Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261150

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

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 12Z TAFs...

Dry across the terminals at this time, with any sprinkles now off
to the east. Expect dry conditions this morning but with chances
for showers and even a period of thunder increasing by early to
mid afternoon. These chances will be tied to current complex
lifting northeast over Iowa, likely continuing into northern
Illinois this afternoon. Should see coverage of precip increase
throughout the day, as forcing arrives and instability increases.
Although higher instability will be just to the south of the
terminals, there will likely be enough to support a 1-2 hour
window of thunder this afternoon. Have included this in the TAF
but did not include as much thunder for RFD, where instability may
be slightly lower. This complex will slowly push east into this
evening, and then soon exit during that time. VFR conditions in
place for most of the day until this precip arrives, when MVFR
conditions will be possible. MVFR briefly lingers this evening,
but with conditions expected to then improve overnight. Light and
variable winds will turn east southeast this morning and then turn
more easterly later this afternoon, at or above 10 KT.



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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