Blizzard and Winter Storm in Alaska, December 10 to 12, 2013

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
557 AM AKST TUE DEC 10 2013

.SHORT TERM...SOMEWHAT QUIET ACROSS THE PANHANDLE THIS MORNING AS
WE ARE GENERALLY DEALING WITH SOME PERIODS OF LIGHT RAIN AND SNOW.
MOST AREAS SOUTH OF ICE STRAIT HAVE TEMPERATURES ABOVE FREEZING SO
MOST PLACES ARE SEEING MORE RAIN THEN SNOW. ANY ACCUMULATIONS HAVE
BEEN LESS THEN 1 INCH IF THAT OVERNIGHT. WINDS HAVE GENERALLY BEEN
LIGHT AS WELL ACROSS THE PANHANDLE WITH MOST PLACES STAYING BELOW
15 KT.

 TODAYS FORECAST IS MORE OF WHAT WE HAVE BEEN SEEING OVERNIGHT.
JUST SOME PERIODS OF LIGHT SNOW OR RAIN AS THE REMAINS OF A WEAK
TROUGH ALONG THE OUTER COAST CONTINUES TO AFFECT THE AREA WHILE A
WEAKENING RIDGE HOLDS OVER THE INNER CHANNELS. A WEAK UPPER LEVEL
VORTICITY MAX THAT IS SLOWLY MOVING NE OVER THE CENTRAL PANHANDLE
AT THE MOMENT WILL PROVIDE A LITTLE BIT OF EXTRA LIFT TO KEEP THE
RAIN AND SNOW GOING OVER THE PANHANDLE TODAY. SNOW ACCUMULATION
WHERE IT IS COLD ENOUGH TO STICK WILL BE LESS THEN 2 INCHES GIVEN
THE WEAK AND DISORGANIZED NATURE OF THE PRECIP.

 THINGS GET REALLY INTERESTING AND TRICKY TONIGHT AS A STRONG
STORM SYSTEM MOVE IN FROM THE GULF. A WARM FRONTAL FEATURE WILL BE
THE FIRST TO MOVE IN GETTING AS FAR NORTH AS ICE STRAIT BEFORE THE
COLDER AIR MASS AT THE SURFACE FORCES IT ALOFT. THIS WILL BRING IN
SOME SOUTHERLY WINDS AND WARMER TEMPERATURES TONIGHT. I THINK THE
WARM AIR ADVECTION WILL BE STRONG ENOUGH THAT TEMPERATURES FROM
GUSTAVUS AND JUNEAU SOUTHWARD WILL EITHER BE SLOWLY WARMING
THROUGH THE NIGHT OR AT LEAST STEADY JUST A FEW DEGREES ABOVE
FREEZING. HAINES AND SKAGWAY WILL BE THE ODD MEN OUT AS THEY WILL
BE STUCK IN THE NORTHERLY WINDS AND LIKELY STAY BELOW FREEZING
THROUGH TONIGHT.

 WITH THE NEAR FREEZING TEMPS ACROSS THE NORTHERN HALF OF THE
PANHANDLE TONIGHT PRECIP TYPE WITH THE WARM FRONT AND SUBSEQUENT
COLD FRONT WILL BE DIFFICULT TO DETERMINE. CURRENT THINKING IS
THAT JUNEAU, GUSTAVUS, AND ELFIN COVE SOUTHWARD WILL BE MOSTLY
RAIN TONIGHT WHILE HAINES AND SKAGWAY WILL BE SNOW. ONE
UNCERTAINTY TO THIS IS IF WINDS IN THE LOWER ATMOSPHERE WILL BE
LOW THEN THE POTENTIAL FOR SNOW MELT COOLING AND THUS SNOW MAKING
IT TO THE SURFACE IS A POSSIBILITY AROUND THE ICE STRAIT CORRIDOR
GIVEN THE HIGHER PRECIP RATES THAT ARE FORECAST TO MAKE IT INTO
THE REGION LATER TONIGHT. ANOTHER UNCERTAINTY IS A COLD UPPER
TROUGH THAT WILL BE DROPPING SOUTHWARD TONIGHT. THIS WILL KEEP
HAINES AND SKAGWAY COLD BUT THE QUESTION IS HOW FAR SOUTH WILL IT
GET TONIGHT. RIGHT NOW IT LOOKS LIKE THE CONVERGENCE BETWEEN THE
WARM SOUTHERLY FLOW AND THE COLD NORTHERLY FLOW WILL BE SOMEWHERE
NORTH OF GUSTAVUS AND JUNEAU AND SOUTH OF HAINES TONIGHT. EXPECTED
SNOW ACCUMULATIONS IN HAINES AND SKAGWAY WILL BE BELOW 3 INCHES IN
A GIVEN 12 HOUR PERIOD THROUGH TONIGHT AS THE HIGHER PRECIP RATES
WITH THIS SYSTEM WILL NOT HAVE MADE IT TO THAT AREA QUITE YET BY
LATE TONIGHT.

