Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 192016

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
316 PM CDT Sun Mar 19 2017

210 PM CDT

Through Monday...

After a pleasant and seasonal (though cool at the lake) Sunday
with partly cloudy conditions and modest winds, our focus will
shift to shower and thunderstorm chances tonight into Monday.

A lead shortwave riding the periphery of a flattening ridge across
the plains is leading to convection along a warm frontal boundary
across Missouri. This activity will dissipate, but additional
shortwave energy will propagate east along the frontal boundary
in an area of increasing moisture return ahead of significantly
increasing southwesterly low level jet. At this point the best
moisture return and theta-e advection will be across our
southwestern counties where the latest high- resolution guidance
is suggesting an expansion of shower and thunderstorm coverage
tonight. Initially this will be focused across Iowa this evening,
but the RAP/HRRR suggest enough moisture convergence north of the
front and some decent elevated instability to suggest a swath of
showers and storms in our area. The favored areas for storms will
generally be south as the night progresses toward daybreak. The
instability is enough to be concerned about some severe hail with
the instability extending through the hail growth region and
decent shear though marginal in the cloud bearing layer, and
therefore the threat is marginal and consistent with the SPC
outlook. The main focus will be after midnight into early Monday,
with better chance of hail largely southwest of the core of the
Chicago metro area based on latest convective allowing model
guidance, but it is certainly a non-zero threat farther north.

The shortwave trough axis will approach Monday morning with the
cold front, which will likely continue the chance of showers
during the morning hours, possibly redeveloping for our northern
areas including the Chicago area. The main warm frontal boundary
will generally slide east along our southern counties, thus we
never truly get into the warm sector ahead of the low. And while
the northern stream cold front will shift into northern Illinois
later morning, it will take a bit longer south which will linger
the shower chances for most of the day for these areas. Expect
cooling in Illinois, especially lake areas in the afternoon. The
instability axis gets shoved farther south but not completely out
of the area until the cold front arrives, so will need to hang
onto the thunder mention into the morning and it is possible the
marginal risk may have to shift north a bit on Monday.



259 PM CDT

Monday night through Sunday...

More energetic shortwave energy and associated precip will shift
south of the CWA by Monday evening, with high pressure expected to
build across the area Monday night into Tuesday. Cold advection
on the backside of this system will return a cooler/drier air mass
to the region, especially by Tuesday. Next chance of precip does
appear to be on Tuesday, although confidence is lower with this
chance owing to some placement and timing issues. Upstream
shortwave anticipated to drop southeast through the region during
this time, as baroclinic zones tightens up. With this setup, a
narrow axis of developing precip seems probable and at this time,
a favored location would be over the southern CWA. Expected
highs/boundary layer conditions would support all liquid at this
time, but will continue to monitor for at least a low potential
for a rain snow mix. With anticipated colder thermal profile
aloft, it looks like if enough forcing were to occur to cool the
entire profile, then snow could be possible. Too early to tell at
this time though, and have maintained only rain for Tuesday. Any
precip then sags southward Tuesday evening with further expansion
of the high to the north. Much colder air filters south and with
this high moving more overhead, Tuesday night is looking to be a
chilly night. However, the CWA may stay on the outer fringe of
this high and tighter gradient. This would allow the winds to
possibly stay more elevated and lessen the extent of the cooling
this night. Dry weather then expected to continue Wednesday into
Wednesday night. Large scale pattern begins to change though, with
upper level ridge building overhead and warmer air returning to
the region. Warmer temps expected Thursday and Friday, but
precip/rain and possibly thunder chances increase by late Thursday
into Friday.



For the 18Z TAFs...

Prime aviation concerns other than the initial east/southeast
winds include thunderstorm chances overnight, with lingering
shower chances tomorrow morning, then a quick shift to nne winds
Monday afternoon.

High pressure is slowly shifting east this afternoon. This is
generally producing a synoptic southeast wind. There is likely
some lake breeze component which at this point is largely at the
lakefront but occasionally will impact ORD/MDW this afternoon, and
this is manifesting itself in considerably variable winds but
largely of an east-southeast component. The MVFR clouds will
continue to thin as well.

Focus shifts to several rounds of convection, most notably
overnight, and then again tomorrow mid to late morning. The main
axis of showers and thunderstorms will spread into northern
Illinois after 7z, and quickly spread east over a couple hour
period, all tied to a leading shortwave and strengthening low
level jet. Highest confidence in predominant thunder is still
south of the terminals, but high in the timing of showers with at
least occasional thunder in the vicinity given the modest
instability expected. Will reevaluate an prevailing or TEMPO
thunder in the 21z or 0z issuance, but timing is outlined fairly

A second round of at least showers, possibly with some thunder, is
expected mid to late morning. This have gone with showers for now,
though this could have some brief thunder with it as well. Lastly,
a decent northeast wind push will come with the cold front Monday
afternoon, and NE winds in excess of 10 kt are expected at
ORD/MDW. Medium on timing.



259 PM CDT

High pressure which has been situated across the lake for much of
the day continues to shift to the east at this time. This trend
will continue this evening and tonight with winds expected to
quickly increase this afternoon. Expect winds of up to 30 KT to be
in place by this evening over the north half with winds of 15 to
25 KT expected over the south half. Winds should stay in the 10 to
20 KT range along the nearshore tonight. The gradient and winds do
weaken by early Monday morning, but anticipate speeds to increase
once again later in the day Monday as a wind shift to the north
occurs. Although some slight variation may occur, expect northerly
winds to persist Monday night into Tuesday with an increase in
speeds expected once again. Speeds Tuesday night into early
Wednesday could reach gales for a time. Confidence is low at this
time, and so have maintained mention of 30 KT in the open water






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