 14z...
* high in IFR ceilings.
* High in drizzle occurring this morning and medium after. High in
IFR visibility at times this morning but low on what visibility
will be this afternoon.
* Medium-high in low-level wind shear tonight.
* High in IFR conditions tonight into Sunday morning. Low on
specific rain timing and visibility within rain.
//outlook for Ord/mdw for 00z Monday-12z Saturday...updated 12z...
Sunday night...rain and IFR probable. Southerly winds.
Monday...chance of rain and snow. Strong west to southwest winds.
Tuesday...MVFR ceilings possible. Westerly winds.
Wednesday...chance of snow and MVFR. Southwest winds.
Thursday...flurries and MVFR possible. Northwest winds.
Friday...VFR probable. Southwest winds.
325 am CST
An active pattern over the lake is expected to continue through the
upcoming week. The southwest gales seen over the lake early this
morning will diminish by middle-late morning as the pressure gradient
somewhat diminishes. Small Craft Advisory criteria winds will
continue though across the Illinois and in nearshores a good part of
the day. It does look like wind speeds will diminish to more 15-25
knots for the evening and then re-increase overnight where another
advisory may be needed.
Moderately strong southerly flow will continue Sunday in advance of
the complex low pressure system organizing to our west. By Sunday
night...low pressure tracking east from the northern plains will
begin to merge with low pressure tracking northeast from the
Southern Plains. Models suggest this phasing of the two lows will
take place very near Lake Michigan Sunday night with a brief period
of lighter winds near the center of the developing storm system. The
low is forecast to continue to rapidly intensify as it moves north
Monday allowing for westerly gales to develop on the lake in its
wake Monday. While model agree in the general idea of a strong low
north of Lake Superior by midday Monday...they vary considerably on
how strong the low will be ranging from 28.5 inches to around 29.0
inches. If the stronger solutions were to verify then a period of
storm force west winds could occur Monday...despite the lack of any
strong high pressure moving in behind the storm. If the weaker
solutions pan out then lower end gales would still likely occur
Monday. It is likely that a gale watch will be issued for the
Monday/Monday night time frame later today. Winds will ease Tuesday
and back to southerly Tuesday night into Wednesday ahead of a
clipper before another shot of strong and possibly gale force north
or northwest winds develop Thursday-ish time frame.
Lm...Small Craft Advisory...nearshore waters until 3 PM Saturday.
Visit US at http://weather.Gov/Chicago (all lowercase)
Follow US on facebook...twitter...and youtube at: