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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
833 PM CST Sat Feb 13 2016

832 PM CST

A few minor tweaks to the forecast this evening.

First...high cloudiness is spreading east across the area and
nudged sky cover up a bit more quickly tonight. As a result of the
cloudiness and developing southeast winds overnight anticipate
only a modest temperature fall from here on out...most pronounced over
eastern County Warning Area where clouds and winds will be latest to arrive. As
clouds thicken and winds pick up temperatures will probably begin a slow
upward rebound prior to daybreak.

Narrow west-northwest-east-southeast band of frontogenetically driven snow is developing
eastward this evening into west central Illinois. Short range
guidance suggests that the frontogenesis will weaken overnight as
it lifts slowly northward. Not uncommon for short range guidance
to weaken such features unrealistically and for them to persist
longer than models suggest. If this were to occur...then could see
snow gradually inch northeast into our southwestern most County Warning Area prior
to dawn. Midnight shift will need to monitor trends closely.

Water vapor imagery early this evening shows powerful shortwave
over the northern High Plains digging southeastward. Nldn has
shown a fair amount of convection breaking out in advance of this
feature over South Dakota. Numerical guidance all agrees that this
shortwave will begin dampening out overnight and particularly on
Sunday as it encounters a more confluent flow on the backside of
departing upper low responsible for our current cold snap.

The million dollar question tomorrow is how quickly the snow in
advance of this feature will weaken in response to the weakening
system. Made no changes to going forecast for tomorrow...though
some of the latest guidance coming in would suggest that snowfall
totals could end up being skewed a bit more toward the lower end
of the forecast ranges.

Steep middle-level lapse could allow some banding features to
develop...even though forcing may be waning with time tomorrow.
Any banding features that do develop would locally enhance
snowfall amounts and could allow some areas to see snowfall
amounts a bit higher than latest guidance supports and more in
line with higher end of our forecast snowfall amount ranges. Given
the range of possibilities see no reasons to make any changes to
forecast at this time.



Short term...

Tonight through Monday...

257 PM...main forecast concern is snowfall amounts Sunday through
Sunday night.

Clipper system to affect the region in two parts with the first
area of snow extending from Iowa into central Illinois late tonight and
then swing east through the County Warning Area during Sunday morning and early
Sunday afternoon. This will likely produce a period of moderate
snow before diminishing some in the afternoon...though still
snowing lightly. As the surface low/trough approaches Sunday
night...there will likely be another prolonged period of light
snow...perhaps briefly moderate. Model quantitative precipitation forecast forecasts have focused
the higher amounts...two to three tenths...more on northwest Illinois
into central Illinois with amounts generally under two tenths across
northeast Illinois and Northwest Indiana. As a result...snowfall totals
now appear to be just a bit lower for the immediate Chicago area
and likely in the 2 to 4 inch range...perhaps closer to 2 inches
across Northwest Indiana. These amounts are also in line with
lower amounts from wpc. But snowfall amounts across the western
and southwest County Warning Area...while also just a bit lower...are still in
the 3-5 inch range and have issued a Winter Weather Advisory
from Boone south to Grundy/Iroquois. If later model runs end up
bringing higher quantitative precipitation forecast amounts further east...its possible the
advisory may need to be extended further east.

As the snow spreads east across the County Warning Area Sunday morning...there
could be a period or burst of moderate snow and this is when
snowfall rates may approach one inch an hour for at least a few
hours. The snow will also be fairly dry/fluffy and increasing
southeast winds...perhaps gusting to 25 miles per hour range...will likely
lead to some blowing and drifting snow.

Temperatures will likely drop into the single digits this evening...
level off and then start slowing rising as cloud cover increases
overnight. But overall trend in temperatures especially this evening is
low. After light winds this evening...southeasterly winds will
likely keep wind chills near zero or slightly below zero overnight
into Sunday morning. Temperatures rise into the lower 20s Sunday and
then end up steady to slowly rising Sunday night into Monday as a
bit more mild air advects in behind this clipper. Despite a new
snow pack and probably extensive cloud cover...thermal fields
suggest low/middle 30s for highs on Monday. Cms


Long term...
308 PM CST

Monday night through Saturday...

Main forecast concern is with another clipper system on Tuesday
and associated precipitation chances and type. In addition...significant
warm-up still appears to be on the way to close out the work week.

Primary feature of note with the Tuesday clipper is a midlevel
shortwave noted on west/v imagery this afternoon over the Gulf of
Alaska that will move ashore onto the British Columbia coast on
Sunday. This wave will then dive southeast...crossing the northern
plains Monday night. Another subtle shortwave trough will move
across the area on Monday night...with dry air and best forcing
passing south keeping conditions dry with just partly/mostly
cloudy skies and temperatures in the 20s.

