Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 131151

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
651 AM CDT Tue Mar 13 2018

240 AM CDT

Through tonight...

The main forecast concern today is the threat for convective snow
showers/squalls over northeastern IL and into portions of
northwestern Indiana into this afternoon.

The main feature of interest for today is the rather stout
looking mid-level disturbance, currently noted in the water vapor
channels shifting southward along the western periphery of the
upper low in place across the Upper Great Lakes. This disturbance,
along with a stronger push of cold air, is expected to shift over
the area this afternoon. As this occurs, this is likely to set up
favorable conditions for convective snow showers over at least
portions of the area today as low-level lapse rates steepen
considerably across the area with this push of colder air aloft.

While some isolated to scattered snow showers and flurries can not
be ruled out across most of the area today, it does appear that
the best chances for more focused convective snow showers may end
up across portions of northeastern IL and into portions of
northwest Indiana. It is in this general region where HiRes
forecast guidance suggests that the northerly winds will shift
north-northeastward off the lake this afternoon, and essentially
set up a lake breeze convergence zone roughly from the Chicago
metro area southward near the IL and IN stateline. If this does
occur, it would likely result in more substantial area of moderate,
to briefly heavy snow showers, with rapid reductions to the
visibility (potentially briefly under a mile) and even some
localized accumulations of snow up to an inch. While this could have an
impact on some of the afternoon commute across the area, there is
uncertainty in how much, if any snow may be able to accumulate on
area roads. We also cant rule out the possibility for some
graupel and lightning in some of the stronger convective cells
this afternoon.

The snow showers should wane with sunset this evening, though some
lake effect snow showers may continue over portions of Porter
county Indiana this evening before the focus shifts to the east

As for temperatures, expected colder conditions today, with highs
in the 30s, coolest near the lake. Lows tonight should drop into
the lower 20s.



252 AM CDT

Wednesday through Monday...

Overall it looks like we will have a quiet, but cool stretch of
weather through much of the period. The main threat of
precipitation looks to remain to our southwest through later in
the week as surface high pressure, and an associated feed of dry
northeasterly flow, over the Great Lakes looks to prevail.
Temperatures are expected to be on the cooler side through late
week, especially near Lake Michgian where onshore flow looks to
result in the coolest daytime highs.

The next substantial chance of precipitation does not appear to
be until early next week. During this period, forecast guidance
suggests that a storm system could shift to our south, possibly
resulting in some rain or snow over portions of the area late
Monday into Tuesday. However, confidence is fairly low with any
real details at this time range.



For the 12Z TAFs...

Scattered clouds will likely increase in coverage early this
morning, with VFR ceilings then likely throughout the day and
into the early evening. Still monitoring the potential for
scattered flurries and snow showers from this morning through
early this evening across all of the terminals. Do have some
concerns for more intense snow showers this afternoon, and did
expand mention of snow in the TAFs. Have included a prevailing
group of reduced vis in light snow showers, but do think periodic
more intense snow will be possible especially for the ORD/MDW/GYY
terminals. In these locations periodic vis of 1 1/2 to 2sm could
be possible this afternoon. Will need to monitor the definite
possibility of even lower vis this afternoon, possibly around 1sm.
Have lower confidence of this possibility at this time though. It
is also possible this afternoon that winds turn to the northeast
near the lake for a time, and have included this in the TAF.



408 AM CDT

A rather tight gradient in place across the lake this morning as
high pressure situated just to the west, and while low pressure
moves across the eastern U.S. In this pattern, winds will
generally be out of the north this morning through much of today
and into this evening. Speeds up to 30 kt are likely over the open
waters and the nearshore and although prevailing gales are not
likely, do think that periodic gale force gusts will be possible
and have made mention of this in the open and nearshore forecasts.
Winds will diminish later tonight, while becoming northwest and
eventually west by early Wednesday. Current small craft advisory
end time still seems on track based on latest wind and wave
forecast. Slightly lighter winds can be expected earlier in the
day Wednesday, however, do expect winds to increase once again out
of the north Wednesday night into Thursday.



LM...Small Craft Advisory...IN nearshore waters until 10 AM

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




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