Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261931

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
231 PM CDT Fri May 26 2017

1249 PM CDT

Well defined MCV lifting ENE across the Mississippi River early
this afternoon is helping to spark some renewed convection along
its southern flank within an increasingly unstable warm sector
just southwest of the LOT CWA. Area observations and satellite
imagery indicate a warm front is draped from near MLI southeast
through BMI towards CMI and is expected to gradually lift into the
southern half of the LOT CWA through mid to late this afternoon.
Within the warm sector, dew points are increasing into the low to
mid 60s which is contributing to 1000+ J/kg MLCAPE. This is
accompanied by 40-50kt 0-6km shear along with a corridor of
enhanced effective SRH in the vicinity of the warm front. This
points to a developing supercell threat across portions of central
Illinois that may possibly continue into east central Illinois
later today, with a non-zero tornado threat particularly near the
warm front. The primary severe threats continue be large hail and
damaging winds. Expected storm motions and Bunkers right moving
supercell vectors are fairly normal to the orientation of the warm
front which will help limit residence time for any individual
storm on the boundary, with the severe threat rapidly diminishing
north of the front. Storms should eventually congeal into a line
with propagation ESE along the instability gradient. A QLCS
tornado threat would still exist given the favorable shear. The
greatest severe threat should start to exit east of the CWA by
early to mid evening.



356 AM CDT

Today and Tonight...

Main short term concern is with timing/coverage and intensity of
precipitation this afternoon and this evening.

Early morning surface analysis depicts weak high pressure ridge
across the eastern cwa. Patchy fog has developed where winds have
gone calm in/near the ridge axis, though areas of mid-high cloud
cover and sunrise now being less than 2 hours away should mitigate
any significant further development. Attention turns back to the
west across the Plains, where a mid-level short wave was evident
in water vapor imagery over portions of eastern Kansas and
Nebraska. Some evidence of a remnant MCV feature from earlier
convection is noted in the vicinity of northwest MO. This short
wave feature is progged to track east-northeast into the area by
this afternoon, with evidence of a surface wave developing into
western IL in synoptic models by 00Z. Various convective allowing
guidance depicts an area of precip associated with the mid-level
circulation spreading into the forecast area by early afternoon,
with an intensification in simulated reflectivity primarily
along/south of the I-80 corridor. Forecast soundings and RAP
analysis depict marginally steep mid-upper level lapse rates,
though also indicate a bit of a warm region in the 700-600 mb
layer which results in weaker lapse rates in low levels. Portions
of the cwa closer to central IL will have the best potential for
stronger destabilization closer to the surface low/subtle warm
frontal region. SPC has included the southern 1/3 or so of the cwa
in a marginal wind/hail severe risk for this afternoon and early
this evening, which seems appropriate given greater surface based
instability and forecast 0-6 km shear in the 40-60 kt range
supporting the potential for organized/longer lived updrafts in
those areas. Low level thermal progs support upper 70`s for
afternoon highs in most areas, though east-southeast flow off lake
will keep northeast IL from the 60`s near the shore to the low-mid
70`s inland.

Shower/thunderstorm threat should linger this evening especially
south/east as mid-level wave completes its transit of the area.
Weak surface high pressure spreads in from the northwest toward



356 AM CDT

Saturday through Thursday...

