Winter Storm in Alaska, March 12, 2013

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
244 PM AKDT MON MAR 11 2013

.SHORT TERM...THE RAIN AND SNOW SHOWERS WILL TRANSITION TO DRIER
AND COLDER CONDITIONS FROM THE NORTH TONIGHT AND TUESDAY. AN
ARCTIC OUTBREAK IS FORMING IN NW CANADA. SMALL ADJUSTMENTS HAVE
BEEN MADE BUT NONE OF THE CHANGES TO THE FORECASTS THROUGH TUESDAY
ARE LARGE. PLEASE NOTE THE WINTER WEATHER ADVISORY FOR HAINES ZONE
IN THE TABLE BELOW. CONFIDENCE IS AVERAGE.

 1014 MB LOW 200 NM S OF CORDOVA IS DRIFTING SE AT ABOUT 8 KT AND
SLOWLY WEAKENING. GOES IR REVEALS SEVERAL CONVECTIVE BANDS, AND
THE ONSHORE TRACK OF THE CELLS WITHIN THESE BANDS WILL WEAKEN ON
THE NORTH GULF COAST WHILE STRENGTHENING ON THE EAST GULF COAST AS
THE LOW CONTINUES THE SE TRACK.

 ANOTHER KIND OF BANDING IS OCCURRING WITH THE SHOWERS:
PRECIPITATION TYPE. THE NORTHERN ZONES WITH UNMIXED SNOW, CENTRAL
SOUTHEAST AND MOST OF THE SOUTHERN PANHANDLE WILL HAVE AN ECLECTIC
PATTERN OF RAIN AND SNOW SHOWERS, AND CLOSE TO DIXON ENTRANCE AS
WELL AS MOST OF PRINCE OF WALES ISLAND WILL BE PREDOMINANTLY RAIN
SHOWERS WITH SNOW MIXED IN AT TIMES.

 MORE ON THE ARCTIC OUTBREAK IS FORMING IN NW CANADA. AN ARCTIC
HIGH PRESSURE DOME WITH A CENTER OF 1040 MB IS ABUT 100 MI NORTH
OF DAWSON YUKON TERRITORY AT THIS TIME. THIS HIGH PRESSURE DOME IS
SPREADING SOUTHWARD. A 510 DAM 1000-500H THICKNESS IS SHOWN ON THE
MODELS AS JUST SOUTH OF DAWSON YUKON TERRITORY. BY 18Z TUESDAY
THIS LOWER THICKNESS WILL HAVE SPREAD TO NORTHERN PANHANDLE.

 THE LOW TRACKING SE THROUGH THE GULF WILL ENHANCE THE OUTFLOW
ASSOCIATED WITH THE ARCTIC HIGH DEVELOPING IN THE YUKON. THE
DRYING CONDITIONS AND LOWERING TEMPERATURES, GIVEN IN THE
FORECASTS, ARE ASSOCIATED WITH THE ARCTIC OUTFLOW MOVING SOUTHWARD
ALONG THE PANHANDLE TONIGHT AND TUESDAY.

 MODELS: THE NAM WAS EMPHASIZED FOR WINDS AND TONIGHT AND TUESDAY,
ALBEIT SEVERAL OTHER MODELS WERE USED TO A LESSER DEGREE. THE NAM
THERMAL PROFILE FOR CENTRAL SOUTHEAST WAS CHECKED AGAINST AN
AIRLINER TEMPERATURE SOUNDING FOR 18Z, AND PROVIDED ADDITIONAL
VERIFICATION THAT THE MODEL IS OFF TO A GOOD START.

 &&

 .LONG TERM...LOW PRESSURE SYSTEM IN THE AK GULF WILL HAVE
WEAKENED AND TRACKED OVER THE SOUTHERN PANHANDLE TUESDAY NIGHT
BECOMING AN OPEN WAVE INVERTED TROUGH WHICH WILL LINGER THROUGH
THURSDAY. ARCTIC HIGH PRESSURE WILL BUILD OVER THE YUKON AND LOOKS
TO LAST THROUGH THE END OF THE WEEK. BY FRIDAY A LOW PRESSURE
SYSTEM WILL MOVE UP FROM THE EASTERN PACIFIC MOVING IN TO THE
SOUTHEAST GULF AND THEN MOVING ALONG A NORTHWESTERLY TRACK. A
SECOND LOW MAY APPROACH THE CENTRAL AK FROM THE ALEUTIANS ON AN
EASTERLY TRACK MONDAY.