 THE OTHER ASPECT OF THIS SYSTEM IS THE WIND. GALE FORCE WINDS ARE
EXPECTED IN THE SE GULF TONIGHT AS THE FRONT INITIALLY APPROACHES.
THE INNER CHANNELS MEANWHILE WILL SEE SMALL CRAFT ADVISORIES
TONIGHT DURING FRONTAL PASSAGE. SOME STRONG WIND HEADLINES HAVE
ALSO BEEN HOISTED FOR ZONES 27 AND 28 AS SOUTHEAST WINDS SHOULD BE
JUST STRONG ENOUGH TO WARRANT IT IN THE WARM SECTOR OF THE STORM.
WIND DIRECTION WILL BE THE HARD PART. LYNN CANAL AND GLACIER BAY
SHOULD REMAIN N WHILE FREDERIC SOUND SOUTHWARD WILL BE FROM THE S
AND E. STEPHENS PASSAGE AND NORTHERN CHATHAM ARE THE TRICKY ONES.
THE PRESSURE GRADIENT IS SUGGESTING A N WIND WHILE GEOSTROPHIC
FLOW IS PUSHING FOR A SOUTHERLY WIND. IN THIS CASE I THINK THE
GEOSTROPHIC FLOW WILL WIN OUT ESPECIALLY TONIGHT WHEN THE WARM AIR
ADVECTION KICKS INTO HIGH GEAR BEHIND THE WARM FRONT.

 SHORT RANGE MODELS WERE GENERALLY IN GOOD AGREEMENT BUT THE DEVIL
IS IN THE DETAILS AS THE MODELS WERE AT ODDS ON WHERE TO PLACE THE
RAIN/SNOW LINE. GFS WAS THE WARMEST PUSHING THE 1290 1000 TO 850
MB THICKNESS LINE AS FAR NORTH AS HAINES BY LATE TONIGHT. ECMWF
AND NAM ARE COLDER, ONLY GOING AS FAR NORTH AS BERNERS BAY. MODEL
SOUNDINGS WERE ALSO AT ODDS WITH EACH OTHER WITH THE NAM KEEPING
JUNEAU, GUSTAVUS, AND HOONAH ALL SNOW TONIGHT WHILE THE GFS IS ALL
RAIN FOR THESE LOCATIONS. NAM IS ALSO A BIT STRONGER WITH THE COLD
UPPER TROUGH THAT IS COMING SOUTH THROUGH THE YUKON TONIGHT. ENDED
UP GOING MORE TOWARD THE NAM AND GFS FOR WINDS AND POPS WHILE
TRIED TO GET A MIDDLE GROUND ON TEMPS BUT TRENDED TOWARD THE
WARMER SCENARIO FOR THE ICE STRAIT CORRIDOR SOUTHWARD. FORECAST
CONFIDENCE IS ABOVE AVERAGE FOR TODAY THEN BECOMES ABOUT AVERAGE
FOR TONIGHT ESPECIALLY IN THE NORTHERN PANHANDLE IN REGARDS TO
PRECIP TYPE.

