Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261740

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1240 PM CDT Fri May 26 2017

1134 AM CDT

Main concern is with convective trends through early evening,
including the risk for severe thunderstorms south of I-80. A
well-defined MCV is noted on radar mosaic and GOES-26 satellite
products crossing the MS River from eastern IA late this morning.
This MCV will ride mean mid and upper flow east-northeast,
spreading stratiform region precipitation across areas north of
I-80. Have been noting embedded lightning in the stratiform, so
anticipate isolated to scattered in cloud to CG activity. As the
MCV initially rides northeast, a sharp gradient in surface based
and ML instability will set up along and just north of the warm
frontal zone that will be draped west-northwest to east-southeast
south of the IL and Kankakee Rivers. Ongoing convection or new
convection should have a tendency to bow east-southeast along the
instability axis.

With relatively steep mid-level lapse rates sampled by 12z RAOBs,
heating ahead of the MCV should yield about 750-1500 j/kg of
MLCAPE along and just north of warm frontal zone. With moderate
mid-level flow already in place and likelihood of some
intensification associated with MCV, westerly deep layer bulk
shear of 40-50 kt will support storm intensification and
maintenance. In addition, it appears there has already been some
modification to lower level winds, with 30 kt southerly 850 mb
flow per recent ILX VWP. Preferred storm mode will in general be
linear clusters/segments, however there could be a window if cells
can break capping ahead of clustered development for supercell
structures. Primary severe threats are wind and hail, but with a
fairly strong LLJ associated with the MCV, as well as proximity of
warm front, there is an isolated tornado risk, supercellular in
any cells ahead of line and QLCS tornadoes embedded in linear
segments/bows. The CAM guidance is fairly consistent in the
overall scenario, with uncertainty on whether storms take off just
west or over the southern CWA or mainly off to the southeast.

The Storm Prediction Center given above mentioned concerns has
placed a corridor from the southern half of LaSalle County to
Jasper County and points south and southeast into a Slight Risk
for severe storms with the 1630z update. This includes a 5%
tornado risk.

Activity should exit fairly quickly to the east this evening with
a dry overnight expected. Have tweaked temperatures downward some,
especially north given increased cloud cover with MCV. Marine
influence liming temperatures to near/around 60 on lakeshore could
spread inland some this afternoon.



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.



421 AM CDT

Monitoring area of fog moving south down the northern portions of
the lake early this morning. Some locations along both sides of
the lake showing some lower visibility, however, confidence with
how dense this fog is low at this time. Could be some pockets of
more dense fog and have continued fog in the forecast this
morning, but will continue to monitor trends. Surface high pushing
across the lake this morning, and providing lighter winds across
the lake. These lighter winds will likely prevail for most of the
day but am monitoring for an outside chance of slightly stronger
winds late this afternoon into the evening. What could support
this is an area of showers/thunderstorms which will move across
the region. Will need to monitor upstream trends with this precip
and surface pattern later this morning into early afternoon, and
adjust if necessary.






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