182 FXAK67 PAJK 182339 AFDAJK SOUTHEAST ALASKA FORECAST DISCUSSION NATIONAL WEATHER SERVICE JUNEAU AK 339 PM AKDT TUE JUN 18 2013 .SHORT TERM...AFTERNOON SATELLITE IMAGE SHOWS A WEAKENING TROUGH OF LOW PRESSURE ACROSS THE CENTRAL AND EASTERN GULF...SLOWLY DRIFTING WESTWARD. NUMEROUS CONVECTIVE CELLS DEVELOPING OVER BRITISH COLUMBIA SLOWLY PROPAGATE WESTWARD CROSS THE BORDER ALONG COAST MOUNTAIN RANGE...INCREASING THUNDERSTORM THREATS OVER THE SOUTHERN PANHANDLE AREAS. A CUT-OFF UPPER LEVEL LOW RESPONSIBLE FOR LAST NIGHT THUNDERSTORMS HAS MOVED OVER THE NORTHERN GULF AND WEAKENED. DAYTIME HIGHS SORE TO THE LOWER 70S AT SKAGWAY AND ANGOON. SEA BREEZES DEVELOPING MANY LOCATIONS. MODELS REMAIN FAIRLY CONSISTENT FOR THE WEAKENING TROUGH OF LOW PRESSURE ACROSS THE GULF OF ALASKA CONTINUES TO MOVE WESTWARD THROUGH WEDNESDAY MORNING BEFORE DISSIPATING BY WEDNESDAY AFTERNOON. MODELS ALSO SHOW A GOOD AGREEMENT FOR A RIDGE OF HIGH PRESSURE BUILDING OVER THE SOUTHERN GULF TONIGHT. THE RIDGE WILL CONTINUE TO EXPAND NORTHWARD ACROSS THE CENTRAL AND EASTERN GULF THROUGH WEDNESDAY. USED THE NAM TO UPDATE FORECAST PACKAGE FOR TONIGHT AND WEDNESDAY. PRESSURE GRADIENT WILL REMAIN RELATIVELY WEAK AND THUS EXPECTING NO SIGNIFICANT WINDS. AS THE RIDGE OF HIGH PRESSURE STARTS BUILDING ACROSS THE GULF...A WEAK INVERTED TROUGH OF LOW PRESSURE WILL REMAIN ALONG COAST MOUNTAIN RANGE. NO SIGNIFICANT SURFACE FEATURES BUT UPPER LEVEL LOW OVER BRITISH COLUMBIA COAST WILL CONTINUE TO PLAY A MAJOR ROLE BRINGING UPPER LEVEL VORTICITY FIELDS WITH INCREASING INSTABILITIES FOR POTENTIAL THUNDERSTORMS INTO THE SOUTHERN PANHANDLE THEN EXPAND NORTHWARD UP TO PETERSBURG THROUGH THE AFTERNOON. .LONG TERM...AS ADVERTISED, A PATTERN CHANGE WILL EMERGE BEGINNING LATE IN THE WORK WEEK, JUST IN TIME FOR THE WEEKEND AS THE EASTERLY FLOW THAT HAS WROUGHT THE WARM TEMPERATURES AND THE CONVECTIVE WEATHER SOUTHEAST ALASKA HAS BEEN EXPERIENCING OVER THE LAST FEW DAYS WILL YIELD TO MORE SOUTHERLY FLOW...ESPECIALLY BY FRIDAY. AN UPPER LOW WILL SCOOT EASTWARD INTO THE WESTERN GULF WHILE THE LOW TO THE SOUTHEAST OF THE PANHANDLE WILL MOVE EAST AS WELL. CONVECTIVE PRECIPITATION WILL BECOME MORE STRATIFORM AND TEMPERATURES WILL COOL JUST IN TIME FOR THE WEEKEND. AT THE SURFACE, A RIDGE WILL