Area Forecast Discussion
Issued by NWS Chicago, IL

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717
FXUS63 KLOT 270207
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
907 PM CDT Wed Jul 26 2017

.UPDATE...
854 PM CDT

Monitoring a line of showers and storms marching east through
northern IL. My overall impression is the line is falling apart,
but slower than I originally thought it would. The latest RAP
analysis has a ridge of 1000+ J/kg through north central IL, but
the 0-6km shear axis has shifted further east. Surface CIN is also
increasing over the Chicago metro area. IR imagery shows cloud top
warming over northern IL and cloud top cooling over central IL.
Therefore, expecting the storms over northern IL to continue to
weaken and remain unorganized as they continue east. However,
cannot rule out an isolated cell or two pulsing up and producing
sub-severe wind gusts.

Also closely monitoring areal and flash flooding as PWAT values
are 1.5-2 inches. Storm motion is the key. If storms begin to
train or slow, flooding is much more likely. Upstream reports from
the Davenport office indicated as much as 2 inches of rain falling
in an hour. Not expecting rates that high since storms are falling
apart, but heavy rainfall still may lead to localized ponding or
flooding.

JEE

&&

.SHORT TERM...
229 PM CDT

Through Tonight...

For the remainder of the afternoon conditions should remain
relatively quiet with increasing mid and high cloudiness in advance
of an area of pcpn extending from srn WI, through ern Iowa and nrn
Missouri.  This activity is largely in response to a modest
shortwave tracking across the middle Mississippi Valley.  Warm,
moist, unstable conditions with MU CAPE of 2000-3000 J/kg,
temperatures climbing into middle to upper 80s and sfc dewpoints in
the middle to upper 70s.  Latest radar trends are for more sct
shra/isold tsra and in spite of a well defined MCV generated by
earlier convective activity over the cntrl plains, Extensive cloud
cover is likely helping to inhibit stronger, more widespread
thunderstorms this afternoon.

Looking into the evening hours, focus will shift to timing and
location of a swatch of heavy rainfall.  The 12Z DVN sounding
indicated pwats of 1.3 inches and the short range guidance indicates
that pwats should climb to slightly in excess of 2 inches as the
shortwave tracks east across the region.  Latest guidance is
suggesting that the shortwave should take a more sely track across
the region, which would suggest that the heaviest pcpn should be
suppressed a little farther south than previously anticipated.  While
locally heavy rainfall is still likely with some of the stronger
storms, the areal average heavier rainfall should be confined to
areas generally south of the I-80 corridor.  The overall severe
threat is also diminishing, but not totally out of the question,
with marginal mid level lapse rates and extensive cloud cover
somewhat limiting sfc based destabilization.  Also, with relatively
weak mid level flow, there primary weather threat will be more for
locally heavy rainfall, with an isolated wind threat more related to
wet microbursts than any kind of upscale thunderstorm development.

&&

.LONG TERM...
222 PM CDT

Thursday through Wednesday...

Main forecast concerns/challenges are with lingering chances for
showers and thunderstorms on Thursday, with a low chance for an
isolated strong storm mainly along/southeast of I-57.

Any lingering showers and thunderstorms Thursday morning will be
confined to areas south of I-80, but will be steadily southeast
out of the CWA through mid morning. Mainly dry conditions expected
for much of the cwa then anticipated through midday. Anticipate
cloudier conditions to continue throughout the day and with these
clouds likely to remain, have maintained similar high temp
forecast of highs around 80 with slightly cooler temps near the
lake.

There remains a chance for additional showers and thunderstorms
by early afternoon mainly for areas along and southeast of I-57.
Higher dewpoints pooling along surface trough still in place
across the southern CWA could serve as focus for additional
development. Coverage should remain widely scattered with any
development, with some isolated development possible further to
the north in northern Illinois. Most development likely to remain
subsevere, however, there is some potential for an isolated
stronger storm. This will be conditional on any clearing
occurring, and if that occurs, isolated areas of higher
instability are not out of the question. Storm chances will
quickly diminish by late afternoon and early evening, as focus
quickly shifts south of the area. High pressure then expected to
build across the region late in the work week into the weekend and
early next week, with no significant weather expected.

Rodriguez

&&

.AVIATION...
For the 00Z TAFs...

648 pm...Main forecast concern is thunderstorm chances this
evening into early Thursday morning.

Scattered thunderstorms near the Mississippi River will continue
moving east/southeast over the next few hours and the overall
trend is for these storms to weaken with the bulk of the activity
moving southeast into central IL. Additional thunderstorms across
southern WI may continue to move south into far northern IL by
mid/late evening and also are expected to weaken. Maintained
thunder at rfd but confidence further east at the Chicago area
terminals is low and trends will need to be monitored over the
next few hours. Some shower activity is possible overnight and
there is a some small chance of showers Thursday morning.

Light southerly winds will turn more southeasterly this evening
but speeds should remain well under 10kts. Winds will likely
become variable overnight and turn northeasterly Thursday morning.
The gradient will tighten and northeast winds will increase along
with some gusts Thursday afternoon into Thursday evening. An
outflow boundary across southeast WI currently will need to be
monitored as it moves into far northeast IL early this evening.

With the moist low levels and light winds...some fog is possible
overnight into Thursday morning along with mvfr cigs. Confidence
is low and trends will need to be monitored later tonight. cms

&&

.MARINE...
222 PM CDT

As surface trough moves across the northern part of the lake this
evening, will likely see a diminishing trend with the winds.
Expect these lighter winds to persist for a time tonight but as
the surface trough/low depart to the southeast late tonight
through early Thursday, will see increasing northerly winds across
the entire lake. Increasing northerly winds will be the trend
across the lake during the day Thursday, with winds of 15 to 25 kt
returning Thursday afternoon and evening. Occasional gusts to 30
kt could still be a possibility. These stronger winds and higher
waves will likely support hazardous conditions for small craft
late Thursday and Thursday night. However, given the time frame,
have not issued a small craft advisory at this time. Northerly
winds will then likely continue into the start of the weekend,
with speeds periodically bouncing from 15 to 25 kt, to 30 kt
during this time. Hazardous conditions for small craft will likely
continue as well.

Rodriguez

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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