Blizzard in Alaska, April 12, 2013

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
150 PM AKDT THU APR 11 2013

.ANALYSIS AND UPPER LEVELS...

A CLOSED UPPER LEVEL LOW RESIDES OVER NORTHWEST CANADA WITH A SHARPLY
DEFINED TROUGH AXIS EXTENDING SOUTH AND WEST FROM THE COPPER RIVER
BASIN TO THE KENAI PENINSULA AND KODIAK ISLAND. A LARGE UPPER RIDGE
ENCOMPASSES THE BERING SEA...WITH NORTHERLY TO NORTHWESTERLY FLOW
EXTENDING FROM THE CHUKCHI SEA TO THE SOUTHWEST COAST AND
SOUTHCENTRAL INTERIOR BEHIND THE TROUGH AXIS.

AT THE SURFACE...A 993 MB LOW RESIDES OVER THE FAR SOUTHERN GULF OF
ALASKA...MOVING EAST-NORTHEAST TOWARD THE SOUTHERN PANHANDLE.

.MODEL DISCUSSION...

MODELS ARE IN DECENT AGREEMENT IN THE SHORT TERM. THE LOW OVER THE
FAR SOUTHERN GULF OF ALASKA HAS AN IMPRESSIVE APPEARANCE ON SATELLITE
IMAGERY...WITH THE GFS MODEL SEVERAL MILLIBARS DEEPER THAN MOST OTHER
MODELS AND LIKELY BEST VERIFYING AT THE MOMENT. MODELS RAPIDLY
DEVELOP GALE TO PERHAPS LOW-END STORM FORCE WINDS ACROSS THE
SOUTH-CENTRAL GULF BY LATE TONIGHT BEFORE THE SYSTEM BEGINS TO WEAKEN
AND MOVE AWAY FROM THE AREA. BEHIND THE TROUGH AXIS DIVING SOUTH INTO
THE GULF THIS AFTERNOON...STRONG NORTHERLY FLOW WILL BEGIN TO
GRADUALLY INCREASE AS SURFACE HIGH PRESSURE SLIDES SOUTH BEHIND THIS
LOW AND STRENGTHENS THE PRESSURE GRADIENT OVER SOUTHERN ALASKA.

MODELS REMAIN IN FAIRLY CLOSE AGREEMENT OVERALL UNTIL THE
WEEKEND...WHERE SIGNIFICANT DIFFERENCES REMAIN IN THE DEPICTION OF A
STRONG CLOSED LOW THAT IS PROJECTED TO DIVE SOUTH AND EAST FROM THE
CHUKCHI SEA TO SOUTHCENTRAL ALASKA. THIS SYSTEM PRESENTS THE NEXT
BEST CHANCE FOR ACCUMULATING SNOW FOR THE ANCHORAGE AREA OVER THE
NEXT SEVERAL DAYS.

THE NAM WAS PREFERRED FOR MOST OF THE EAST DOMAIN...WHILE THE GFS WAS
PREFERRED OUT WEST TOWARD THE BERING SEA.

.SHORT TERM FORECAST...

SOUTHCENTRAL...

DIURNAL SNOW SHOWERS ARE EXPECTED THIS AFTERNOON AND EARLY EVENING
OVER HIGHER TERRAIN MAINLY EAST OF ANCHORAGE ALONG THE CHUGACH
RANGE...THEN NORTHERLY OUTFLOW WINDS PICK UP LATE TONIGHT AS A STRONG
PRESSURE GRADIENT DEVELOPS. THOMPSON PASS DOT STATION REPORTS 16
INCHES OF TRANSPORTABLE SNOW LEFT OVER FROM THE HEAVY SNOW EARLIER IN
THE WEEK...WITH WINDS EXPECTED TO INCREASE TO 30 TO 50 MPH WITH
HIGHER GUSTS BY LATE MORNING FRIDAY. THIS HAS NECESSITATED THE
ISSUANCE OF A BLIZZARD WATCH FOR THOMPSON PASS...WHICH WILL LIKELY BE
UPGRADED WITH THE AFTERNOON FORECAST PACKAGE AT 4 PM. WINDS WILL
EASILY MAKE IT ALL THE WAY INTO THE TOWN OF VALDEZ AND CAUSE SOME
OCCASIONAL BLOWING SNOW PROBLEMS AS WELL. OUTFLOW WINDS WILL ALSO BE
QUITE STRONG FOR WHITTIER AND SEWARD AS WELL.

