Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 281854

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
154 PM CDT Sun May 28 2017

151 PM CDT

Through Monday...

A cold front has pushed across northern Illinois and northwest
Indiana earlier today with convection early this afternoon in the
vicinity of the front, primarily over northern Indiana, but
clipping Benton County. Behind the front, northwest winds have
overspread most of the CWA with modest instability in place, but
weakly capped or uncapped per RAP soundings. There will be two
areas to keep an eye on through the afternoon and evening for
potential showers and thunderstorms. The first will be associated
with the lake Breeze in and around the Chicago Metro area this
afternoon. A strong thunderstorm has already developed on the
boundary and additional isolated showers and storms will be
possible through the afternoon. Farther west, a secondary surface
trough is evident in obs while a cu field has developed within the
vicinity of the trough that stretches from central WI into far NW
IL an eastern IA. There is potential for isolated to widely
scattered convection as this area moves across the CWA mid to late
this afternoon and into the early evening. With only 500-700 J/kg
MLCAPE and 20-30kt deep layer shear in place, severe threat is
not a big concern, but cannot rule out the potential for a few
storms to become strong. Cloud to ground lightning and gusty winds
would be a concern for any outdoor events.

Threat for precipitation should wane diurnally with dry conditions
expected overnight into Monday morning. Upper low is expected to
pivot across the western Great Lakes tomorrow while a sheared vort
lobe drops into northern Illinois. Forecast soundings tomorrow
are not too terribly different than today indicating steep low
level lapse rates with modest instability and little or no
convective inhibition. Given the favorable diurnal timing of the
shortwave, anticipate widely scattered showers and thunderstorms
will once again be a possibility. Temperatures will also be
fairly similar tomorrow with highs in the mid 70s. Deep mixing and
breezier conditions will be in place though, so there is a lower
likelihood of a lake breeze.



252 AM CDT

Monday through Saturday...

Closed upper low is progged to drift slowly east across the
northern Great Lakes region through mid-week, with the forecast
area within the southern periphery of the associated large scale
upper trough. A series of sheared, low-amplitude short wave
disturbances are depicted moving through the west-northwest flow
aloft, which will bring periodic isolated-scattered
shower/thunderstorms primarily during the diurnally-favored midday
through late afternoon hours. Monday appears to be likely to see
the greatest coverage, with a somewhat stronger wave apparent in
guidance as well as a surface trough/secondary cold front which
may provide some focus for showers and storms by afternoon.
Gradual cool-advection in the low-levels by Tuesday should provide
more of a mid-level (650 mb or so) cap/warm-nose, resulting in
weaker instability and lower coverage of weak convection. Guidance
generally indicates the last the series of waves rotating across
the area Tuesday night into early Wednesday.

Medium range model solutions indicate a brief period of rising
heights Wednesday, as the trough becomes elongated north of the
Lakes. This allows weak surface high pressure to slide southeast
across the Midwest through Wednesday night, after a period of
breezy west-northwest cyclonic low-level flow earlier in the
period. Guidance is in some agreement in dropping additional mid-
level energy across the upper Midwest and Great Lakes region late
in the period, though there are significant differences in timing
and placement as another closed upper low forms from the northern
Lakes to New England by Friday and Saturday. For the local area,
there is general consensus that the weak surface high will move
southeast of the forecast area Thursday, with modest south-
southwest return flow developing as the ridge moves away and
surface low pressure which passes north of the Lakes Thursday
night. A cold front trailing this low eventually sags into the
area by Friday or Friday night, though with the aforementioned
model differences there is spread in the timing and placement of
the surface front too. Overall, this scenario would bring the next
chance of organized precip to the region in that late Thursday-
Friday, and perhaps into Saturday given the ECMWF`s solution of
hanging the front up across the area.

Temperature wise, after highs in the mid-70`s Monday, slightly
lower than average temps are expected especially Tuesday-Wednesday
with highs in the upper 60`s to lower 70`s mid-week. West winds
should preclude any lake cooling however. Temps moderate back into
the low-mid 70`s across the cwa later in the week, and could be
warmer Friday if the cold front arrives later rather than sooner.



For the 18Z TAFs...

A cold front has pushed across the terminals today with northwest
winds picking up behind the front early this afternoon. Showers
and thunderstorms are expected to redevelop along the front this
afternoon from roughly a VPZ to RZL to TIP line and areas east. A
few isolated showers and thunderstorms may redevelop farther
northwest over Wisconsin and northern Illinois, passing near the
terminals, but coverage is expected to remain on the lower side
and confidence of precip on station is too low for more than a
VCSH mention at this time.

Other concern today will be the movement of a lake breeze that has
developed and pushed inland over northwest Indiana...currently
through GYY and on the doorstep of MDW with veered winds
east/north of the boundary. Some of the high-res models do
initialize well with the boundary near MDW early to mid afternoon,
but with deeply mixed and somewhat breezy conditions in place,
leaning against any lake breeze passage or wind shift for ORD
which is supported by the models. Expect the boundary to
eventually retreat back over MDW with west winds resuming, though
confidence in timing is low.



300 AM CDT

Only real marine forecast concern for the period is with a period
of modestly breezy west-southwest winds Monday into Tuesday, with
gusts around 25 kt possible.

Low pressure over northwest Ontario will gradually deepen as it
drifts slowly southeast and then east to near James Bay through
mid-week. A weaker low, currently over central IL, will lift
northeast into lower Michigan later today, and eventually will
merge into the deepening circulation north of the Lakes. This will
result in winds generally becoming west-southwesterly by tonight,
as some variability in direction today within a weaker gradient.
The gradient will tighten up Monday as the low deepens north of
the Lakes, with breezy/gust west-southwest winds spreading off of
the adjacent land areas of WI/IL. Relatively mild air over the
cool lake waters will make for stable conditions further out over
the lake, with the strongest gusts Monday afternoon largely along
the WI/IL shore areas. The gradient will continue to tighten
Monday night into Tuesday, particularly across the northern half
of the lake, increasing winds a bit even well away from shore. The
low over Ontario will eventually drift east across James Bay and
into northwestern Quebec by mid-late week, allowing the gradient
to ease and winds to diminish. Weak high pressure will pass south
of the lake Wednesday night, with winds eventually turning south
again Thursday and Thursday night. A cold front is then expected
to push south across the lake sometime Friday or Friday night.






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