Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 202000
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
300 PM CDT Sun May 20 2018

.SHORT TERM...
256 PM CDT

Through Monday...

Generally quiet conditions are expected through this evening as
broken cloud cover persists in the wake of this morning`s precip.
Low-level moisture, a lowering cloud deck, and weak isentropic
ascent should support some patchy drizzle and fog late this evening
through much of the overnight hours.

A fairly potent mid-level shortwave over OK this afternoon will
become convectively enhanced as it tracks northeastward tonight.
With it will come a possibly decaying shield of rain with embedded
thunder. Arrival time for the southwest CWA is expected around 2-
4AM, and around 6-8AM for the Chicago metro. Even without
thunderstorms, this shield of rain will likely have pockets of heavy
rain that may induce some minor impacts for the morning commute.

The trough is expected to slow as it encounters a ridge currently
across the western Great Lakes. This will allow extensive cloud
cover and some showers to linger through much of the day Monday
across the northern tier of the CWA. Have thus lowered max temps
to the mid/upper 50s in Lake County (IL) and northeast Cook
County, to around 60 for the remainder of the Chicago Metro.
Farther south, clearing in the afternoon will support mid 70s to
possibly 80F.

Across the southeastern CWA in the afternoon, showers and some
thunderstorms may initiate as destabilization occurs ahead of a
surface trough. MLCAPE values surpassing 1000J/kg with 0-6km shear
to 30kts will support a conditional isolated severe thunderstorm
risk for large hail and strong winds.

Kluber

&&

.LONG TERM...
258 PM CDT

Monday night through Sunday...

The forecast items of interest in the long range are lake-induced
cooling midweek with warming temperatures to above normal inland,
then potential for even warmer with gradually increasing
thunderstorm chances for Friday into the first half of the
holiday weekend.

At the beginning of this period (early Monday evening), depending
on where the above mentioned surface low and attendant front are,
scattered thunderstorms could be as far west still as northwest
Indiana. The instability and shear combination ahead of the front
would support a conditional strong to severe threat. Any
convective activity will get a push eastward as the upper short
wave disturbance shifts into the Great Lakes. Showers associated
with the upper low are likely to remain mainly north of the area,
possibly grazing far northern Illinois.

Surface high pressure will drift over the area Tuesday with
gradual return flow ensuing thereafter for the week. With the
high overhead on Tuesday, a thermally-induced lake breeze should
surge inland during the early-mid afternoon, especially if morning
cloud cover has departed. Lakeside temperatures likely will drop
through the 50s while 70s forecast further inland. Wednesday will
have warmer temperatures areawide with 80+ inland, while more 60s
lakeside.

The upper level pattern features by Wednesday onward will be a rex
block west while a building ridge across the middle of the county,
with multiple lead short waves across the central Plains into the
Upper Midwest and possibly some parts of the Corn Belt states. The
main support for ascent and foci of low-level jets would point
toward much of the storms remaining west and north of the CWA
through Thursday. Also trying to consider that the models may be a
bit fast with eastward progression of precipitation given the
blocked pattern forecast. Warm advection aloft though and possibly
some spilling of decaying, likely ill-defined MCSs, does warrant
low chances of rain in the northern and western CWA during that
time. The most recent GEFS supports a sharp gradient in
precipitation chances just west and north of the area.

The better chances for convection look to be Friday into Saturday
at this time, as heights start to break down with correlated
ascent, and the low-level thermal and moisture ridge spill into
the area. If convective clouds do not interfere, highs on Friday
and Saturday could be around 90 in parts of the CWA given 850mb
temperatures of 17-20C. The pattern favors multiple rounds of
storms in the general region, impossible to say if over the CWA,
but have broad chances in the forecast during those two days.
Looks like more of a shower and isolated thunder potential the day
after the surface low passes as the northwest-flow upper low
drifts over the region.

MTF

&&

.AVIATION...
For the 18Z TAFs...

IFR ceilings should gradually lift into the low to mid-MVFR range
through this afternoon in response to diurnal heating. However,
lower ceilings may persist a bit longer into the afternoon at
ORD/DPA with increased moisture off Lake Michigan. Ceilings are
expected to lower into IFR levels and potentially LIFR levels by
the early overnight hours. Vsby may also drop as DZ likely forms.
A shield of SHRA will then spread across all sites from the SW
beginning around daybreak Monday. Confidence was too low to
include any mention of TS in the TAF at this point, but the
leading edge of the SHRA may have an isolated embedded TS or two.
IFR ceilings should then gradually lift into MVFR levels late
morning into the afternoon.

Kluber

&&

.MARINE...
248 AM CDT

A surface trough/front has pushed south through much of the lake
early this morning, with the northerly winds observing an increase
to the 15 to 25 kt range over much of the lake. Although winds up
to 30 kt are not expected, a few gusts approaching these speeds
may still occur early this morning. While northerly winds will be
present today across the lake, for the nearshore waters, highest
speeds this morning should remain mainly along the IL nearshore
with speeds and waves slightly lower over the IN nearshore. Winds
are then expected to turn more to the east tonight, while speeds
and waves lower.

Rodriguez

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...Winthrop Harbor to Burns Harbor until
     9 PM Sunday.

&&

$$

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