Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 242327

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
627 PM CDT TUE MAY 24 2016

233 PM CDT

Through Tonight...

Deep layer moisture in on the increase across the region with a
fetch of Gulf moisture riding on SSWLY flow.  Temperatures across
the region have risen into the middle to upper 80s with sfc
dewpoints in the middle to upper 50s over the western portions of
the CWA.  Dewpoints over NWRN Indiana remain a bit lower, in the
40s, but as upper level troughing slowly shifts EWD, the moist air
will overspread the entire area tonight.  Pcpn remains split to the
north, over Wisconsin and to the south, over eastern Missouri into
southern Illinois.  Have been battling with PoPs for the afternoon
and into the evening hours.  An area of showers have been developing
along a weak sfc trough/moisture axis in Wisconsin.  Some showers or
a few thunderstorms could sag south into the northern tier counties
south of the Wisconsin border, but expect that coverage would be
sparse.  In general, feel that much of the guidance has been
generating too much pcpn with this feature, so have tried to keep
PoPs in the low chance or slight chance range.  A pair of MCVs
moving through Missouri have been the focus of thunderstorms through
the day.  The short range guidance is in generally decent agreement
on continuing to indicate some convective development with these
features through the night. With the general SWLY flow upper ridging
building along the Mississippi Valley, there is a chance that some
thunderstorms may track into the southern portions of the CWA.  So
have increased PoPs over the southwestern portions of the CWA with
the area from LaSalle, to Pontiac to Gibson City having the greatest
chances for pcpn during the pre-dawn hours. With general increase in
moisture across the region, overnight lows will not be quiet as low
as in recent days.  Expect overnight lows tonight in the lower to
middle 60s.



256 PM CDT

Wednesday through Tuesday...

Main forecast concerns/challenges are with periodic chances for
thunderstorms through the period...with warm/muggy conditions
appearing to stay in place through the period.

Did make some changes to pop trends on Wednesday, beginning with
lowering pops in the morning. Although some guidance is hinting at
the possibility for scattered precip in the morning, don`t think
precip will be widespread during this time. Strongest forcing
associated approaching shortwave energy and stronger LLJ will be
focused to the west of the CWA. That being said, can`t completely
rule out some isolated thunderstorms in the morning as there is
the potential for weaker energy to drift across the area and while
surface trough/boundary to the north approach. So have maintained
low chance pops in the morning but then lower to mainly slight
chance by late morning into midday, to account for
diminishing/exiting development that does occur. Late tonight into
Wednesday morning, instability will increase significantly across
the entire cwa including steepening mid level lapse rates. If
anything does occur Wednesday morning dont anticipate any
strong/severe storms given that lack of large scale ascent.
However, this increasing instability would allow for small hail or
heavy downpours. Think most locations will stay dry midday into
the early/mid afternoon while any focus for development remains
west of the CWA. With this in mind, most locations will likely
stay in more partly cloudy to partly sunny skies and with
continued WAA, raised high temps Wednesday. Low to mid 80s
definitely possible for most locations and if any location gets
into sunnier skies, can`t rule out upper 80s for some places.
Conditions near the lake will be tricky as surface pattern will
support more onshore flow for areas in northeast Illinois. This
will be due in part to the lingering trough axis/boundary in the
area and have fairly good confidence with winds in the morning,
but confidence does lower in the afternoon. Some guidance keeping
this onshore flow in place, while other guidance showing a push to
southwest/west winds. With this lower confidence in place,
maintained the idea of some cooling near the lake but it is still
not out of the question for these areas to get into those warmer
temps at least a brief time in the afternoon and early evening.

Although I delayed the highest pops on Wednesday to the late
afternoon and evening, maintained likely pops for most locations.
However, did confine them more into northern Illinois where most
guidance is indicating stronger ascent will be in the afternoon.
With lower confidence with respect to where the boundary will be,
have lower confidence where best focus at the surface will be too
which would owe to coverage and intensity. However, with dewpoints
still expected to rise well into the mid/upper 60s, MLCAPE
approaching 2000 J/KG and with steeper lapse in place on
Wednesday, at least isolated to scattered thunderstorms will
remain possible over much of the area. Especially with short wave
energy is expected to progress through the area in the afternoon.
With flow aloft rather weak, bulk shear will be low, and dont
anticipate any widespread severe threat. The presence of these
steeper lapse rates and possible better focus at the surface could
allow for an isolated strong to severe storm though, with hail and
winds the main threats. At this time, areas over the
west/southwest half of the CWA appear to have the higher threat of
this potential. Will also need to monitor for the likelihood for
heavy rainfall with any storm that does develop as current higher
PWAT axis of 1.5 inches or more over the Plains/Mid Mississippi is
anticipated to swing overhead. Couple that with the higher
instability and slow storm motion owing to weaker flow aloft,
heavy downpours will be likely.

Anticipate thunderstorm intensity to weaken with time in the
evening and overnight hours Wednesday. However, with additional
support aloft, do think thunderstorm development will continue and
have maintained likely pops. It looks like the area gets into a
more subsident environment on Thursday with precip chances staying
on the low side. Guidance is still showing a stronger signal for
some precip development in the afternoon over the southern CWA,
and continued this trend in the grids with low chance pops. A warm
and muggy day expected on Thursday with this pattern, that appears
to remain in place through the remainder of the forecast period.
With this warm/moist environment in place, periodic thunderstorm
chances will continue as well.




The main forecast concern will be with the timing and chances of
thunderstorms over the area...especially later Wednesday into
Wednesday night.

A broken line of showers...with a few rumbles of thunder...continue
to shift eastward across southern Wisconsin...with more isolated
activity farther south into northern Illinois. While a brief shower
or rumble of thunder could impact KRFD here through 01 to 02
UTC...any activity would be quick and of low coverage. Also cant
rule out an isolated shower getting close to the Chicago area
terminals this evening...but chances are too low at the present to
include in the TAF.

Aside for the potential of some lower visibilities in fog later
tonight...mainly outside the main Chicago appears the
thunderstorm threat will remain low overnight. It appears the better
chances of thunderstorms at the main terminals will enter the
picture later Wednesday afternoon and evening. Model guidance is in
good agreement in shifting an upper level disturbance over the area
into the early evening...and with increased low level moisture...we
should have a good chance of thunderstorms over the area. Because of
this...I have converted the PROB30 for TS to a tempo group.

There remains uncertainty on the exact timing of thunder at each
terminal. However...the late afternoon through at least the early
evening hours is appearing to be a good window for thunderstorm
activity. Additional thunderstorms will remain possible over the
area Wednesday evening and night...possibly as other
thunderstorms develop to our west and shift into northern Illinois.
However...confidence on timing of any additional thunderstorm
activity is low at this time.



325 PM CDT

Frontal boundary is expected to push down the late tonight into
Wedneday morning, providing a change to a more northerly direction
for mainly the north half. This change looks to be brief Wednesday
until transition back to more of a southerly direction over the
entire lake occurs by Wednesday night and Thursday. Expect
southerly winds to then be in place for the remainder of the
forecast and with this expected, a warmer more moist airmass will
likely move across the lake. This will help setup the possibility
for fog, some of which could be dense through this week. Will need
to monitor trends, for this possible dense fog.






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