BY THE WEEKEND...ATTENTION TURNS TO THE NEXT UPPER LEVEL LOW DIVING
SOUTH. THE NAM WAS THE PREFERRED MODEL ON THE EAST DOMAIN...AND SHOWS
A VERY STRONG VORTICITY MAX PROVIDING FOR STRONG DYNAMIC LIFT BY
SUNDAY AFTERNOON BUT WITH LIMITED MOISTURE. IF THIS SOLUTION
VERIFIES...WE WOULD LIKELY BE LOOKING AT A PERIOD OF LIGHT SNOW
ACROSS THE SOUTHCENTRAL REGION WITH THE POTENTIAL FOR VERY MINOR
ACCUMULATIONS. TEMPERATURES WILL REMAIN WELL BELOW SEASONAL NORMALS
BUT WILL BEGIN TO SLOWLY APPROACH NORMAL BY EARLY NEXT WEEK.

SOUTHWEST ALASKA...

CLEAR SKIES WILL ACCOMPANY OFFSHORE FLOW OVER MOST OF SOUTHWEST
ALASKA. THE EXCEPTION MAY BE ALONG KUSKOKWIM DELTA...WHERE NORTHWEST
FLOW OVER THE NORTHERN BERING WILL LIKELY PUSH SOME LOW-LEVEL
MOISTURE SOUTH AND EAST TOWARD BETHEL LATE TONIGHT. THE NEXT BEST
CHANCE FOR PRECIPITATION WILL COME IN THE FORM OF SNOW WITH THE UPPER
LOW DIVING SOUTHEAST ON SUNDAY SKIRTING THE NORTHERN BOUNDARY OF THE
REGION.

BERING SEA/ALEUTIANS...

HIGH PRESSURE WILL BE THE PREDOMINANT FEATURE FOR THE NEXT FEW DAYS.
IN THE NEAR TERM...SNOW SHOWERS BEHIND THE SHARP TROUGH TO THE EAST
WILL DOMINATE THE EASTERN BERING...INCLUDING THE AKPEN...EASTERN
ALEUTIANS...AND PRIBILOF ISLANDS...AS VERY COLD AIR POURS OFF OF THE
ICE EDGE OVER THE RELATIVELY WARMER SEA THROUGH TONIGHT. A FRONT
MOVING NORTH THROUGH THE WESTERN ALEUTIANS TONIGHT INTO FRIDAY WILL
GRAZE ATTU AND SHEMYA WITH SOME HIGH-END SMALL CRAFT ADVISORY
WINDS...WITH GALES JUST TO THE WEST OVER ADJACENT COASTAL WATERS.

.LONG TERM FORECAST...

UPPER RIDGING IS POISED TO STRENGTHEN INTO A CUT-OFF HIGH OVER FAR
NORTHEASTERN RUSSIA AND THE NORTHERN BERING BY NEXT WEEK. THE UPPER
LOW WHICH WILL DIVE SOUTH AND EAST TO SOUTHCENTRAL ALASKA BY LATE
THIS WEEKEND WILL THEN LIKELY MEANDER BACK WEST OR SOUTHWEST...LIKELY
RE-ESTABLISHING SOUTHERLY OR SOUTHWESTERLY FLOW AT LEAST OVER COASTAL
SECTIONS OF SOUTHCENTRAL...AS WELL AS LIKELY RE-INTRODUCING CLOUDS
AND A CHANCE OF SNOW AND/OR RAIN FOR THE EARLY TO MIDDLE PART OF NEXT
WEEK ESPECIALLY OVER COASTAL AREAS. TEMPERATURES SHOULD SLOWLY
APPROACH NORMAL FOR HIGHS BY THE MIDDLE OF NEXT WEEK...WITH OVERNIGHT
LOWS STILL BELOW NORMAL.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...BLIZZARD WATCH...131
MARINE...GALES...127 130 176
         HVY FRZG SPRY...121 126 127 128 129 130 138 140 150 160 165
                         179
FIRE WEATHER...NONE.

29 APR 13