391 
FXUS63 KILX 271918
AFDILX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
218 PM CDT FRI APR 27 2012

.DISCUSSION...
ISSUED 200 PM CDT FRI APR 27 2012

WATER VAPOR LOOP SHOWING TWO DISTINCT UPPER WAVES TO OUR WEST.
THE STRONGER OF THE TWO SHORTWAVES WAS LOCATED OVER SOUTHWEST
MONTANA WITH THE SECONDARY WAVE ACRS WESTERN KS...WHICH IS BEING
DRAWN NORTHWARD AS IT ENCOUNTERS A BLOCKING RIDGE ACRS THE
NORTHERN PLAINS. FURTHER EAST...A LARGE CANADIAN HIGH WAS ANCHORED
NORTH OF THE GREAT LAKES EARLY THIS AFTERNOON HELPING TO DRAWN
DOWN SOME COOL AND VERY DRY AIR SOUTH INTO OUR AREA.

SHORT TERM...TONIGHT THROUGH MONDAY NIGHT

SEVERAL CONCERNS THIS FORECAST PERIOD INCLUDING...TIMING OF RAIN
ACROSS THE FORECAST AREA THIS EVENING AND THEN CONVECTION CHANCES
INCLUDING SEVERE THREAT AROUND THE I-70 CORRIDOR TOMORROW AFTERNOON.
AFTER THAT...A SERIES OF WEATHER SYSTEMS MOVE OUT OF THE SOUTHERN
PLAINS AND INTERACT WITH A FRONTAL BOUNDARY WHICH WILL BE SHIFTING
SLOWLY NORTH AS THE LARGE CANADIAN HIGH FINALLY EDGES OFF TO OUR
EAST. THIS WILL BRING ADDITIONAL CHANCES FOR SHOWERS AND THUNDERSTORMS
THROUGH EARLY NEXT WEEK. 

FORECAST SOUNDINGS SHOWING ONLY A SLOW MOISTENING OF THE LOWER LEVELS
THIS EVENING AS THE SURFACE LOW PRESSURE AREA ASSOCIATED WITH THE KANSAS
UPPER WAVE MOVES INTO NORTHERN MISSOURI BY LATE TONIGHT. IT APPEARS
WE WILL SEE OUR BEST THREAT FOR SHOWERS AND ISOLATED THUNDERSTORMS LATE
THIS EVENING WEST...AND THEN SHIFTING NORTHEAST OVER THE REMAINDER
OF THE AREA LATE TONIGHT...BEFORE EDGING OUT OF OUR AREA BY LATE SATURDAY
MORNING AS THE SURFACE WAVE SHIFTS INTO EAST CENTRAL MO/SOUTHWEST ILLINOIS
BY NOON. LOOKS AS IF THE NAM-WRF HANGS ON TO THE IDEA OF A CLOSED SFC
WAVE TOO LONG BASED ON THE UPPER SYSTEM PULLING WELL OF TO ITS NW DURING
THE MORNING. 

LOCAL HI-RES MODELS SUGGEST THE WARM FRONT WILL PUSH TO JUST SOUTH
OF I-70 BY EARLY TOMORROW AFTERNOON WITH STEEP MID LEVEL LAPSE RATES
OF UP TO 8 DEG/C AND MIXED LAYER CAPES OF 1500-2000 J/KG ALONG AND
JUST SOUTH OF THE WARM FRONT. SCATTERED STORMS ARE EXPECTED TO DEVELOP
IN THE 18Z-21Z TIME FRAME JUST EAST OF THE WEAKENING SFC WAVE AND 
ALONG/JUST SOUTH OF THE WARM FRONT. AT THIS TIME...IT APPEARS THE PRIMARY
MODE OF CONVECTION TOMORROW AFTERNOON WOULD BE INITIALLY SUPERCELLUAR
WHICH WOULD QUICKLY TRANSLATE MORE INTO A LINEAR MODE BASED ON THE
UNIDIRECTIONAL WINDS FORECAST IN SE IL. PRIMARY THREAT WOULD BE LARGE
HAIL AND DAMAGING WINDS...WITH AN ISOLATED TORNADO POSSIBLE...ESP JUST
EAST OF THE SFC WAVE AND ALONG THE WARM FRONT.

ANY LINGERING CONVECTION SHOULD COME TO AN END BY 00Z ACRS SE IL WITH
THE BOUNDARY GETTING NUDGED BACK SOUTH AGAIN AS THE CANADIAN HIGH 
BUILDS BACK INTO THE AREA SATURDAY EVENING. WILL CONTINUE TO HOLD 
ON TO CHANCE POPS ACRS SOUTHERN IL AS THE BOUNDARY WILL NOT BE THAT
FAR AWAY. MEANWHILE...FURTHER TO THE NORTHWEST...THE UPPER LOW SHIFTS
SLOWLY NORTH INTO SOUTHERN CANADA AS THE RELENTLESS BLOCK HOLDS FIRM
ACRS THE NORTHERN PLAINS AND WESTERN GREAT LAKES. MODELS BEGIN TO 
BUILD THE HEIGHTS ACRS THE AREA IN RESPONSE TO THE RETREATING UPPER
WAVE WHICH SHOULD ALLOW THE LARGE CANADIAN HIGH TO EDGE FURTHER
EAST ON SUNDAY/SUNDAY NIGHT. THE FRONTAL BOUNDARY TO OUR SOUTH WILL
BEGIN TO PUSH BACK NORTH DURING THIS TIME FRAME WITH ALL MODELS
INDICATING WEAK DISTURBANCES MOVING OUT OF THE SOUTHERN PLAINS. ONE
SUCH WAVE WILL PUSH ALONG THE BOUNDARY AND BRING AN INCREASING
THREAT FOR SHOWERS/THUNDERSTORMS LATE SUNDAY INTO A PORTION OF MONDAY
BEFORE SHIFTING OFF TO OUR EAST MONDAY NIGHT. 

LONG TERM...TUESDAY THROUGH FRIDAY

A WARMER AND ACTIVE WEATHER PATTERN CONTINUES TO BE DEPICTED ON
MEDIUM RANGE MODELS DURING THIS FORECAST PERIOD...HOWEVER...TIMING
OF INDIVIDUAL SHORTWAVES ACRS THE AREA AS THE PATTERN DEAMPLIFIES
CONTINUES TO BE A CHALLENGE WITH RESPECT TO PRECIP CHANCES EACH DAY.
PATTERN TRANSITIONS INTO MORE OF A ZONAL FLOW EARLY IN THE FORECAST
PERIOD...THEN THE UPPER FLOW SHIFTS TO SOUTHWESTERLY LATE IN THE WEEK
AS A TROF SHIFTS INTO THE WEST COAST. MOST OF OUR AREA WILL BE ALONG
THE SOUTHERN PERIPHERY OF THE STRONGER WINDS ALOFT...WHICH USUALLY
TRANSLATES TO PERIODIC CONVECTION CHANCES THIS TIME OF YEAR...AND
UNFORTUNATELY...THIS IS THE CASE THRU THE EXTENDED PORTION OF THE
FORECAST. ALTHOUGH TEMPERATURES WILL WARM WELL INTO THE 70'S TO
LOWER 80S STARTING TUESDAY...THE PATTERN CONTINUES TO SUPPORT A
LEAST MENTIONABLE POPS IN THE GRIDS EACH DAY.

SMITH
&&

.AVIATION...
ISSUED 1230 PM CDT FRI APR 27 2012

LARGE AREA OF MID-LEVEL CLOUDS OVER THE TAF SITES AT MIDDAY. RADAR
SHOWING SOME ECHOES ALONG THE MISSISSIPPI RIVER FROM KUIN-KSTL
MOVING EASTWARD...BUT WITH VERY DRY AIR AT THE LOWER LEVELS...AM
ONLY EXPECTING VIRGA AT TIMES THIS AFTERNOON. MAIN CONCERN BEGINS
MID TO LATE EVENING...AS SHOWERS AND ISOLATED THUNDERSTORMS
INCREASE ALONG A WARM FRONTAL BOUNDARY LIFTING NORTHWARD TOWARD
CENTRAL ILLINOIS. EXISTING PROB30 GROUP AT KSPI/KDEC/KCMI FOR
THUNDER MENTION STILL APPEARS WARRANTED...BUT CONVECTIVE CHANCES
FURTHER NORTH APPEAR SHAKIER AND HAVE REMOVED MENTION OF THUNDER
AT KPIA/KBMI FOR NOW. FORECAST SOUNDINGS AND SREF CEILING PROBABILITY
GRAPHICS ALL SUPPORT MVFR CEILINGS DEVELOPING LATE TONIGHT WITH
THE PASSAGE OF THE WARM FRONT. THESE SHOULD PERSIST THROUGH THE
MORNING...WITH CONDITIONS IMPROVING AROUND KSPI TOWARD THE END OF
THE FORECAST PERIOD.

GEELHART
&&

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

$$