Issued by NWS Northern Indiana
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000 FXUS63 KIWX 090559 AFDIWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE NORTHERN INDIANA 158 AM EDT SAT MAY 9 2009 .AVIATION... VFR CONDS WILL HOLD THROUGH THE PD. MAIN CONCERN LIES W/STG GRADIENT WIND POTENTIAL AS INTENSE SW TROUGH OVR NE IA ACCELERATES EWD THIS MORNING AND DEEPENING DRY SLOT ADVTS EWD ACRS NRN HALF OF CWA IN CONJUNCTION W/STRONGEST ISOLLABARIC MOMENTUM PUSH. THAT SAID...BOTH TERMINALS WILL SEE A SVRL HR PD OF STG PK SFC GUSTS UP ~ 40 KTS FM MID MORNING INTO EARLY AFTN AIDED BY INSOLATION WITHIN DRY SLOT. OTHERWISE FNTL SURGE AND RENEWED DEEP CONVN DVLPG ONCE AGAIN THROUGH THE MID MS/TN VALLEYS SUGGEST SUGGEST FNTL SCALE CIRC WILL FALL APART COMPLETELY BY SUNRISE W/LTL SUSTAINED FOCUS FOR THUNDER AND HAVE DROPPED ALL MENTION. && .SHORT TERM.../TONIGHT THROUGH SATURDAY NIGHT/... ANOTHER INTERESTING SHORT TERM WITH MAIN CONCERN BEING STRONG WINDS ON SATURDAY AFTER A FRONTAL PASSAGE TONIGHT. STRONG WESTERLY FLOW ALOFT WILL RAPIDLY PROGRESS AN H5 SHORTWAVE AND ATTENDANT SFC LOW THROUGH SOUTHERN MICHIGAN AND ASSOC SFC FRONT WILL BARREL THROUGH THE CWA TONIGHT. THIS WAVE CURRENTLY LOCATED OVER NW IOWA BEGINNING TO GENERATE WIDESPREAD CONVECTION WHICH THE MODELS HAVE BEEN CONSISTENTLY STEERING JUST TO OUR NORTH. CLOUD SHIELD TODAY COMING OFF LARGE COMPLEX CENTERED OVER SOUTHERN ILLINOIS HAS CURTAILED INSTABILITY OVER THE CWA AND CLEARING THAT IS SLOWLY MOVING IN FROM THE WEST WILL HAVE LITTLE TIME TO ACT BEFORE INSOLATION WANES THIS EVENING. TO ADD TO THIS...BEST DYNAMICS REMAIN NORTH AROUND COMPACT DEEPENING LOW TONIGHT AND SATURDAY. MINOR TWEAKS MADE TO POP GRIDS WITH SCHC THIS EVENING ONCE LIGHT RAINS CLEAR THE SOUTHERN COUNTIES..AND CHC POPS AS FRONT PASSES OVERNIGHT. PRESSURE GRADIENT TIGHTENS RAPIDLY AS LOW DEEPENS AND MOVES EAST. MAGNITUDE OF THE WIND THREAT HEAVILY DEPENDENT ON MODEL OF CHOICE WITH THE NAM BEING THE STRONGEST AND THE 12C ECMWF THE WEAKEST. THE NAM APPEARS OVERDONE...AND PREFER TO SIDE WITH THE MIDDLE GROUND GFS AT THIS TIME WHICH PRODUCES A VERY WINDY AFTERNOON. 925 WINDS 35-40KTS SHOULD TRANSFER TO THE GROUND VERY WELL BY LATE MORNING WITH STRONG COLD ADVECTION ONGOING...PRODUCING SFC GUSTS TO NEAR 35 KNOTS. GIVEN THE SPEED OF THIS FEATURE...EASTERN PORTIONS OF THE CWA WILL BE PREFERRED AS BETTER MIXING LAGS STRONGEST WINDS ALOFT IN WESTERN ZONES. WITH GUSTS CURRENTLY EXPECTED TO FALL JUST SHORT OF THE 40 KNOT THRESHOLD...HAVE NOT ISSUED AN ADVISORY AT THIS TIME. WITH STRONG CAA ON SATURDAY TEMPS FAIL TO RECOVER MUCH BY LATE AFTERNOON LEAVING HIGHS ON AVERAGE NEAR THE 60 DEGREE MARK. AS SFC RIDGE BUILDS WITH DRIER AIR FOR SATURDAY NIGHT A GENEROUS TEMP DROP SHOULD BE REALIZED. GUIDANCE CURRENTLY DROPS TEMPS INTO THE UPPER 30S IN NORTHERN ZONES...AND PATCHY FROST WORDING HAS BEEN SPREAD TO COVER ALL OF THE MICHIGAN COUNTIES FOR SUNDAY MORNING. && .LONG TERM.../SUNDAY THROUGH FRIDAY/... A PRONOUNCED REX BLOCK WILL FORM