Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200940

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
340 AM CST Wed Feb 20 2019

337 AM CST

Taking a bit longer to saturate, such that our window of snow will
be a bit more limited farther north and towards the city of
Chicago, as the better forcing shifts through the Ohio Valley and
across NW Illinois. Still expecting a quick shot of snow that
will be accumulating before we transition to freezing drizzle,
though this window is narrowing with the dry slot aloft rapidly
approaching. Therefore brief moderate snows are expected, but
accumulations will be tempered.




Through Thursday...

Impressive look to water vapor imagery this morning depicting our
upper low in lee of the Rockies, warm/moist conveyor belt over the
Mississippi Valley and a strong dry slot pushing in right behind
it. Precipitation out ahead of this system has generally been
following a snow to sleet/freezing rain and eventually a freezing
drizzle or drizzle. No significant changes to the forecast theme
after looking at the 0z raobs and forecast suite.

Temperatures are warming as warmer air attempts to shift into the
area. In spite of temps getting near the freezing mark in some
spots, evaporative cooling should prevent most area from staying
at or above freeing before precipitation ultimately begins. There
are some bands of snow embedded in a generally light snow radar
picture. These will shift north through the area in the coming
hours, and the ring of drier air around KLOT radar should fill in

NAM/GFS Cobb output are consistent with ensemble guidance of
somewhere around 0.2" of liquid equivalent precipitation while we
are in the period of wintry precipitation, maybe a bit more if the
transition to just plain rain occurs a bit later, as QPF mean for
total QPF is up around 0.2 to 0.3" liquid. Upstream observations
are consistent with the idea of several hours of snow, with a
period of of a wintry mix of Sleet and/or Freezing Rain then
freezing drizzle and eventually drizzle. Guidance here is painting
slightly lower snow/sleet totals than that which has been
observed in areas in SW IL/SE MO where QPF and sleet will likely
be a bit higher than observed here. Generally feel snow amounts of
0.5-2" are expected given a brief surge with the the elevated
warm front/frontogenesis which could introduce some moderate to
briefly heavy snowfall rates at/shortly after onset. The best FGEN
signals shift through Indiana and also through NW/NC Illinois, and
given looks at the issues with saturation and approach of the dry
slot, feel that our snow window is going to be somewhat brief.
Most of the snow will occur late tonight into very early
Wednesday. Following this in the morning, the transition period
will occur from south to north, where up to 0.1" of ice or so is
still possible, with variance hinging on the time of the mix

Regardless, things will be messy for the commute period this
morning as snow will likely be mixed with sleet and freezing rain.
Current end times for the advisory have a little cushion on the
latter end in a few spots, and given the trends in surface
temperatures, at this point do not anticipate these will need to
be extended. Warmer surface air should ultimately send
temperatures above freezing with a surface low that will track
toward the Minneapolis area, as low level winds will shift to S
and even SW and keep the ice window somewhat narrow.

The low will strengthen and track to Lake Superior tonight, and
this will result in breezy southwest winds. We will get clipped
by the colder air associated with the upper low. Following this, a
broad surface high will make for dry day on Thursday with seasonal
readings and sunshine.



156 AM CST

Thursday night through Tuesday...

Fair weather conditions are expected locally under the influence
of  surface ridge axis centered over the Great Lakes region
Thursday night and Friday. Dry weather will prevail through the
day Friday as the ridge shifts towards New England. Temperatures
are expected to warm into the mid to upper 30s north of I-80 and
low 40s south.

Meanwhile, a deep longwave trough will be centered over the Great
Basin Thursday night and Friday as a potent shortwave rounds the
base of this trough over the Desert Southwest and ejects out over
the plains. Surface low will deepen in response and lift from the
central Great Plains Friday night to Iowa on Saturday. Cloud cover
increases later in the day Friday in advance of the low as
moderately strong mid level warm air advection overspreads the
Upper Midwest. Models remain in reasonable agreement with precip
chances moving into the local are around or after midnight Friday
night with 850mb temps already to around 4-8C by this point, with
further warming aloft thereafter. There has been a slight
southward shift on the low track from last night`s model runs
with the GFS and GEM clipping NW Illinois and EC bringing the low
across Chicago. With melting aloft fairly likely at this point,
surface temperatures overnight will be important to figuring out
the threat for ice. Temp trends should be upward late in the
evening and overnight, just how quickly is the question. For this
forecast package, tried to limit the extent of any accumulating
ice to the left of the ECMWF low track, but additional southward
trends would pose a higher threat for icy conditions farther south
Friday night.

Temperatures continue to warm through the day Saturday with highs
in the 40s north to 50s south and precip transitioning to rain
area-wide. A cold front will sweep across the area Saturday night
with precip tapering off for most of the area in its wake. The
exception will be across far northern Illinois which may be
glanced by some of the wrap-around moisture bringing a little bit
of snow before precip ends midday Sunday.

Yet another system is progged to move across the region around
Tuesday. At this distance, there are still large differences in
the strength and track of this next wave. Very generally, expect
undulating temperatures early through the middle of next week as
this wave traverses the region and additional precip chances, but
model spread is too large to have any confidence in specifics.



For the 06Z TAFs...

No major changes to the forecast thinking late this evening. Radar
continues to show increasing returns spreading northward into NW
and north central IL. Snow is still working to overcome lower
level layer of dry air but have seen a few recent nearby obs
reporting snow so saturation is occurring. Some light snow should
start to occur at RFD over the next 1-2 hours before steadier snow
arrives. Snow will take a little longer at the Chicago area
terminals closer to 09z. Intermittent snow may be moderate to
heavy at times over a 2-4 hour window after it begins and 1-2
inches may accumulate prior to 12z. Cigs will lower to MVFR as the
snow arrives. Snow will likely result in IFR vsby around 1SM and
may be fall to around 3/4SM at times. Warmer air aloft will move
into the area and combine with a layer of drier air bringing a
transition from snow to freezing drizzle after about 12z in the
Chicago area and a bit later at RFD. Did speed this up by an hour
in the TAFs. Ceilings and vsby will likely remain IFR and
possibly LIFR at times through Wednesday morning. Surface air
temperatures should warm above freezing by midday allowing FZDZ to
become DZ in the Chicago area but RFD may hold on to FZDZ through
much of the afternoon.

Winds will be east to southeast tonight and more southeasterly
during the day Wednesday. Winds will shift to the southwest late
Wednesday afternoon bringing in drier air and an end to the
drizzle. Vsby will increase with IFR ceilings scattering/improving
at a slower rate Wednesday evening.



IL...Winter Weather Advisory...ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-
     ILZ032-ILZ033-ILZ039 until noon Wednesday.

     Winter Weather Advisory...ILZ003-ILZ004-ILZ008-ILZ010-ILZ011
     until 6 PM Wednesday.

     Winter Weather Advisory...ILZ005-ILZ006-ILZ012-ILZ013-ILZ014
     until 3 PM Wednesday.

IN...Winter Weather Advisory...INZ001-INZ002-INZ010-INZ011-INZ019
     until noon Wednesday.

LM...Small Craft Advisory...nearshore waters
     until 9 AM Thursday.




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