Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201750

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1250 PM CDT Thu Apr 20 2017

1055 AM CDT

Lifting shortwave energy ahead of main upper level trough to the
west, along with surface low and associated boundaries are
supporting scattered convection this morning. With the cold front
pushing through northern Illinois at this time, precip axis is
along an Ottawa to Chicago line moving east northeast. This will
be the farthest northwest of this precip axis, with additional
development late this morning through mid afternoon developing
along and ahead this front in east central Illinois and northwest
Indiana. Shower and thunderstorm development have struggled to
grow in intensity this morning, however, instability axis with
steep lapse rates and decreasing CIN ahead of this front will
likely support strengthening development over the next one to two
hours. Shear remains marginal, but cannot rule out a stronger
severe storm, given the steep lapse rates in place. Still
monitoring a hail and damaging wind threat. The window for this
possibility will be from now through mid afternoon, with the
highest threat expected to be southeast of the Kankakee river in
east central Illinois and northwest Indiana.

Temps have observed several swings this morning, with lifting warm
front and now with the cold front. Current warm temps in the 70s
will see a falling trend over the next couple of hours, as all
locations cool behind this front this afternoon.



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 18Z TAFs...

As surface cold front pushes through the area, any showers and
thunderstorms continue to shift east away from all of the
terminals. This drying trend will remain, with dry conditions
expected for the remainder of the forecast period. Westerly winds
are settling in and will persist through much of tonight, with
gusts also continuing. Ceilings are bouncing around from VFR to
MVFR but with more widespread MVFR ceiling coverage headed towards
the terminals, have gone with MVFR prevailing conditions.
Ceilings will likely remain overhead tonight into Friday, with
ceilings once again, bouncing around from MVFR to VFR. As surface
high shifts towards the area on Friday, more northerly winds will
shift to the northeast by midday with speeds staying at or above
10 KT.



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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