Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
611 PM CST Sat Dec 21 2013
331 PM CST
..winter storm...Through tomorrow afternoon...
Headline notes...will allow the Freezing Rain Advisory to expire at
4pm for east central Illinois counties.
The Winter Weather Advisory from LaSalle County to Chicago will be
left as is to expire at 10pm. However...the evening shift may need
to consider extending at least parts of the advisory to Sunday
morning pending trends with sleet/freezing rain or drizzle
this evening followed by light snow accumulations late tonight
through Sunday morning.
The Winter Storm Warning and northern portion of the Winter Weather
Advisory (lake and kane) will now be started at 6pm as most
significant impacts are not expected until later this evening.
Interestingly enough...guidance continues to be split on evolution
of this system even at this late hour...with the 18z NAM shifting
southeast with track of upper level and surface low closer to
GFS/European model (ecmwf) solutions. However...the overall theme remains consistent
in focusing the heaviest snows into north central and portions of
far northeastern Illinois despite this continued variance.
The warm conveyor belt precipitation will continue to stream into
areas primarily along and south of Mendota to Chicago line into
early this evening...but just skirting the northern County Warning Area. Thermal
profiles are quite complex...with a warm nose of over 4c evident on
a recent Ord aircraft sounding just below 800 mb. As the
precipitation works its way north...this will cool some with
saturation but still enough to at least partially melt ice crystals.
So areas west of Chicago are in line for sleet or freezing
rain/plain rain this evening where it is likely to remain above
freezing/including downtown Chicago. Ice accumulations possibly up
to one tenth of an inch in some areas will result in hazardous Road
conditions. Farther south and east...the entire column will remain
above freezing resulting in plain rain continuing. Far
southeastern portions of the area could see some nuisance type
ponding and small stream flooding. The other concern is for areas
of fog...locally dense...continuing into the middle or late evening
north of a pnt to gyy line.
In the winter storm and northern portion of Winter Weather Advisory
area...with heaviest precipitation remaining south into the evening...only
anticipating light snow and sleet accumulations into this evening.
Here it will be somewhat borderline for all snow with a less
pronounced warm layer gradually cooling with time to below freezing.
The farther south and east in this area you go can expect more of a
threat for light ice accumulations due to the potential for full
melting of ice crystals.
During the overnight hours...a pronounced midlevel dryslot with the
upper system clearly evident on west/v imagery will envelop the
southeastern half to two thirds of the area. This will cut off the
steadier precipitation and result in periods of light rain and
drizzle/fzdz...with possibly some light sleet mixed in. At this time
is also when the trowal feature/deformation axis will most greatly
impact north central and far northeastern Illinois. Guidance
continues to suggest strong midlevel frontogenesis and lift through
the dendritic growth zone...so periods of heavy snow are likely with
snowfall rates occasionally in excess of 1 inch per hour.
Again...trends will need to need to be monitored closely for the
trajectory of the heaviest banding. This is the time of the most
significant impacts and when Road crews could have trouble keeping
up with the snow accumulations. We are also concerned that the quantitative precipitation forecast
the models are showing under the dry slot could be a steadier
drizzle and thus freezing drizzle for areas below freezing. This
could necessitate the extension of the Winter Weather Advisory from
La Salle County to Chicago as mentioned above. Though note that the
city itself should remain above freezing and mitigate impacts.
Finally...into Sunday morning...the deformation axis will pivot
across the northern County Warning Area. The big question is how pronounced the dry
slot is at that time...with the GFS much more bullish than the other
models in returning saturation to areas close to Chicago. This will
impact how much snow falls during the morning southeast of the
heaviest snow axis in our County Warning Area when thermal profiles are finally
supportive of all snow for most of the area. At this time...have
leaned toward the dry slot making it tough for much accumulation in
the city and points just south...but if the GFS solution comes to
fruition...amounts could be a bit higher than currently forecast. On
the other hand...with the best forcing still to the north and
departing northeast...would not expect any heavy snowfall rates.
The overall idea in forecast snowfall amounts remains very similar
to previous forecasts...highest in the 6 to 8 inch range north of a
Woodstock to Dixon line and quickly tapering to the southeast...with
less than inch in downtown Chicago and little or no accumulation
points south and east.
By Sunday afternoon...system snows will exit to the north as the
surface low moves to the eastern Great Lakes. A northern stream
trough moving overhead could keep some light snow going...with
little or no accumulation anticipated. As colder air arrives with
gusty north/northwesterly winds...there could blowing and drifting
snow in open areas of north central and far northeastern Illinois.
