Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 240133

Area Forecast Discussion
National Weather Service Lincoln IL
833 PM CDT FRI SEP 23 2016

Issued at 833 PM CDT Fri Sep 23 2016

Upper level ridge building into the area will provide subsidence
for clear skies tonight with just some scattered high clouds
possible mainly north of I-74. Meanwhile...a frontal boundary
moved in from the north earlier today and has made very slow
progress since arriving roughly at a Schuyler county to Paris
orientation west-east across central IL. Main impact of the
boundary should be a good chance for some fog to form overnight
along and behind the front in the cooler moister air with
dewpoints in the upper 60s to around 70 along with good cooling
due to the clear skies. Even to the south of the
boundary...dewpoints this afternoon were still fairly moist in the
mid 60s and could result in patchy fog with similarly effective
radiative cooling. Otherwise...expect lows to reach the mid 60s
tonight. No major updates needed to the forecast tonight.


.SHORT TERM...(This evening through Saturday)
ISSUED AT 310 PM CDT Fri Sep 23 2016

A surface boundary/back door cold front just south of I-74 this
afternoon will continue to slowly sag sw across central IL into
tonight, with 1029 mb surface high pressure over Ontario ridging
into the Great Lakes region. Latest run of HRRR is now showing
some convection from I-74 northeast late this afternoon and early
evening but feel its coverage is too high and earlier runs of HRRR
were dry. Current radar mosaic shows just isolated very light
showers ne of Vermilion county IL which latest RUC model shows, so
will keep pops at 10% over northeast CWA through 00Z/7 pm today.
Dewpoints in the 60s across CWA and models show some increasing
low level moisture near and northeast of the front tonight
especially overnight into early Saturday morning allowing patchy
fog to form with light wind regime.

The latest forecast models continue to have a strong upper level
ridge over IL on Saturday with 500 mb heights around 589 dm. This
should bring another very warm summerlike day to the area with
highs 85-90F and dewpoints in the mid to upper 60s.

.LONG TERM...(Saturday night through Friday)
ISSUED AT 310 PM CDT Fri Sep 23 2016

The main focus for this forecast cycle will be the frontal passage
and associated precipitation for this weekend. The 12z models seem
to have better agreement on the a more progressive theme to the
upper level pattern progression. The Canadian NH no longer cuts off
a low behind the front, for later passage mid-week. The consensus
indicates the occluded surface and upper low pressure centers will
move from the western Dakotas Saturday afternoon northeast into
Manitoba and western Ontario by Sunday evening. Then the upper
trough is projected to deepen toward the southeast, into the central
Great Lakes region. This will provide the reinforcing push of cooler
air for our mid-week cool down.

As for the sensible weather during this scenario, we are looking for
increasing clouds Saturday night as the cold front moves half way
across Iowa. We included a narrow ribbon of slight chance PoPs along
our far western counties in Knox, Fulton and Schuyler counties. The
strongest forcing for precip will generally remain north of our
forecast area on Sunday as the surface and upper lows lift into
western Ontario. As a result, the front circulation and low level
moisture convergence will most likely be weakening during FROPA
Sunday afternoon and early evening. Support for any thunderstorms
will come from high boundary layer moisture with dewpoints around 70
in the pre-frontal air mass. Instability params will support a line
of storms, with MUCAPES of 1500-2200 J/kg Sunday afternoon. Bulk
shear will be marginal, in the 20-25kt range. The better agreement
on frontal timing has increased confidence in precip on Sunday
afternoon/eve, and PoPs were increased to Likely /60-70%/ north of
I-72, progressing from west to east between Noon and midnight.

Precip chances will linger into Monday as a secondary lobe of energy
rotates through the upper trough before it departs to the east on
Tuesday. A much cooler and drier air mass will settle into Illinois
for Tuesday through Thursday. Highs will be below normal in the low
70s on Tuesday, then warm to near normal in the mid to upper 70s for
Thursday, with upper 70s across the board on Friday. No
precipitation is expected from Monday night through Friday, and
possibly into next weekend.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 613 PM CDT Fri Sep 23 2016

Dry conditions will continue over the next 24 hours for the
central IL terminals as a high pressure ridge builds over the
area. At the surface a weak frontal boundary has pushed into the
area from the north bringing additional moisture and a shift to
northeast winds. Behind and along the boundary the moisture should
be adequate for fog to develop overnight. This boundary looks to
stall in the vicinity of I-72 overnight, so have included
conditions dropping to IFR vsby for the I-74 terminals KPIA-KBMI-
KCMI while only MVFR vsby for the I-72 terminals KSPI-KDEC due to
uncertainty in the impacts there. Fog should diminish by 14Z.
Scattered cumulus cloud cover around 5000 ft AGL should develop
by late morning Saturday across the area.


.ILX Watches/Warnings/Advisories...


LONG TERM...Shimon
AVIATION...Onton is the U.S. government's official web portal to all federal, state and local government web resources and services.