Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
904 PM CST Monday Dec 9 2013
718 PM CST
Only issues thus far have been temperatures taking a nose dive.
Rfd dropped to 3 degrees already with other stations under clear
skies tanking as well. As lowered low temperatures for tonight with
outlying areas dipping just below zero. Based on trends...temperatures
may fall a bit lower than currently forecast for areas north of a
LaSalle to ohare Airport line. Basically where the sky is clear.
Used hrrr trends with a few hand edits to get temperature trend tonight
since areas under the cloud cover will cool very slowly.
Issued a Special Weather Statement to highlight the cold temperatures and wind chills since
this is the coldest we have been this season. Min wind chills
of -10 to -15 are expected for areas in and north of a LaSalle
County to Will County line. Isolated areas north of I-90 may even
seen min wind chills approaching -20. However the area of -20 will
not be widespread enough to warrant a Wind Chill Advisory.
Otherwise made minor adjustments to sky cover as the
rest of the forecast is on track. Still expecting temperatures to rise a
bit around sunrise as cloud cover associated with the next clipper
253 PM CST
It appears we are stuck in mother natures winter pattern...and for
the next several days it does not appear we are looking at a break
from this setup. Long-term pattern is beginning to show just a brief
respite...then a return to unseasonably cold setup.
It's The Gift that keeps on giving Clark.
This afternoon/evening...will remain relatively quiet for the
forecast area...however that will change towards daybreak Tuesday.
Surface ridge has become centered across the middle-Missouri
Valley...beginning to stretch east through the Ohio Valley. The
departing surface low that brought the fresh blanket of snow has lifted
northeast of Lake Huron...meanwhile a clipper is beginning to dip
into the northern plains late this afternoon. Infrared imagery shows a
channel of clouds over Saint Louis stretching northeast through
Southern Lake Michigan...associated with a frontal boundary that
continues to slowly slide east/southeast across the forecast area.
Temperatures ahead of this boundary have held steady in the low 20s...while
behind this boundary temperatures have already fallen into the middle/upper
teens. Across the Stateline into Iowa...temperatures have held in the
single digits above zero.
Guidance has trended slightly further south with the first clipper
tonight...and as a result have increased probability of precipitation across the northern
County warning forecast area. The pressure gradient does increase this evening...and will
persist through daybreak Tuesday. This will likely result in some blowing
snow. As noted about increased probability of precipitation...this has resulted in light
snow expected just before daybreak Tuesday for the northern half of the
County warning forecast area.
Using the Cobb technique would suggest a few points between 09-12z
could pick-up a couple tenths of an inch of snowfall. With the
breezy conds expected...and cold air in place...temperatures will likely
tumble to the single digits above zero and wind chill values well
below zero and nearing -15 in a few locations west of the Fox
Tuesday through Wednesday...
500mb trough axis has trended stronger...which has also displaced
the clipper poised to arrive across the region further south Tuesday
morning. As a result have introduced probability of precipitation to accommodate this through
18z Tuesday. But in traditional fashion...this clipper should quickly push
east by early afternoon with weak surface ridging pushing back into the
forecast area. Some weak warming may occur with this system
early...allowing temperatures to warm into the teens to possibly low 20s along
the southern County warning forecast area Tuesday afternoon.
Then yet another clipper is prognosticated to traverse the region late Tuesday
ngt/Wed...which has continued to trend further south and stronger.
With the surface ridge drifting south into the tenn valley...this will
likely allow the wave to push across The Heart of the County warning forecast area. Quantitative precipitation forecast of
around 0.10" for this second clipper snow ratios will hover around 15:1
to 20:1. The best Omega/lift is pushing into the favored dgz Wednesday
morning...which could help produce larger dendrites and increase
snowfall accumulate rates. At this point have accums generally between
1-3"...and could see a few locations between the I-80 and I-88
corridor could pick up close to 4" by late Wednesday morning.
Then high pressure builds back into the region Wednesday night...setting the
stage for a very cold night into Thursday morning. The ridge center has
also trended slightly further south...which will likely keep some
west/northwest wind component. Despite this temperatures should still radiate
into the single digits below zero.
Confidence in accumulating snowfall Wednesday...high.
Thursday through Sunday...
previous guidance had held the surface ridge over the forecast area Thursday
morning...with dry weather. However as has been the case with the
prior couple of clippers...and placement of the 500mb trough
axis...guidance has now introduced yet another clipper slicing
southeast across the northern plains early Thursday...and could bring a
period of light snow/flurries into northern Illinois Thursday morning. This
may result in temperatures Thursday morning not getting as cold...but that will
hinge upon timing and cloud cover from the aforementioned clipper.