OVER THE CENTRAL PACIFIC THIS WEEKEND ALLOWING TROUGHING TO STRENGTHEN OVER THE GULF OF ALASKA...WITH THIS TROUGH MOVING EAST NEAR THE US/CANADIAN BOARDER THROUGH THE EXTENDED PORTION OF THE FORECAST. COOL NORTHWEST FLOW WILL DOMINATE THE SUNDAY-MONDAY TIME FRAME WHILE RISING HEIGHTS AHEAD OF THE TROUGH WILL BRING A DRY/MODERATING TREND BY TUESDAY. THE ARRIVAL OF THE TROUGH ITSELF BRINGS OUR NEXT CHANCES OF RAIN BY MIDWEEK. A COLD FRONT WILL ACCOMPANY THE TROUGH LATE IN THE WEEK WITH THE POTENTIAL FOR COOLER AND DRIER CONDITIONS TO RETURN BY FRIDAY. THE GFS/ECMWF ARE IN REASONABLE AGREEMENT WITH THE OVERALL EVOLUTION OF THE PATTERN...WITH THEIR BIGGEST DIFFERENCES BEING WITH 1-TEMPERATURE OF THE COOL AIRMASS FOR SUNDAY-MONDAY AND 2-THEIR TIMING OF SHORTWAVE ARRIVAL TUESDAY NIGHT-WEDNESDAY. HAVE USED A COMPROMISE THAT LEANS A LITTLE TOWARDS THE ECMWF IN LINE WITH HPC/S THINKING...AND ALSO DOESN/T STRAY THAT MUCH FROM THE PREVIOUS FORECAST. FOR THE SUNDAY-MONDAY TIMEFRAME WE/LL BE UNDER COOL NORTHWEST FLOW ALOFT WITH T850 BETWEEN +2 AND +4C. THE GFS CONTINUES TO BE COOLER THAN THE ECMWF/NAM AND A COMPROMISE SUGGESTS HIGHS IN THE LOWER 60S..AROUND 5F BELOW SEASONAL NORMS. WITH PRETTY COLD TEMPS ALOFT...DO EXPECT QUITE A FEW CU TO DEVELOP AND WOULDN`T BE COMPLETELY SURPRISED IF THERE WAS A SPRINKLE/LIGHT SHOWER. HAVE UPPED AFTERNOON CLOUD COVER BOTH SUNDAY/MONDAY BUT HAVE DECIDED TO HOLD OFF ON ADDING ANY POPS...GIVEN VERY WEAK FORCING ALOFT AND A PRETTY DRY LLEVEL AIRMASS. WE/LL ALSO HAVE TO WATCH FOR FROST AGAIN SUNDAY NIGHT GIVEN THE COOL/DRY AIRMASS IN PLACE ALONG WITH LIGHT FLOW. HAVE PENCILED IN SOME PATCHY FROST OVER MY MICHIGAN COUNTIES...AS WELL AS FAR NORTHWESTERN OHIO. H85 RIDGE CRESTS ON TUESDAY...WITH MODERATING TEMPS ALOFT AND SUBSEQUENTLY LESS CLOUD COVER. T850 SHOULD REBOUND ABOVE +5C ALLOWING HIGHS BACK INTO THE UPPER 60S. SHORTWAVE AND IT/S ATTENDANT COLD FRONT ARRIVE WEDNESDAY/WEDNESDAY NIGHT. ECMWF DEVELOPS QUITE A BIT MORE IN THE WAY OF PRECIP AS GULF MOISTURE SURGES BACK INTO THE REGION AHEAD OF THE WAVE. AFTER THIS...THE 12Z GFS/ECMWF ARE IN PRETTY GOOD AGREEMENT WITH FRONTAL PASSAGE...ALTHOUGH THE ECMWF BUILDS MORE INSTABILITY AHEAD OF THE COLD FRONT. WILL BUMP POPS UP...BUT MAINTAIN THEM IN THE CHC CATEGORY FOR THE WEDNESDAY-THURSDAY PERIOD. WILL ALSO KEEP THUNDER MENTION GIVEN STRENGTH OF THE UPPER TROUGH...AND AN OPEN GULF OF MEXICO AS IT ARRIVES. AT THIS POINT SEVERE POTENTIAL BEARS WATCHING GIVEN WHAT LOOKS TO BE A DECENT FRONTAL TIMING WEDNESDAY AFTERNOON. HIGHS A LITTLE ABOVE 70 LOOK GOOD GIVEN T850 AROUND +8 TO +10C BUT LIKELY SOMEWHAT LIMITED SUNSHINE. THINGS QUIET DOWN AS WE RETURN TO DRIER NORTHWEST FLOW ON FRIDAY BEHIND THE DEPARTING TROUGH. TEMPS LOOK SEASONAL WITH HIGHS IN THE UPPER 60S AS T850 FALLS BACK TO AROUND +6C. && .IWX WATCHES/WARNINGS/ADVISORIES... IN...NONE. MI...NONE. OH...NONE. LM...GALE WARNING FROM 6 AM TO 5 PM EDT SATURDAY FOR LMZ043-046. && $$ SHORT TERM...LUD LONG TERM...ARNOTT AVIATION...HOLSTEN
