Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 271747

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1247 PM CDT SAT AUG 27 2016

1113 AM CDT

There have not been any large changes to the afternoon forecast
of a chance of isolated/scattered thunderstorms along with patches
of sunshine and the humid air mass warming to the low to mid 80s
from north to south. Do think the chance of isolated storms exists
somewhat equally across the forecast area during the mid afternoon
through early evening.

Late morning water vapor satellite imagery indicates continued
moist southwesterly flow which is mainly laminar. Precipitable
waters were observed at 1.7 and 2.0 inches on the 12Z DVN and ILX
soundings. One small somewhat sheared short wave disturbance
across south central Iowa is tracking northeast. RAP analysis
indicates slight 850mb-600mb cooling with this which based on the
DVN sounding would allow for a convective temperature around 81
degrees. Hand surface analysis show the warm front to be somewhat
diffuse across the area with its northern edge located near I-88
and inching gradually northward. This should become slightly
better defined this afternoon with diabatic enhancement. The near-
surface wind flow remains fairly weak though, so the frontal
convergence may be somewhat limited in the northern CWA, but
possibly enough with the aforementioned forcing to see isolated
to scattered storms. While the CAMs remain fairly broad in
solutions, a majority do generally indicate some activity in the
WI/IL border region later today or early this evening. With
effective shear of 30-40 kt, if near-surface destabilization can
reach northward...which is a fairly big if still as noted in
previous AFD...cannot completely rule out that strong to severe
storm or two. Main threat would be gusty winds.

As for south of there in the CWA, slightly confluent south-
southwest flow may support isolated storms as well in the northern
end of the warm sector. Convective allowing guidance generally is
not excited about this and have maintained the low chances.

Widely isolated showers may linger into tonight across mainly the
central and eastern CWA.



316 AM CDT

Through tonight...

Water vapor imagery very telling this morning showing a plume of
moisture flowing from the Gulf of Mexico across the Plains and
into the Upper Midwest. Broad area of low/mid level warm air
advection underneath the right entrance region to the upper jet is
resulting in numerous thunderstorms across the region early this
morning. Precipitation is expected to continue across northern
Illinois and northwest Indiana through late this morning, then the
better forcing will lift to our north and should allow coverage to
diminish from the south. Most of the hi-res CAMs are in decent
agreement showing the bulk of the precip out of the CWA by 16-18Z
with spotty activity thereafter.

The forecast details become a bit murky as we head into the
afternoon. RAP/HRRR runs don`t develop much of anything this
afternoon, but NAM/GFS continue to hone in on afternoon showers
and thunderstorms affecting portions of the CWA, particularly
along/east of a Pontiac to Chicago line where guidance indicates
there will still be some upper level divergence in the entrance
region to the jet. Unsure if any appreciable instability will be
able to redevelop today, but if we do see partial clearing then
moderate instability could result. This will allow any
thunderstorms that redevelop to become strong to severe with mid
level flow of 35 to 40 kt in place providing sufficient shear for
an organized severe weather threat. Confidence in this occurring
is low.



316 AM CDT

Sunday through Friday...

Warm, humid, and unstable conditions will be in place Sunday
through Tuesday with periodic chances for showers and
thunderstorms through this timeframe. On Sunday, potent shortwave
is expected to lift across the northern Great Lakes region with
upper ridging building into portions of the Upper Midwest.
Meanwhile, the low level ridge axis will extend from the New
England coast southwest into the lower/mid Mississippi Valley with
convection expected in the unstable air mass on the periphery of
the ridge. This pattern should generally persist through Tuesday
with afternoon highs in the mid 80s and dewpoints in the 70s
contributing to moderately strong instability across much of the
Midwest. No upper level support and little in the way of forcing
should limit thunderstorm coverage, but do anticipate some widely
scattered pop-up thunderstorms, diurnally favored, Sunday and
Monday. Lake breezes appear possible each day and could serve as
the focus for thunderstorm development. Weak mid-level flow will
limit any severe threat through this period. A cold front is
expected to push across the region on Tuesday which will bring a
better chance for showers and thunderstorms.

Stage is set for a pattern change for the latter half of the week
favoring drier and cooler weather. Through midweek, upper ridge
is expected to amplify over the midsection of the country which
will allow cooler air to spill into the Great Lakes region. The
850mb trough is expected to push across the region Tuesday night
with temps falling into the low teens Celsius in its wake, and
possibly even touching single digits briefly per the 12Z ECMWF.
Continue to see model differences in how far southwest the cold
air will spread, though the 00Z ECMWF came into a little better
agreement with the GFS keeping the coolest air just to our east.
Nudged temps down slightly for Wednesday and Thursday. Thursday
should be the coolest day, with afternoon highs only in the low to
mid 70s. Surface high will build across the Great Lakes Thursday
and Friday providing dry weather, then the upper ridge follows
suit Friday into Saturday allowing a gradual warming trend to
commence as we head into the weekend.



For the 18Z TAFs...

The concerns with the TAFs are with slowly departing MVFR cigs
along with the potential for them to return later tonight. There
also may be areas of fog late tonight/early Sunday morning.
Finally, there is the possibility of isolated SHRA/TSRA later this
afternoon and evening.

A warm front is draped west-east from near Rockford to the north
side of Chicago. This front is fairly broad and slow moving so
ceilings have been very slow to lift. Would expect a continued
gradual trend upward through the MVFR category and eventually
into VFR by late afternoon. These clouds have and likely will
prevent significant destabilization, so any afternoon and evening
showers/storms are expected to be isolated in coverage. Cannot
rule them out at any of the airports, but the probability is low.

Southerly winds with occasional gusts to around 15 kt this
afternoon will turn southwest and lighten this evening. The
direction should become variable late tonight as a weak surface
trough inches into the area. Given the most column, would expect
there to be some MVFR cigs to develop in this light wind regime,
though confidence on cloud base height tonight into Sunday morning
is low and cannot rule out IFR, especially at RFD. If clouds are
limited in their persistence tonight, then fog could be a greater



253 AM CDT

No major marine concerns through at least early next week other
than a few periods of thunderstorms, most notably today and again
possibly Tuesday. Weak high pressure currently in place across
the Great Lakes will quickly shift east of the region today as
quick moving low pressure will shift northeast across central Lake
Michigan. Low pressure across the Canadian Rockies will cross
north central Canada early next week as high pressure returns
Sunday-Tuesday. The aforementioned low will continue east across
James Bay Tuesday and to the Canadian Maritimes later in the week.
This low will send a cold front cruising south through Manitoba
and Ontario, with a portion of this front impacting Lake Michigan
beginning later Tuesday. This will shift winds to northerly before
high pressure returns for mid to late week.






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