Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 221933
AFDILX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
233 PM CDT FRI MAY 22 2015

.SHORT TERM...(TONIGHT)
ISSUED AT 233 PM CDT FRI MAY 22 2015

HIGH PRESSURE CENTERED OVER THE TENNESSEE RIVER VALLEY WILL CONTROL
THE WEATHER ACROSS CENTRAL ILLINOIS TONIGHT...PROVIDING DRY
CONDITIONS.  MEANWHILE...A VIGOROUS SHORT-WAVE NOTED ON 19Z/2PM
WATER VAPOR IMAGERY OVER WESTERN NEBRASKA WILL LIFT INTO THE
NORTHERN PLAINS AND WILL REMAIN WELL WEST OF THE AREA.  MID/HIGH
CLOUDS FROM THIS FEATURE ARE ALREADY SPILLING ACROSS ILLINOIS THIS
AFTERNOON AND WILL BECOME THICKER/MORE WIDESPREAD TONIGHT.  DUE TO
THE CLOUD COVER...OVERNIGHT LOWS WILL BE A FEW DEGREES WARMER THAN
LAST NIGHT...WITH MOST LOCATIONS REMAINING IN THE LOWER TO MIDDLE
50S.  A FEW SPOTS NEAR THE INDIANA BORDER WHERE SKIES WILL REMAIN
MOSTLY CLEAR LONGER MAY DROP INTO THE UPPER 40S.

&&

.LONG TERM...(SATURDAY THROUGH FRIDAY)
ISSUED AT 233 PM CDT FRI MAY 22 2015

A MAJOR PATTERN SHIFT WILL OCCUR EARLY NEXT WEEK...AS UNSEASONABLY
DEEP UPPER LOW OVER QUEBEC SHIFTS EASTWARD AND A SECOND LOW OVER THE
DESERT SOUTHWEST CROSSES THE ROCKIES.  THIS WILL ALLOW A SIGNIFICANT
RIDGE TO BUILD ACROSS THE SOUTHEAST CONUS...WHICH IN TURN WILL PLACE
THE MIDWEST IN A WARM/UNSETTLED SOUTHWESTERLY FLOW FOR MUCH OF NEXT
WEEK.

AS IS OFTEN THE CASE WITH PATTERN CHANGES...MODELS HAVE BEEN TOO
FAST WITH EJECTING THE DESERT SOUTHWEST UPPER LOW EASTWARD INTO THE
STRENGTHENING RIDGE.  12Z MAY 22 MODELS CONTINUE THEIR RECENT
SLOWING TREND WITH THIS PROCESS...WHICH SEEMS PRUDENT BASED ON PAST
EXPERIENCE.  WHILE BOTH THE NAM AND ECMWF STILL TRY TO BRING SOME
LIGHT QPF INTO THE FAR WESTERN KILX CWA AS EARLY AS SATURDAY
NIGHT...THE LATEST GFS AND GEM ARE NOW COMPLETELY DRY.  HAVE ONCE
AGAIN SCALED BACK POPS...AND NOW ONLY HAVE A SLIGHT CHANCE FOR
SHOWERS/THUNDER IN THE ILLINOIS RIVER VALLEY AFTER MIDNIGHT.  HAVE
ALSO SLOWED THE EASTERN PROGRESSION OF RAIN ON SUNDAY...SINCE
SEVERAL MODELS ARE NOW SUGGESTING EAST-CENTRAL ILLINOIS MAY REMAIN
DRY THROUGH THE ENTIRE DAY.  WILL CARRY LIKELY POPS IN THE ILLINOIS
RIVER VALLEY...TAPERING DOWN TO JUST SLIGHTS ALONG/EAST OF I-57.

