Storm Warning 25, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
200 PM AKDT MON SEP 21 2009

.SHORT TERM...994MB LOW CONTINUES TO SPIN JUST SOUTH OF PWS WITH
SECOND FRONTAL BAND ONSHORE ALONG THE NORTHERN GULF COAST. TWO
LIGHTNING FLASHES NOTED THIS MORNING EAST OF PAYA WITH FROPA.
SHOWERS AND MVFR CIGS ACROSS THE PANHANDLE. WINDS GENERALLY
15 MPH OR LESS ACROSS THE PANHANDLE WITH THE EXCEPTION OF PAGY
WHERE ENHANCED GRADIENT IS SLOW TO DROP.

 FOCUS OF ATTENTION FOCUSED ON 1006MB DEVELOPING LOW MOVING
NORTHEAST ALONG POLAR FRONT IN THE EASTERN GULF NEAR 52N AND
139W. PRESSURE FALLING RAPIDLY AT 46184. TRACK OF STORM CENTER
MOVES THE LOW ONSHORE OVER PASI AROUND 03Z TONIGHT WITH FRONT
APPROACHING OUTER COAST LATE THIS AFTERNOON. LOW RAPIDLY FILLS
AND WEAKENS TONIGHT AFTER LANDFALL AND CONTINUES TO MOVE
NORTHEAST AS A TROFF BY TUESDAY MORNING. LOW LEVEL RIDGE BUILDS
NORTHWARD OVER QUEEN CHARLOTTE ISLAND PROVIDING FOR TIGHT
PRESSURE GRADIENT SOUTH OF FREDERICK SOUND. BOTH 12Z GFS AND 12Z
NAM12 DEPICTING 50 TO 60 KTS AT 850MB LEVEL WITH FAVORABLE
SOUTHWEST ORIENTATION FOR ADVISORY LEVEL WINDS IN ZONES
23...26...27...28 AND 29. GALES ALSO POSTED FOR MARINE ZONES
41...42 AND FOR SOUTHERN CHATHAM STRAIT AND CLARENCE STRAIT.

 CATEGORICAL POPS FOR SOUTHERN ZONES TONIGHT AND LIKELY TO
CATEGORICAL POPS FOR CENTRAL AND NORTHERN ZONES TONIGHT. 6HR QPF
AMOUNTS RANGE FROM ONE AND A HALF INCHES ALONG THE COASTAL
MOUNTAINS IN THE SOUTHERN ZONES TO AROUND ONE QUARTER OF AN INCH
IN THE NORTHERNMOST ZONES OVERNIGHT. QUICK MOVING SYSTEM WILL
MOVE INTO THE YUKON TOMORROW AS RIDGE BUILDS INTO SOUTHERN ZONES
BRINGING BRIEF REDUCTION OF POPS INTO THE CHANCE AND LIKELY RANGE
 BEFORE THE NEXT SYSTEM MOVES INTO THE AREA ON WED.

 INHERITED GRIDS WERE UPDATED WITH A BLEND OF 12Z GFS AND 12Z
NAM12. NAM12 INITIALIZED BOTH POSITION AND STRENGTH OF EASTERN
GULF LOW BETTER THAN BOTH THE 12Z GFS AND 12Z ECMWF. ECMWF WAS
THE WEAKEST AND TRACKED THE LOW FURTHEST SOUTH AND WAS NOT USED.
THE GFS WAS VERY GOOD WITH THE LOW TRACK. THE BLEND OF 12Z GFS
AND 12Z NAM12 PROVIDED STRONG TRACK...POSITION AND TRACK.

