Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 230208
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
908 PM CDT Fri Jun 22 2018

.UPDATE...
908 PM CDT

Only a few minor tweaks to going forecast this evening, mainly to
add some fog across the southeast after midnight, and to lower
temps slightly north suburbs and along the lake.

Evening GOES vapor imagery indicates our upper level low centered
over central Indiana, with a northeasterly drift. While a few
isolated to scattered lighter showers continue affect the forecast
area, most of the stronger convective showers were focused near
and ahead of the low center to our southeast. Coverage of showers
should continue to slowly diminish overnight as the low begins to
accelerate off to the east-northeast. As indicated above, have
added or increased fog mention across the southern cwa after
midnight tonight, with much of the high-res model guidance
developing lower visibilities there mid-60`s dew points and
lighter winds will linger.

Of course, most area rivers will continue to rise after heavy
rains of the past few days. Evening river stage forecast updates
are in progress at this time. Updated digital/text forecast
products already available.

Ratzer

&&

.SHORT TERM...
221 PM CDT

Through Saturday...

A stacked low pressure system will continue to pinwheel south of
the CWA through tonight. As of 19Z, the surface low center was
located near Danville, with the mid-level low displaced just one
county south per satellite imagery. A concentrated area of
convection on the NW side of the low is expected to continue into
the early evening primarily south and east of a line from southern
Livingston County to Kankakee to Michigan City, IN. Some concern
for flooding remains across far southern portions of the CWA given
PWAT values still over 1.5 inches in conjunction with the slow-
moving convection near the pivot point of the low. Also, a non-
zero brief and weak tornado chance remains across Iroquois County
and NW Indiana south of the Kankakee River. A recent report of a
funnel cloud near Watseka indicates this environment remains
favorable for a very brief and weak spin-up.

Otherwise through tonight, isolated to scattered showers across
the remainder of the CWA should gradually end as upper forcing
drifts east of the area. Though thick cloud cover will remain
across the area, the combination of weak lift, abundant low-level
moisture, and even minor cooling may support some reduced
visibilities overnight. Anticipation is for an environment more
favorable for haze (e.g. 3-5mi visibility) rather than fog, but
would not be surprised to see some patchy fog develop by daybreak
Saturday. Additionally, along the immediate lakeshore, some fog
over Lake Michigan may shift onshore at any point through the
night.

Saturday: After starting the day with a fairly substantial low-level
stratus deck, expectations are for clouds to lift into a broken
cumulus deck by the afternoon. Though deep-layer forcing will have
shifted east of the area, a weak low-level trough lingering SW
into northern IL will keep fairly minor forcing present across
primarily the SE half of the CWA into the afternoon. With
sufficient low-level moisture present, convective cloud depths
within the cumulus field of around 3kft may be enough to generate
a few light showers or sprinkles during the mid to late afternoon.
The focus for this activity will be across NW Indiana and east-
central Illinois, wrapping NW along the Lake Michigan lake breeze
into the Chicago metro generally along and west of I-294. Have
introduced a slight chance of showers during the afternoon for
this area. Farther north and west, dry air advecting into the
region aloft should entrain into the boundary layer and limit
cloud depth/coverage.

Kluber

&&

.LONG TERM...
318 PM CDT

Saturday night through Friday...

Saturday night through Monday: Any isolated diurnal showers across
the SE half of the CWA as noted in the short term discussion should
quickly diminish during the early evening. Upper ridging will then
edge across the area through Sunday night. Guidance has continued to
back-off on precip chances Sunday as the ridge suppresses the
influence of a trough shifting SE across the eastern Great Lakes.
Though no precip is anticipated at this time, any convectively
enhanced vort max rounding the building ridge would have the
potential to sneak a couple storms into the western CWA during the
afternoon.

Monday through Tuesday: Pronounced troughing across the central
Great Plains will gradually shift eastward across the area during
this time, bringing periodic chances of showers and storms Monday
night through Tuesday night. WAA should support some elevated
convection spreading northward across the area Monday night.
Convective trends on Tuesday will then hinge heavily on how the
early morning convection evolves and if any nocturnal MCS activity
muddles the atmosphere across the area. There is a conditional
threat for severe convection if minimal contamination of the ambient
atmosphere by prior convection across the mid-Mississippi Valley
Tuesday afternoon and evening. This period will need to be monitored
into early next week.

Wednesday through Friday: Precip should quickly exit to the east by
Wednesday morning. A slightly zonal mid-level flow will set-up across
the region through Thursday, then begin to break down on Friday as
troughing develops across the western CONUS. This may allow a weak
trough embedded in the large-scale flow to bring precip chances to
the area Thursday night into Friday. Otherwise, the area will find
itself on the northern extent of a building heat dome across the
central CONUS. With heavy recent rainfall, this warm and humid
airmass may build into the area by next weekend.

Kluber

&&

.AVIATION...
For the 00Z TAFs...

Upper low continues to spin over the region resulting in mainly
light rain and drizzle at times for the terminals. Convergence at
925mb will continue to result in some spotty precip over the next
few hours, but expect precip to wind down late this evening as the
low continues to shift east away from the area and winds aloft
back to the north. Earlier model runs were suggesting that IFR
ceilings would return to the terminals overnight, but have trended
higher suggesting primarily MVFR ceilings instead. Fog will be a
concern overnight as we remain in high humidity air.

Through the day Saturday should maintain some type of northerly
wind direction. Some models are suggesting an afternoon lake
breeze, and went with that scenario in the TAFs showing slightly
backed wind early becoming northeast in the afternoon. Skies
should scatter out, though there is low confidence on when this
would occur. Erring on the slower side of guidance for the latest
forecast.

BMD

&&

.MARINE...
318 PM CDT

An upper-level low pressure over west-central Indiana will drift to
the eastern Great Lakes tonight through Saturday night. Expect a
persistent period of 15-25 knot N to NE winds through Sunday. High
pressure will then build across the Great Lakes region Sunday night
into Monday night, bringing a period of generally NE winds to 20
knots. A low pressure trough approaching the from the west on
Tuesday will then support S to SE winds to 25 knots late Monday
night through Tuesday night. Periods of thunderstorms will accompany
the trough during this time.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement...ILZ006-ILZ014 until 4 AM Saturday.

IN...Beach Hazards Statement...INZ001 until 4 AM Saturday.

LM...Small Craft Advisory...nearshore waters
     until 4 AM Saturday.

&&

$$

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