Small Craft Advisory 339, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
354 PM AKDT THU MAY 12 2011

.SHORT TERM...A WEAK TROUGH IS MOVING THROUGH THE NORTHERN
PANHANDLE TODAY BRINGING LIGHT RAIN AND OVERCAST SKIES TO THE
AREA. THE MAIN VORTICITY FEATURE IS LOCATED IN NORTHERN BRITISH
COLUMBIA AND MOVING NW. SO THE MOST INTENSE AREA OF LIFT IS OUT
OF OUR AREA, BUT WE ARE STILL RECEIVING LIGHT RAIN AND CLOUDS
FROM FREDERIC SOUND NORTHWARD. THIS IS ALSO BRINGING SOME HIGHER
WINDS TO LYNN CANAL, HAINES AND SKAGWAY. THE PRESSURE GRADIENT
AHEAD OF THE TROUGH IS PRODUCING 30 KT NORTHERLIES AT ELDRED ROCK
WITH BREEZY CONDITIONS AT HAINES AND SKAGWAY. THESE WINDS SHOULD
CALM DOWN THIS AFTERNOON AS THE TROUGH PASSES BY. MEANWHILE, THE
WEAKENING LOW WEST OF HAIDA GWAII (FORMERLY CHARLOTTES) IS
CONTINUING TO DRIFT TO THE SE, BUT IT IS STILL SENDING THE THREAT
OF SHOWERS TO THE SOUTHERN PANHANDLE TODAY. A WEAK RIDGE OVER THE
SOUTHERN PANHANDLE IS KEEPING THE SHOWERS FROM GETTING MUCH
FARTHER NORTH THAN THE EXTREME SOUTHERN AREAS.

FOR TONIGHT AND TOMORROW THE THREAT OF SHOWERS CONTINUES FOR THE
NORTHERN AND SOUTHERN PANHANDLE. FOR THE SOUTH THIS WILL COME FROM
CONTINUING WEAK FRONTAL BANDS ROTATING NORTH FROM THE DIEING LOW
IN THE NE PACIFIC. MEANWHILE THE NORTH WILL STILL HAVE LINGERING
SHOWERS FROM THE RETREATING TROUGH AS IT MOVES INTO THE YUKON. IN
BETWEEN, WEAK EASTERLY FLOW WILL DEVELOP DRYING OUT THE CENTRAL
PANHANDLE TONIGHT. THAT DRY SLOT WILL CONTINUE TO SPREAD TOMORROW
AS TODAY'S TROUGH MOVES INTO THE YUKON.

MODELS WERE IN RELATIVELY GOOD AGREEMENT THROUGH THE PERIOD, BUT
THERE WERE SOME MINOR DIFFERENCES THAT SHOWED UP. NAMELY WAS THE
HANDLING ON HOW LONG IT WOULD TAKE FOR THE PRECIP FROM THE TROUGH
TO DIMINISH IN THE NORTHERN PANHANDLE. NAM IS HANGING ON TO IT THE
LONGEST KEEPING LIKELY TO CATEGORICAL POPS IN THE NORTH INTO FRI
AFTERNOON. GFS, GEM, AND EC ON THE OTHER HAND ARE EJECTING IT MUCH
FASTER PARTICULARLY THE EC WHICH HAS ZERO POPS FOR MOST OF THE
NORTH BY FRI AFTERNOON. WENT WITH A MORE MIDDLE OF THE ROAD AND
STUCK WITH CHANCE TO SLIGHT CHANCE POPS FROM ICE STRAIT NORTHWARD
FRI AFTERNOON. OTHER THAN THAT USED MAINLY A NAM/EC BLEND FOR THE
FORECAST UPDATES TODAY. FORECAST CONFIDENCE IS FAIR.

