Small Craft Advisory 152, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
350 PM AKST TUE FEB 17 2015

.ANALYSIS AND UPPER LEVELS...SOUTH TO SOUTHWESTERLY UPPER LEVEL
FLOW CONTINUES ACROSS THE MAINLAND OF ALASKA WITH EMBEDDED
WEAK DISTURBANCES HELPING TO INITIATE SHOWER ACTIVITY...MOST
NUMEROUS OVER THE SOUTHWEST MAINLAND. WINDS CONTINUE TO DIMINISH
OVER SOUTHCENTRAL AS SURFACE HIGH PRESSURE BUILDS OVER THE
NORTHERN GULF AND PRINCE WILLIAM SOUND. UPPER LEVEL HIGH PRESSURE
REMAINS OVER WESTERN NORTH AMERICA WITH A CLOSED UPPER LEVEL LOW
MOVING NORTH ALONG THE WESTERN COAST OF THE MAINLAND. BEHIND THE
RETREATING LOW...A NEW RIDGE OF HIGH PRESSURE ALOFT IS RAPIDLY
BUILDING NORTH AND WILL RE-DEVELOP AND RE- STRENGTHEN THE RIDGE BY
MID-WEEK OVER THE EASTERN GULF THROUGH THE ALCAN BORDER REGION OF
ALASKA/CANADA.

&&

.MODEL DISCUSSION...MODELS ARE IN RATHER TIGHT AGREEMENT FOR THE
SHORT TERM (24HRS)...SO THE FORECASTERS WILL BE RELYING ON THE
HIGHER RESOLUTION MODELS. BEYOND THAT...SOME RATHER SIGNIFICANT
DISCREPANCIES BEGIN TO APPEAR WITH HOW THE MODELS HANDLE THE NORTH
PACIFIC LOW APPROACHING THE EASTERN ALEUTIANS AND ALASKA PENINSULA
WED THROUGH FRI. THE PRIMARY MODELS IN GENERAL ELONGATE THE
SURFACE LOW ACROSS THE BERING AND NORTH PACIFIC AS THE SYSTEM
MOVES NORTH...BLOCKED BY THE DOMINANT RIDGE TOWARD THE PANHANDLE
AND ALCAN BORDER REGIONS LATE THIS WEEK THROUGH THE WEEKEND. THE
DETAILS WILL BE FINE-TUNED AS WE GET CLOSER TO THIS EVENT...BUT
FOR NOW WILL MAINLY STICK WITH WPC GUIDANCE IN THE EXTENDED TIME
FRAME FROM LATE THIS WEEK THROUGH EARLY NEXT WEEK. THIS WILL MEAN
THAT THE NEXT SIGNIFICANT STORM SYSTEM WILL BEGIN TO IMPACT THE
STATE LATE THURSDAY AND PERSIST THROUGH MOST OF THE WEEKEND.