Also...any remaining moisture on roads and sidewalks could
quickly freeze later in the afternoon.
For the expended forecast period...from Monday night through
Saturday...including the Christmas Holiday...
The sensible weather remain largely benign for the extended forecast
period...with the primary concerns being temperature trends and the
chance for some light snow from Tuesday evening through Thursday
morning. For this period...the larger scale models are in very good
agreement on a very high amplitude...quickly progressive pattern over
North America. What this will translate into to northern Illinois
and Northwest Indiana will be a roller coaster ride of a temperature
trend with dips to well below zero fahrenheit to comparatively mild
highs...though even the highest of the highs will still be below
For Monday...the day will start out on the cooler side as a deep
upper trough digs into the Mississippi Valley as high pressure
builds across the northern plains and spreads eastward. There will
actually be little diurnal temperature chance on Monday with highs
only a couple degrees higher than the morning lows as the region
sits under persistent strong cold advection. The coldest conditions
of the period will come for Tuesday morning as the center of the
high slides into Missouri while the upper level pattern amplifies
further while the longwave pattern wavelength decreases. The
building ridge will extend from the south High Plains toward Hudson
Bay...setting up a pattern that will draw cold Arctic air into the
region. Additional cooling will be aided by clearing skies and
weakening winds...which will permit some radiational cooling to
complement the advective cooling. Also...a weak pressure gradient
will remain in place as the center of the high will settle into
Missouri. Air temperatures expected to drop well below zero over
portions of the County Warning Area north of a line from Chicago to Pontiac...with
some of the low lying...sheltered areas dropping as low as
-10f...even the expected relatively light winds of 5-10kt will be
sufficient to generate wind chill reading of -20f to -25f.
Locations south of the Chicago to Pontiac line are expected to
remain slightly warmer with low in the single digits above zero...so
wind chill reading will "only" be in the -10f to -15f range.
So...while the wind chills over the northwest half of the County Warning Area will
likely reach the Wind Chill Advisory criteria levels...given the
ongoing winter weather...will hold off on issuing any type of
headline at this time.
The other significant concern will be the chances for some light
snow for Christmas evening through Christmas day. Following the passage
of the high pressure off to the east...the upper ridge will also
progress to the east of the local area while a broad upper trough
carves out over the northern and Central Plains with a series of
embedded shortwaves. While the shortwaves and associated surface
troughs/weak cold fronts through the period from Christmas evening
through boxing day...will provide ample forcing for generating
precipitation...and deep layer cold air in place will guarantee that the precipitation
will be all snow...the missing ingredient for any significant snow
will be deep layer moisture. From Tuesday night through
Thursday...the County Warning Area will be cut off from any Gulf moisture by a ridge
of surface high pressure extending from the middle Atlantic region through
the Red River valley. So...while it is looking like there should be
periods of some light snow falling through the Christmas Holiday
period...actual additional accumulations should be light.
For the remainder of the extended forecast period...the roller
coaster ride of temperatures will continue as the patter remains
quickly progressive. Temperatures should moderate some on Wednesday
with highs in the upper 20s to around 30f and lows Thursday morning
ranging from the lower teens over the Rockford area to the lower 20s
over the southeastern portions of the County Warning Area...another shot of Arctic air will
intrude on the area through the day on Thursday...with only a few
degrees of warming above morning lows...while Friday morning...
temperatures should drop below zero over the northwestern portions of the
County Warning Area...reaching down to -5f over the Rockford area. However...since
the track of the surface high will be through Iowa and Illinois...
leading to lighter winds and less of a wind chill factor. On the
other hand...if there is more sky clearing to go along with the
lighter winds...there may be more radiative cooling than would be
suggested at this time and min temperatures for Friday morning may be lower
than currently anticipated and the County Warning Area could be flirting with
advisory criteria wind chills again. Other than the chance for some
light snow around Christmas...no other precipitation is expected through the
remainder of the extended forecast period.
//Ord and mdw concerns...updated 00z...
* low probability of rain mixing with sleet/freezing rain tonight.
* IFR/LIFR ceilings into early Sunday. IFR improving to low-MVFR
late Sunday/Sunday evening.
* A period of snow Sunday morning with minor accumulation.
* Potential for additional light snow/drizzle Sunday afternoon and
light snow Sunday evening.
* Northeast winds 10-15 knots becoming north-northwest Sunday morning.
