Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 311115

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
615 AM CDT TUE MAY 31 2016

351 AM CDT

Today through Wednesday night...

Primary forecast concern in the near term is timing of rain and
thunderstorms across the area...which should become more numerous
later today through Wednesday evening.

Goes water vapor imagery depicts a deep upper trough over the
western Dakotas early this morning...with deepening surface low
pressure over southeastern North Dakota per 07z analysis. An
occluding cold frontal boundary trails south-southwest into the
central Plains...while a subtle warm frontal boundary extended from
the triple point south of the low...southeast across Iowa and into
central Illinois. North of the warm front...isolated to scattered
thunderstorms have festering across parts of northwest/north central
Illinois within the elevated baroclinic zone and within a region of
moist ascent and convergence on the downstream flank of a 30-40 kt
low level jet. These storms have been increasing in coverage and
intensity over western parts of the cwa over the past couple of
hours...with relatively slow north-northeastward movement. None of
the various convection-allowing models appear to be doing very well
with the ongoing storms across our cwa...though the high-res 1km NMM
has the closest simulation with storms over southern WI. Taking the
suite of CAMs and their general trend into account...expectation is
that elevated convection will continue to bubble from north central
IL into southern WI/far northeast IL...with some eventual weakening
through sunrise or shortly after as flow begins to back slightly
ahead of the deepening northern Plains trough. Locations roughly
along and north of a Peru to Waukegan line appear most likely to
experience an early morning isolated thunderstorm.

After a lull in the convective action during the morning...more
showers and thunderstorms are expected to develop toward midday as
a short wave trough currently evident in water vapor imagery over
Missouri lifts northeast atop weakly unstable low level air mass
across the Mississippi Valley. CAMs indicate initiation will likely
take place across western/central IL by early afternoon...with
storms increasing in coverage as they spread northeast across much
of northern IL. Given relatively light winds through the column...
storms are expected to pose minimal severe risk with mainly locally
heavy rainfall amounts...small hail and perhaps 40-50 mph wind gusts
from the stronger storms. Once the short wave lifts across the area
by early appears the rain/thunderstorm focus will
shift back across western parts of the cwa...and the low level
moisture feed ahead of the the approaching cold front. Have
carried highest pops across the western cwa with a gradient
toward lower pops in the far east tonight. Rain/thunderstorm
chances then maximize across the area Wednesday as the cold front
moves east across the area...and the upper trough propagates
across the upper Midwest and northern Great Lakes region.
In addition to a locally heavy rain threat on Wednesday...the
potential for organized strong/severe storms will also increase
as an 80 kt upper level jet streak translates across the region
late in the day. 0-6 km bulk shear increases to 30-40 kt by late
afternoon/early evening as the right entrance region of the jet
moves overhead...and the surface cold front pushes through.
Rain and thunderstorms will end from west to east Wednesday
evening as the front clears the cwa.

Very warm temperatures will continue today and into Wednesday
with low level thermal fields supporting low-mid 80s across the
forecast area today...and upper 70s west to the 80-85 degree
range east on Wednesday. A southeast component to the surface
winds will allow a weak lake breeze and minor cooling along the
Illinois shore today.



352 AM CDT

Thursday through Monday...

The extended forecast period starts out dry and still mild
Thursday in the wake of the departing upper trough and surface
cold front combo. Large scale flow is initially westerly...
with temperatures generally in the mid-70s to around 80 on
Thursday as weak high pressure spreads across the area.

The upper pattern undergoes a significant shift toward the end of
the week strong upper ridging builds along the west
coast...inducing downstream troughing across the central and
eastern portions of the CONUS. Guidance is reasonably similar in
digging a strong short wave across the Great Lakes region Friday
night...which helps to carve out a deep long-wave trough across
the eastern U.S. through next weekend. In addition to showery
precipitation...this brings cooler temperatures to the area as the
trough deepens just off to our east. Highs across the eastern
parts of the cwa...especially downwind of Lake Michigan across
northwest Indiana...may not reach the 70 degree mark on Sunday.
Inland areas and locations farther west will likely see low-mid
70s rather than readings closer to 80 as earlier in the week.



For the 12Z TAFs...

The main concerns during the period will be the potential for a
period of showers and thunderstorms this afternoon at the

Isolated to widely scattered showers and storms developed over
north central Illinois earlier this morning along the leading edge
of better moisture and instability. These storms briefly impacted
KRFD...but have since dissipated. Recently additional showers
have begin developing just north of Bloomington IL. This will
likely continue to be the trend through the morning and afternoon
hours today as an upper level disturbance shifts over Illinois
today. This feature will likely lead to an uptick in thunderstorms
over portions of western Illinois by early afternoon as diurnal
instability increases. The focus of these storms should gradually
shift eastward through the afternoon. However...due to the fact
that the storms could be on a weakening trend as they begin to
approach a less favorable environment over far northeastern
Illinois late in the day...I have continued with only a PROB30
mention for ORD...MDW and GYY at this time. Sites farther west
will have a better potential this afternoon.

A cold front is expected to move across the region on Wednesday.
Additional thunderstorms are expected with this feature. Timing
of these thunderstorms is difficult this far out...but I have
included a second PROB30 mention at the end of the current 30 hour
ORD Tab and at the end of the 24 hour RFD TAF to advertise this
potential. This timing may need to be shifted with later updates.

Winds through the period will remain east-southeasterly in the 5 to
10 KT range.



150 AM CDT

A cold front is currently dropping southward over northern
portions of the lake this morning...and this is setting up some
higher northerly winds up around 20 KT at times. This front will
stall out across the central portion of the lake today...with
east- southeast winds expected to the south. Tonight...the winds
will shift southeastward up around 20 KT lake-wide as a storm
system moves into the upper Midwest. This storm system is forecast
to shift eastward into southern Ontario by late Wednesday. This
will push a cold front over the lake Wednesday night...before
ending up near James Bay on Thursday. An associated cold front is
expected to move across Lake Michigan Wednesday night. It still
appears that a period of thunderstorms will occur ahead of this
front Wednesday into Wednesday evening. Following the frontal
passage expect lighter westerly flow to set up for Thursday.

In the extended period it appears that another storm system could
take aim on the western Great Lakes by Saturday. This could result
in the next chance of showers and thunderstorms along with stronger






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