Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200837

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
337 AM CDT Thu Apr 20 2017

315 AM CDT

Through tonight...

In the short term, watching low pressure moving into NE IA early
this morning, with the lake enhanced front over central IL finally
showing signs of lifting back north. However, MCS that moved
across WI has left a healthy cold pool in place over southern WI
and with outflow from this MCS moving into our northern CWA. The
southern flanks of this convective cold pool could delay the
likely northward surge of the warm air this morning, possibly to
the point that our NW CWA never makes it back into the warm
sector or only does so briefly.

By midday, the cold front is likely to bisect our CWA from NE to
SW with warm and seasonably humid air ahead of the front from
Chicago to Pontiac and points SE, which less humid and gradually
cooler air will be filtering in behind the front. The frontal
passage (FROPA) timing has slowed from what models depicted
yesterday, so there will be more opportunity for temps to heat up
(especially SE half of the area) and also more opportunity for
destabilization prior to FROPA. While instability is not forecast
to become terribly strong over NW IN and EC IL, it should be
sufficient (MLCAPE 500-1000 J/Kg) to support robust convective
development along the front, likely by late morning. Initially,
mid and upper level flow is not forecast to be all that impressive
this morning resulting in fairly marginal deep layer shear
profiles this morning. However, by early afternoon, stronger mid-
upper level flow (and resultant stronger shear) will translate
east and become juxtaposed overtop the destabilizing warm sector.
Given the modest instability and improving shear profiles with
time, anticipate storm intensification to be gradual in nature and
while it does seem likely they will become severe, that may not
take place until they get east of our CWA. It could be close
though and SPC`s marginal/5% risk looks spot on with depicting the
risk area in our CWA.

Frontal convection will shift east of our CWA during the late
afternoon as front works across the area, resulting in falling
temps this afternoon over SE CWA, while temps NW largely hold
close to steady most of the day. The cooler and drier air mass
will continue to filter into the area tonight and Friday. By
Friday afternoon the sfc high will spread far enough east to turn
the gradient N to NNE and allow our several day period of chilly
temps near the lake to begin. Highs could briefly reach 50 along
the lake before falling into the 40s in the afternoon, while areas
away from the lake will only top out in the 50s.

- Izzi


315 AM CDT

Friday through Wednesday...

Saturday looks to be the chilliest day of the upcoming stretch as
a storm system tracks well to our south, but close enough to
stiffen up the NE gradient some and provide for some cloud cover.
Temps likely to spend the day in the 40s near the lake and even
inland a bit, while areas well away from the lake toward I-39
potentially inch their way up into the mid-upper 50s. Heart of the
cold air mass will shift east of the area Sunday allowing for
moderating temps and a lighter/later lake breeze should even allow
lake adjacent locations to briefly get into the 50s before
afternoon lake breeze knocks temps back down.

Further warming will take place Monday as low pressure moves out
into the northern plains. Enough of a SSE gradient should exist
for an IL/northern suburbs lake breeze in the afternoon, but
elsewhere temps will warm well into the 70s. Tuesday looks windy
and fairly warm with highs making at a run at the 70s many areas.
Another sharpening frontal boundary is forecast to set up in the
region later in the week which will probably cool temps down
assuming it settles south of the area, like is climatologically
typical this time of year. By the nearby strong baroclinic zone
does suggest at least a chance of big temp swings. Rain chances
will also ramp up heading into the middle of next week in the
vicinity of the front.

- Izzi


For the 06Z TAFs...

The main forecast concerns for the period will be convective
and wind trends associated with a cold front moving across over
the region. A secondary concern will be cig/vis restrictions
associated with the convective activity.

At issuance time, a warm front is lifting north through the
region, with winds veering from ely to sely. As the warm front
lifts north, a low pressure center over nern Iowa will track
slowly newd into Wisconsin while the associated cold front pushes
ewd. At 06z, much of the tsra/shra activity was over Wisconsin
with the mid level shortwave providing the greater forcing. Some
isold shra has developed in the warm sector over nrn IL in a zone
of warm advection and isentropic lift, but forcing remains weak
and any activity developing in advance of the cold front is likely
to be scattered in coverage. Given the latest radar trends, have
delayed the vcts mention at the Chicago area terminals by an hour
or so. Will continue to remain a bit conservative with the TS
forecast at the terminals given the ongoing uncertainty as to the
TS coverage in nrn IL/IN ahead of the cold front. Feel that the
better opportunity for prevailing thunder will be invof the cold
front, but given the current scattered nature to the convection
over nwrn IL and that the activity is already pushing out an
outflow boundary into nwrn IL, feel that convection will be on a
weakening trend as it reaches the terminals, especially the
Chicago area terminals. So, while confidence in an extended period
of prevailing TS is low, confidence is relatively high that there
should be at least some sct tsra/shra invof the terminals between
arnd 07z and 10z.

winds should remain generally sely ahead of the cold front, though
there will likely be some variability due to convective outflows
and invof any tsra that may develop around the terminal areas.

With strengthening sly to swly winds at the top of the mixed
layer, with winds arnd 40-45 kt range overnight so, will keep the
mention for LLWS for a few more hours until sfc winds strengthen
and become more gusty along the front and pot-frontal. Following
the frontal passage, winds should veer to wly and then nwly with
sustained speeds of 15-20 kt and gusts of 25-30 kt through the
afternoon hours. There is some uncertainty regarding cloud bases
in the post-frontal air mass. While high pressure should
eventually build in behind the exiting system, satellite imagery
shows a large area of stratiform cloud cover on the wrn/swrn
periphery of the low and there is a chance that the post-frontal
cloud cover may be in mvfr ranges than the lower end vfr that is
currently being carried in the TAFs.


330 AM CDT

Active weather is expected across the Great Lakes for the next
several days as a series of lows move across the country. Low
pressure over northeastern Iowa will lift across central Lake
Michigan this afternoon. Moderate south flow to 30 kt will be in
place across the south half of the lake today while a window of
easterly gales is expected across northern Lake Michigan north of
the low track. West to northwest winds to 30 kt will overspread
the lake behind the departing low Thursday night and could be near
gale at times as colder air overspreads the lake. Moderate north
flow persists Friday into Saturday as a strong high builds into
the Upper Midwest. Winds over northern and central portions of
Lake Michigan should briefly diminish Saturday as the ridge axis
settles over that area, but another area of low pressure moving
east across Ontario Saturday night will push a cold front across
Lake Michigan Sunday night resulting in winds picking up once
again late in the weekend.


LM...Small Craft Advisory...IL nearshore waters until 11 AM Friday.

     Small Craft Advisory...IN nearshore waters until 10 PM Friday.




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