Main aviation concerns continue to be precipitation type with
winter storm tonight into Sunday...and IFR/LIFR ceilings into
Sunday morning. Gradual improvement through IFR to low MVFR
expected late Sunday/Sunday evening.
Low pressure was deepening across the lower Mississippi and lower
Ohio valleys early this evening...in association with a deep upper
trough which was lifting northeast from the Southern Plains. The
low will track northeast along a strong frontal zone into Ohio by
early Sunday morning...spreading precipitation across the area.
Precipitation type concerns revolve around current amdar
observations which depict a warm layer aloft several degrees
warmer than those in model forecast soundings. Thus areas which
were anticipated to be more of a rain/freezing rain/sleet/snow mix
are seeing rain at this time...with freezing rain and a little
sleet mainly west of Chicago where surface temperatures were at or below
freezing. For the Chicago terminals...this has reduced the
potential for rain to mix with sleet tonight...and freezing rain
also appears to be a minimal threat for Ord/mdw/gyy with surface
temperatures expected to hold just above freezing through the
night. Dry slot developing aloft later tonight will likely allow
precipitation to diminish to drizzle after midnight. For
rfd...which is further into the cold air...amdar soundings are
also warmer aloft than forecast though enough cooling is still
expected to occur there which will result in mixed precipitation
eventually changing over to a period of moderate-heavy snow late
tonight/early Sunday morning.
Extensive IFR/LIFR ceilings across the region have shown some
improvement into IFR over northern Indiana and the southeast parts
of the Chicago area with some of the slightly heavier rain areas.
The expectation however is that conditions will settle back to
LIFR overnight especially with the dry slot moving in after
midnight and drizzle/fog. IFR/LIFR conditions expected to continue
early Sunday with a period of snow...gradually improving to low-
end MVFR by Sunday evening.
Winds will be northeast 10-15 knots overnight as the low moves into
the Ohio Valley...shifting to the north-northwest early Sunday and
to the northwest during the day Sunday. Winds may gust 15-20 knots at
times during the day.
//Ord and mdw confidence...updated 00z...
* medium confidence in precipitation remaining mainly rain/drizzle
* Low confidence in any significant sleet/freezing rain for
* Medium-high confidence in prevailing IFR/LIFR ceilings into
* Low confidence in snow amounts/intensity Sunday morning.
* High confidence in winds through period.
//outlook for Ord/mdw for 12z Monday-12z Saturday...updated 00z...
Monday...chance of shsn. MVFR/IFR possible.
Wednesday...chance of light snow. MVFR/IFR possible.
Thursday...slight chance of light snow. MVFR possible.
237 PM...low pressure over Arkansas will move northeast to near
Lake Erie by Sunday morning and continue northeast across New
England Sunday and Sunday night. At the same time high pressure
will remain nearly stationary across Manitoba and northern Ontario
through Sunday. As this low approaches the lakes region
tonight...the gradient will tighten with northeast winds to 30 kts
developing this evening and continuing into Sunday. The gradient
will likely weaken some across the far south end of the lake
Sunday morning. A weaker trough of low pressure still appears like
it will remain across the lakes region Sunday night. This may make
for a problematic wind forecast...specifically speeds. It appears
winds may diminish into the 10-20kt range over much of the lake.
The ridge noted above will merge with a stronger high that will
move south across the northern plains Sunday night. This high will
then turn across the Ohio Valley Monday night into Tuesday. Thus
the gradient may increase again Monday into Monday night...
especially over the southern part of the lake. After this high
moves east...winds will quickly turn southerly Tuesday afternoon/
evening and increase to 30 kts...ahead of a large trough of low
pressure that will move across the region Wednesday night into
Thursday. There could be a prolonged period of 30 kts Thursday
into Thursday night as winds shift westerly behind this trough/
Fog will still be possible into this evening across the southern
end of Lake Michigan and there is some potential for locally
dense fog...just not too confident on duration or coverage. Cms
Illinois...Winter Storm Warning...ilz003-ilz004-ilz005-ilz008-ilz010-ilz011
until 10 am Sunday.
Winter Weather Advisory...ilz006-ilz012 until 10 am Sunday.
Winter Weather Advisory...ilz013-ilz014-ilz019-ilz020-ilz021-
ilz022 until 10 PM Saturday.
Lm...Small Craft Advisory...Illinois nearshore waters until 3 PM Sunday.
Small Craft Advisory...Indiana nearshore waters until 4 am Monday.
Visit US at http://weather.Gov/Chicago (all lowercase)
Follow US on facebook...twitter...and youtube at: