Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
421 PM CST Wednesday Dec 4 2013
340 PM CST
Active weather pattern to persist into the weekend with fog
persisting and showers this evening...much colder air and light
snow possible for the southeast County Warning Area Thursday night into
Friday...as well as another system to move across the area late
this weekend into early next week.
In the near term...the County Warning Area remains in the warm sector as low
pressure to the west continues to lift north/northeast across the
region. In this much warmer airmass...well above normal temperatures in
the 50s have settled in for most areas with high dewpoint air
still streaming north across the region. Due to this warm and
moist airmass...areas of fog and dense fog persist at this hour.
Although there has been some improvement over the last several
hours...this improvement has occurred outside of the advisory
area. In the advisory area...north central and northeast
Illinois...dense fog with visibility at a quarter of a mile or
less is still in place. The most dense fog appears to be closer to
the Illinois/Wisconsin border...as well as for areas near the lake
where both locations are observing visibility near zero. Once
again...am not expecting any changes with conditions over the next
2-3 hours with the current advisory/location very much valid at
this time. Outside of this advisory...locations could still
observe areas of fog with visibility dropping as low as 1-3 miles.
All fog/low visible should improve around the 00z time frame as frontal passage
occurs in the 00-03 time frame...and with any denser fog lingering
across the Illinois nearshore to begin pushing to the
north/northeast as winds veer to the southwest and west this
evening. Weak large scale lift owing to strong warm air advection and subtle weak
middle level features will continue to aid in light shower
development...and especially with frontal passage this evening. Although
anything that falls should be light and more sporadic in nature.
Due to this...have left low chance probability of precipitation for this afternoon and
early evening...with chances for showers diminishing after 9 PM
tonight. After the departure of the fog/showers this evening...a
much colder airmass will begin working its way east across the
region and result in low temperatures in the middle-upper 20s and low 30s
After above normal temperatures today...below normal temperatures around 30 can
be expected for most areas on Thursday as this much colder airmass
continues to push southeast across the area through the afternoon.
Expect little improvement in cloud cover with some areas across
northern Illinois trying to become more partly cloudy/partly sunny
on Thursday. Although...broad southwest flow aloft will allow for
a continue stream of short wave troughs tomorrow and even into
Friday. These features as well as resultant low/middle level
baroclinic zone will allow for developing precipitation axis to the south
to inch its way closer to the far southern County Warning Area. Latest trends with
model guidance would keep any precipitation/snow just to the south of the
County Warning Area...but with some guidance still hinting that the far northern
extent of this precipitation could reach our southern tiers of the County Warning Area.
So trended towards lowering probability of precipitation Thursday night into
Friday...mainly for areas south of a line from Gibson City in
Illinois northeast to Rensselaer in Indiana...but did not
completely remove them. With the column sufficiently cold now at
this time...any precipitation would be all snow. If this does reach the
County Warning Area...current thinking/grids would suggest at most a half inch to
an inch of possible snowfall. Well below normal temperatures and quiet
weather will then be observed Friday night into Saturday
night...with steering flow likely to keep any lake effect precipitation
just outside of the area. Then continue to monitor the next system
to move through the region...which would be Sunday into Monday.
Guidance appears to have a fairly decent handle on this system and
if current trends continue...most areas could observe accumulating
snowfall with several inches of snowfall possible.
//Ord and mdw concerns...updated 22z...
* ceilings...mainly remaining IFR. However...there looks to be a
few hour window of improving ceilings above 1,000 feet above ground level after 00
UTC. Ceilings look to drop back around 7-800 feet following the cold
frontal passage around 04 UTC.
* Visibility...prevailing IFR to low MVFR improving significantly
after the frontal passage this evening.
* Patchy -dz/-ra.
* Wind shift with cold frontal passage this evening bringing
moderately gusty winds into Thursday.
another warm front is lifting northward across central sections of
the state late this afternoon. Ceilings and visible are improving nicely
in the wake of this front...and this front should lift across the
eastern terminals after 00 UTC. This could lead to a few hours of
better visible and ceilings. However...once the cold front pushes
through...around 04 UTC expect ceilings to drop again around 700-800
feet for several hours tonight before things improve later
Surface low becoming better organized 50sm S of msp at 17z with
elongated pressure fall center from east Minnesota across west central and north
WI... Michigan u.P. And Lake Superior. Models take this low north-northeast to far
east central Minnesota at 21z and to to far southeast Minnesota by 18z then NE to northwest-north
central WI by 00z.
Southeast-south-southeast low level winds across the local area continue to advect
warmer but also higher dew point air with surface dew point
depressions of 0-1 degree f over all but extreme S portions of Illinois
and in. Continued feed of this low dew point depression air
expected to maintain restricted visibility and low ceilings until the
surface winds veer a bit to S-south-southwest late afternoon-early evening as
the low moves to north WI. This slight west component to the low level
flow starts to advect in somewhat greater dew point depressions
which is expected to result in some increase in visibility and
A few patches of -dz/-ra currently moving NE and far north central
Illinois. Models continue to show persistent and gradually increasing
but still somewhat weak upward vertical velocity down into the low level moisture
through early evening as a middle level short wave trough rotates NE
over the middle MS valley. Expect current low coverage light liquid
precipitation to become somewhat broader by middle afternoon in response to
strengthening low level isentropic lift.
With the low prognosticated to move to West Lake Superior by 06z the
trailing cold front is expected to push west to east across the local
area early-middle evening. Strong cold air advection and pressure
rises will result in increasing and moderately gusty winds behind
the front. Currently...upstream winds behind the front are out of
the northwest but with the low moving north-northeast and away from the area tonight
expect local winds to be WSW-W. Subsidence and drying following
frontal passage should allow ceilings to rise to MVFR as current observed
from NE OK to southeast Nebraska.
Another short wave trough prognosticated to be extending from north central
WI SW to S central Iowa at 05.18z but with no middle level moisture to
work with and the continued decrease in low level moisture am
anticipating no precipitation with this disturbance and only cirrus
ceilings by sunrise Thursday.
//Ord and mdw confidence...updated 22z...
*medium confidence in ceiling trends.
*Medium- high confidence in visibility trends.
*High confidence in winds through period including timing of wind shift
with frontal passage +/- 1hr.
//outlook for Ord/mdw for 00z Friday-12z Wednesday...updated 12z...
Thursday night through Saturday...VFR.
Sunday...chance snow...with snow likely Sunday night.
Monday...slt chance snow. MVFR possible.
157 PM CST
The very active weather pattern for Lake Michigan will continue for
the next several days. A deepening low pressure system is lifting
northeast through the middle Mississippi Valley. An associated warm
front extends east across the northern portions of the lake with
strong east flow over the far northern portion of the lake with
modest southeasterly-southerly flow over the remainder. Unseasonable warm...moist
air has overspread relatively cooler lake water...leading to areas
of dense fog over the nearshore and open waters. A dense fog
advisory will remain in effect for much of Lake Michigan while it
remains under the influence of the warm...moist generally southerly
flow. As the low tracks through Wisconsin and across Lake Superior
tonight...it will deepen rapidly as a strong cold front sweeps
across the lake. Winds will quickly shift from southerly to westerly/northwesterly
ushering in much colder air overnight tonight and through the day
tomorrow. Not only will the wind shift and influx of cooler...drier
air help dissipate the fog...the strongest cold advection will
coincide with the most rapid deepening of the parent low. While southerly
winds will strengthen to just under gale force ahead of the front...
winds are expected to reach 35 knots...to possibly 40 knots...gales over
the northern portions of the lake where pressure rises will be
strongest in the wake of the low as it continues to the northeast.
So...will upgrade the gale watch to a warning. The pressure
gradient and pressure rises over the southern portions of the lake
will not be quite as strong...with another Arctic high building out
of southwestern Canada and into Central Plains tonight. Also...a weak ridge
axis associated with the Arctic high will poke over the southern
portions of the lake late tonight and tomorrow...which will also
help keep winds below gale force. However...the gradient should be
strong enough to support 20-30kt winds...so will keep the Small
Craft Advisory in effect for the nearshore waters. There will be
another potential period of gales for Friday night and into Saturday
as the Arctic high builds across the middle Mississippi Valley and a
middle level impulse drops out of Canada and tracks across the upper
Great Lakes. This potential gale event is a little less certain as
it would be highly dependent on the track of the high. If the high
center slides more over the Ohio Valley...there will be a better
chance for northwesterly gales...but if the high takes a more northerly
track...then winds would be lighter. Will not issue a headline for
this second potential gale event...but will carry the possibility in
the text of the forecast.
Illinois...dense fog advisory...ilz003-ilz004-ilz005-ilz006-ilz008-ilz010-
ilz011-ilz012-ilz013-ilz014-ilz019-ilz020 until 6 PM
Lm...dense fog advisory...lmz080-lmz261-lmz362-lmz364-lmz366-lmz563-
lmz872-lmz874-lmz876-lmz878 until 9 PM Wednesday.
lmz868...9 PM Wednesday to 9 PM Thursday.
Small Craft Advisory...lmz740-lmz741-lmz742-lmz743...11 PM
Wednesday to 3 PM Thursday.
Small Craft Advisory...lmz744-lmz745...1 am Thursday to 4 am
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