 AS THE CURRENT LOW MOVES TO THE SOUTH PRECIP WILL DIMINISH FROM
NORTH TO SOUTH WITH CLEARING SKIES. THE TROUGH OVER THE SOUTHERN
PANHANDLE WILL KEEP CHANCE POPS THERE FOR MOST OF THE WEEK. THE
DEVELOPING LOW IN THE SE GULF BY FRIDAY WILL BRING ANOTHER ROUND
OF PRECIP MOVING FROM THE SOUTH THE THE NORTH. BY THE START OF
NEXT WEEK THE INCOMING LOW FROM THE ALEUTIANS WILL AGAIN BRING
CHANCE OF PRECIP TO THE REGION.

 INCREASING PRESSURE GRADIENT FROM THE ARCTIC HIGH AND THE
SOUTHERN TROUGH WILL BRING NORTHERLY WINDS UP TO AT LEAST GALE
FORCE THROUGH FRIDAY FOR THE NORTHERN INNER CHANNELS. WITH COLDER
AIR HAVE KEPT THE MENTION OF FREEZING SPRAY. OUTFLOW WINDS FOR
YAKUTAT, HAINES NEAR LYNN CANAL, SKAGWAY AND DOWNTOWN JUNEAU WITH
GUSTS TO 40 MPH OR MORE WILL DEVELOP AS THE ARCTIC FRONT MOVES
SOUTH. WHILE THERE WILL BE STRONG OUTFLOW WINDS FOR DOWNTOWN
JUNEAU FOR NOW NOT EXPECTING A TAKU EVENT AS THE CRITICAL LEVEL IS
NOT OPTIMAL.

 THE NORTHERLY FLOW WILL POUR THE CONTINENTAL POLAR AIR MASS FROM
THE YUKON AND BRITISH COLUMBIA INTO THE PANHANDLE. CONTINUED THE
TREND OF PREVIOUS FORECAST AND LOWERED TEMPERATURES A FEW MORE
DEGREES. MAY STILL NEED TO DROP THE CENTRAL AND NORTHERN REGIONS
DOWN A FEW MORE DEGREES DUE TO THE ARCTIC AIR MASS AND RADIATIONAL
COOLING FROM CLEAR SKIES BUT THE INVERTED TROUGH AND CLOUD COVER
OVER THE SOUTHERN PANHANDLE MAY LIMIT COOLING THERE AS INDICATED
BY MODELS SUCH AS THE SREF. IF THE COLD AIR REMAINS IN PLACE FOR
THE CENTRAL AND NORTHERN PANHANDLE AS THE NEXT SYSTEM MOVES INTO
THE SE GULF COULD SEE AN OVER RUNNING SNOW EVENT WITH THE PRECIP
SPREADING NORTH. BUT FOR NOW NOT MUCH CONFIDENCE IN THIS. BY THE
WEEKEND AS THE NORTHERN GRADIENT DECREASES AND MID LEVEL FLOW
SHIFT TO THE SOUTH EXPECT TEMPERATURES TO WARM AND RETURN TO
SEASONAL NORMALS.

 MODELS HAVE BEEN SHOWING RATHER LARGE RUN TO RUN INCONSISTENCIES
LATELY. IN ADDITION MODEL SPREAD WITH THE THURSDAY INTO FRIDAY
SYSTEM INCREASES. THE NAM/GEM GLOBAL WERE SHOWING BEST AGREEMENT
WITH THE ECMWF CLOSER THAN THE GFS BUT STILL BECOMES OUT OF PHASE
BY DAY 4. GFS WAS NOT HANDLING THE PRESSURE GRADIENT OVER THE
NORTHERN PANHANDLE AS WELL AS THE OTHER MODELS. USED A 60/20/20
BLEND INHERITED/NAM/GEMGLB THROUGH DAY 4. USED NEW WPC (FORMERLY
HPC) GUIDANCE FOR THE REMAINDER OF THE FORECAST, WHICH WAS MOSTLY
A BLEND OF ECMWF AND ENSEMBLE ECMWF. CONFIDENCE THROUGH DAY 3 IS
ABOVE AVERAGE BUT BELOW AVERAGE FOR THE REMAINDER OF THE FORECAST
DUE TO INCREASED MODEL SPREAD AND RUN TO RUN INCONSISTENCIES.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER WEATHER ADVISORY UNTIL 10 PM AKDT THIS EVENING FOR AKZ019.
         STRONG WIND FROM TUESDAY MORNING THROUGH TUESDAY EVENING FOR
     AKZ018.
MARINE...GALE WARNING FOR PKZ011>013-051-053.
         SMALL CRAFT ADVISORY FOR PKZ021-022-031-032-043-052.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ041-042.

&&

JBT/PRB