Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 172238

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
438 PM CST Sat Feb 17 2018

135 PM CST

Through Sunday...

Convective nature of incoming snow per visible satellite, 35 dBz
echoes on radar, and sounding analysis of elevated mid and higher
level lapse rates suggests a burst of moderate heavy snow this
afternoon. Upstream in Iowa near 1 inch per hour rates have been
observed. While snow will not last long, roads will quickly
become snow covered, particularly less traveled roads. Fortunately
our surface temperatures have warmed which will only briefly limit
pavement accums, but low dewpoints suggest as soon as precip
arrives wet bulbing will lower conditions. Therefore snow will be
the dominant precipitation type in all locations. This means a
significant increase in the snow forecast south of I-80. Be
prepared for a rapid deterioration with snow. There will be areas
that pick up more than 1 inch of snow given these rates.

Conditions will quiet down fairly quickly this evening as the
strong upper disturbance associated with this band of snow will
quickly shift east. Breezy northwest winds behind the wave will
bring temperatures back down through the 20s tonight with clearing
skies. High pressure will glide by to our south tonight as well.
Winds will slacken overnight and shift to south/southeast, which
may promote some fog or possibly some very low cloud over the
fresh snow.

Given the anticipated fresh, though wet snow pack, model guidance
has come down a bit on the anticipated temperatures on Sunday, but
we will still head back to around 40 north, a bit warmer farther
south. Otherwise, southerly winds appear to be the main weather
maker for Sunday as they ramp up ahead of deepening low pressure
across the upper Midwest.



320 PM CST

Sunday night through Saturday...

Multiple concerns in a very active long term period,including
heavy rain potential Monday through mid week, which could
introduce flooding issues given still frozen ground, melting of
remaining snow cover and ice cover on area rivers. During Sunday
night through late Tuesday, thunderstorms will also be possible,
some of which could be strong to severe Monday PM and Tuesday for
parts of the area. Next, as colder air filters back into the area
on Tuesday night, a wintry mix (including freezing rain and sleet)
is possible for portions of the area if precipitation is
still occurring at that time, which is a bit uncertain. Finally,
additional rounds of precipitation are possible at times Wednesday
night through Saturday, with confidence low on specific details.
Temperatures look to be on quite the roller coaster ride as well,
with very mild temperatures Monday through Tuesday, cooling back
to closer to seasonal mid week and then likely warming back above
normal to close the work week and into next weekend.

Sunday night will start tranquil as surface high pressure pulls
farther away from the area. Not expecting temperatures to fall
all that much on Sunday night and if winds stay up more than
forecast, they could rise through the night. Of bigger concern is
the threat for showers and even thunderstorms overnight into
Monday morning in strong warm advection pattern. While mid-level
lapse rates don`t look to be steep, the pattern looks to be fairly
classic in terms of cold season nocturnal convection where the
models don`t indicate much in the way of instability but the
strong WAA drives enough for isolated thunderstorms, with the best
chance of this occurring mainly along/south of I-80.

A secondary warm front is progged to lift north across the area on
Monday as a ~1000 mb surface low or trough lifts northeast toward
eastern IA/NW IL/southern WI area by early Monday evening. There
is some variance in specifics with the surface pattern by this
time, but overall decent agreement. With noted trend of surface
low pressure systems this season to verify weaker and southeast
from modeled a few days out as well as some uncertainty with how
much snow cover will be left entering Monday, this will be
something to watch with respect to how far north warm front gets.
For now, have continued with thinking that the front makes it
through most or all of the CWA.

Waves of showers and isolated to possibly scattered thunderstorms
should continue into Monday afternoon. Very strong low level warm
and moist advection and dew points surging into the 50s behind
the warm front will support temperatures rising into the 50s to
low-mid 60s even with clouds and precip. Have discounted 12km NAM
temperature and dew point profiles, which are contaminated by the
model thinking snow cover will remain throughout Monday. As
mentioned in summary, synoptic pattern recognition of Monday PM
into Tuesday is one that is concerning for at least some risk for
strong to severe thunderstorms in the area based on previous
winter season severe episodes. Southwesterly mid and upper flow
will undoubtedly be strong enough for high levels of deep layer
wind shear, with main question how much instability can be
realized in terms of whether any strong/severe storms occur, as
well as exact specifics of pattern at the surface and aloft.

The threat for heavy rain through Monday night and into Tuesday
is of higher confidence and of greater concern for now due to
this higher confidence, especially with any thunderstorms, as the
the deep layer southwest flow will transport highly anomalous
precipitable water values into the area of 1 to 1.50 inches, which
is 4 to 5 standard deviations above climatological normal for
February. For reference, the highest observed PWAT value in the
month of February for ILX/PIA sounding point is 1.34 inches. The
first round of potentially heavy rain should set up near warm
front position by Monday evening over northern Illinois, where
WPC has indicated a Slight Risk of excessive rainfall. The looming
question mark then in terms of the magnitude of the heavy rain
and resultant flooding risk and how long it persists is the
handling of the pattern through mid week.