 &&

 .LONG TERM...GALE FORCE LOW IN THE SOUTH CENTRAL GULF WILL
CONTINUE TO TRACK EASTWARD THROUGH THE THURSDAY WHILE WEAKENING. A
TRIPLE POINT SURFACE FEATURE ALONG THE ASSOCIATED FRONT FORMS IN
THE NORTHERN GULF THEN RETROGRADES TOWARD PRINCE WILLIAM SOUND.
SURFACE HIGH PRESSURE OVER THE YUKON UNDER UPPER LEVEL TROUGH
REMAINS IN PLACE THROUGH THE WEEKEND. A 975 MB LOW FORMS SOUTH OF
THE ALEUTIANS THURSDAY THEN TRACKS NORTHEASTWARD TOWARDS PRINCE
WILLIAM SOUND BY THE WEEKEND. THE OCCLUDED WEATHER FRONT
ASSOCIATED WITH THIS LOW CROSSES THE AK GULF FRIDAY THEN MOVES
OVER THE PANHANDLE SATURDAY INTO SUNDAY WHILE THE PARENT LOW
WEAKENS OVER THE GULF.

 COMPLEX SPLIT FLOW PATTERN WITH CONFLUENCE REGION BETWEEN
NORTHERN AND SOUTHERN STREAMS PRODUCING REGION OF PRECIP BANDING
OVER THE CENTRAL AND NORTHERN PANHANDLE. STILL SOME UNCERTAINTY
WITH SNOW AMOUNTS DUE TO THE POSITION OF THE ARCTIC FRONT AND
PRECIPITABLE WATER FROM THE ADVANCING FRONT. THE FRONT AND GULF
LOW IN SOUTHERN FLOW WILL BRING IN AMPLE MOISTURE WITH TROPICAL
CONNECTION TO THE PANHANDLE. HIGHEST QPF VALUES ARE OVER THE
SOUTHERN PANHANDLE. WITH WARMER TEMPERATURES IN PLACE FOR
LOCATIONS SOUTH OF FREDERICK SOUND PRECIP SHOULD FALL ALL AS RAIN.
STILL SOME QUESTIONS HOW FAR NORTH THESE QPF VALUES MOVE WHICH
WILL DETERMINE SNOW ACCUMULATIONS...ESPECIALLY OVER THE NORTHERN
PANHANDLE HIGHWAYS. MEANWHILE THE TROUGH PUSHING DOWN FROM THE AK
INTERIOR AND YUKON WILL MOVE COLD ARCTIC AIR OVER THE REGION. THE
RESULTING RAIN-SNOW LINE AS OF NOW SHOULD BE SOMEWHERE ALONG A
CORRIDOR FROM GUSTAVUS TO JUNEAU WEDNESDAY INTO WEDNESDAY NIGHT.
RAIN OVER DOWNTOWN JUNEAU WITH HEAVY SNOW NORTH OF THE MENDENHALL
VALLEY WILL BE POSSIBLE DEPENDING ON THE POSITION OF THIS FRONT.
FACTORS STILL IN QUESTION FOR THIS EVENT ARE A WEAK TROUGH AHEAD
OF THE FRONT WITH MID LEVEL FLOW DIRECTION POSSIBLY KEEPING DOWN
PRECIP AMOUNTS. IF PRECIP RATES ARE HEAVY ALONG THE RAIN SNOW LINE
THERE MAY DEVELOP AND ISOTHERMAL LAYER DUE TO MELTING SNOW COOLING
AT LOWEST LEVELS ALLOWING PRECIP TO REMAIN AS SNOW. INDICATIONS
THAT COLD AIR WILL CONTINUE TO PUSH FURTHER SOUTH THROUGH THE END
OF THE WEEK BUT WITH DIMINISHED POP AND QPF SNOW TOTALS WILL BE
LOWER. AS OF NOW LOOKING AT POSSIBLE 24 HOUR SNOW TOTAL FOR ZONES
18 AND 19 IN THE 8 TO 15 INCH RANGE...HEAVIEST ALONG THE BORDER.
POSSIBLE SNOW AMOUNTS IN THE 1 TO 6 INCH RANGE FOR ZONE 20 AND 25
WILL BE HIGHLY DEPENDENT ON THE RAIN-SNOW LINE LOCATION.