&&

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

THE REMAINING SOUTHEASTERLY WINDS ALONG TURNAGAIN AND KNIK
ARMS...AS WELL AS SOUTHWEST ANCHORAGE SHOULD FINALLY DIMINISH THIS
EVENING AS THE EASTERN GULF LOW CONTINUES TO APPROACH THE GULF
COAST. AS THESE WINDS DIMINISH...COLD AIR ADVECTION BEHIND THE LOW
WILL ALLOW TEMPERATURES THROUGHOUT SOUTHCENTRAL TO FALL BELOW
FREEZING TONIGHT...ALLOWING FOR REFREEZE ON THE ROADS AND
SIDEWALKS AROUND SOUTHCENTRAL AFTER TODAYS RECORD WARMTH AND
RESULTING SNOW MELT. THE EASTERN GULF LOW HAS ALSO TRENDED
SLIGHTLY FURTHER WEST THAN WAS EXPECTED YESTERDAY...WHICH WILL
ALLOW FOR ACCUMULATING SNOW TO FALL FROM THOMPSON PASS INTO THE
EASTERN COPPER RIVER BASIN THROUGH WEDNESDAY MORNING. SKIES
THROUGHOUT THE AREA WILL BEGIN TO CLEAR ON WEDNESDAY WITH LIGHT
WINDS ALLOWING FOR THE FORMATION FOR PATCHY DIURNAL FOG IN MOST
LOW LYING AREAS THROUGH THURSDAY MORNING. BY THURSDAY
AFTERNOON A NORTH PACIFIC FRONT WILL BEGIN TO APPROACH THE
SOUTHERN GULF AND KODIAK ISLAND. THIS FRONT WILL CONTINUE TO
SPREAD NORTH AND BECOME THE FOCAL POINT FOR THE WEATHER THROUGHOUT
THE SOUTHERN MAINLAND INTO LATE THIS WEEK.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
SOUTHWEST ON SHORE FLOW WILL KEEP SHOWERS OVER THE SOUTHWEST
THROUGH MUCH OF WEDNESDAY. THESE SHOWERS WILL START OFF PRIMARILY
AS RAIN BUT AS THE LOW NEAR NUNIVAK ISLAND PULLS IN MORE COLD AIR
THE PRECIPITATION WILL TRANSITION OVER TO SNOW. THIS WILL ALSO
HELP TEMPERATURES DROP BEFORE THE NEXT WARM FRONT COMES ON
THURSDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
AN UNSTABLE AND MOIST ATMOSPHERE WILL LEAD TO SHOWERS FOR THE
ALEUTIANS WITH SOUTHERLY AIR OUT EAST KEEPING THOSE SHOWERS RAIN.
BUT WITH NORTHERLY FLOW OUT WEST THE SHOWERS WILL LIKELY BE SNOW.
LATE ON WEDNESDAY A FRONT APPROACHING FROM THE SOUTH WILL BRING
MORE RAIN AND WARMING TO THE EASTERN ALEUTIANS AND ALASKA
PENINSULA. THIS PUSH OF WARMER AIR WILL LAST THROUGH THE REMAINDER
OF THE WEEK.

&&

.LONG TERM FORECAST (THURSDAY THROUGH TUESDAY)...

LONGWAVE RIDGING OVER THE EASTERN GULF OF ALASKA WILL BUILD
EASTWARD OVER THE CONTINENTAL UNITED STATES LATE THIS WEEK AND
HEADING INTO THIS WEEKEND. THIS RIDGE WILL ACT AS A BLOCKING
PATTERN THAT WILL PREVENT THE JET STREAM THAT IS CURRENTLY LOCATED
OVER THE CENTRAL PACIFIC OCEAN TO MOVE TO A MORE SEASONAL
LOCATION NEAR ALASKA. MULTIPLE AREAS OF LOW PRESSURE WILL DEVELOP
IN THE UPPER LEVEL TROUGH EXTENDING FROM NEAR THE KAMCHATKA
PENINSULA TO THE ALASKA PANHANDLE. EACH OF THESE LOWS WILL BRING A
BLAST OF SUBTROPICAL MOISTURE AND WARMER TEMPERATURES TO ALASKA
THROUGH MUCH OF THE WEEKEND. BY MONDAY...MODELS ARE BEGINNING TO
HINT AT A POTENT SHORTWAVE DEVELOPING IN THE CENTRAL PACIFIC AND
MOVING RAPIDLY TOWARD THE ALASKA PANHANDLE. THIS COULD POSSIBLY
FLATTEN OUT THE RIDGE OVER SOUTHEAST ALASKA AND SHUNT IT EASTWARD
ALLOWING FOR A COLDER AIR MASS OVER RUSSIA TO BRING COOLER (BUT
STILL ABOVE NORMAL) TEMPERATURES TO WESTERN ALASKA BY TUESDAY.
REGARDLESS...THE PERIOD OF WELL ABOVE NORMAL TEMPERATURES IS
LIKELY TO PERSIST THROUGH THE LONG TERM FORECAST WITH NO
APPRECIABLE CHANGE IN SIGHT.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES 155 AND 172.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CC
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...MC