MAIN PRECIP EVENT IS STILL ON TARGET FOR SUNDAY NIGHT INTO
MONDAY...AS CLOSED SOUTHWEST UPPER LOW GRADUALLY LIFTS INTO THE
NORTHERN PLAINS AND WEAKENS.  CORRESPONDING SURFACE LOW WILL DEVELOP
IN THE LEE OF THE ROCKIES ACROSS EASTERN COLORADO ON
SATURDAY/SUNDAY...THEN TRACK INTO THE NORTHERN GREAT LAKES BY
TUESDAY.  WITH UPPER HEIGHTS RISING DUE TO THE BUILDING SOUTHEAST
RIDGE...FRONTAL BOUNDARY WILL REMAIN WELL TO THE WEST/NORTHWEST AND
WILL NEVER PASS THROUGH CENTRAL ILLINOIS.  MODELS CONTINUE TO SHOW A
CORRIDOR OF DEEP-LAYER MOISTURE WELL AHEAD OF THE SURFACE/UPPER
SYSTEM THAT GRADUALLY SHIFTS EASTWARD INTO ILLINOIS SUNDAY NIGHT
INTO MONDAY.  WITH SURFACE DEWPOINTS RISING INTO THE MIDDLE TO UPPER
60S...PRECIPITABLE WATER VALUES WILL CLIMB INTO THE 1.75 TO 2.00
INCH RANGE...WHICH IS ABOUT 2 STANDARD DEVIATIONS ABOVE NORMAL FOR
THIS TIME OF YEAR.  GIVEN THE COPIOUS AMOUNTS OF MOISTURE IN THE
ENVIRONMENT...THINK LOCALLY HEAVY RAINFALL WILL BE THE PRIMARY
CONCERN EARLY NEXT WEEK.  AS CORRIDOR OF SHOWERS/THUNDERSTORMS
SHIFTS EASTWARD INTO INDIANA BY LATE MONDAY AFTERNOON...AIRMASS WILL
BEGIN TO DESTABILIZE.  MUCAPE VALUES WILL GENERALLY BE AROUND
1000J/KG...BUT WILL REACH 1500 TO 2000J/KG IN THE ILLINOIS RIVER
VALLEY.  DESPITE THE DEVELOPING INSTABILITY...0-6KM BULK SHEAR
REMAINS MARGINAL AT 30-35KT AND SYNOPTIC FORCING WILL BE ABSENT.
THE BIG QUESTION WILL BE WHETHER OR NOT ANY MESOSCALE BOUNDARIES
LEFT BEHIND BY EARLY DAY CONVECTION CAN TAKE ADVANTAGE OF THE
INSTABILITY...RESULTING IN RENEWED DEVELOPMENT ACROSS WEST-
CENTRAL ILLINOIS LATE IN THE DAY.  NO MODEL IS CURRENTLY SHOWING
THIS OCCURRING...BUT THIS WILL NEED TO BE MONITORED CLOSELY OVER
THE NEXT COUPLE OF DAYS AS LATE MONDAY AFTERNOON/EVENING MAY BE
THE BEST SHOT AT SEEING STRONG TO SEVERE STORMS WITHIN THE
FORECAST PERIOD.

SOUTHWESTERLY FLOW WILL CONTINUE TO DOMINATE ACROSS THE MIDWEST FOR
THE REMAINDER OF THE WEEK.  GFS/ECMWF ARE BOTH SHOWING A WEAK UPPER-
LEVEL WAVE LIFTING NORTHEASTWARD OUT OF TEXAS LATE TUESDAY INTO
WEDNESDAY...WHICH WILL PROVIDE ENHANCED LIFT AND A RENEWED CHANCE
FOR CONVECTION.  HAVE INCREASED POPS TO HIGH CHANCE/LIKELY TUESDAY
AFTERNOON INTO WEDNESDAY ACCORDINGLY.  AFTER THAT...FORCING FOR
ADDITIONAL CONVECTION WANES AND RAIN CHANCES DIMINISH.  HAVE
THEREFORE GONE WITH A WARM/DRY FORECAST WEDNESDAY NIGHT THROUGH
THURSDAY NIGHT.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SATURDAY AFTERNOON)
ISSUED AT 1205 PM CDT FRI MAY 22 2015

VFR CONDITIONS ON TAP THROUGH THE PERIOD. HIGH CLOUDS WILL SPREAD
IN THIS AFTERNOON AND GRADUALLY LOWER TO AROUND 10KFT THIS
EVENING...WHICH WILL PERSIST THE REMAINDER OF THE PERIOD. WINDS
WILL BECOME LIGHT/VARIABLE TONIGHT...BEFORE A SOUTHERLY WIND PICKS
UP LATE SATURDAY MORNING.


&&

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

$$

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


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