Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 260857

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
357 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 06Z TAFs...

VFR conditions in place early this morning, while conditions
staying dry at this time. Latest radar imagery is still depicting
some isolated sprinkles approaching from the west and although
there may be some brief sprinkles observed early this morning, did
leave out of the TAF. Mid clouds will steadily move overhead
through the morning, slowly lowering through the day. Guidance is
varying with precip potential later this afternoon. Area of
interest are the the showers in northern Kansas and southern
Nebraska, which guidance lifts northeast towards the terminals
later this afternoon. Wonder if guidance which brings this over
the terminals may be a bit aggressive with the northward push
later today. However, this guidance continues to be rather
consistent with this signal. So with lower confidence with how
this afternoon will unfold, have left the precip and timing alone
in the TAFs but will continue to monitor trends through the
morning. If this precip were to materialize, could see MVFR
ceilings for a time later this evening. Light and variable winds
will turn east southeast this morning and then turn more easterly
later this afternoon, at or above 10 KT.



253 PM CDT

A moderate northerly gradient in place over Lake Michigan this
afternoon will gradually ease as low pressure over the Upper Ohio
Valley departs towards New England Friday. As winds ease, the
small craft advisory will be allowed to expire this afternoon for
the Illinois nearshore waters, but will continue a few more hours
into the evening as waves gradually diminish in the Indiana zones.
A transient ridge will pass over the lake Friday which will allow
for generally light flow across the lake and for afternoon lake
breezes to develop. Flow should remain relatively light through
the remainder of the weekend as the western lakes fall between two
areas of low pressure -- one moving east across the Canadian
Prairies and another lifting from the Southern Plains to the
central Great Lakes. Fresh west flow will develop early next week
as these two lows merge over Ontario and slowly lift to near James






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