Modest height rises of 20-40 meters are depicted across the area
Saturday morning, in the wake of the mid-level short wave trough
which propagates across the region late Friday and Friday night.
This is reflected at the surface as weak high pressure which
lingers into the early afternoon hours, with the surface frontal
boundary suppressed well downstate from central Missouri into
southern IL/IN. This region of generally subsident air coincides
with a relative minima in QPF from various guidance across the
forecast area during the day, suggesting much of the cwa will see
many dry hours Saturday. 925 mb temps ranging from +17 to +21C
support afternoon high temps in the upper 70`s in most areas
Saturday, with 60`s along the lake and a few lower 80`s across
our southern counties. Potential for rain then increases Saturday
night into Sunday, as a weakening surface cold frontal trough
approaches the forecast area from the northwest. Large scale
height falls develop across the upper Midwest Saturday night atop
the advancing front, in response to short wave energy rounding the
base of the upper trough. Several models also depict a small
amplitude short wave rippling across Missouri and central IL
Saturday evening, with the potential for convective-enhancement or
MCV formation with thunderstorms perhaps affecting southern
counties of the cwa during the evening hours. The 3km WRF seems
to have a conceptually reasonable depiction of strong convection
occurring close but largely south of the cwa Saturday evening,
with convection then developing after midnight along the boundary
northwest of the cwa and moving in during the pre-dawn hours of
Sunday morning. Front then continues to transit the eastern cwa
through midday Sunday, with the focus for organized storms moving
east of the area in the afternoon. Guidance does depict weakly
unstable lapse rates (<500 J/kg MLCAPES) lingering across the
area Sunday afternoon however beneath the mid-level trough axis
which swings across the region. This may continue to support a
threat of isolated/scattered showers or isolated thunderstorms
into Sunday afternoon, though with low confidence in much
coverage. Loss of a couple of degrees C at 925 mb behind the front
Sunday looks to support afternoon highs generally in the
mid-70`s, with westerly winds limiting threat of lake cooling.
Pesky upper low drifts slowly east across Ontario Monday, with
another lobe of short wave energy rotating across the upper
Midwest. Forecast soundings depict some weak (<500 J/kg again)
diurnal instability by Monday afternoon which could support some
isolated/scattered coverage showers/thunderstorms.

Medium range models are generally in agreement in slowly drifting
the upper low east of the Great Lakes region through the middle of
next week, though with troughing lingering across the upper
Midwest through the period. This leaves IL/IN in a region of
west-northwest mid-upper level flow in which a series of weak
disturbances track. Models really show some differences in surface
features by days 6-7 with the ECMWF developing weak high pressure
across the upper Mississippi Valley, though with weak trough
lingering aloft in all guidance have somewhat low confidence in
the fine details at this distance. Temps generally at or a little
below normal appear reasonable based the pattern.



For the 18Z TAFs...

A convective complex is crossing the Mississippi River and will
impact the terminals from the early/mid aftn through the early
evening. The best chance for strong to severe storms will be south
of the terminals. Have low confidence in thunder being reported
at RFD so only have a VCTS there, but have higher confidence in
thunder at the eastern terminals so they each have a 2 hour TEMPO
for thunder.

Cigs under the convective complex are mainly VFR right now, but
guidance continues to suggest that MVFR cigs will form as the
complex pushes east. Since guidance is doing such a good job
capturing the complex and current cigs, I decided to follow their
lead and keep lower cigs in the forecast. Dry conditions return
in the early evening. MVFR cigs scatter out and east winds become
north-northeast. East winds less than 10 kt are expected Saturday
along with dry, VFR conditions.



227 PM CDT

Will keep the marine fog advisory going as web cams indicate
haze/fog remains over the open waters.

A high pressure ridge over the lake will continue east. A weak
surface low over the plains will pass south of the lake tonight and
continue across the Ohio Valley Saturday. The low may cause wind
speeds to increase to 10-20 kt over the far southern end of the lake
this evening. One model would suggest gusts up to 25 kt, but do not
have enough confidence to go that high.  High pressure passes over
the lake early Saturday morning and light winds will vary in
direction across the lake.  Variable winds are expected over the
southern end and southerly winds over the northern end.

The next low forms over the plains Saturday and reaches the Great
Lakes region Sunday morning.  The low moves over Michigan Sunday
afternoon and winds become west 10-20 kt behind the low.  The low
over Michigan merges with another Canadian low to the north over
Ontario Sunday night. The low is forecast to remain relatively
stationary over Ontario early next week which may lead to a an
extended period of westerly winds of 15-25 kt. High pressure moves
in behind the low mid to late next week.






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