Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 170501

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1101 PM CST Fri Feb 16 2018

910 PM CST

No major changes to the going forecast for tonight. However, I did
make a few adjustments to forecast for Saturday afternoon, mainly
to add some better timing resolution and to cool hourly
temperatures during the onset of precipitation. While
temperatures on Saturday will warm into the 30s as warmer air
begins to advect over the area via southwesterly winds, it does
appear that as the precipitation onsets across the area during
the afternoon, wetbulb cooling will allow temperatures to quickly
drop into the low 30s. Because of this, any precipitation that
falls over the area is expected to be mainly in the form of snow
showers, though a mix of rain and snow will be possible across my
far southern counties.

While this is not expected to be a big snow maker for the area,
it does appear there will be a about a 2 to 4 hour window tomorrow
afternoon for a period of light to possibly briefly moderate snow
showers. At the present time it appears this timing will be
between about 2 pm through 6 pm for the area as an mid-level
disturbance shifts across the area and warm air advection
(isentropic ascent) moistens the column from top down.

Given the quick movement of this system, snowfall amounts are
likely to remain less than an inch. The snow showers will end
from west to east during the late afternoon and early evening



106 PM CST

Through Saturday...

Quiet and cold conditions will be theme for tonight, followed by
at least some scattered snow showers Saturday afternoon.

High pressure is centered across the central plains this
afternoon, and this has maintained a cool but dry northwest flow
area wide. The high will shift across central Illinois tonight.
While winds do not appear to completely subside, the high will
be close enough to allow for decent decoupling. With initial
clear skies and warm advection not kicking in immediately, expect
a decent drop off in temperatures this evening, with teens
spreading in fairly quickly west to east through the evening.

Our next weather maker is out over the northern rockies/plains. We
get into modest warm advection ahead of this system. The main
moisture transport remains focused well south of the region, but
the northern stream shortwave seems potent enough to produce some
showers Saturday afternoon. Wet bulb thermal profiles suggest the
main precipitation type will be snow, with a mix possible farther
south. It is a pattern that suggests some areas don`t get much,
but some areas see a burst of moderate snow showers, given the
presence of fairly steep lapse rates aloft ahead of the incoming
shortwave. Surface temperatures will be initially marginal for
accumulation, and therefore even the higher end QPF on the EC
(around 0.05") would struggle to produce much more than a few
tenths. Grassy surfaces appear favored given the marginal surface
temperatures, but pavement temps may not warm a ton given the
colder night tonight and clouds arriving tomorrow, so some road
impacts are possible.



315 PM CST

Saturday night through Friday...

Multiple concerns in the 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 at least
Tuesday, thunderstorms will also be possible, some of which could
be strong to severe Monday PM and possibly even Tuesday for parts
of the area. Finally, as colder air gradually returns to the area
by mid week, a wintry mix (including sleet and freezing rain) is
possible for portions of the area if precipitation is occurring at
that time, which is quite uncertain.

Saturday night into Sunday will be tranquil as surface high
pressure departs the area. Increasing southerly winds with gusts
up around 25 mph on Sunday afternoon will transport in a much
milder air mass, with remaining snow cover fairly likely to fully
melt outside of larger piles. Temperatures should warm into the
mid to upper 40s for much of the area, with 50 possible south. 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 to scattered thunderstorms.

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. Waves of showers and possibly
thunderstorms should continue into Monday afternoon. Very strong
low level advection and dew points surging into the 50s behind the
warm front will support temperatures rising into the 50s and 60s
even with clouds and precip. As mentioned in summary, synoptic
pattern recognition of Monday PM into Monday night 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.

The threat for heavy rain through Monday night and into Tuesday
is of 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 1.25 inches. For
reference, the highest observed PWAT value in the month of
February for ILX/PIA sounding point is 1.34 inches. The looming
question mark 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.

The GFS in particular is much quicker to spread in expansive
1035-1040 mb high pressure over the Plains toward the area on
Tuesday, while the ECMWF has been insistent on hanging up the
surface trough and thus keeping warm front nearly stationary
through much of Tuesday before finally surging cold front
southeast on Wednesday. The ECMWF and its ensemble mean are in
pretty good agreement with a swath of 2-3" of rain/liquid
equivalent across the area by Wednesday, indicative of less spread
in ECMWF ensemble members. The GFS ensemble on the other hand has
much more spread in its members in the timing of the cold front,
with therefore very large spread in QPF among members with a mean
around 1" for ORD. The global Canadian model is somewhat of a mid
way compromise between the operational GFS and ECMWF. Suspicion
is that the low level cold will be able to get in quicker similar
to GFS, however at this vantage point it`s certainly too early to
rule out more concerning ECMWF scenario especially considering its
internal consistency last several runs.

Should front get hung up, then Tuesday could be even warmer than
forecast for part of area (and vice versa), with a non-zero risk
for strong to severe thunderstorms continuing in the warm sector.
Finally, depending on if it is still precipitating while the low
level cold ooozes in Tuesday night into Wednesday, thermal
profiles on ECMWF and GEM/CMC indicate 850 mb level could remain
several degrees above 0 Celsius, which could then mean freezing
rain and sleet potential for parts of area (likely more so than
any snow). A quiet period looks to finally occur at some point
next week as surface high pressure moves in, with timing ranging
from Tuesday PM on early side to Wednesday evening on late side as
currently modeled operationally. Another threat for precipitation
could then arrive on Thursday night into Friday, with flow pattern
across the CONUS remaining active and strong southeastern CONUS
mid-upper ridging likely to continue.



For the 06Z TAFs...

Main concerns through the period continue to focus on the
increasing potential for a short period of light to moderate
snowfall this afternoon across the terminals. Not much has
changed with the previous thinking.

High pressure will shift over the region overnight resulting in
light winds. As this high shifts east of the area today the winds
will turn southwesterly across the area. Overall, this will help
transport atmospheric moisture back northward over the area into
the afternoon in advance of an approaching mid-level disturbance
and setting the stage for a period of snow. Currently it looks
like this will be a quick hitting snowfall (only 2 to 4 hour)
event during the mid to late afternoon hours (roughly from 20z
through 00z at the Chicago area terminals). While snow amounts
are likely to only be an inch or less, it could briefly fall at a
moderate rate, which drive down visibilities to, or just below a

Expect the snow to quickly end by early this evening, with winds
turning westerly.



400 PM CST

A fairly active pattern will persist over the lake. 25 to 30 kt
southwest winds are expected tonight into Saturday over the open
waters ahead of a cold front. A brief period of gale force gusts
to 35 kt is possible Saturday morning on the north half of the
lake. A brief period of slightly lighter west and southwest winds
will follow Saturday night into Sunday, with a shift to south and
quick increase to 30 kt Sunday afternoon and continuing through
Sunday night. As a surface low/trough crosses the lake later
Monday through Monday night, south winds are likely south of the
associated warm front, with brisk northeast to north winds north
of the front. A complex and uncertain surface pattern Tuesday through
Wednesday lowers confidence in prevailing wind directions and
speeds. For the nearshore waters, the next period of concern for
hazardous wind speeds for small craft is on Sunday afternoon into
Sunday evening with brisk south-southwest winds likely. A similar
scenario could occur for portions of the nearshore Monday
afternoon through Monday evening.






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