There is fairly significant spread on the operational and
ensemble guidance regarding the track of the clipper low well as its strength. NCEP based guidance has been
running north of foreign counterparts....though 12z Canadian and
European model (ecmwf) (and ensembles of both) did come into slightly better
agreement with GFS. Gefs trended a bit flatter with wave and
weaker with slope mean. Because of aforementioned differences in low
track and strength...confidence is relatively low in the evolution
of the system and local sensible weather impacts. It's possible
that downstream trough and surface low to the east could also
force a weaker...farther south track.

With this being a nod to 12z guidance trends...bumped
up probability of precipitation to chance range area wide. Thermal profiles are
marginal...especially with southward extent. Operational 12z
NAM/GFS indicated a warm layer at around 925 mb and with expected
highs above freezing...indicated chance light rain/snow south of
the Illinois and Kankakee rivers and light snow north. Strength of the
low as it passes near or south of central Illinois will also help
dictate magnitude of forcing...with potential for decent fgen on
north side of a stronger low and changing any mix to snow. If this
scenario occurs...some wet snow accumulation would be possible
while on the other hand...weaker low would result in little to no
snow accums County Warning Area wide. Sampling of the parent midlevel wave on
Sunday by the radiosonde observation network in Canada should help sort out the
differences in the guidance some.

In the wake of this system...a brief shot of colder air will
follow on Wednesday. Middle/upper pattern will then quickly shift to
a strong ridge in the midsection of the country in response to
trough to west and also one digging to our east. This will set the
stage for strong warm advection aloft and also surface warm
advection on Thursday into Friday between expansive high pressure
to the east and troughing over the plains. Extent of any snow
cover will affect how quick temperatures can warm on Thursday. But then
on Friday...prognosticated thermal profiles support highs well into the
40s and 50s. Some light rain/showers are possible in vicinity of
cold front trailing from surface low over the northern lakes.
Airmass will be cooler but likely still mild behind front on



for the 00z tafs...

Strong clipper system will track southeast and weaken as it moves
into the region tomorrow. While the system will be weakening as it
moves through the is still expected to produce a
prolonged period of snow with prevailing IFR conditions a good bet
from late morning through the afternoon. High degree of
uncertainty pinpointing the timing of the heaviest snow Sunday
but it is Worth noting that any heavier snow bursts could briefly
knock visibility down below 1sm at times. A large part of the
uncertainty hinges on how quickly the snow weakens as it is moving
east...with heavier snow expected farther west where system is
expected to be stronger.

Southeast winds will increase Sunday morning in advance of the
system and probably become gusty to...if not a bit above 20kt at
times Sunday afternoon.

Snow should taper off Sunday evening with flake size likely
decreasing or possibly transitioning over to some drizzle or
snizzle (snow/drizzle mix) as cloud top temperatures warm and clouds lose
ice needed for snow production.



209 PM CST

High pressure has arrived over the western Great Lakes this
afternoon...however it is expected to continue to drift east
overnight and allow the light west winds to steadily turn southwest
to south by daybreak Sunday. Another low pressure system will slide
south from central Canada into the northern plains late
tonight...and continue to slide east towards Iowa/central Illinois
by Sunday morning. This will allow an increasing pressure gradient
to develop and increase winds over the lake during the day Sunday.
Current indications are that winds will be generally between 15 to
25 knots...however there could be a period of 30kt winds for the open
waters sun during the day. Low pressure then slides east across the
central/southern portions of Lake Michigan...allowing a brief lull
in the winds for the central/southern half of the lake although
along the northern portions of the lake winds will persist from the
northeast. The low is expected to lift northeast across lower
Michigan towards Lake Huron by midday Monday allowing winds to
mainly be westerly to southwesterly but with a lighter
gradient...winds will settle to a 10-20kt range Sunday night into

Late Monday night into Tuesday another low pressure system will
quickly slide south from central Canada into the northern plains and
move east towards the Ohio Valley Tuesday. This will cause winds to
turn north/northeasterly over the lake...with a tightening pressure
gradient along the southern half of the lake. It is possible that a
period of hazardous winds to small craft may occur...and with the
northeast/east wind direction...this could also result in hazardous
waves along the Illinois/Northwest Indiana nearshore waters Tuesday
afternoon/evening. High pressure then arrives Wednesday and lingers
over the Great Lakes through Thursday with much lighter winds and
less wave action expected.



Lot watches/warnings/advisories...
Illinois...Winter Weather Advisory...ilz003-ilz004-ilz008-ilz010-ilz011-
ilz019-ilz020-ilz021-ilz032-ilz033-ilz039...6 am Sunday to
midnight Monday.



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