 GALE FORCE WINDS OVER THE GULF AND STRONG WINDS OVER THE
PANHANDLE WILL SUBSIDE THROUGH WEDNESDAY INTO WEDNESDAY EVENING
WITH THE PASSING FRONT. TIGHT NORTHERLY PRESSURE GRADIENT BETWEEN
THE YUKON HIGH AND GULF LOW WILL KEEP STRONGER WINDS OVER THE
NORTH PANHANDLE THROUGH THURSDAY.

 FOR THE WEEKEND THE NEXT LOW ADVANCING FROM THE WEST WILL AGAIN
INCREASE WINDS OVER THE GULF AND PANHANDLE. WITH TROPICAL
CONNECTION HEAVY PRECIP EVENT IS LIKELY. WHILE COLDER AIR WILL
HAVE PUSHED FURTHER SOUTH FOR OVERRUNNING EVENT SET UP THE COLD
AIR MASS DOES NOT LOOK TO BE A WELL ESTABLISHED PLUS THE LOW LOOKS
TO TRACK FURTHER NORTH WITH WARMER AIR MASS MOVING NEAR ICY
STRAIT.

 MODELS WERE IN FAIRLY GOOD AGREEMENT FOR SYNOPTIC PATTERN FOR
BOTH THE MID WEEK AND WEEKEND LOW PRESSURE SYSTEMS. LIKED 12Z
ECMWF/GFS THROUGH DAY 4 THEN KEPT WITH WPC THROUGH DAY 8. OVERALL
CHANGES TO FORECAST GRIDS WERE MINOR. SOME SLIGHT VARIATION IN
POSITION OF THE LOW CENTER AND STRENGTH STILL BUT NEW RUNS ARE
FALLING MORE IN LINE. MAIN DIFFERENCES AMONG MODELS...WHICH ARE
THE MAIN CONCERNS FOR THE WEDNESDAY SNOW EVENT...ARE EXTENT OF
COLD AIR MOVING SOUTH AND QPF. NAM WAS COLDEST AMOUNT THE MODELS
PUSHING THE ARCTIC FRONT AS FAR SOUTH AS FREDERICK SOUND. GFS
LOOKED TO KEEP THERMAL GRADIENT BIT FURTHER NORTH KEEPING JUNEAU
AS RAIN LONGER. ECMWF WAS BETWEEN THE THERE TWO. WHILE THE NAM WAS
THE COLDEST IT IS BIT LOWER FOR QPF AND SNOW TOTALS ESPECIALLY
ALONG THE HIGHER ELEVATION NORTHERN HIGHWAYS. IN ADDITION HIGHEST
QPF VALUES SEEM TO FALL WHEN TEMPERATURES ARE STILL WARM ENOUGH
FOR MOSTLY RAIN TO FALL. FOR NOW KEEPING THE WINTER STORM WATCH IN
PLACE AS THERE IS ENOUGH UNCERTAINTY IN EXACT SNOW FALL AMOUNTS.
FORECAST CONFIDENCE IS ABOVE AVERAGE FOR SYNOPTIC PATTER BUT STILL
BELOW AVERAGE WITH MESOSCALE DETAILS.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND FROM LATE TONIGHT THROUGH WEDNESDAY MORNING FOR
     AKZ027-028.
         WINTER STORM WATCH FROM LATE TONIGHT THROUGH LATE WEDNESDAY
     NIGHT FOR AKZ018-019.
MARINE...GALE WARNING FOR PKZ041-043.
         SMALL CRAFT ADVISORY FOR PKZ012-013-021-022-031-033>036-042-051-
     052.

&&

EAL/PRB