Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 162118

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

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 is the handling of the pattern through mid

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. 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 18Z TAFs...

-High End MVFR dissipating
-Snow showers timing and coverage Saturday afternoon
-MVFR/IFR Visibility occurrence in snow showers

- High on MVFR clearing times
- Medium on exact timing for snow showers. High on there being
  snow showers around during the 18z-0z time frame Saturday.
- Medium on Visibility impacts. High on MVFR where showers occur.
  Low-Medium on IFR lasting any significant period of time.

The remnants of MVFR clouds will shift southeast this afternoon,
lingering longest along and east of I-57, though expect VFR at
most sites by mid afternoon. NW winds will shift to SW and weaken
this afternoon into tonight. High pressure will move east of our
area toward daybreak. A band of snow showers will develop across
the area Saturday afternoon from west to east. While not all areas
may have snow, this is the setup that can produce brief though
modest snow showers. Surface temperatures will be borderline, but
a quick dusting will be possible in the stronger showers and
MVFR/brief IFR. Winds will remain SW in the showers, which could
be mixed with rain mainly south of the terminals.



325 AM CST

Strong high pressure is building into the northern Plains early
this morning with north winds to around 25 kt in place over Lake
Michigan. Winds will ease to 10 to 20 kt this afternoon and
evening as the high center builds towards the region, passing
just south of Lake Michigan tonight. Winds back to southwest
late this evening and tonight behind the ridge axis and will
increase to 25 to 30 kt ahead of an approaching cold front. Winds
may briefly tag gale force across northern Lake Michigan Saturday
morning. Winds will drop off late Saturday night into early
Sunday morning for a few hours as another ridge axis quickly moves
across the region, then moderately strong south winds gusting to
around 30 kt will return by midday Sunday as another low takes
shape over the Great Plains.






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