&&

.LONG TERM...MEDIUM RANGE MODELS ARE IN GENERAL AGREEMENT WITH A
WEAK UPPER LEVEL TROUGH MOVING INTO THE EASTERN GULF TUESDAY NIGHT
INTO WEDNESDAY MORNING WHILE THE 12Z RUN GFS SHOWING A BIT FASTER
TREND WITH LESS AMPLIFIED THAN OTHER MODELS. THE NAM SHOWS A BIT
MORE AMPLIFIED. IN GENERAL...THE 12Z RUN GFS IS PRETTY CONSISTENT
WITH THIS UPPER TROUGH AND THE 00Z RUN ECMWF HAS A SIMILAR TREND
TOWARD A LITTLE LESS AMPLIFIED TROUGH DURING THE EARLIER PORTIONS
OF THE LONG TERM FORECAST PERIODS. THE INITIAL CONDITIONS SHOW THE
12Z NAM AN OUTLIER WHILE COMPARING WITH OTHER MODELS. PREFER TO
BLEND WITH THE 12Z GFS AND 00Z ECMWF INTO THE INHERITED GRIDS
THROUGHOUT THE LONG RANGE PERIODS. THE HPC GUIDANCE FOR SOUTHEAST
ALASKA ALSO TAKES A MEAN POSITION OF THE GFS AND ECMWF.

 THIS WILL RESULT THE WEAK TROUGH AND ASSOCIATED SURFACE FRONT
SHEARING OUT EASTWARD AND WEAKENING ACROSS THE LOCAL REGION
THROUGH WEDNESDAY NIGHT. FOR NOW IT LOOKS LIKE A 994 MB LOW
ENTERING THE SOUTHERN OUTSIDE WATERS EARLY WEDNESDAY AFTERNOON
THEN MOVING ONSHORE ALONG THE CENTRAL PANHANDLE COAST BY EARLY
WEDNESDAY EVENING THEN QUICKLY PROGRESSING FARTHER INLAND THROUGH
LATE WEDNESDAY EVENING THEN DISSIPATING EARLY WEDNESDAY NIGHT.
EASTERN PERIPHERY OF THE LOW WILL GENERATE SMALL CRAFT WINDS AND
ROUGH SEAS ACROSS THE SOUTHERN OUTSIDE WATERS AND OCEAN ENTRANCES.
A BIT LOWER POPS ARE EXPECTED WITH THIS TREND ACROSS THE SOUTHEAST
ALASKA AND A SHORT BREAK OF RAIN FREE FOR YAKUTAT ON WEDNESDAY AND
WEDNESDAY NIGHT.

 THE GFS AND ECMWF ARE ONCE AGAIN IN PRETTY GOOD AGREEMENT WITH A
WEAK RIDGE REBUILDING ACROSS THE EASTERN GULF AND SOUTHEAST ALASKA
ON THURSDAY MORNING AND CONTINUING THROUGH FRIDAY MORNING AS THE
NEXT LOW ACROSS THE ALEUTIANS MOVES INTO THE WESTERN GULF. THIS
RIDGE WILL BUILD RAPIDLY BEHIND THE DECAYING TROUGH AND GRANT MOST
OF THE LOCAL AREAS LOWER POPS THROUGH THURSDAY MORNING. THE NEXT
LOW SYSTEM INCLUDES THE REMAINS OF TYPHOON CHOI-WAN AND GENERATE
ANOTHER ROUND OF STRONG WINDS AND ROUGH SEAS ACROSS THE COASTAL
WATERS THROUGH SATURDAY AS WELL AS BREEZY TO WINDY CONDITIONS OVER
INLAND COASTAL AREAS FROM ELFIN COVE TO CRAIG. THE RIDGE WILL FLAT
DOWN AS THE LOW IN THE WESTERN GULF MOVES A LITTLE TO NORTHEAST
BEFORE WEAKENING OVER THE ALASKA PENINSULA THROUGH THE WEEKEND AND
INTO EARLY NEXT WEEK. EXPECT CONTINUED WET WITH A LITTLE LOWER
POPS FOR EARLY NEXT WEEK.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WIND ADVISORY UNTIL MIDNIGHT AKDT TONIGHT FOR AKZ023-026-027.
         WIND ADVISORY UNTIL 3 AM AKDT TUESDAY FOR AKZ028.
         WIND ADVISORY FROM 6 PM THIS EVENING TO MIDNIGHT AKDT TONIGHT
         FOR AKZ029.
MARINE...GALE WARNING FOR PKZ033-036-041-042.
         SMALL CRAFT ADVISORY FOR PKZ022-031-032-034-035-043-051-052.

&&

CCC/AHN