Heavy Snow in Alaska, March 6 to 7, 2015

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
346 PM AKST FRI MAR 6 2015

.ANALYSIS AND UPPER LEVELS...
UPPER LEVEL FLOW IS PROGRESSIVE WITH A 150KT UPPER LEVEL JET
EXTENDING FROM THE WESTERN ALEUTIANS TO MAINLAND ALASKA. AN UPPER
LEVEL TROUGH AND WEAKENING SURFACE LOW OVER THE NORTHWEST BERING
SEA ARE RIDING THE JET NORTHEASTWARD TOWARD NORTHWEST ALASKA. MORE
IMPORTANTLY FOR WEATHER ACROSS THE SOUTHERN MAINLAND...A FRONTAL
SYSTEM IS PUSHING ONSHORE ALONG THE SOUTHWEST COAST. THERE IS GOOD
SUPPORT FOR AREAS OF INTENSE PRECIPITATION WITH THIS SYSTEM...WITH
GOOD UPPER LEVEL JET DYNAMICS...DEEP MOISTURE TRANSPORT...AND A
STRONG LOW LEVEL JET (WHICH WILL ENHANCE PRECIP IN UPSLOPE AREAS).
THE STRONG SOUTHERLY WINDS ARE CAUSING TEMPERATURES ALONG THE
SOUTHWEST COAST TO RISE TO NEAR OR ABOVE FREEZING...WITH A CHANGE
IN PRECIP TYPE TO RAIN IN SOME AREAS. THIS GENERAL TREND FOR
COASTAL AREAS IS EXPECTED TO CONTINUE AS THE SYSTEM HEADS
EASTWARD TO SOUTHCENTRAL TONIGHT...MEANING THE HIGHEST SNOW
ACCUMULATION WILL BE INLAND AND AT HIGHER ELEVATIONS.

LARGE SCALE COLD AIR ADVECTION IS OCCURRING ACROSS THE WESTERN
AND CENTRAL BERING SEA BEHIND THE FRONTAL SYSTEM. OF NOTE IS A
SECONDARY SURFACE TROUGH OVER THE WESTERN BERING SEA WITH EVEN
COLDER AIR BEHIND IT. SATELLITE IMAGERY SHOWS CLOUD STREETS
SETTING UP OFF THE KAMCHATKA PENINSULA (INDICATIVE OF THE STRONG
OFFSHORE WINDS AND SNOW SHOWER DEVELOPMENT AS COLDER AIR MOVES
OVER THE RELATIVELY WARMER WATER).

CONDITIONS ARE RELATIVELY CALM OVER SOUTHCENTRAL OUT AHEAD OF THE
MAIN STORM SYSTEM. A WEAKER UPPER TROUGH WHICH MOVED THROUGH LAST
NIGHT HAS NOW EXITED TO THE YUKON LEAVING BEHIND SOME LINGERING
SHOWERS...MAINLY ALONG WINDWARD SIDE OF THE MOUNTAINS.

&&

.MODEL DISCUSSION...
MODELS REMAIN IN EXCELLENT OVERALL AGREEMENT OVER THE NEXT FEW
DAYS. THE FORECAST CHALLENGE CONTINUES TO BE PRECIPITATION TYPE
ALONG AND NEAR THE COAST. THE GFS HAS BEEN BY FAR THE WARMEST
SOLUTION...BUT THAT IS FAIRLY NORMAL AND IT HAS A WELL RECOGNIZED
WARM BIAS. THE OTHER SOLUTIONS HAVE TRENDED WARMER WITH LOW LEVEL
THICKNESSES. THIS IS REFLECTED IN MOS GUIDANCE WHICH HAS TRENDED
HIGHER WITH SURFACE TEMPERATURES AND DEWPOINTS AND LOWER WITH
PROBABILITY OF SNOW FOR SOME KEY COASTAL LOCATIONS. THIS INCLUDES
DILLINGHAM...KENAI...ANCHORAGE...AND VALDEZ. STILL THINK PRECIP
WILL BE HEAVY ENOUGH IN ANCHORAGE TO DRAG SNOW LEVEL DOWN TO SEA
LEVEL OVERNIGHT...BUT IT MAY BE TOO LATE TO ACCUMULATE MUCH SNOW.
IN GENERAL HAVE TRENDED TOWARD MORE RAIN AND LESS SNOW FOR COASTAL
LOCATIONS. FORECAST CONFIDENCE REMAINS MUCH HIGHER FOR SIGNIFICANT
SNOW ACCUMULATION AS YOU HEAD INLAND AS WELL AS TO HIGHER ELEVATIONS
(INCLUDING THE ANCHORAGE HILLSIDE).

