Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 291730
AFDILX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
1230 PM CDT SAT AUG 29 2015

.UPDATE...
ISSUED AT 1018 AM CDT SAT AUG 29 2015

SCATTERED SHOWERS CONTINUE TO PUSH ACROSS AREAS MAINLY EAST OF
I-55 THIS MORNING...WITH SOME SPRINKLES STARTING TO MOVE INTO
PARTS OF SOUTHEAST ILLINOIS AS WELL. MORNING SOUNDING FROM OUR
OFFICE SHOWED QUITE A BIT OF DRY AIR BELOW 12000 FEET...WHICH HAS
HELPED KEEP MOST OF THE RAIN LIMITED TO A FEW HUNDREDTHS OF
ACCUMULATION. LATEST RAP MODEL SOUNDINGS SHOW SOME MOISTENING OF
THE COLUMN DOWN TO ABOUT 5000 FEET OVER THE NEXT FEW HOURS...
HOWEVER SOME DRIER AIR IS ALSO EVIDENT ON WATER VAPOR IMAGERY
DROPPING SOUTHWARD AHEAD OF A FAST MOVING SHORTWAVE NOW LOCATED IN
NORTHERN IOWA...WHICH SHOULD KEEP THE NORTHWEST PART OF THE
FORECAST AREA DRY FOR A COUPLE MORE HOURS. HIGHER RESOLUTION MODEL
GUIDANCE SUGGESTS SOME ADDITIONAL SHOWERS SURGING NORTHEAST OUT OF
SOUTHERN MISSOURI AS A WEAK FRONTAL BOUNDARY PUSHES EAST. EARLIER
GRID UPDATES HAD INTRODUCED THIS TREND AND ONLY REQUIRED MINOR
UPDATES...MAINLY TO NUDGE TEMPERATURES DOWN A BIT.

&&

.SHORT TERM...(TODAY)
ISSUED AT 312 AM CDT SAT AUG 29 2015

07Z/2AM SURFACE ANALYSIS SHOWS A WEAK FRONTAL BOUNDARY EXTENDING
FROM WISCONSIN SOUTHWESTWARD TO THE TEXAS PANHANDLE.  A PLUME OF
MAINLY MID-LEVEL MOISTURE AHEAD OF THE FRONT HAS RESULTED IN
SCATTERED LIGHT SHOWERS/SPRINKLES ACROSS THE WESTERN HALF OF THE
KILX CWA EARLY THIS MORNING.  00Z UPPER AIR SOUNDING SHOWED PLENTY
OF DRY AIR BELOW 850MB...SO PRECIP HAS BEEN HAVING A HARD TIME
REACHING THE GROUND IN MANY AREAS.  FORECAST SOUNDINGS SHOW A
GRADUAL MOISTENING OF THE PROFILE TODAY...HOWEVER DEEP-LAYER
MOISTURE REMAINS LACKING.  IN ADDITION...WIDESPREAD CLOUD COVER WILL
LIMIT INSTABILITY...WITH GFS SBCAPE VALUES ONLY REACHING THE 500 TO
1000J/KG RANGE.  END RESULT WILL BE SCATTERED SHOWERS WITH PERHAPS A
RUMBLE OF THUNDER.  THINK AREAL COVERAGE OF PRECIP WILL REMAIN
LOW...SO WILL ONLY CARRY 30-40 POPS.  HIGH TEMPERATURES WILL RANGE
FROM THE UPPER 70S ALONG/NORTHWEST OF THE ILLINOIS RIVER...TO THE
MIDDLE 80S ALONG/SOUTHEAST OF I-70 WHERE CONDITIONS WILL REMAIN DRY
UNTIL A FEW SHOWERS ARRIVE DURING THE AFTERNOON.

