Winter Storm in Alaska, March 11 to 12, 2015

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
433 PM AKDT WED MAR 11 2015

.SHORT TERM...DUE TO ADJUSTMENT IN PROCEDURES TODAY WAS ABLE TO
SPEND MOST OF THE DAY LOOKING CLOSELY AT RAIN/SNOW LINE AND ITS
EVOLUTION...AND QPF AMOUNTS...AS A STRONG LOW ENTERS THE GULF
FROM THE SOUTH JUST AFTER YESTERDAYS ESTABLISHMENT OF CONTINENTAL
AIR MASS OVER THE NORTHERN HALF OF THE PANHANDLE. COLD AIR BECAME
FIRMLY ENTRENCHED OVER THE NORTHERN HALF LAST NIGHT AND THIS HAS
IMPLICATIONS FOR SNOWFALL PERSISTING LATER FOR SOME LOCAL AREAS. UTILIZED
PRIMARILY 12Z EC AND GFS UPDATES ALONG WITH 12Z/18Z NAM DATA ALONG
WITH OBSERVED TIMING OF SNOW AND RAIN EVOLUTION IN THE CENTRAL AND
SOUTHERN PANHANDLE TODAY. WARM ADVECTION IN SOUTHERLY FLOW IS
FIGHTING WIDESPREAD NORTHEASTERLY WINDS...AND CHANNELED WINDS...ON
THE COLD SIDE AND THE LATTER ARE STRONG UP ON THE NORTHERN FRINGE.
CAME UP WITH A SCENARIO WHICH NECESSITATED A WINTER STORM WARNING FOR
HAINES/SKAGWAY ZONES FOR 6-12" LATE TONIGHT TO TOMORROW AFTERNOON
CHANGING TO RAIN AFTER MID-AFTERNOON. ALSO BEST ESTIMATE FOR A
CHANGE OF RAIN TO SNOW IN THE JUNEAU/GUSTAVUS AREA WAS AROUND
SUNRISE...THAT BRINGS THESE ZONES INTO AN ADVISORY FOR 3-6". LESS
SNOWFALL ANTICIPATED TONIGHT PRIOR TO RAIN IN ZONES 21...22 AND
24. MODEL TREND WAS TO BRING THE PRECIP NORTH QUICKER AND THAT HAS
VERIFIED WITH SNOW REACHING NORTH OF JUNEAU CURRENTLY...APPEARS
PETERSBURG HAS A COUPLE INCHES ON THE GROUND BUT 34 DEGREES
NOW...ALSO ANGOON HAS HAD BOUTS OF MODERATE SNOW OVER THE LAST
COUPLE OF HOURS. FEEL THAT THAT AREA WILL LIKELY END UP WITH 2-4".
VARIOUS MODEL SCENARIOS INCLUDED A CHANGEOVER IN THE JUNEAU AREA
FROM BETWEEN 09 AND 15Z AND THE SKAGWAY/HAINES AREA BETWEEN 15 AND
00Z. BASED ON CURRENT OBS AND SIMILAR PREVIOUS SYSTEMS THIS WINTER
WENT WITH 11-15Z FOR JUNEAU/GUSTAVUS AND 17-01Z FOR SKAGWAY/HAINES
AT SEA LEVEL...TRANSITION WILL BE RAPID TO 35-40 DEGREES AT THE
SURFACE WITHIN A FEW HOURS AT THESE LOCATIONS.

OVERRUNNING SCENARIO IS THE PRIMARY FORCING FOR INITIAL PRECIP
OVER THE NORTHERN HALF OF THE PANHANDLE TONIGHT AND QPF VALUES
PER 6 HRS WILL GRADUALLY INCREASE TONIGHT AND TOMORROW AS DYNAMICS
OF THE FRONT/LOW SYSTEM BECOME LARGE. MANY AREAS ALONG THE
NORTHERN AND EASTERN SIDES OF THE PANHANDLE GET INTO GREATER THAN
A HALF INCH IN 6 HRS BY 00Z FRI. (WELL AFTER THE CHANGEOVER TO
RAIN FOR MOST AREAS). WIND ISSUES ARE CRITICAL WITH THIS SYSTEM
BUT THESE WERE COVERED BY THE LONG TERM FORECASTER TODAY.

