Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 212336

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
636 PM CDT Wed Mar 21 2018

239 PM CDT

Through Thursday...

Expect mainly clear/sunny skies and dry weather this forecast
period, as high pressure builds across the region. However, could
see continued scattered clouds spilling off the lake this evening,
mainly into northwest IN. As the gradient relaxes, winds will
further diminish this afternoon and evening. Under clear skies and
light winds, expect temps tonight to fall well below normal, with
lows in the low to mid 20s. Given the setup, could see slightly
lower temps, and will monitor the potential for a few locations to
fall into the teens tonight. High clouds may try to increase on
Thursday, however, think sunny skies will prevail. Temps will
rebound well into the 40s on Thursday, with locations near the
lake observing slightly lower temps. This extent of this cooling
will be tied to lake breeze timing, that may not develop and then
push inland until later in the day.



258 PM CDT

Thursday night through Wednesday...

Main forecast concern in the extended will be the potentially
significant snowfall Friday night into Saturday.

Ridge of high pressure will begin to shift to our east Thursday
night into Friday morning while low pressure deepens over the
central plains. Southerly flow from the gulf will bring moisture
rich air north across the Mississippi River Valley with 1 inch
PWat contour spreading into portions of northern Illinois. PWats
of around 0.8 inches and mid level mixing ratios of 4.5 to just
over 5.0 g/kg will move into the local CWA by Friday evening
around the same time as deep corridor of isentropic ascent
overspreads the region resulting in quickly saturating mid levels.
Timing of low level saturation and onset of precipitation a
little more uncertain with a dry boundary layer flow out of the
east to northeast, but thinking this should occur sometime mid to
late evening for areas southwest of a Rockford to Rensselaer
line. Somewhere just north of this line, there will likely be a
sharp cutoff in snow totals.

Late Friday night into Saturday morning, forcing is expected to
peak with a number of features aligning to provide deep ascent
across the region. Synoptically, right entrance region to a strong
jet digging across the mid-Atlantic coast will move across the
western Great Lakes while the nose of a weaker jet moves into the
mid Mississippi Valley. The coupling of these two jets along with
increasing DCVA ahead of a shortwave trough poised to cross the
local area around midday Saturday will provide broad ascent across
the region.

Guidance also signaling the potential for strong mesoscale forcing
as a sloped region of strong F-gen overspreads portions of the CWA
topped by a region of negative EPV and steep mid/upper level
lapse rates which will allow for a deep vertical response to the
forcing. The combination of strong forcing and instability will
set the stage for heavy snowfall rates, which will likely be in
the form of large aggregates at times as soundings show a deep
region of saturation cutting across a broad temperature range.

Main forecast uncertainty will be pinning down the exact location
of the axis of heaviest precipitation, and, for some locations,
there will be p-type concerns in our far south. At this distance,
there is reasonable agreement between the main global models
indicating the axis of highest QPF (around 1.5 inches liquid
equivalent) falling from eastern Iowa southeast across central
Illinois, including Ogle/Lee counties southeast through
Ford/Iroquois counties. Amounts will taper to the northeast with a
likely sharp gradient down to near nothing somewhere over the
northeastern counties in the CWA, including possibly portions of
the Chicago Metro area. As is typical with events driven by strong
mesoscale features, wouldn`t be surprised to see shifts in the
precip axis 100 miles or so in either direction over the next day
or two, but in the meantime there is a good signal for some
locations seeing in excess of 6 inches of snow. SREF plumes show a
very broad range of possible values. Near Sterling, IL for
example, plumes have a range of nothing up to a foot with a mean
around 3 inches. While there is some decent clustering in the 1-4"
range, there is also a secondary cluster around 6-8" presumably
with models catching onto the mesoscale banding. This may be a
decent starting point for much of the CWA with this system
producing a broad area of 1-3 inches with a narrow corridor
perhaps 50-100 miles wide with the potential for 6+ inches. Exact
details on placement still to be worked out, but for now the
latest forecast shows a Dixon, IL to Remington, IN line as the
axis of heaviest snowfall.

Meanwhile, in our far south along a Peru, IL to Fowler, IN line,
thermal profiles are supportive of a wintry mix including freezing
rain and sleet. Will have to keep an eye out on the potential for
minor ice accumulations within this corridor.

Saturday night through Monday, ridging will re-establish over the
region providing quiet weather. Another system may impact the
region by the middle of next week with a warming trend ahead of
the system and primarily rainfall Tuesday into Wednesday.



For the 00Z TAFs...

Benign weather in place for the next 24-30 hours. Northeast winds
closer to the lake, including ORD and MDW, will hold at or above
9-10 kt for a few hours supporting an east operation for ORD.
Winds ease tonight and likely shift back to the NNW/NW overnight
into Thursday morning. Decent signal for a lake breeze/lake
enhanced NE wind once a weak surface high passes overhead and
shift to Lake Michigan in the afternoon. Medium confidence on
timing in the 21-22z time frame at ORD/MDW and medium confidence
on generally lighter speeds, but could briefly push 9-10 kt.



258 PM CDT

High pressure will remain over northern Ontario or northern Quebec
through the remainder of the week and into the weekend while low
pressure traverses the Great Plains. Northerly flow will prevail
ahead of the ridge axis through Thursday and much of Friday, then
winds should begin to veer to the east later Friday into Saturday
as low pressure forms over the central Great Plains. Winds looks
to peak at around 30 kt out of the east Saturday morning. The low
will track across the mid Mississippi Valley Saturday and weaken.
Another low will form over the plains late in the weekend and move
to southern Lake Michigan around midweek.



LM...Small Craft Advisory...IN nearshore waters until 5 AM Thursday.

     Small Craft Advisory...IL nearshore waters until 10 PM




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