.LONG TERM...CHANGES TO THE LONG RANGE FORECASTS ARE SMALL,
INCLUDING THE GEN DRY CONDITIONS AND SAT AND SUN DAYTIME HIGHS
AROUND 60F SHOULD OCCUR IN SEVERAL INLAND LOCATIONS ALONG THE
PANHANDLE. THE LARGE-SCALE DRYING MECHANISMS ARE ASSOCIATED
WITH A VERTICALLY STACKED LOW THAT IS CURRENTLY ABOUT 300 NM SW
OF PAKT, AND A 500H RIDGE CURRENTLY EXTENDING N FM THE NPAC OVER
PACD AND NNE TO OVER PABR. CONFIDENCE IS AVERAGE.

THE LOW TO THE S AND THE RIDGE ALOFT TO THE WEST ARE PART OF
A LARGE SCALE BLOCKING PATTERN. THIS BLOCKING PATTERN IS POORLY
ORGANIZED BUT SLOW TO BREAK DOWN.

THE BLOCKING PATTERN WE HAVE IS ALMOST THE SHAPE OF THE GREEK
LETTER "OMEGA" AND THEREFORE WAS GIVEN THE NAME "OMEGA BLOCK".
TRUE OMEGA BLOCKS CAN EASILY LOCK INTO PLACE FOR MANY DAYS AT A
TIME. THE IDEAL OMEGA SHAPE INCLUDES A CENTRAL RIDGE ALOFT WITH
LOWS ALOFT ON EITHER SIDE THAT "PINCH" THE RIDGE ON THE SIDES
ENOUGH TO CREATE A WEAK CLOSED HIGH IN THE RIDGE AND A COL OR
SADDLE POINT BETWEEN THE LOWS. THE EASTERN LOW OF THE OMEGA
HAS A VERTICALLY-STACKED CENTER ABOUT 300 NM SW OF PAKT.

OUR PRESENT BLOCK IS A POORLY FORMED OMEGA AND WILL NOT HOLD
UP WELL. THE MAIN PROBLEMS ARE THE COMPLEX STRUCTURE OF THE
EASTERN LOW, THAT A LOW OVER WRANGELL ISLAND WILL INTERFERE WITH
THE "WESTERN" LOW OF THE OMEGA, AND THAT THE LOW TO THE WEST AND
THE RIDGE ALOFT ALSO BEING COMPLICATED BY THE LARGE LATENT HEAT
AND WATER VAPOR CONTRIBUTION FROM A TROPICAL INJECTION TO THE
POLAR FRONT ZONE. TWO FACTORS ARE GIVING THE LONG RANGE MODELS A
DIFFICULT TASK: FIRST IS A BLOCKING MECHANISM THAT NEEDS TO HAVE
ITS BREAKDOWN PREDICTED, AND SECOND IS THE WATER VAPOR PLUME THAT
HAS ITS ORIGIN FROM TROPICAL DEPRESSIONS NEAR JAPAN.

MODEL CONSENSUS FOR MSLP AND WINDS HAS IMPROVED AND THE EC IN
PARTICULAR HAS SETTLED ON SOLUTIONS THAT VARY LESS FROM RUN TO
RUN. NOTE THE EC GUIDANCE FOR MSLP AND WINDS DOES DEPICT WINDS FOR
NRN LYNN CANAL FOR TUE. MODEL GUIDANCE FOR MAX TEMPERATURES WAS A
FEW DEGREES TOO COOL AND WAS ADJUSTED UPWARD 2-4F, ALBIET
DIFFERENTIAL HEATING NEAR SHORELINES WILL HOLD THE TEMPS DOWN FOR
SUCH LOCATIONS. THE MODEL CONSENSUS BETWEEN THE EC AND GFS HAVE
IMPROVED FOR OUR AOR. EVEN WHEN THE SOLUTIONS FOR 500H FEATURES S
OF THE PANHANDLE SPREAD OUT ABOUT 00Z TUE, THEY COME BACK INTO
LINE BY 00Z WED.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.

&&

EAL/JBT