Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 311757

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1257 PM CDT TUE MAY 31 2016

1200 PM CDT

Storms across north central Illinois have been slow-moving and
pulsey...and generally unorganized. This is not surprising given
the moist and unstable air mass that is void of any real deep
layer shear. Steep low-level lapse rates and associated DCAPE
values around 800 J/kg has allowed outflow from the storms to
quickly develop and progress outward. Gusts have been generally
30-40 mph on some of this. In our CWA...this outflow is
progressing east into marginally unstable air. While it is likely
some showers and possibly storms may fester into the western
Chicago metro early to mid afternoon...the severe threat is low.
The now fairly large stratiform rain area over north central
Illinois and temperatures in the mid 60s are indicators of a
fairly sizable cold pool that will limit the severe threat there
over the next few hours too.

A weak surface convergent axis remains from roughly Dubuque IA to
Springfield IL and looks to ever so slowly move east this
afternoon. This general zone should remain a focus for some
deeper updrafts through the afternoon and early evening as a weak
upper level disturbance seen on water vapor drifts overhead.
Stronger downdrafts capable of 40+ mph winds and torrential
downpours in concert with slow storm movement will be the main
concerns during the mid-late afternoon. There was what appeared
to be a weak non-supercell tornado or landspout in far eastern IA
earlier /near Elvira/ and there is a non-zero threat of a widely
isolated one near that low-level convergent axis.



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 18Z TAFs...

The main concerns during the period will be the timing and
coverage of showers and thunderstorms through the period.

an area of rain with isold/sct embedded thunderstorms continues
over the area. RFD is the only terminal directly impacted at
issuance time, but expect the area of pcpn to slowly work to the
east. Latest timing suggests arrival of pcpn at ORD/MDW by arnd
19z. radar imagery indicates an outflow boundary tracking to the
east which should pass through DPA by arnd 18z and ORD/MDW by arnd
19z. Expect the pcpn to begin arnd the time of the passage of the
outflow boundary. Also, there will likely be a temporary wind
shift to SWLY with ocnl gusts to 25 kt following the passage of
the outflow boundary. The airmass is quite moist over the region
and there is the potential for periods of heavy rainfall with some
of the stronger storms reducing visibility to IFR levels for brief
periods. The pattern will continue to be unsettled through the
afternoon. A warm front extending from nern Illinois into wcntrl
Indiana will be the focus for additional development. Latest vis
satellite imagery shows an area of towering CU along the warm
front and shra/tsra are quickly developing over nwrn Indiana. This
activity should continue to expand through the afternoon, keeping
chances for tsra over the terminals into the early evening hours.
The activity associated with the warm front should be largely
diurnal in nature, so there could be a lull in the activity from
late evening through the early morning hours. a resurgence of tsra
chances will begin again durg the early morning hours as a
stronger cold front moves across the region. The models are in
relatively decent agreement on the general timing of the beginning
of the next round of tsra tomorrow, beginning durg the late
morning hours. There is less certainty on the ending time as the
push of the front is relatively weak. A slow frontal progression
would suggest that there could be an extended period of tsra/shra
tomorrow, with the main uncertainty being the extent of coverage.



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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