Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 300143 AAA

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
843 PM CDT WED JUN 29 2016

842 PM CDT

Only minor adjustments made to the going forecast. Given the
surface ridge placement over the area and the dry strong
subsidence inversion on the 00Z DVN sounding, along with what was
observed this morning for area lows, did bump down forecast lows a
few degrees. East of I-57 seems the area where a few sites could
dip below 50 degrees since some locations still have dew points
in the 40s.

Observational and latest model trends continue to support any
rain in the CWA holding off to after mid-afternoon on Thursday,
though mid to high cloud are likely to have spread across north
central Illinois prior to that time.



228 PM CDT

Through Thursday...

Fair weather cumulus clouds continue to sprout across the region
while winds are generally light and variable under the surface high.
The cooler and pleasant conditions continue overnight with winds
becoming southeast as the high shifts east.  Lows will be in the mid
to upper 50s away from the lake, and in the 60s in Chicago and along
the lake.

Cloud cover increases from west to east Thursday ahead of the next
system.  Thinking precip will stay close to the forcing from the
upper level trough and cold front. Therefore, expecting a mostly dry
day. Precip chances increase in the afternoon with only a chance
along and west of I-55.  Have low confidence in how far east showers
and thunderstorms will travel Thursday afternoon, and the whole CWA
may be dry through Thursday afternoon.

The cloud cover and precip will play a part in high temps Thursday.
Thinking north central IL will see slightly cooler temps and
struggle to get out of the 70s tomorrow due to increased cloud
cover.  The rest of the CWA should be around 80.



249 PM CDT

Thursday night through Wednesday...

Period will begin fairly active across much of the CWA Thursday
evening, as thunderstorm chances increase with a few isolated
stronger storms not out of the question over north central

Guidance consistent with well defined short wave trough dropping
southeast through the upper Midwest and Great Lakes region
Thursday night. Out ahead of this feature, surface trough/low will
shift east through the region, while thunderstorm chances increase
across northern Illinois by early to mid evening. Greatest focus
for development in the evening will once again be across northern
Illinois where higher dew point and instability axis will reside.
Then, as this low and associated front continues east, expect this
precip to swing east through the remaining areas in northeast
Illinois and northwest Indiana through the late evening. Highest
thunderstorm chances, and potential for more robust development,
during this period will reside in northern Illinois in the evening
with thunderstorm coverage likely diminishing throughout the
evening for remaining areas. Not overly concerned with a
widespread strong/severe threat. However, with increasing
instability and steep low level lapse rates ahead of this stronger
forcing, can`t rule out an isolated stronger storm and isolated
wind threat. Limiting factors such as possible lower dew point air
than currently advertised, tall/skinny CAPE, and weak shear will
keep this development isolated and brief especially as instability
quickly falls. Have increased to likely pops in the evening for
much of the CWA and then lower them to chance and slight chance
overnight, but its possible for most areas to be on the drier side
after midnight.

High pressure will then build southeast through the region Friday
into Friday night with fair conditions returning. Air mass Friday
into Saturday supportive of below normal conditions, with 70s
forecast for most locations. Not overly confident with the next
precip chances late in the weekend as guidance struggling with the
extent of the northward push of precip, associated with advertised
approaching shortwave energy. It does appear that
shower/thunderstorm chances will try and inch north Sunday and
Sunday night, with the higher potential well south of I80.
Continue this trend of higher pops across the southern CWA on
Monday, but will need to monitor for a possible northward shift of
this precip to remaining areas in the CWA.



For the 00Z TAFs...

The easterly lake breeze winds will begin to abate early this
evening, then gradually shift to the south-southwest tonight. The
winds on Thursday will continue out of the south-southwest up to
around 10 KT out ahead of an approaching cold front. This cold
front, and an associated upper level disturbance, will move over the
area Thursday evening, likely resulting in a period of showers and
thunderstorms. Current timing for the eastern terminal sites is
between 02 to 05 UTC Thursday evening. Farther west towards RFD, the
thunderstorm window looks to be between 00 UTC through mid Thursday
evening. Given this anticipated timing, the main thunderstorm
chances will be beyond the current TAF period, with the exception of
the 30 hour ORD TAF. These thunderstorms will likely need to be
added to the other TAF sites at a later time.



312 PM CDT

High pressure over the region will shift to the east tonight,
while surface low/trough approach from the northwest. As this
occurs, winds will shift to a southerly direction, and continue on
Thursday as this next system continues to move toward the lake.
With the low likely moving across the lake in the evening, there
could be a brief period of variable winds but do expect a
transition to a northerly direction through the night. Increasing
winds Thursday night into Friday morning will allow waves to build
across the lake, with the highest waves expected on the southern
half. The nearshore will be impacted with conditions hazardous to
small craft likely by early Friday morning, and then continuing
into the afternoon.






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