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...

THE SIGNIFICANT SNOW EVENT ADVERTISED ACROSS THE SOUTHERN
MAINLAND IS CURRENTLY UNDERWAY AS A SURFACE LOW OVER THE NORTHERN
BERING HAS A FRONT ALONG THE SOUTHWESTERN MAINLAND. THIS FRONT
MAKES IT TO THE WESTERN ALASKA RANGE AND SUSITNA VALLEY THIS
EVENING. HEAVY SNOW ACROSS THE NORTHERN SUSITNA WILL BE
ACCOMPANIED BY GUSTY WINDS AS A 40 TO 50 KT JET ACCELERATES FROM
THE COOK INLET NORTH. BLOWING SNOW IS EXPECTED TO REDUCE
VISIBILITIES LATE TONIGHT FOR THIS AREA AND ALSO ACROSS THE
NORTHERN COPPER RIVER WITH ANOTHER SPEED MAX DUE THE TIGHTENING
PRESSURE GRADIENT BETWEEN LOW PRESSURE IN THE NORTHWEST AND HIGH
PRESSURE TO THE EAST. PRECIPITATION MAY START OUT BRIEFLY AS RAIN
ACROSS AREAS OF SOUTHCENTRAL AS TEMPERATURES HOVER IN THE MID
30S...HOWEVER THE COLUMN IS EXPECTED TO COOL QUICKLY CHANGING
PRECIPITATION OVER TO SNOW. THE ONLY AREAS EXPECTED TO SEE A
CONTINUED MIX OF PRECIPITATION ARE ALONG THE GULF...INCLUDING THE
KENAI WITH ALL RAIN ACROSS KODIAK ISLAND.

WITH THE PASSING OF THE FRONT ON SATURDAY...WESTERLY WINDS
INCREASE ACROSS THE GULF THROUGH CHANNELED TERRAIN. PRECIPITATION
IS EXPECTED TO TAPER OFF ACROSS THE COPPER RIVER VALLEY BY
SATURDAY NIGHT. A MUCH COLDER AND DRIER AIR MASS FILTERS INTO THE
AREA FOR THE END OF THE WEEKEND.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THE FRONTAL SYSTEM THAT HAS BEEN PUSHING INTO SOUTHWEST ALASKA
TODAY WILL CONTINUE EAST TONIGHT WITH SNOW AND AND AREAS OF
BLOWING SNOW THE MAIN IMPACTS NORTH OF THE NORTHERN SHORE OF
BRISTOL BAY. TO THE SOUTH WARMER TEMPERATURES WILL KEEP
PRECIPITATION MIXED OR ALL RAIN. STRONG SOUTHERLY WINDS ALONG THE
WEST COAST WILL SHIFT TO SOUTHWEST TONIGHT AND MORE WESTERLY
SATURDAY MORNING BUT WILL KEEP BLOWING WELL INLAND ACROSS THE
KUSKOKWIM DELTA.