RISKS TO THE CURRENT HIGHLIGHTS INCLUDE A POTENTIAL EARLIER CHANGE
TO RAIN OVER BOTH THE ADVISORY AND WARNING AREAS...CURRENTLY FEEL
THAT THIS RISK IS RELATIVELY LOW BUT YOU JUST NEVER KNOW
ESPECIALLY DURING THE SPRING WHEN SUBGRID-LEVEL CONVECTIVE PROCESS
ARE INVOLVED. THEN AGAIN THE LOCAL NORTHEASTERLY COLD WINDS COULD
PERSIST LONGER THAN WE THINK...LEADING TO MORE SNOWFALL...BUT THIS
CURRENTLY APPEARS TO BE THE LONG SHOT AT THIS POINT. RECOMMEND
WITH THE EVENT ONGOING IN THE ADVISORY AREA AND SOON-TO-DEVELOP
OVER THE WARNING AREA CLOSE MONITORING OF OBSERVATIONS AND
REACTING WHERE NECESSARY.

.LONG TERM...ACTIVE WEATHER PATTERN TO CONTINUE THROUGH THE LONG
TERM PERIOD. STRONG NORTH WINDS THROUGH THE INNER CHANNELS WILL
SHIFT TO THE SOUTH THROUGH THURSDAY MORNING AND AFTERNOON. CAN
EXPECT GALE FORCE WINDS OVER THE GULF NEAR THE LOW CENTER AS WELL
AND GALE FORCE GUSTS THROUGH MANY OF THE INTERIOR PASSES. ONCE THE
FRONT/LOW MOVE INLAND TOMORROW AFTERNOON THOSE SOUTHERLY WINDS
WILL JUMP UP AS WILL THE TEMPERATURES AND ENSURE THE CHANGEOVER TO
RAIN DISCUSSED ABOVE.

ANOTHER LOW/WAVE WILL QUICKLY PASS OVER THE AREA FROM SOUTH TO
NORTH EARLY FRIDAY. THIS WILL CAUSE A PERIOD OF LIGHT AND VARIABLE
WINDS OVER THE NORTHERN HALF OF THE REGION...AND STRONG SOUTHERLY
WINDS OVER THE SOUTH. THIS FEATURE WILL ALSO INCREASE RAINFALL
AMOUNTS OVER THE SOUTHERN PANHANDLE WITH A LONG FETCH OF MOISTURE
POINTED AT POW ISLAND AND THE KETCHIKAN REGION THROUGH FRIDAY NIGHT.

MEANWHILE ONCE THIS 2ND LOW PASSES SURFACE PRESSURE BECOMES ZONAL
AND COLD AIR WILL SPREAD ACROSS THE GULF FROM THE WEST. THIS WILL
CAUSE CONVECTION OVER THE GULF TO DEVELOP. COOLING TEMPERATURES
ALOFT WILL MAKE FOR MOSTLY SNOW SHOWERS...EXPECT THESE THROUGH
SATURDAY. SOME OF THEM COULD BE ROBUST WITH ICE PELLETS AS NAM
MODEL CAPE RISES UP TO 500J/KG EARLY SATURDAY MORNING JUST OFF THE
COAST OF ELFIN COVE. BROAD HIGH PRESSURE WILL ALLOW FOR SUNNY
BREAKS SATURDAY. SUNDAY THE RIDGE WILL SHIFT TO THE EAST AND HELP
TO KEEP THE SOUTHERN PANHANDLE A LITTLE LONGER. THE NORTHERN AREAS
WILL START TO BE AFFECTED BY THE NEXT SYSTEM MOVING IN FROM THE WEST.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND FROM 10 PM AKDT THIS EVENING THROUGH THURSDAY
     EVENING FOR AKZ027.
         WINTER WEATHER ADVISORY UNTIL 7 AM AKDT THURSDAY FOR AKZ020-025.
         STRONG WIND FROM 10 PM AKDT THIS EVENING THROUGH THURSDAY
     AFTERNOON FOR AKZ022.
         STRONG WIND FROM 10 PM AKDT THIS EVENING THROUGH THURSDAY
     AFTERNOON FOR AKZ023.
         STRONG WIND UNTIL 7 PM AKDT THIS EVENING FOR AKZ028.
         STRONG WIND FROM THURSDAY MORNING THROUGH THURSDAY EVENING FOR
     AKZ028.
         STRONG WIND FROM 10 PM THIS EVENING TO 1 AM AKDT THURSDAY FOR
     AKZ019.
         WINTER STORM WARNING FROM MIDNIGHT TONIGHT TO 4 PM AKDT THURSDAY
     FOR AKZ018-019.
         STRONG WIND THROUGH THURSDAY MORNING FOR AKZ018.
MARINE...GALE WARNING FOR PKZ012-013-021-022-031>036-041>043.
         SMALL CRAFT ADVISORY FOR PKZ011-051-053.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ052.

&&

WESLEY/FERRIN

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