Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 241758
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1258 PM CDT SUN JUL 24 2016
1102 AM CDT
Focus remains on thunderstorm chances and coverage this afternoon
as a cool front slides southeast across the area. Surface analysis
shows the cool front stretching from near Madison, WI southwest to
about Cedar Rapids, IA and points southwest. Front lines up well
with leading edge of a narrow stratus deck on visible satellite
which has aided in being able to time the southeastward movement
over the next few hours. Satellite also shows another area of
clouds which seem to correspond with a mid level thermal gradient.
Weak low level convergence of south winds ahead of southwesterly
winds may be aiding this cloud development, though it has eroded
somewhat across northern IL in the last hour or so. RAP analysis
shows plenty of mixed layer instability available with inhibition
beginning to erode.
Have adjusted pops through the afternoon to reflect the movement
of the front. The front will act as convergence and also to erode
any capping. Coverage of storms is still a bit of a question mark
but do expect to see some spotty activity develop early this
afternoon with an increase in coverage late this afternoon.
Initiation time is tricky and it is possible that it may not occur
until the front is at least partially into the forecast area so
may still be a little overdone on pops across the northwest third
of the area or so. Areas from Chicago southwest to LaSalle and
points south likely have a more favorable chance for scattered
storms this afternoon so have a bit better confidence in pop
values there. Gusty winds and torrential downpours remain the main
threats with any storms that develop. The front is expected to
slow across the southern forecast area this evening leaving a
continued chance for thunderstorms.
Otherwise have made few tweaks to temperatures and dewpoints.
Therefore made no changes to the heat advisory. Only other tweak
at the moment will be to shift winds behind the front more
northwesterly this afternoon.
318 AM CDT
Feeling much more confident with the today`s forecast versus the
past couple of days due to clearer forcing for storms. First things
first, issued a heat advisory for the I-88 corridor south including
northwest Indiana. Heat indices will reach 105-110 degrees quickly.
On to convection...a few showers are visible over southern LaSalle
and Grundy counties this morning with the rain from the evening MCS
shifting east into central and eastern Indiana. Additional
convection over eastern WI and along the MN/WI state line will
remain north of us. The latest CR-HRRR suggests that the convection
over MN will build south and pass over the IL/WI state line this
morning. I am not buying this progression because storms are not
building south and storm movement is due east.
Thinking the atmosphere will recover this morning with no precip
expected. Meanwhile the cold front currently stretching from
eastern MN through IA will shift east reaching the Rockford area
early this afternoon. The atmosphere should recover enough for
storms to sprout along the cold front. Guidance features CAPE
around 3000 J/kg, 0-6 km bulk shear of 20-30 kt, and PWATs around 2
inches. Steering flow is a bit faster today around 30 kt. Therefore
expecting a scattered storms to develop ahead of the cold front with
a more solid line of storms along the front. Hail and damaging winds
are the main severe threats along with localized heavy rain. SPC
mentions that supercells may be possible in their discussion, but
thinking supercells will be more common to our north in WI where the
shear is greater. The line of storms shifts southeast through
318 AM CDT
Monday through Saturday...
Any lingering storms dissipate early Monday morning as cooler and
drier air spread across the region along with high pressure. Have a
dry forecast through Tuesday with high temps only in the mid 80s.
Cooler conditions are expected along the lake.
An upper level wave passes over southern WI and northern IL
Wednesday. Thinking the majority of storms will form over WI, so
only have a low chance of storms along the IL/WI state line. A
stronger upper level wave arrives late Wednesday night into Thursday
morning bringing showers and storms to the entire region. The
storms should also limit warming Thursday leading to highs only in
the low 80s.
The pattern remains busy through the end of the week with chances of
showers and storms. However, the good news is the cooler temps with
highs in the low 80s continue through late this week as well.
For the 18Z TAFs...
Focus continues to be on potential thunderstorm development this
afternoon with the approach of a cool frontal boundary. The front
stretches from south-central WI southwest across eastern IA and is
making steady southeastward progress. The front, as well as
several additional old boundaries from earlier storms over Lake
Michigan, will be the focus for storm development as they converge
on the local area. Expect scattered clusters of storms to develop
over the next few hours and have good confidence in timing of
VCTS/TEMPO TSRA in the latest TAFs. Best coverage may be across
the Chicago metro and points south and east through the afternoon
with RFD having a more limited threat with lesser coverage
expected in that area.
Outside of the storms, some sct-bkn MVFR stratus looks to move in
for a few hours behind the front before dissipating or clearing.
South-southwest winds will turn southwesterly as the front
approaches then west-northwest behind the front. Storms may
provide outflow winds which will result in erratic wind direction
as storms depart. Winds will turn a little more northwesterly this
evening, possibly earlier than in the latest TAFs. Steady
northwest winds are expected Monday. A lake breeze will likely
develop along the lakeshore but is not expected to reach ORD/MDW.
However, GYY will likely see a shift to northeasterly winds Monday
thanks to the lake breeze.
252 AM CDT
Southwest winds will increase today to around 25 kt ahead of low
pressure that will move east through northern Ontario today. A
cold front behind this low will move south across the lake tonight
with modest northwest winds behind it. High pressure will take
control through mid week with fairly benign conditions across Lake
Michigan. Thunderstorm chances exist today and then return later
in the week.
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