Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221918

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
218 PM CDT SUN MAY 22 2016

138 PM CDT

Through Monday Night...

Sunshine and warmth will be the main story in the near term as high
pressure is sprawled across the Great Lakes and portions of the mid
to upper Mississippi Valley. The ridge axis is centered just to
our north this afternoon allowing a light northeast breeze to keep
the lake front cooler while temperatures inland warm into the low
80s. The ridge axis will slide to our southeast overnight
allowing southerly flow to overspread the forecast area tomorrow.
Temperatures tomorrow are expected to be similar to today...except
the warmth should spread across much of the Chicago metro with
south flow. Parts of Lake County and northern Cook County will be
the main areas of concern where an afternoon lake breeze will
keep temperatures cooler.

Monday night...low level jet ramps up across portions of the mid
Missouri Valley which will be the focus for initial nocturnal
convection.  As the low level jet veers late into the overnight
hours...several models indicate that convection will spread into
northwest and north central Illinois late Monday night. With the
main instability axis focused well to our west and veering/weakening
low level jet by the time precip approaches the local area...would
anticipate any convection to be weakening if and when it enters the
CWA. Maintained some slight chance/low chance PoPs mainly along and
west of the I-39 corridor and focused them towards the pre-dawn
hours early Tuesday morning.



217 PM CDT

Tuesday through Sunday...

Main forecast concerns/challenges are with periodic chances for
thunderstorms through the period.

The period will begin with approaching thunderstorm chances
Tuesday morning, as upper level ridge shifts to the east and as
steering flow helps to shift over more energetic flow across the
region. The stronger forcing coinciding with surface reflection
and frontal boundary will be further to the north over northern
Wisconsin Tuesday morning and even Tuesday afternoon. However,
guidance showing some thunderstorm development pushing through
eastern Iowa into west/northwest Illinois in the morning. This
scenario seems reasonable during that time, given eastward
shifting instability and moisture axis. Guidance continues to vary
to the extent of this development in the afternoon as it pushes
further east, with some guidance dry for most of the CWA in the
afternoon. Confidence lowers in the afternoon with thunderstorm
chances as it would appear any lingering development would follow
instability/moisture axis to the south of the cwa. Decent
instability and moisture in place for much of northern Illinois
would support any guidance showing thunderstorm chances for the
remaining cwa in the afternoon, and so have maintained slight
chance/chance wording for the entire cwa Tuesday afternoon. Flow
in the mid/upper levels is rather weak during this time, and think
thunderstorm organization would be limited if it occurred but with
dewpoints reaching the lower 60s and increasing instability, will
continue to monitor for the possibility for isolated strong storms
with any development Tuesday afternoon. With low confidence on
precip chances Tuesday, lower confidence with high temps as well.
However, airmass would support lower 80s, and have maintained that
in the grids.

After a possible lull in activity Tuesday evening, approaching
boundary from the north and increasing LLJ/shortwave energy will
likely support development once again later in the night into
Wednesday morning. Guidance showing frontal boundary continuing
south into northeast Illinois by Wednesday morning, with this
providing highly varible temp conditions Wednesday. Did lower high
temps for areas near the lake in northeast Illinois Wednesday but
if front were to have a stronger push, then think temps could be
even lower than the low to mid 60s now forecast. Higher confidence
for thunderstorm development on Wednesday now with a frontal
boundary anticipated to in be in place across the CWA. Large scale
ascent appears to be present for most of the day with rather
energetic flow aloft and have maintained likely pops for the day,
but possible ongoing thunderstorms in the morning is providing
lower confidence for thunderstorm intensity later in the day. With
dewpoints likely pooling along boundary, any location south of the
boundary that sees clearing will see instability ramp up. So
despite lower confidence, will need to continue to monitor for
additional chances for at least strong thunderstorms on Wednesday.
No real significant changes to remainder of the forecast into late
in the work week and weekend with mild/moist conditions and
energetic mid levels supportive of continued chances of




Very quiet weather expected to continue through the remainder of
the TAF period with VFR conditions and generally light flow
under the influence of high pressure. Ridge axis is currently
centered just north of the terminals stretching from near James
Bay southwest into the Upper Mississippi River Valley and allowing
light northeast flow across northern Illinois and northwest
Indiana. The ridge axis will settle to our southeast overnight
into Monday allowing southerly flow to overspread the region.
Outside of the winds gradually veering...conditions will remain
unchanged with Mostly Sunny/Clear skies and VFR.



217 PM CDT

Ridge of high pressure in place across the region providing
relatively lighter winds across the lake this afternoon. This high
will begin to shift east of the lake this evening/overnight with
winds turning more southerly. With this high just to the east and
with low pressure northwest of the area, will see an increase in
winds across the lake on Monday with the highest increase expected
over the north half of the lake. This trend looks to continue
Monday night into Tuesday, although, with some slight diminishing
trend in the southerly winds. Southerly winds will turn to the
north over the lake Tuesday night into Wednesday as a frontal
boundary pushes through the region. However, don`t anticipate the
northerly winds to be overly strong at this time.






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