Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251130

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

323 AM CDT

Through Tonight...

The focus for today and tonight was on timing the highest
potential for storms and their coverage as we enter an unsettled
summer-ish pattern. Storms still appear likely during the mid-
afternoon through early evening period or so...but several factors
present uncertainty on what the coverage will well as
storm chances for the remainder of tonight. The severe threat
appears marginal namely due to the absence of stronger and
widespread forcing.

Early this morning multiple MCSs are tracking eastward across the
PLains and Missouri River Valley regions with an MCV present
between Omaha and Kansas City that is on a track for our area this
afternoon. Satellite IR imagery has indicated warming cloud tops
with this area and expect a gradual convective weakening to
continue through the morning as this area outruns deeper forcing.

On the note of broader forcing...the synoptic lift over our CWA
this afternoon and evening is fairly minimal with stronger
upper/mid level flow and height falls remaining mainly west and
northwest. Thus convective coverage and any potential for stronger
storms seems hinged on the aforementioned MCV and associated short
wave disturbance maintaining themselves as they track northeast.
High-resolution and convective allowing models /CAMs/ do indicate
this tracking over the region near peak heating time which should
help at least scattered storms...especially given dew points
having increased into the 60s and PWATS in excess of 1.2 inches.
With forecast highs in the lower to mid 80s yielding MLCAPEs of
1000 to 1500 J/kg...have leaned toward the scenario of some re-
intensification of a broken line or scattered storms into north
central Illinois by mid-afternoon and advancing east from there.
The further east the less ideal of an air mass so confidence
lessens on coverage toward Chicago and northwest Indiana and it
is possible that storms become isolated east of I-55 late this
afternoon into evening. A couple storms may spike up to near-
severe or severe levels given the instability/shear/PWAT spectrum
forecast...with a marginal threat of wind or hail.

Behind this wave a 40 kt + southwesterly low-level jet under an
EML advancing into the area will continue the potential for
festering scattered storms. This seems most favored south of I-88
in our area as the nose of this LLJ gradually veers to west-
southwesterly through the night. A few of these could be strong to
severe if they can develop. The setup of moisture
transport/convergence is decent for locally heavy rain in any
convection...but again confidence on coverage is quite low and
have mainly just maintained broadbrushed chance PoPs.

With temperatures potentially being knocked down late this
afternoon into early this evening with some storms...and dew
points holding steady or rising...fog development may occur



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.




630 am...Two main concerns this period are thunderstorm chances/
timing and wind directions today.

Upper wave associated with decaying convection across the plains
will move across the area this afternoon into this evening. Most
guidance remains in fair agreement for thunderstorms to form along
a line or a broken line across western IL early this afternoon
and then continue northeast across the terminals through early
this evening. Current tempo timing remains on track...but as
trends emerge later today...this timing can be better refined.

There will likely be a lull in precip mid/late this evening but
additional development remains possible overnight. Confidence of
this activity is increasing as low level moisture continues
northward into the area but location remains it
could be southwest of the terminals. Have introduced a longer
prob30 for the overnight hours for now but changes with later
forecasts may be needed.

Southerly winds to start are expected to turn more to the
southeast this morning at the Chicago area terminals and likely
remain southeast until thunderstorms arrive...with speeds in the
10-15kt range. Wind directions will likely switch to some westerly
form with/behind expected thunderstorms and then settle to light
southeasterly by early/mid evening. cms



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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