Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181942

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
242 PM CDT Tue Apr 18 2017

230 PM CDT

Through Wednesday...

Breezy and warm conditions will continue this afternoon.  Winds
diminish this evening and cloudy skies are expected.  Expecting
overnight lows to drop into the upper 50s to around 60.

The cold front currently stretching from southern Minnesota through
eastern Nebraska will slide over the region early Wednesday morning.
A diminishing line of showers and storms will shift east across
the forecast area. Thinking the line will be mainly scattered to
isolated showers and storms so capped precip chances in the low
end chance range.

The cold front stalls out as an east west boundary along or just
south of I-88 by mid to late Wednesday morning. By afternoon the
boundary, lifting north as a warm front ahead of a center of low
pressure approaching from the central Plains, becomes the focus
for convection. Timing for convection depends on how quickly we
recover from morning precip, but thinking we should see the next
round of precip begin in the late morning/early afternoon. The GFS
and 12Z ECMWF are the most aggressive with QPF focused along and
north of the boundary. Other sources of forcing will be a weak
upper level shortwave moving through and a low level jet
approaching the west central forecast area.

As far as severe weather is concerned, the best threat is still over
Iowa as depicted in the SPC Day 2 forecast. However, we cannot rule
out a few strong to severe storms.  The big question will be how
well we recover from earlier activity. Guidance is hinting that
areas north of the boundary will struggle since highest CAPE
values of upwards of 2000 J/kg will be south of the boundary. Deep
shear values are favorable across the region though so if storms
can tap the CAPE, we could see large hail as the main threat. 0-1
km shear increases to a favorable 30-40 kt by 00Z Thursday so the
tornado threat will not be zero.

The GFS suggests more convection will fire south of I-80 Wednesday
afternoon, but have low confidence in this solution given that most
of the forcing will be either west or east of the region by that
time.  Kept a chance of showers and storms just in case though.

The other issue will temperatures. South of the boundary,
temperatures will be pushing 80. However winds become northeast off
of the lake on the north side with temps falling in the afternoon.
Lake front temps will likely remain in the 50s.



230 PM CDT

Wednesday night through Tuesday...

For Wednesday night and Thursday...

Deterministic models are in decent agreement with the position of
the surface low just north of the area as of daybreak Thursday. The
associated cold front would just be moving into the area at this
time, making for a mild and moist morning with dewpoints in the mid
50s to lower 60s and temperatures only a couple of degrees warmer.
In southern locations the lows Wednesday night might be more typical
of normal afternoon highs for this time of year. Locations in far
northeast Illinois are likely to see lows occur in the evening and
then rising temperatures into Thursday morning.

Given the degree of moisture and temperature advection, Wednesday
evening into Thursday morning would appear to be the period with the
best chance for significant precipitation over the next week.
Somewhat surprisingly, and likely due in part to the unfavorable
diurnal timing, model QPF fields keep the warm sector of the low
mostly dry and confine the best precip chances to the northeast
quadrant of the low and along the cold front.  Moreover, models do
not show the cold front becoming widely active until Thursday
afternoon when it has already cleared the local area.  Given these
trends, will plan to maintain likely pops in northern portions of
the area and slight chances in southern locations, with relatively
light precip amounts under a half inch up north and only a trace
south. By morning most of this activity should be clear of the area,
leaving cool and gusty post frontal winds with falling afternoon

For Thursday night and Friday...

A broad surface ridge builds into the upper Mississippi Valley
behind the departing low on Thursday night into Friday.  Effects of
the cool north to northeasterly advection ahead of the ridge Friday
afternoon will be most pronounced along the Lake Michigan shoreline,
and also in southern portions of the area which otherwise had
managed to climb near or above 70 degrees all week. At this range it
appears highs for most portions of the forecast area will remain in
the 50s on Friday.

For Saturday...

As the upper Mississippi ridge slides into the Great Lakes, cool and
breezy northeasterly flow continues on Saturday, keeping areas along
the lake in the 40s and outlying locations again in the 50s. There
have been ongoing questions regarding the track of a low pressure
system that models show digging out of the southern Rockies and
lifting into the Tennessee Valley. Previous runs of the GFS and
ECMWF suggested the northern extent of the precip shield around this
low could spread into the local area, but more recent trends show
the Great Lakes ridge inhibiting development of rainfall this far
north. Given the continued uncertainty in this track, we will
maintain slight chance POPs Saturday afternoon and evening but
mostly for our southern counties.

For Sunday through Tuesday...

As the Tennessee Valley low makes its way into the mid-Atlantic and
then eventually moves offshore somewhere around the Carolinas,
surface and upper level ridging becomes reestablished over the local
area for the end of the weekend and early next week.  Dry weather
and a return to seasonable highs appear to be the trend at least
until Tuesday when the next system approaches from the northern

Another consideration for the cooler period on Friday through Sunday
is the possibility for frost in the predawn hours.  Overall the
threat does not appear to be widespread, but outlying locations
toward the Wisconsin line may see lows falling into the mid 30s and
the corresponding possibility of patchy light frost, especially on
Saturday morning when winds are lightest.



For the 18Z TAFs...

Gusty south winds continue through this afternoon with high level
cirrus spreading from west to east. Winds diminish this evening
and a decaying line of showers and storms will shift through early
Wednesday morning along a cold front. Have low confidence in the
coverage of showers and storms especially at the eastern
terminals. Guidance features convection in various stages of
decay; therefore, did not make any changes to the PROB30 for early
Wed morning. Guidance features MVFR cigs with the early morning
convection. Since guidance has been overdoing low level moisture
and low clouds lately, kept sites at VFR.

A boundary will lie east to west across northern IL Wednesday and
another round of thunderstorms is expected to form. Have low
confidence in the timing and coverage of these storms as well.
First, convection initiation time will depend on how quickly we
recover from the morning convection. Second, where exactly will
the boundary be? Lastly, I have low confidence in coverage. So
went with another PROB30 for the most likely time for thunder.
Showers and storms may persist through the afternoon. In addition
to thunder, winds will become northeast to east on the north side
of the boundary and wind speeds are expected to reach 10+ kt at
the eastern terminals. Locations south of the boundary will retain
southwest winds.

Guidance also keeps a thin band of MVFR cigs along the boundary
and over ORD. Kept all of the eastern sites at MVFR through the
morning and afternoon due to low confidence in the exact placement
of the boundary.



230 PM CDT

Two centers of low pressure will cross the lake this week, the first
across the far northern end tonight and the second across mid-lake
on Thursday.  The second will be more potent in terms of its wind
and weather impacts, mainly due to being a little stronger and
tracking directly over the lake. A brief period of gales is possible
north of the second low on Thursday morning. Both lows will support
the chance of thunderstorms along with multiple wind shifts.
Following the second low, high pressure building across the lake
will bring quieter conditions from Friday into early next week.






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