.LONG TERM...(TONIGHT THROUGH FRIDAY)
ISSUED AT 312 AM CDT SAT AUG 29 2015

FRONTAL BOUNDARY WILL SLOWLY PUSH EASTWARD LATER TODAY INTO
TONIGHT...WITH MODEL CONSENSUS SHOWING IT REACHING A CHICAGO TO
KIRKSVILLE LINE BY 12Z SUN. SCATTERED SHOWERS WILL BE ONGOING
AHEAD OF THE FRONT ACROSS MUCH OF THE AREA THIS EVENING...THEN
WILL SHIFT TO MAINLY LOCATIONS ALONG/EAST OF I-57 OVERNIGHT. MAIN
WEATHER CONCERN FOR TONIGHT WILL BE THE POTENTIAL FOR FOG ACROSS
THE W/NW CWA. AS THE SLOW-MOVING FRONT ARRIVES...WINDS WILL BECOME
LIGHT/VARIABLE ACROSS THIS AREA. DUE TO THE NEARLY CALM
WINDS...ADDED LOW-LEVEL MOISTURE FROM SHOWERS TODAY...AND PARTIAL
CLEARING LATE TONIGHT...THINK AT LEAST PATCHY FOG WILL FORM. BOTH
FORECAST SOUNDINGS AND NUMERIC GUIDANCE SUGGEST A REDUCTION IN
VSBY OVERNIGHT. CURRENT OBSERVATIONS IN THE VICINITY OF THE FRONT
ACROSS IOWA SHOW WIDESPREAD FOG...WITH A FEW VSBYS DOWN TO AROUND
1/4 MILE. THINK THIS SAME SCENARIO WILL UNFOLD FURTHER EAST
TONIGHT...SO HAVE INCLUDED PATCHY FOG IN THE FORECAST WEST OF I-55
AFTER MIDNIGHT.

THE FRONTAL BOUNDARY WILL SLOWLY DISSIPATE ON SUNDAY...HOWEVER
ENOUGH UPPER-LEVEL SUPPORT WILL PERSIST TO SUPPORT AT LEAST A
MENTION OF A FEW SHOWERS/THUNDERSTORMS.  MOST MODELS PRODUCE LIGHT
QPF ACROSS NEARLY THE ENTIRE CWA AS THE BOUNDARY HAS SLOWED SLIGHTLY
FROM PREVIOUS RUNS.  AS A RESULT...HAVE PULLED POPS FURTHER WEST TO
INCLUDE SLIGHT CHANCES AS FAR WEST AS THE ILLINOIS RIVER SUNDAY
AFTERNOON/EVENING.

AFTER THAT...THE REMAINDER OF THE EXTENDED LOOKS WARM AND DRY AS
UPPER-LEVEL RIDGING DOMINATES THE EASTERN CONUS THROUGH THE END OF
THE COMING WEEK.  A SLIGHT WEAKNESS IN THE RIDGE APPEARS TO DEVELOP
ACROSS ILLINOIS INTO THE OHIO RIVER VALLEY...POTENTIALLY TRIGGERING
ISOLATED AFTERNOON/EVENING CONVECTION TUESDAY THROUGH FRIDAY.
HOWEVER...MID-LEVEL WARMING BENEATH THE RIDGE WILL LIKELY LEAD TO
CAPPING AND AN OVERALL SUPPRESSION OF CONVECTION.  AS A
RESULT...WILL MAINTAIN A DRY FORECAST WITH HIGH TEMPERATURES
CLIMBING INTO THE UPPER 80S TO AROUND 90 DEGREES EACH DAY.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SUNDAY AFTERNOON)
ISSUED AT 1230 PM CDT SAT AUG 29 2015

MAINLY VFR CONDITIONS OVER CENTRAL ILLINOIS AT MIDDAY...WITH A FEW
SHOWERS EAST OF KBMI-KSPI. WATCHING AN AREA OF MVFR/IFR CEILINGS
DROPPING DOWN FROM IOWA AND NORTHWEST ILLINOIS AHEAD OF AN UPPER
LEVEL DISTURBANCE...AND MVFR CEILINGS ARE NOT TOO FAR AWAY FROM
KPIA. SOME QUESTION AS TO THE SOUTHWARD PROGRESS...WITH THE RAP
MODEL KEEPING KPIA JUST ABOVE VFR LEVEL AND THE NAM DIPPING INTO
MVFR TERRITORY. HAVE INCLUDED SOME TEMPO CEILINGS AROUND 2500 FEET
THERE INITIALLY AND WILL CONTINUE TO EVALUATE THE PROGRESSION OVER
THE NEXT HOUR OR SO. MORE OF THE CONCERN FOR MVFR CONDITIONS OVER
CENTRAL ILLINOIS WILL BE AFTER 06Z...AS A WEAKENING FRONTAL
BOUNDARY BECOMES NEARLY STATIONARY OVERHEAD. THE WIDESPREAD LOW
CLOUDS CURRENTLY TO OUR NORTHWEST ARE NOT LIKELY TO COMPLETELY
ERODE AND MORE LIKELY WOULD EXPAND OVERNIGHT...WITH THE RAP
HINTING AT POTENTIAL IFR/LIFR CONDITIONS AT KPIA LATE IN THE
NIGHT. HAVE HIT CONDITIONS AT KPIA THE HARDEST WITH TEMPO CEILINGS
AND VISIBILITIES THAT LOW...BUT MAINTAINED MVFR CONDITIONS
ELSEWHERE LATE TONIGHT INTO SUNDAY MORNING.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...GEELHART
SHORT TERM...BARNES
LONG TERM...BARNES
AVIATION...GEELHART


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