FXUS63 KAPX 200833 AFDAPN AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GAYLORD MI 425 AM EDT THU MAY 20 2004 .SHORT TERM...LATEST SURFACE PLOT PLACES WARM FRONT OVER EASTERN WISCONSIN STRETCHING SOUTHEAST INTO SW LOWER MICHIGAN AND INTO THE OHIO VALLEY. MAIN FORECAST CONCERN TODAY WILL BE CONVECTIVE CHANCES. SURFACE COLD FRONT WILL MOVE THROUGH THE FORECAST AREA THIS AFTERNOON. FORCING WITH THE COLD FRONT NOT IMPRESSIVE AS BEST 800-500MB Q-VECTOR CONVERGENCE WILL MOVE OFF TO THE NORTHEAST BEFORE THE FRONT ARRIVES ACROSS THE FORECAST AREA. MODEL SOUNDINGS SUGGEST THAT BEST INSTABILITY WILL LIE ACROSS THE SOUTHEAST PORTION OF THE FORECAST AREA GENERALLY SOUTH OF A LINE FROM APN TO HTL WITH CAPES OF 1500-2000J/KG AND LI/S UP TO AROUND -6 BASED ON A 76/65 PARCEL. UNIDIRECTIONAL 850/500MB WINDS (50KTS@850MB AND 25KTS@850MB) SUGGEST THAT MAIN SEVERE THREAT WILL COME FROM BOWING SEGMENTS. CANNOT RULE OUT AN ISOLATED SUPERCELL OCCURRING BEFORE FRONTAL PASSAGE WITH BULK RICHARDSON NUMBERS IN THE TEENS ALONG SAGINAW BAY. BUT WITH WEAK FORCING...DISMAL LAPSE RATES (AROUND 6C/KM)...AND POSSIBLY LIMITED SUNSHINE TO FURTHER DESTABILIZE THE ATMOSPHERE THE SEVERE THREAT MAY NOT MATERIALIZE AGAIN. WILL GO WITH LIKELY POPS THIS MORNING BUT SCALE DOWN TO SCATTERED SHRA/TSRA THIS AFTERNOON. .LONG TERM...OVERVIEW...COLD FRONT EXITS THE SE CWA EARLY THIS EVENING WITH REMNANT SHOWERS AND THUNDERSTORMS ENDING WITH A SOUTHWARD PROGRESS OF THE FRONT. THE FRONT MAKES IN DOWN INTO SRN LOWER MICHIGAN/NRN IL/IN...SIMILAR TO PREVIOUS RUNS. THIS BOUNDARY RETURNS AS A WARM FRONT TO CENTRAL MICHIGAN FRIDAY NIGHT. SHORTWAVES ALOFT AND SUBSEQUENT SFC LOW PRESSURE CIRCULATIONS WILL FOLLOW ALONG THIS BOUNDARY...CREATING SHOWERS AND THUNDERSTORMS ALONG AND NORTH OF THE FRONT...WITH HEAVY RAINFALL CERTAINLY A POSSIBILITY FOR NORTHERN LOWER MICHIGAN. THERE ARE TWO MAIN SFC WAVES TO IDENTIFY. THE FIRST RIDES THROUGH FRIDAY NIGHT AHEAD OF THE PARENT SFC CIRCULATION THAT WILL REMAINED ANCHORED NEAR KANSAS FRIDAY THROUGH SATURDAY. THE BOUNDARY WILL SLIP SOUTHWARD AFTER THIS WAVE PASSES... MORE INTO SRN/SC LOWER MICHIGAN. THE MAIN SFC CIRCULATION (FROM KANSAS) THEN EJECTS OUT WITH THE MAIN UPPER SHORTWAVE AND MAKES IT INTO WISCONSIN SUNDAY AFTERNOON. THE FRONT WILL RETURN NORTH AGAIN...AND PRECIP CHANCES RISE AGAIN. THIS LOW PROGGED TO MOVE ACROSS THE SAGINAW BAY AREA EARLY MONDAY. TRICKY WEATHER PATTERN WHEN A FRONTAL BOUNDARY HANGING AROUND IN THE VICINITY...BUT FRIDAY NIGHT AND SUNDAY AFTERNOON THROUGH SUNDAY NIGHT LOOK LIKE BEST CHANCES FOR RAIN. MODEL DISCUSSION...MODELS ARE CONTINUE WITH FAIRLY GOOD AGREEMENT ON THE UPCOMING WET WEATHER PATTERN. WITH THE COLD FRONT RETURNING AS A WARM FRONT FRIDAY AFTERNOON THEN LINGERING AROUND THE AREA THROUGH SUNDAY WITH A SERIES OF WEAK DISTURBANCES MOVING ALONG IT...TIMING OF EACH PERIOD OF RAINFALL WILL BE DIFFICULT. HOWEVER A COUPLE THINGS CAN BE GLEANED FROM THE MODELS...GFS POTENTIALLY SHOWING SOME TYPICAL BIASES WITH LOWERING SFC PRESSURES TOO MUCH IN FRONTAL SYSTEM AHEAD OF EJECTING UPPER TROUGH IN THE ROCKIES...REFERRING TO THE SFC PRESSURES IN THE NRN PLAINS ON FRIDAY. WILL CONTINUE CLOSER FORECAST TO THE ETA...WHICH DOES NOT INDICATE THIS...AND KEEPS FRONTAL BOUNDARY FURTHER SOUTH...AHEAD OF SFC LOW WHICH WILL BE IN NEBRASKA. IN ADDITION...GFS SHOWING POTENTIALLY MORE CONVECTIVE FEEDBACK ISSUES...ALTHOUGH THE ETA SHOWING SOME AS WELL. THERE IS GOOD AGREEMENT ON THE EJECTION OF THE MAIN SHORTWAVE AND SFC LOW THAT MOVES OUT OF KANSAS INTO WISCONSIN SUNDAY. WILL GO WITH A BLEND OF THE ETA/GFS THIS TIMEFRAME. FORECAST CONCERNS ARE TIMING OF MAIN PRECIP EVENTS...POTENTIAL FOR HEAVY RAINFALL AND THUNDERSTORMS FRIDAY NIGHT. TEMPERATURES TO SOME DEGREE...DUE TO SOME UNCERTAINTY WITH EXACT POSITION OF FRONTAL BOUNDARY OVER THE WEEKEND. THURSDAY NIGHT...IN THE EARLY EVENING...FRONT WILL BE ACROSS THE FAR SE CWA FROM HOUGHTON LAKE TO OSCODA. SFC BASED CAPES WILL REMAIN IN THE 1500-2000 J/KG...LI'S TO -2C. WILL KEEP CHANCE FOR SHOWERS AND THUNDERSTORMS GOING HERE. WILL ALSO KEEP ISOLATED SHOWERS IN FOR NRN LOWER MICHIGAN...AS DEEP RH (>85% H8-H5) LINGERS. OTHERWISE...FRONT SLIDES SOUTHWARD TO JUST SOUTH OF THE MICHIGAN BORDER INTO NRN IL/IN. A COOLER AND MORE STABLE NORTHERLY LOW LEVEL FLOW MOVES IN. LOW LEVEL MOISTURE TRAPPED BELOW ROUGHLY 900MBS DOES NOT LOOK TOO IMPRESSIVE. WILL KEEP THE PARTLY CLOUDY SKY FORECAST GOING...BUT DO EXPECT A PERIOD OF SOME LOW CLOUDS TO AFFECT NRN MICHIGAN. WITH A PERIOD OF CLEARING...DO SEE TEMPS DROPPING INTO THE 30S IN EASTERN UPPER AND PARTS OF NRN MITT. .FRIDAY...FRONT STILL PROGGED TO CLIMB BACK NORTH...BUT MORE QUICKLY. HAVE BUMPED UP POPS TO A HIGHER CHANCE ACROSS MAINLY THE WESTERN HALF OF THE CWA...DUE TO H8 LLJ OF 35-40KT AND BEST THETA-E ADVECTION POKING INTO THE AREA. THIS ALSO WHERE MINIMAL H8 ELEVATED CAPE RESIDES. THE THUNDERSTORM CHANCE STILL LOOKS BEST HERE...BUT WILL ALSO INCLUDE THE SOUTHERN TIER OF THE CWA...DUE TO FASTER FRONTAL PUSH. .FRIDAY NIGHT...LLJ AND H5 WINDS INCREASE TO 50-55KTS AND LAY OUT OVER SAGINAW BAY IN A WSW FASHION. IF THE LLJ WERE ORIENTED MORE PERPENDICULAR TO THE FRONTAL BOUNDARY...WOULD EXPECT MORE EXPLOSIVE CONVECTION SUCH AS AN MCS. NEVERTHELESS...SHOWERS AND THUNDERSTORMS ARE LIKELY ALONG AND NORTH OF THE FRONT. THERE IS STRONG H3 DIVERGENCE OVER THE BOUNDARY THROUGH THE EVENING...AND THE HELP OF A SHORTWAVE RIDING INTO THE AREA...ALL SUGGESTIVE OF VERY GOOD CHANCE FOR RAIN. THE RAIN COULD BE POTENTIALLY HEAVY AS WELL...THE BOUNDARY IS FORECAST TO REMAIN ACROSS THE SOUTHERNMOST TIER OF COUNTIES THROUGH THE NIGHT. CONVECTION WILL BE FIRING ALL ALONG THE FRONT WELL BACK TO OUR WEST. STORMS COULD CONTINUALLY TRACK ALONG THIS BOUNDARY ESSENTIALLY HITTING THE SAME AREAS THROUGH THE NIGHT. TO EARLY TO ISSUE ANY FLOOD WATCHES...ESPECIALLY WITH SOME DEGREE OF UNCERTAINTY IN THE EXACT POSITION OF THE FRONT...BUT WILL HAVE TO BE WATCHED CLOSELY. THE FINER RESOLUTION META SUGGESTING THAT A WEAK SFC CIRCULATION MOVES THROUGH THE SOUTHERNMOST COUNTIES WHICH INCREASES THE SRH VALUES SIGNIFICANTLY IN THE EVENING. WITH UNIDIRECTIONAL WINDS THROUGH A LARGE DEPTH OF THE ATMOSPHERE (H8-H5) WIND THREAT MORE LIKELY...HOWEVER COULD BE SOME LOW LEVEL ROTATION TO THE STORMS...ON POTENTIAL BOWING SEGMENTS. THIS WILL BE MORE OF A CONCERN IF THE WARM SECTOR DOES IN FACT REACH THE SOUTHERN COUNTIES. .SATURDAY...MODEL DISCREPANCIES THIS PERIOD CONCERNING THE FRONTAL POSITION. GOING MORE WITH THE ETA...THE FRONT DRIFTS SOUTHWARD THROUGH THE DAY...TO SRN MICHIGAN...TAKING MUCH OF THE THUNDER CHANCE/INSTABILITY ALONG WITH IT. WILL DOWNPLAY THE THUNDER MENTION FOR NOW...BUT KEEP HIGH CHANCES FOR SHOWERS..AS THERE IS SHORTWAVE ENERGY RIDING OVERHEAD AND H8-H5 MOISTURE REMAINS GREATER THAN 90%. .SATURDAY NIGHT AND SUNDAY...MAIN UPPER TROUGH/SHORTWAVE EJECT TOWARD THE WESTERN GREAT LAKES. INITIAL SHORTWAVE MOVES IN SATURDAY NIGHT WHICH LOOKS LIKE CONVECTIVE FEEDBACK...SO QUESTION IT SOMEWHAT. THE MAIN SFC LOW FROM KANSAS WILL SHIFT INTO EASTERN IOWA REGION LATE SATURDAY NIGHT...AND THE LLJ STRENGTHENS INTO NW IL. THIS IS A MORE FAVORABLE SETUP FOR MCS DEVELOPMENT. CYCLONIC CONVERGENCE WITH STRONGER SFC LOW AT H8...UPPER DIVERGENCE/THETA-E CONVERGENCE/MAIN SHORTWAVE AND INSTABILITY GRADIENT ALL IN PHASE IN THIS REGION. THE FRONTAL BOUNDARY WILL BE SHIFTING BACK NORTH THROUGH SATURDAY NIGHT...AND ANY CONVECTION WILL FOLLOW THE H8-H3 THICKNESS RIGHT INTO NRN MICHIGAN. IF THIS PATTERN HOLDS...THIS CERTAINLY LOOKS BEST FOR STRONGER CONVECTION. STORMS THAT DO MOVE IN WILL HOLD TOGETHER AS THEY WILL ENCOUNTER A DESTABILIZING AIR MASS IN NRN MICHIGAN. THIS WILL HAVE TO BE MONITORED AS WELL. ACTUAL SFC LOW ARRIVES SUNDAY NIGHT...AND GOOD CHANCES FOR RAIN PERSIST THROUGH THEN. UPDATES TO SUNDAY NIGHT MAY BE NEEDED...BUT WILL NOT CHANGE THIS PART OF FORECAST ATTM. NO CHANGES LATER PERIODS. SMD && .APX...WATCHES/WARNINGS/ADVISORIES...NONE. && $$