Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 202009

Area Forecast Discussion
National Weather Service Lincoln IL
309 PM CDT SAT AUG 20 2016

.SHORT TERM...(This evening through Sunday)
ISSUED AT 309 PM CDT Sat Aug 20 2016

Cold front looks to have reached I-57 this afternoon although
not well defined as it crosses the area. Instability rapidly
diminishing behind the front with the remaining chance of any
significant thunderstorms to the east. Could be an isolated
lightning strike to the west as the most vigorous of the scattered
showers west of the front have pushed a bit above the freezing
level...but chances low. Expecting the front to reach the
Indiana border around 6 p.m...perhaps around 9 p.m. south of I-
70. Fairly stiff pressure gradient across central IL will bring
breezy West winds 10-15 mph and gusts over 20 mph through the
afternoon/early evening today...then expecting a somewhat steady WNW
wind around 10 mph overnight. 10-15 mph WNW winds to continue
Sunday. Skies to clear out after sunset behind the front...but
should see scattered cumulus developing for Sunday afternoon.

Lows to range from around 55 near Galesburg to 60 south of I-
70...cooling 10-15 degrees from this morning. Highs only mid to
upper 70s Sunday.


.LONG TERM...(Sunday night through Saturday)
ISSUED AT 309 PM CDT Sat Aug 20 2016

The models remain in good agreement that the upper trough axis will
make steady progress east of Illinois Sunday night. That will start
a gradual warming trend in the mid-upper levels. However, light
winds and clear skies Sunday night should allow for favorable
radiational cooling conditions, and lows dropping to the low to mid
50s, which will be our coldest low temps in several months. The NAM
and GEM continue to advertise even colder lows, but continued to
lean toward the warmer ECMWF by using a GEFS mean compromise.

A dry period is expected Monday through Tuesday, as the surface
ridge keeps a relatively dry mid-upper level airmass in place. The
rising heights and ridging aloft will translate into 850mb temps
climbing from 10-11C at 00z/7pm Sunday to 19-20C Wednesday night at
06z/1am. High temps should respond by climbing a couple degrees each
day from Monday to Wednesday, with highs reaching the low to mid 80s
on Wed. Dewpoints will also respond to the southerly low level flow
by climbing from the low to mid 50s Monday morning, to the muggy
lower 70s Wednesday afternoon/evening.

Precipitation chances will increase as early as Tuesday night ahead
of an approaching cold front. Increased moisture transport will
produce PWAT values up into the 1.75" to 2" range by 12z/7am
Wednesday, with those levels continuing through Thursday morning,
per the GFS. There are model differences in the location and track
of the surface low Wed through Thursday, but there is better
agreement that a cold front will pass across IL on Thursday. Pre-
frontal shortwaves will help to trigger likely rain/storms on
Wednesday and Wed night, with chances progressing eastward on
Thursday with the FROPA reaching the Indiana border by 00z/7pm
Thursday. Instability parameters point toward some potential for
severe storms Wednesday afternoon, with MUCAPES of 1500-2500 J/kg
and bulk shear values of 30-35kt. The high PWAT values will support
locally heavy rainfall with any storms as well.

The cold front will usher in colder temps once again to close out
the week and start the weekend. Dry conditions are expected Friday
and Saturday, with highs in the upper 70s and lows in the upper 50s
to low 60s.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 105 PM CDT Sat Aug 20 2016

Cold frontal zone is currently centered over central IL with the
strongest pre-frontal thunderstorm band already east of central IL
terminals...while a few post-frontal shower bands remain just to
the west. Initially broken ceilings in MVFR category are affecting
all terminals or expected to shortly move in. Expecting ceilings
to lift and scatter out somewhat with afternoon heating. After 21Z
have increased ceilings to just above MVFR and cleared skies after
00-02Z. Included VCSH in TAFs, but kept out mention of thunder at
this point since probability is quite low. Nevertheless, the
higher probability of thunder will be at KDEC-KCMI eastward.
Winds WSW 10-15 kts gusting to 20-25 kts becoming WNW around 10
kts after 00Z.


.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.