Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201747

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1247 PM CDT Tue Jun 20 2017

1037 AM CDT

Through Wednesday...

Main focus in the short term is on area of showers/embedded
thunderstorms associated with fairly vigorous upper level
shortwave trough over WI. Impressive wave especially for June with
pocket of cold air aloft and steepening lapse rates advecting SE
along upper Mississippi and into north central IL later this
afternoon. Noting a drying trend in showers/radar echoes along the
leading edge as the entire precipitation shield moves SE and into
the lower dew point air/dryer low levels per forecast sounding

Expect thunderstorms to develop along/ahead of differential
heating boundary associated with aforementioned showers moving
into the area... and lake breeze convergence boundary early this
afternoon. Graphical NOW recently issued depicts current thinking
of timing of development across the area. With increased shear
later in the afternoon ahead of the upper shortwave remained
concerned for discrete storm development with perhaps some
isolated storms taking on low topped supercell characteristics.
Deep mixing through 600 mb per forecast soundings particularly
across the southern sections of the CWFA suggest strong wind gust
potential with storms... especially if storms can organize into
linear segments later in the afternoon. Could also see a few
quarter (or larger) size hail reports given decent CAPE given
steeper lapse rates ahead of the shortwave. Short range CAMs
supportive of this thinking as well.

Expect storms to diminish northwest to southeast across the area
this evening with most activity winding down by 8-830 pm with both
passage of upper wave as well as loss of better instability with
both wave and diurnally. Skies then clear overnight with
pleasantly cool conditions for this time of year. Fair weather
expected Wednesday under the influences of departing high pressure
before a returning warm front late in the day brings increased
chances of rain.

Ed F


309 AM CDT

Wednesday through Monday...

Beyond today, not much change in thinking from previous forecasts.
Still appears as though a transition to a more zonal flow pattern
will take place with a pretty good shots at showers/storms. First,
there is a chance of showers/storms Wednesday night as warm front
lifts north, though better chances look to be north of our area
across WI/MI and kept our chance pops focused mainly over our
northern CWA.

Blended model guidance keeps rain chances going all day Thursday,
but anticipate that the warm front will make it far enough north
that Thursday will be dry over most, if not all of our CWA during
the day Thursday. Left the chance pops from the blended model
guidance in the grids for now, but if later forecasts continue to
keep the warm front safely north into WI, should be able to lower
if not remove precip chances for Thursday in later forecasts.

Probably our best chance of rain looks to be Thursday night
possibly lingering into Friday morning as frontal boundary sags
back south into the area. As day shift alluded to, significant
moisture pooling along and ahead of the boundary should boost
PWATs to seasonably high values. Low level jet is not forecast to
be overly strong Thurs night, but what LLJ there is should veer
westerly and with deep westerly flow fairly parallel to the front
suggesting that the front and convection may not be in a big hurry
to move south. Certainly looks like there will be a threat for
heavy rainfall in the region Thurs night and something to keep an
eye on.

Front will sag south of the area with a couple nice, mild summer
days Friday and Saturday before the next trough digs into the
Upper Midwest bringing increasing threat of showers and perhaps
some thunderstorms Sat night into Sunday. GFS and especially the
ECMWF bring a pretty impressive slug of cold air down with this
trough later Sunday into Monday, but the 00z operational runs keep
this trough fairly progressive with quick moderating trend next

- Izzi


For the 18Z TAFs...

Line of showers dropping across the metro causing winds to shift
north - 360 at 8-9 kt... expect winds to increase in the hour
following to 11-12 kt and occasionally G 17-18 kt. Have resolved
this in the TAFs. Still looking for scattered thunderstorms to
then develop mid afternoon with best window for any thunder at the
TAF sites coming between 3-5 pm local time. Upper level
disturbance and surface cold front move south of the area after
6-630 pm and expect with that any rain/storm chances at the
terminals to end by that time. Winds remain a challenging forecast
behind the convection but expect NNW to NE synoptic flow to
establish itself during the evening with winds dropping below 10
kt by 00z as well.

Ed F


205 AM CDT

West winds become chaotic due to convection this afternoon.  Could
see a period of east to northeast 10-20 kt winds over the far
southern end of the lake this afternoon.  Light winds become south
to southwesterly tonight as high pressure builds overhead. The high
shifts east Wednesday and winds become south behind it.  A low
shifts east across southern Canada through the latter half of this
week, and the tightened pressure gradient over the lake may lead to
15-25 kt late Thursday morning. Winds become west behind the low`s
cold front Thursday night and continue to veer to northwest or north
Friday evening. An extended period of north winds is possible over
the weekend, and lake breezes are possible.






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