Then a relatively quiet period for Friday with the 500mb flow trying to
flatten...but this will only be brief. Guidance is now beginning to
trend towards an inverted trough developing in the wake of the surface
ridge Friday...with a strong shortwave taking shape across the Southern
Plains Friday evening. This system will need to be closely monitored
for the upcoming weekend...as this could spread accumulating
snowfall through the Ohio Valley into northern Illinois/northwest in.
The latest GFS indicates an 850mb low associated with this inverted
trough...lifting northeast across central Illinois then northeast into
northcentral in midday Sat. Low level flow ahead of this feature will be
turning northeast Sat...setting up the potential for a hybrid system
of lake effect snow and potentially synoptic snow Sat.
Temperatures should begin to warm to near 30 degree...or possibly the freezing
point Friday through Sat. Then with yet another Arctic airmass poised to
push behind this system sun.
//Ord and mdw concerns...updated 03z...
* low chance for snow showers middle/late Tuesday morning with brief
IFR visibility possible but coverage may be low.
* Southwest wind gusts 25-30 knots by middle morning turning west and
increasing to 30-35 knots early afternoon.
* Accumulating snow chances increase Tuesday night...IFR likely.
Strong west winds are beginning to ease with gust frequency
diminishing as well. VFR will continue through the night with
winds turning southwest late tonight. Wind speeds will increase by
middle Tuesday morning with gusts of 25-30 knots developing as an
Alberta clipper reaches the area. The bulk of the snow with this
system will be to the north but expect that there will be at least
a chance for some snow showers to extend toward the terminals
during the morning. Coverage may not be all that high and
confidence in occurrence is somewhat limited but will continue
with vcsh/prob30 mention for now. Should have better confidence
going into the overnight as the system tracks across the Dakotas
and Minnesota/Iowa. Should snow showers cross the area then
intensity could be such that IFR visibility could occur for a short
time. The system will quickly exit by early afternoon turning
winds westerly and allowing wind gusts to increase to 30-35 knots
before diminishing Tuesday evening. Attention will then turn to a
stronger clipper system which will cross the area Tuesday night
and Wednesday morning bringing much better chances for
accumulating snow and IFR conditions.
//Ord and mdw confidence...updated 03z...
* medium confidence that there will be some snow showers in the
area middle/late Tuesday morning. Low-medium confidence that if
snow showers do occur then brief IFR visibility is possible.
* High confidence in wind forecast Tuesday.
* High confidence in snow chances increasing Tuesday night and
medium confidence in at least some IFR.
//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.
145 PM CST
Main marine forecast concerns are a couple of periods of gale force
winds over Northern Lake Michigan this afternoon/evening...and again
Tuesday and Tuesday night across the entire lake. Have upgraded gale
watch to a Gale Warning for Tuesday and Tuesday night.
Strengthening low pressure was located just north and northeast of
Lake Superior this afternoon...and will continue to deepen as it
lifts toward James Bay this evening. As the low deepens...a period
of tightening surface pressure gradient was resulting in low-end
west gales across mainly Northern Lake Michigan with Grand Traverse
light gusting 34 kts as of noon CST. Gales will linger tonight as
the low pulls away...gradually easing after midnight across the far
northern portion of the lake as weak high pressure ridging moves
across the western lakes. The lull looks to be brief however...as a
fast moving clipper system currently moving across far southern
Saskatchewan travels quickly to the upper Mississippi Valley
overnight...and across the northern/western Great Lakes region on
Tuesday. This will induce a quick re-tightening of the surface
pressure gradient across Lake Michigan Tuesday morning...with strong
surface-based mixing and cold advection expected to produce westerly
40 knots gales based on model forecast soundings. Gales look to persist
into Tuesday evening across the south half of the lake...and past
midnight farther north. High pressure then builds southeast from the
Central Plains through the middle/lower Missouri and lower Ohio River
valleys Wednesday and Wednesday night...allowing gales to ease
though winds remain westerly in the 20-30 knots range in the tighter
gradient north of the surface high and south of low pressure which
lingers over Hudson Bay. West winds will linger through Thursday as
the high spreads east through the Ohio Valley...and another weak low
crosses the northern lakes.
In the near term...small craft advisories are in effect for the
Illinois and Indiana near shore waters through early Tuesday... with
winds/waves diminishing more quickly along the Illinois shore
tonight. The Gale Warning Tuesday/Tuesday evening includes the near
Lm...Gale Warning...lmz261-lmz362-lmz364-lmz366-lmz563-lmz565 until 3
Gale Warning...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...nearshore Winthrop Harbor to Gary in until 10 PM
Visit US at http://weather.Gov/Chicago (all lowercase)
Follow US on facebook...twitter...and youtube at: