000 FXUS63 KAPX 210753 AFDAPX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GAYLORD MI 353 AM EDT SAT JUN 21 2008 .DISCUSSION...03Z SURFACE RUC ANALYSIS AND IR/WV/RADAR IMAGERY SHOW NORTHERN MICHIGAN CONTINUING TO LIE WITHIN NW FLOW ALOFT BETWEEN RELATIVELY FLAT RIDGING OVER THE WESTERN UNITED STATES...AND TROUGHING OVER THE EAST. WITHIN THIS NW FLOW...A MYRIAD OF SHORTWAVES ARE PROGRESSING ACROSS THE GREAT LAKES THAT ARE TRIGGERING CONVECTION AT THIS HOUR. ONE IMPULSE IS HEADING ACROSS SOUTHERN LAKE MICHIGAN WITH A FEW THUNDERSTORMS AHEAD OF IT...PERHAPS GETTING A BOOST FROM A SURFACE TROUGH THAT EXTENDS OVER EASTERN LAKE SUPERIOR TO SW WISCONSIN. A FEW OTHER SHOWERS HAVE FIRED ALONG THIS TROUGH OVER GREEN BAY AND EASTERN UPPER. FURTHER WEST...MORE POTENT SHORTWAVE ACTIVITY IS DIVING ACROSS THE ARROWHEAD OF MN AND ALSO WESTERN ONTARIO WITH SOME SEMBLANCE OF 850MB THETAE RIDGING/MOISTURE POOLING AHEAD OF IT. AS THESE FEATURES MOVE INTO NORTHERN MICHIGAN...THUNDERSTORM POTENTIAL IS THE MAIN FORECAST CONCERN OVER THE NEXT COUPLE DAYS. TODAY...SURFACE TROUGH OVER WESTERN LAKE MICHIGAN WILL SLOWLY SHIFT EAST DURING THE MORNING ACROSS THE CWA. PER RADAR TRENDS...HARD NOT TO ENVISION SOME VERY SPOTTY SHOWERS ALONG IT...WITH AN ISOLATED RUMBLE OF THUNDER. THEN...THE SHORTWAVE OVER THE ARROWHEAD OF MN WILL MAKE A RUN TOWARDS NORTHERN MICHIGAN BY AFTERNOON...HELPING TO PUSH ALONG THE 850MB THETAE AXIS AT THE SAME TIME. INCOMING LARGE-SCALE FORCING/HEIGHT FALLS ALONG WITH INCREASED LOW-LEVEL CONVERGENCE VIA WESTERLY SYNOPTIC FLOW BUTTING HEADS WITH LAKE HURON LAKE BREEZE WILL SET THE STAGE FOR DEEP CONVECTION THIS AFTERNOON EAST OF A LINE FROM LAKE CITY TO CHEBOYGAN. HOW MUCH INSTABILITY DEPENDS ON THE AMOUNT OF MIXING THAT TAKES PLACE THROUGH EARLY AFTERNOON. UPSTREAM DEWPOINTS WITHIN THE H850 THETAE AXIS FELL INTO THE UPPER 40S TO LOWER 50S ACROSS SOUTHERN MN/WESTERN WI YESTERDAY AFTERNOON...WHICH BELIEVE WILL IMPACT INSTABILITY ACROSS NW LOWER. HOWEVER...BELIEVE MOISTURE POOLING ALONG THE LAKE BREEZE WILL HELP MAINTAIN DEWPOINTS IN THE MID 50S JUDGING BY TODAYS OBS. A 76/54 SURFACE PARCEL NEAR WEST BRANCH GENERATES ML CAPES AROUND 1000-1200 J/KG BY MID-AFTERNOON. LESS INSTABILITY TO THE TUNE OF 500 J/KG WILL BE FOUND ACROSS NW LOWER AND EASTERN UPPER. THEREFORE...BELIEVE BEST THUNDERSTORM COVERAGE WILL RESIDE ACROSS NE LOWER WHERE BL CONVERGENCE IS MAXIMIZED THIS AFTERNOON. FAIRLY UNIFORM WIND FIELDS WITH 0-6KM BULK SHEAR VALUES AROUND 25KTS WILL LIMIT STORM ORGANIZATION AND DAMAGING WIND POTENTIAL...BUT CANNOT RULE OUT COIN SIZE HAIL THREAT. TONIGHT...850MB THETAE AXIS SLIDES EAST OF THE REGION...WHILE SHORTWAVE TROUGH MOVES OVERHEAD. LOOKING BACK UPSTREAM...SPOTTY CONVECTION CONTINUED TO FIRE WITHIN THE MID-LEVEL COLD POOL WELL INTO THE EVENING HOURS. WILL KEEP HIGHEST CHANCE OF PRECIP OVER AREAS ADJACENT TO SAGINAW BAY WHERE LAKE BREEZE CONVERGENCE AND ANY COLD OUTFLOW BOUNDARIES WILL HAVE A BETTER PROBABILITY OF TRIGGERING STORMS. BUT ELSEWHERE...COVERAGE MAY START OUT AS SCT (IF NOT WIDELY SCT) EARLY IN THE EVENING...THEN DIMINISH TO ISOLATED AS SURFACE BASED INSTABILITY DECREASES WITH LOSS OF HEATING. SUNDAY AND SUNDAY NIGHT...SHORTWAVE OVER WESTERN ONTARIO SETTLES OVER THE AREA WITH H500 TEMPS AROUND -19C. SOME INDICATIONS OF A SECONDARY SURFACE TROUGH...WHICH WILL PASS ACROSS EASTERN UPPER DURING THE MORNING...AND NORTHERN LOWER DURING THE AFTERNOON...MAY ACT TO TRIGGER SHOWERS AND STORMS...ESPECIALLY DURING THE AFTERNOON WHEN PEAK HEATING OCCURS AND ADDITIONAL CONVERGENCE IS PRESENT VIA LAKE BREEZE. NOT QUITE AS MUCH INSTABILITY WILL BE AVAILABLE TO WORK WITH DUE TO COOLER TEMPS AND LOWER H850 DEWPOINTS...BUT MODIFYING A SOUNDING FOR NE LOWER GIVES ML CAPES BETWEEN 600-800 J/KG. 0-6KM BULK SHEAR IS ONLY 10KTS...SO AGAIN WE ARE LOOKING AT PULSY...MARGINAL HAIL THREAT STORMS. WILL CONTINUE TO ADVERTISE GREATER CHANCE OF PRECIP IN THE AFTERNOON...AND IN THE VICINITY OF NE LOWER. ANY STORMS HAVE POTENTIAL TO LINGER INTO THE EVENING BEFORE INSTABILITY DIMINISHES ONCE AGAIN. FULL BLOWN LARGE-SCALE TROUGH WILL BEGIN TO PULL AWAY OVERNIGHT AND NOT TOTALLY CONFIDENT THAT A DRY FORECAST IS THE WAY TO GO WITH THE COLD POOL VERY CLOSE BY...BUT WILL LET A DRY FORECAST RIDE AND SEE HOW IT PANS OUT. MPC MONDAY THRU WEDNESDAY...HIGH PRESSURE WILL BUILD INTO THE WRN GREAT LAKES...PROVIDING MAINLY DRY WEATHER AND NEARLY NORMAL TEMPERATURES FOR THE FIRST HALF OF NEXT WORKWEEK. A WEAK DISTURBANCE WILL SLIDE SOUTH OF OUR REGION ON WEDNESDAY...WITH JUST ENOUGH LIFT... INSTABILITY AND MOISTURE DEVELOPING ACROSS NRN LOWER MICHIGAN ALONG THE NRN EDGE OF THIS SYSTEM FOR A MINIMAL CHANCE OF SHRA/TSRA. CURRENT GRIDS ARE HANDLING THIS SCENARIO WELL. EXPECT HIGH TEMPS IN THE 70S AND LOWS IN THE UPPER 40S TO MID 50S ALL THREE DAYS. && .PREV DISCUSSION... /ISSUED 100 AM EDT SAT JUN 21 2008/ AVIATION...UPSTREAM DISTURBANCE MOVING ACROSS UPPER MI MAY SUPPORT A FEW SHOWERS NEARING TVC/PLN TOWARD MORNING. THIS ACTIVITY IS EXPECTED TO BE ISOLATED...AND NOT WORTH INCLUDING IN THE TAFS AT THIS TIME. MAIN CONCERN IS THUNDERSTORM ACTIVITY DURING PEAK HEATING THIS AFTERNOON AND EVENING. BEST COVERAGE WILL BE AWAY FROM LAKE MI...AND HAVE TEMPO-D IN SOME THUNDER AT APN AFTER 3 PM. SHORT-LIVED STRONG STORMS ARE POSSIBLE. ACTIVITY QUIETS DOWN WITH LOSS OF EVENING BY LATE EVENING...THOUGH FOG MAY DEVELOP WHERE IT RAINED EARLIER IN THE DAY. JZ && .APX WATCHES/WARNINGS/ADVISORIES... MI...NONE. LM...NONE. LH...NONE. LS...NONE. && $$