Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251933

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
233 PM CDT WED MAY 25 2016

232 PM CDT

Through Tonight...

No real significant changes with thoughts for thunderstorms this
afternoon into the evening, with scattered development likely
impact much of the area.

Latest radar imagery depicting isolated to widely scattered
storms developing just west of I39, focused along weak convergence
over northwest Indiana. Increasing instability supported by
steepening lapse rates and with diminishing CIN will continue over
the next couple of hours as large scale ascent further increases
with the approach of mid level energy. Most short term guidance
still indicating further expansion of what is occurring at this
time over the next 2-3, while likely shifting east with time and
feel fairly confident with this solution. Also feel fairly
confident with timing, once again, highlighting the 20/22z time
frame over north central Illinois and then the 21/23z period for
northeast Illinois and northwest Indiana for increasing coverage
and intensity of thunderstorm development. This also includes the
possibility for isolated strong to maybe severe storms across the
entire area, with hail/wind still the likely hazards. Overall
development should likely diminish by mid evening, however, some
guidance continues to develop additional thunderstorms beyond this
period likely owing to an increasing LLJ with large scale ascent
still overhead. Do think the guidance that is showing this
development is a bit overdone with coverage at that time, which
could be associated to boundaries that more than likely wont be in
place. Nonetheless, still maintained low chance to slight chance
pops wording overnight, but focus it more south with time as the
focus shifts south tonight.



335 AM CDT

Thursday through Tuesday...

Upper ridging to our immediate southeast is expected to try to
push back into the area Thursday while an overall warm and moist
air mass continues to reinforce itself in deep southwesterly
flow. While it is possible a residual boundary or two is draped
over or just south of the area to start the day...most of the
daytime period Thursday looks capped and have maintained only low
chances for storms. If this is the case and not much for cloud
cover...then temperatures should easily reach mid-upper 80s for
highs. Storm chances look to increase into the nighttime as
another low-level jet strengthens under improving upper difluence.
Any storms look to be elevated with a possible isolated hail

As the upper low over the four corners region shifts into the
Plains Friday...improved upper jet positioning/forcing within
high PWAT air should increase shower and storm likelihood across
much of the Mississippi River Valley regions. Guidance disagrees
on just how far east this will the western CWA has
higher chances for multiple rounds on Friday than the eastern CWA.
Likewise temperatures may be much warmer further east...mid to
upper 80s...than west where they could be held at or below 80
depending on timing/coverage of showers. Instability looks
marginal and wind profiles are not particularly favorable for any
severe storms.

Confidence is high in above normal temperatures for the Holiday
weekend and fairly high that any showers/storms are scattered
more so than widespread given once again nebulous forcing. Have
only made small tweaks to the guidance blend at this time.




Winds have already begun to shift to the southeast in response to
the surface pattern, but think a further eastward shift will be
possible this afternoon with lake breeze interaction. However, do
think winds will trend back more southerly later this evening into
Thursday morning. VFR conditions expected to prevail for most of
the forecast period. This will be outside of likely precip this
afternoon/evening and then possible fog overnight. Lower
confidence with fog trends tonight so have left it out of the TAFs
at this time. Did maintain thunder mention though, with a period
of scattered thunderstorms still likely to impact the
thunderstorms late this afternoon into the early evening. Did
delay timing, as well as shorten up the tempo, given latest
trends. Also maintained mention of additional precip late tonight
into Thursday morning, however, have lower confidence with this




128 am...A cold front over northern Lake Michigan will move south
to central portions of the lake this morning and then become
stationary. Low pressure moving northeast across the plains
tonight in Thursday will lift this front back north as a warm
front tonight into Thursday morning. The gradient will tighten
some behind the front with the potential for easterly winds as
high as 15-25kts on the far north end of the lake this evening. A
large trough of low pressure is expected to persist across the
plains through early next week with southerly winds prevailing
across the western lakes region. As warmer and more moist air
spread across the cooler waters...fog will likely develop with
some dense fog possible. cms





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