A MUCH COOLER AIR MASS WILL ARRIVE ON SATURDAY AS AN UPPER LEVEL
TROUGH SWINGS INLAND. THIS COLD UNSTABLE AIR MASS WILL KEEP
NUMEROUS TO SCATTERED SNOW SHOWERS GOING THROUGH MUCH OF THE DAY
SATURDAY THEN GRADUALLY TAPERING OFF SATURDAY NIGHT. ANOTHER UPPER
LEVEL SHORTWAVE WILL SWING THROUGH ON SUNDAY BRINGING MORE COLD
AIR ALOFT AND ANOTHER SHOT OF SNOW SHOWERS.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
A STRONG UPPER LEVEL TROUGH IN THE NORTHWESTERN BERING WILL SWING
EAST TONIGHT AND SATURDAY BRINGING STRONG WESTERLY FLOW AND A MUCH
COOLER AIR MASS TO THE BERING. FURTHER TO THE SOUTH ALONG THE
ALASKA PENINSULA AND CENTRAL AND EASTERN ALEUTIANS SURFACE WINDS
WILL SHIFT NORTHWESTERLY ON SATURDAY AS THE COLD AIR APPROACHES
WITH COLD OUTFLOW GUSTS DEVELOPING THROUGH BAYS AND PASSES. ON
SATURDAY NIGHT AND SUNDAY A PACIFIC LOW PRESSURE SYSTEM WILL
APPROACH THE WEST AND CENTRAL ALEUTIANS FROM THE SOUTH.

&&

.LONG TERM FORECAST (DAYS 4 THROUGH 7)...
SKIES OVER THE SOUTHERN MAINLAND WILL BEGIN TO CLEAR BY LATE THIS
WEEKEND AS OFFSHORE FLOW TAKES OVER. OUTFLOW WINDS ALSO DEVELOP
THROUGHOUT THE NORTH GULF COAST ON SUNDAY EVENING AS A WEAK LOW
WILL DEVELOP IN THE VICINITY OF PRINCE WILLIAM SOUND. THESE WINDS
WILL CONTINUE INTO THE MIDDLE OF NEXT WEEK AS THE LOW MOVES INTO
THE EASTERN GULF...LEAVING A TIGHT AND PERSISTENT PRESSURE
GRADIENT IN ITS WAKE.

OVER THE BERING...A NORTH PACIFIC LOW ENTERING THE WESTERN
ALEUTIANS ON SUNDAY WILL SLOWLY MOVE EASTWARD INTO THE EASTERN
ALEUTIANS...BUT RIDGING OVER THE EASTERN BERING WILL PREVENT IT
FROM TRACKING ANY FURTHER TO THE EAST. IMPACTS FROM THIS LOW
CONTINUE TO BE EXPECTED PRIMARILY OVER THE ALEUTIANS...WITH
RAIN/SNOW AND GALE FORCE WINDS. THIS SYSTEM WILL EVENTUALLY
RETREAT BACK TO THE NORTH PACIFIC ON TUESDAY AND BECOME ABSORBED
IN A COMPLEX LOW THAT IS EXPECTED TO APPROACH THE GULF LATE NEXT
WEEK. THIS COMPLEX LOW REMAINS A CHALLENGE IN THE LONG TERM
FORECAST AS THE MODELS ALL CONTINUE TO PAINT A SIGNIFICANTLY
DIFFERENT PICTURE FOR MOST OF THE STATE. THE GFS HAS CONSISTENTLY
BEEN MUCH DEEPER AND FURTHER WEST WITH THIS SYSTEM BY BRINGING A
WARM...DEEP...AND MOIST LOW TOWARDS THE KENAI PENINSULA. THE EC ON
THE OTHER HAND REMAINS NOTABLY WEAKER AND FURTHER EAST...BRING A
MAJORITY OF THE PRECIPITATION AND WARM AIR TO THE PANHANDLE WITH
RIDGING OVER THE SOUTH MAINLAND. THE LONG TERM FORECAST CONTINUES
TO USE WPC GUIDANCE HEAVILY DURING THIS PERIOD WHICH IS MORE
SIMILAR TO THE EC SOLUTION...AND KEEPS BELOW NORMAL TEMPERATURES
AND LOW CHANCES OF PRECIPITATION FOR THE SOUTH MAINLAND THROUGH
NEXT WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...HEAVY SNOW WARNING 145.
         SNOW ADVISORIES 101 111 141 152 155 161.
MARINE...GALE WARNING 119 120 130 131 139 179 180 181 185.
         HEAVY FREEZING SPRAY 180 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...DEK