Heavy Snow in Alaska, January 17 to 18, 2009

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
219 PM AKST SAT JAN 17 2009

.SHORT TERM...MODERATE TO HEAVY RAIN CONTINUES ACROSS THE
YAKUTAT AREA THIS MORNING AS PERSISTING WARM AND MOIST AIR FROM
THE SOUTH MOVES OVER TOP OF THE HIGH AMPLITUDE RIDGE. MORNING
SATELLITE LOOP SHOWS A LONG FETCH OF ABUNDANT TROPICAL MOISTURE
PLUME ACROSS THE EASTERN AND CENTRAL GULF. TO THE WEST OF THE
RIDGE...A DEVELOPING LOW MOVES NORTHWARD INTO THE SOUTH WESTERN
GULF FROM THE NORTH PACIFIC. MODELS ARE IN FAIR AGREEMENT WITH A
HIGH PRESSURE PERSISTS OVER BRITISH COLUMBIA FOR THE NEXT COUPLE
OF DAYS AND A SERIES OF FRONTAL WAVE IN THE EASTERN GULF SLOWLY
MOVE ACROSS THE NORTHERN PANHANDLE TONIGHT AND SUNDAY. WILL USE
THE NAM SOLUTIONS TO MAINTAIN A MAXIMUM CONSISTENCY OVER THE
INHERITED GRIDS.

 COORDINATED WITH THE RFC TO EXTEND THE AREAL FLOOD ADVISORY FOR
YAKUTAT AREA THROUGH LATE SUNDAY AFTERNOON. THE 12Z NAM QPF HINTS
A BIT LESS RAINFALL OVER YAKUTAT BUT STILL HIGH ENOUGH TO CARRY
THE FLOOD ADVISORY THROUGH SUNDAY MORNING. OTHER CONCERNS INCLUDE
MONTANAN CREEK AND TAKU RIVER WHERE THE WATER LEVELS MAY REACH TO
ACTION STAGES ON SUNDAY OR MONDAY. WILL NEED TO KEEP AN EYE ON
FOR ANY POSSIBLE DEVELOPMENT AND A SPECIAL WEATHER STATEMENT HAS
BEEN ISSUED FOR NORTHERN PANHANDLE VALID THROUGH SUNDAY MORNING.
THE MOISTURE PLUME WILL CONTINUE TO MOVE NORTH BUT CONFINE THE
RAIN BAND WEST OF A LINE FROM TAKU INLET TO PORT ALEXANDER
THROUGH SUNDAY. MODERATE TO HEAVY RAIN WILL CONTINUE ACROSS THE
YAKUTAT AREA WITH OCCASIONAL MODERATE RAIN OVER THE NORTHERN
PANHANDLE. SNOW LEVEL WILL FALL SIGNIFICANTLY OVER NORTH OF
SKAGWAY TONIGHT. EXPECT SNOW ABOVE 500 FEET AND QUITE HEAVY
SNOWFALL ACCUMULATION NEAR WHITE PASS. WILL ISSUE A WINTER STORM
ADVISORY FOR ZONE 18 ABOVE 500 FEET TONIGHT THROUGH SUNDAY
MORNING.

 VISIBILITY IS NOT RESTRICTED AT THE JUNEAU AIRPORT BUT SHALLOW
LAYER OF PATCHY FOG REMAINS IN THE MENDENHALL VALLEY. UNDERNEATH
THE HIGH AMPLITUDE RIDGE...SOUTHERN HALF OF PANHANDLE WILL REMAIN
UNDER MODERATE SUBSIDENCE. ANNETTE MORNING SOUNDING SHOWS A VERY
SHALLOW MOISTURE BOUNDARY NEAR THE GROUND. EXPECT PATCHY FOG
RETURNS TO THE AREA DURING THE NIGHT AND MORNING HOURS. AS A
SERIES OF WAVES PASSING ACROSS THE EASTERN GULF AND PANHANDLE
COASTAL ZONES...BREEZY TO WINDY CONDITIONS WILL CONTINUE BUT
REMAIN BELOW THE ADVISORY LEVEL GUSTS. SOUTHEAST GALES AND ROUGH
SEAS WILL CONTINUE ACROSS THE OUTSIDE WATERS. WILL DROP THE WIND
ADVISORY FOR YAKUTAT BY LATE THIS AFTERNOON.

.LONG TERM...THE LONG TERM MODELS ARES IN FAIR AGREEMENT THROUGH
MON AFTERNOON THEN THEY DIVERGE WITH THE CHANGE IN THE UPPER
LEVEL PATTERN. SO THERE IS LOW FORECASTER CONFIDENCE FOR MOST OF
THE WEEK IN INDIVIDUAL SYSTEMS BUT THE OVERALL PATTERN IS BEING
HANDLES WELL WITH MORE NORMAL TEMPS AND PRECIP. THE EC HAS BEEN
THE MOST CONSISTENT MODELS FOR THE PAST COUPLE OF DAYS SO BLENDED
THE 12Z EC WITH A LITTLE OF THE 12Z GFS THROUGH TUE NIGHT THEN
FOR DAYS 4 THROUGH 7 USED HPC AS IT IS A GOOD COMPROMISE WITH THE
LARGE SPREADS IN THE MODELS AND ENSEMBLES.

THE SYNOPTIC PATTERN WITH THE LARGE UPPER HIGH OVER THE WEST COAST
OF NOAM SENDING SIGNIFICANT WARM MOIST AIR OVER THE GULF AND SE AK
THROUGH TUE WILL BEGIN TO WEAKEN. THIS IS CAUSED BY UPPER LOWS OVER
THE WESTERN GULF LIFTS INTO THE GULF AS STRONG UPPER LOWS DRIFTS
OUT OF THE NORTHWEST PACIFIC AND DIGS SOUTH OF THE CHAIN. AT THE
SURFACE THE BAROCLINIC ZONE OVER THE GULF WILL PRODUCE HEAVY RAIN
OVER THE NORTHERN GULF NEAR PAYA AND COASTAL AREAS OF THE
PANHANDLE THROUGH TUE. THIS FRONTAL ZONE WILL PUSH TO THE EAST
WITH WEAK SURFACE LOWS LIFTING INTO THE GULF THROUGH THE WEEK.
COLD AIR WILL BEGIN TO FUNNEL BACK INTO THE INTERIOR THROUGH THE
WEEK TO BUILD A HIGH PRESSURE. THIS WILL INCREASE THE PRESSURE
GRADIENT OVER BETWEEN THE INTERIOR AND THE PANHANDLE TO INCREASE
THE WINDS OVER THE NORTHERN INNER CHANNELS BEGINNING WED. THIS
NORTHERLY WINDS WILL ALSO BRING COLDER AIR BACK INTO THE PANHANDLE
AND INCREASE THE CHANCE OF SNOW OVER THE NORTHERN HALF OF THE
FORECAST AREAS. SO WITH THAT SAID CONTINUE TO LOWER TEMPS FROM MID
WEEK INTO THE NEXT WEEKEND.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO 12 PM AKST
     SUNDAY FOR AKZ018 ABOVE 500 FT.
         AREAL FLOOD ADVISORY UNTIL 5 PM AKST SUNDAY FOR AKZ017.
         WIND ADVISORY UNTIL 6 PM AKST THIS AFTERNOON FOR AKZ017.
MARINE...GALE WARNING FOR PKZ043-051-052.
         SMALL CRAFT ADVISORY FOR PKZ011-021-022-031-033>036-041-042.

&&

SJA/ABJ