Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210903

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

255 AM CDT

Through Thursday Night...

An impressive and large synoptic scale system from top to bottom
continues to take shape in the Mid-Atlantic region this morning.
The cold conveyor belt associated with this deepening system is
wrapped over the Great Lakes into the Ohio River Valley, with
850mb temperatures on regional soundings last evening running
around the 10th percentile for late March. This colder air above
and into the boundary layer combined with clouds at least through
midday will provide temperatures 5 to 10 degrees below normal for
northern Illinois and northwest Indiana. The coolest values that
will stay shy of 40 degrees look to be in lake adjacent counties.

Winds will finally diminish and quickly late this afternoon and
early this evening due to high pressure building in. Some lake
effect clouds may linger into northwest Indiana. Lows of lower to
mid 20s are forecast for much of the CWA.

The light winds continue into Thursday with a lake breeze likely.
Apart from this lake adjacent cooling, temperatures may rebound
as much as to near 50 for highs, but that will depend on any high
cloud thickness associated with a short wave dropping southeast
over the Mississippi River Valley.



334 AM CDT

Friday through Tuesday...

The dry pattern over the past week looks to come to an end to
start the weekend, but quite possibly for only a portion of the
forecast area. How much of the area in what conceptually should
be a tight gradient in precipitation is the challenge, with the
most favored portion being the southwest half (south of a
Rockford to Joliet to Rensselaer line) during later Friday Night
and Saturday. Even if accounting for a warmer evolution than
guidance, at least some wintry precipitation including snow is
likely with this system into north central Illinois and possibly
northeast Illinois and northwest Indiana.

The culprit for this late season wintry precipitation potential is
the next in what has been a series of deepening upper waves moving
into the Ohio Valley. This neutrally tilted one takes more of a
southeast trajectory into that region, thus taking it closer to
the local area and on 00Z model solutions impinging the elevated
baroclinicity and warm advection near/over the western and
southern CWA. The persistent northeast low-level flow will be in
place on Friday ahead of this system and be a probable slowing
factor for onset and progression Friday night, as well as just how
far northeast the precipitation does spread. While there is
fairly sound agreement in the synoptic evolution from the 00Z
GFS/EC/GEM and the 00Z GEFS members, the details of the gradient
in appreciable precipitation show a high amount of spread that on
almost all guidance is draped across some part of the CWA.

The drier air and the onset time of after dark on Friday night
allows for the likelihood of wet-bulb cooling, and as forecast now
the potential for dynamic cooling late Friday night and Saturday
morning into the western and southern CWA. This is when snow
would be most favored just north of the warm nose aloft. This warm
nose has been indicated to be deep enough for at least partial
melting, and given the northeast low-level flow and amount of
refreezing energy forecast, there is concern for sleet and maybe
even freezing rain depending on surface temperatures which should
at the least be near freezing.

In place in an axis from Iowa into the northern half of Illinois
Friday night into Saturday are parameters for moderate to even
heavy warm advection precipitation. Those are quasi-geostrophic
forcing for lift, strong moisture transport and convergence, and
high moisture values of 4.5 g/kg and PWATs just shy of one inch.
These look to be in place for about 12-15 hours on current
guidance and could yield several inches of wet snow. While
mentioned above was the gradient in precipitation on the northeast
side, there very likely could be an extremely sharp gradient of
any snowfall/sleet on the southwest side of the precipitation
swath too. All this adds to the system challenges and these are
likely to continue during the next couple days of the forecast,
although refinement in details will certainly be made.

This system will pull away from the region late day Saturday into
the evening with below normal temperatures fittingly forecast to
finish out the weekend. The blocking pattern in the northern
Atlantic to Greenland region is forecast to break down some next
week, allowing for more progressive flow in Canada/northern U.S.,
and ridging forecast to build back into the region within this.
This will mean some milder air though not screaming warmer than
normal at least not for any sustained amount of time. Part of that
is due to the next chance for rain looking to move in already by
the Monday night-Wednesday time frame on current global guidance.



For the 06Z TAFs...

Primary forecast concern for this period will continue to be
winds. Winds which should remain generally northerly through the
remainder of the night and through the daytime hours tomorrow.
With an inverted trough extending from low pressure over the Upper
Ohio Valley through north-central IL and a weak finer scale ridge
along the west coast of Lake Michigan, wind direction will be a
little difficult to pin down exactly if it will be a little west
of north or a little east of north. At 06z, the dividing line
between NNW and NNE is between ORD/DPA and MDW/GYY. Expect that
there may be some variation to wind direction through the late
morning hours when the low over the Upper Ohio Valley will begin
to merge with the stronger low off of the Mid-Atlantic coast and a
ridge of high pressure begins to build across the Mid Mississippi
Valley. This will, ultimately, lead to a weaker pressure gradient
over nrn IL/IN and set up the possibility for a lake breeze to
develop, or perhaps, just some lake influence, leading to more of
a true NE wind direction in the afternoon. In general, confidence
in wind direction is only low-to-moderate through the morning
hours. Through the late afternoon and evening hours, the ridge
will build east across region, leading to a trend toward lgt/vrbl
winds which should then persist through the overnight hours.

Otherwise, the only other concern is some lingering mvfr cigs for
the remainder of the overnight hours. Latest obs indicate the
low-level clouds scattering, but there could still be brief
periods of mvfr cigs. Otherwise, cloud cover will trend to
operationally insignificant through the morning and then become
skc for tomorrow night.


400 AM CDT

Brisk north to northeast winds remain over Lake Michigan as low
pressure slowly moves through the Upper Ohio Valley and strong
high pressure remain parked over Ontario.  The strongest pressure
gradient is over the southern portions of the lake, where winds up
to 25 kt persist over the Open and Nearshore waters.  Given the
persistent N-NE fetch, the Small Craft Advisory will continue and
have extended the portion of the Advisory for the Indiana
nearshore waters until 10Z Thursday as the latest wave guidance
suggests that the higher waves will persist a bit longer than
previously anticipated.  The portion of the Advisory for the
Illinois nearshore waters remains unchanged. A ridge of high
pressure is expected to build across the lake Wednesday night and
Thursday, allowing for a period or relatively light winds over
the lake. Strengthening E-SE winds are expected for Friday night
and Saturday as deepening low pressure tracks just south of the


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

     Small Craft Advisory...IL nearshore waters until 4 PM Wednesday.




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