Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210539

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1239 AM CDT Mon May 21 2018

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.



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

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.



For the 06Z TAFs...

Low ceilings and precip chances will continue this forecast
period, while an east northeast wind persists. In the near term,
ceilings range from VFR to MVFR, to IFR/LIFR across the TAF sites.
Do think a downward trend will be observed for all sites over the
next 1-2 hours, with similar trends that MDW has just observed of
going from MVFR ceilings straight to IFR. These lower ceilings
will then remain for much of the morning, and possibly longer into
the day than currently forecast. Some improvement may occur later
today, but do think another night of low ceilings will once again
be possible. Dry conditions in the near term, but think upstream
precip development across west central IL will track northeast
across the terminals early this morning and last through midday.
Have not included thunder for early this morning, but will
continue to monitor this possibility of thunder moving across the



305 PM CDT

Gusty northeast to north winds across the southern part of the
lake will gradually ease this evening. Small Craft Advisory waves
will ease by mid to late evening. Low pressure will lift northeast
over the northern Indiana/southern Michigan region during Monday
evening, with northeast to north winds during the afternoon into
the evening leading to near Small Craft Advisory criteria waves
across the far south.

Midweek will see a lake breeze pattern in the nearshores, while
southerly flow begins to take over across the open water.
Thunderstorm chances look small during the midweek time before
increasing late in the week.






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