DEVELOP BEYOND THE AFOREMENTIONED WEAK TROUGH MENTIONED IN THE SHORT-TERM WEDNESDAY NIGHT INTO THURSDAY. IN ADDITION, SLIGHT WARM AIR ADVECTION FROM BC OVER SE AK THURSDAY WILL ONCE AGAIN ESTABLISH A THERMAL TROUGH OVER SE. WHILE THINK AREAS OF FOG AND LOW CLOUDS WILL BEGIN APPEARING OVER THE OFFSHORE AND NORTHERN OUTSIDE WATERS, THE THERMAL TROUGH SHOULD SHUNT ANY ENCROACHMENT ONSHORE OFF TO THE SOUTHEAST...AT LEAST THIS IS THE THINKING FOR NOW. BOTH NAM AND GFS INDICATED CAPE VALUES ARE STILL HIGH ENOUGH FOR AN ISOLATED RISK OF THUNDERSTORMS FOR THE SOUTHERN THIRD OF THE PANHANDLE, ZONE 26, 28, AND 29 FOR WEDNESDAY EVENING. AS PREVIOUS SHIFT HAS MENTIONED, THIS LIKELY IS ASSOCIATED WITH A LOBE OF POSITIVE VORTICITY MIGRATING WESTWARD ACROSS THE AREA. WHILE THE LOW MIGRATES INTO THE MIDDLE NORTHERN HALF OF THE PANHANDLE EARLY THURSDAY MORNING, NOT CONFIDENT ENOUGH TO HOLD TOGETHER CONVECTION THAT LATE IN THE NIGHT, FOR SUCH A POORLY DEFINED FEATURE. FOR THURSDAY, TEMPERATURES MAY BE SLIGHTLY WARMER, BUT CAPE VALUES NOT AS IMPRESSIVE AND THUS HAVE DECIDED NOT TO CONTINUE THUNDERSTORMS FOR NOW. WHILE MINOR SHORTWAVES WILL ROTATE AROUND A MAIN UPPER LEVEL LOW FEATURE FROM FRIDAY THROUGHOUT THE WEEKEND GIVING SOUTHEAST ALASKA CHANCES OF RAIN, EVEN MAKING RAIN LIKELY FOR FRIDAY INTO FRIDAY NIGHT, MODELS ARE INDICATING THE MAIN LOW WILL MIGRATE EASTWARD EARLY NEXT WEEK WITH MORE CHANCES OF RAIN. WHILE TEMPERATURES INDICATED NOW ARE MORE FITTING FOR SOUTHEAST CONVECTIVE FLOW, LATER SHIFTS WILL LIKELY NEED TO COOL TEMPERATURES FOR THE WEEKEND EVEN FURTHER AND ESPECIALLY FOR NEXT WEEK AS WELL, AS TREND LOOKS MORE LIKELY TO BE A MORE MARINE INFLUENCED AIR MASS. USED A BLEND OF NAM, ECMWF, AND GFS TO HANDLE THE BUILDING THERMAL TROUGH WEDNESDAY NIGHT AND THURSDAY. FOR PRECIPITATION, DECIDED EXISTING FORECAST LOOKED GOOD FOR WEDNESDAY NIGHT AND THURSDAY, BUT INSTEAD EXPANDED LIKELY POPS FOR FRIDAY AND FRIDAY NIGHT WITH SHORTWAVE MOVING NORTH BECOMING MORE PROBABLE. MADE FEW CHANGES TO LONG TERM, DID EMPLOY SOME WPC TO SMOOTH PATTERN FOR MONDAY AND TUESDAY. CONFIDENCE IN FORECAST JUST AVERAGE SINCE PATTERN CHANGES SOMETIMES ARE DIFFICULT TO NAIL. .AJK WATCHES/WARNINGS/ADVISORIES... PUBLIC...NONE. MARINE...SMALL CRAFT ADVISORY FOR PKZ012. && $$ AHN/JWA |