While the GFS remains on quicker side of guidance in spreading in
expansive 1030-1035 mb high pressure over the Plains toward the
area on Tuesday night, the trend in guidance has been toward
consistent ECMWF scenario of hanging up the surface trough and
thus keeping warm front nearly stationary through much of Tuesday
before finally surging cold front southeast on late Tuesday into
Wednesday. Exactly where the trough and front sets up on Tuesday
is uncertain/lower confidence, but it should bisect northern

There is still time for changes in how Tuesday plays out, but
trend for anomalously moist air mass getting hung up looks to
support additional waves of showers and thunderstorms and heavy
rainfall/flooding potential. Flow aloft parallel to surface trough
could result in training of convection. All in all, ensembles
have come into better agreement on total rainfall amounts of about
1.5 to 3", with less spread noted than yesterday and less members
under 1". Certainly can`t rule out rain amounts of greater than
3" through mid week, as indicated by some deterministic
operational runs. Will continue with an ESF for the area to
emphasize the heavy rain and flooding risk.

With front more likely to get hung up on Tuesday, another very
mild day is in store for the warm sector (upper 50s to as high as
67 deg in east central IL/west central IN. Finally, depending on
if it is still precipitating while the low level cold oozes in
Tuesday night into early Wednesday, thermal profiles on ECMWF and
GEM/CMC indicate 850 mb level will remain a few to several degrees
above 0 Celsius, which could then mean freezing rain and sleet
potential for parts of area. This will also be a period to monitor
closely for travel impacts.

After Wednesday dries out with seasonably cool temperatures,
latest ECMWF indicates yet another round of precip Wednesday
night, which could be in form of snow. Another threat for
precipitation will likely arrive sometime late in the work week
into Saturday, with variance on timing among the guidance. The
flow pattern across the CONUS will remain quite active as strong
southeastern CONUS mid- upper ridging and western troughing likely
continues, supporting large temperature swings and possibly
precipitation type issues at times.



For the 18Z TAFs...

A period of moderate to heavy snowfall is on track for later this
afternoon. Expect LIFR to even brief VLIFR in the snow.

Upstream visibilities have generally been anywhere from 1/2SM to
1 1/2SM, and do feel we will see similar conditions for our
terminals. The snow is convective in nature, especially south of
the terminals, but brief 1/4SM visibility is not out of the
question at ORD/MDW/RFD. At this time confidence is too low in
this lasting any long period of time to include yet in the TAF.
Main time frame is 20-22Z at RFD, and 21-23Z for ORD/MDW. Things
could start a tad sooner than in the TAF given current radar
trends. Accums to around 1 inch are possible, likely less for
most areas. Surface temps are climbing to at or above freezing
outside of north central IL, but expect them to hold or drop once
snow arrives, and some mix is possible closer to central IL.

Precip will end quickly around 0z or maybe a tad later. Expect
initially gusty WNW winds behind the snow to ease and shift to
SSE toward daybreak Sunday. Some increase in the SSE wind will
occur by mid morning, with gusty S winds then expected Sunday



435 PM CST

Brisk southwest winds of 25 to 30 kt will diminish and veer
westerly late tonight as a weak surface high pressure ridge moves
across the lake. Winds will then ramp up quickly again from the
south or south-southeast Sunday morning. Model forecasts suggest
another period of southerly 35-40 kt gales Sunday afternoon and
evening, and have issued a Gale Warning for that time period. The
highest winds will be focused on the northern portion of the lake,
with gales generally confined to the northern 2/3 of the lake, but
with only a small portion of the south half possibly sub-gale have
included the entire lake in the Gale Warning. Have also issued a
Small Craft Advisory for the entire nearshore for the brisk
southerly winds Sunday afternoon through Sunday night.

A cold front sags across the lake late Sunday night, becoming
stationary Monday morning. A surface low pressure wave translating
along the front is expected to enhance winds a bit (southerly on
south end, and northerly on the north) to around 25 to 30 kts. A
Small Craft Advisory may be needed for parts of the nearshore from
Monday afternoon through much of Monday night for the brisk
south-southwest winds. North winds up to around 25 kts will then
develop more fully late Tuesday as frontal zone shifts to the
south. Winds then diminish late Wednesday night into early
Thursday as another weak surface high pressure ridge drifts across
the lake.



LM...Gale Warning...LMZ777-LMZ779...noon Sunday to midnight Monday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...noon Sunday to 3 AM Monday.




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