Small Craft Advisory 991, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
350 AM AKST WED DEC 11 2013

.ANALYSIS AND UPPER LEVELS...

A HIGHLY AMPLIFIED UPPER AIR PATTERN PERSISTS OVER THE REGION THIS
MORNING...WITH A LARGE BLOCKING HIGH CENTERED OVER WRANGEL ISLAND
NORTH OF EASTERN SIBERIA IN A REX BLOCK FASHION. A DEEP LONGWAVE
TROUGH EXISTS DIRECTLY OVER THE DATELINE TO 30N...WHICH IS
INTERACTING WITH A PLUME OF SUBTROPICAL MOISTURE. OVER MAINLAND
ALASKA...A POLAR STREAM OF AIR IS DEEPENING OVER SOUTHCENTRAL
ALASKA...EVIDENCED BY CONTINUED DRYING THROUGH THE MID LEVELS OF THE
ATMOSPHERE BASED ON WATER VAPOR SATELLITE TRENDS. AN UPPER LEVEL
TROUGH AND ASSOCIATED LOW IS SLOWLY TRACKING INTO THE GULF OF ALASKA.
COMBINED WITH THE COLD VORTEX FORMING OVER SOUTHCENTRAL...COLD AND
DRY OUTFLOW WINDS WILL CONTINUE TO ACCELERATE THROUGH FAVORED GAPS
AND EXPOSED TERRAIN AS THE PRESSURE GRADIENT TIGHTENS THROUGH TODAY.

WITH TIME...THE BLOCKING HIGH OVER EASTERN SIBERIA WILL SLIDE OVER
THE BEAUFORT SEA AND NORTHERN ALASKA AND CONSTRICT...EFFECTIVELY
ENHANCING THE DEVELOPMENT OF THE UPPER LEVEL VORTEX OVER AND ALONG
THE ALASKA/ALEUTIAN RANGES. THE AFOREMENTIONED UPPER TROUGH OVER THE
DATELINE WILL TRACK EAST...WITH A DEEPENING PACIFIC LOW FORMING ALONG
THE BASE OF THE TROUGH...EVENTUALLY TRACKING NORTHEAST INTO THE GULF
OF ALASKA BY LATE WEEK.

.MODEL DISCUSSION...

ALL THE MODELS ARE IN FAIR AGREEMENT WITH THE LARGE SCALE PATTERN
THROUGH THE WEEKEND WITH THE EXCEPTION OF THE NAM. THE NAM REMAINS A
SUBSTANTIAL OUTLIER...FORMING A DEEP AND CONSOLIDATED LOW/WARM FRONT
THAT TRACKS ALL THE WAY INTO SOUTHCENTRAL ALASKA BY THIS WEEKEND. THE
NAM ALSO DEPICTS THE UPPER VORTEX RETROGRADING SEVERELY TO THE
WEST...AND EVENTUALLY SOUTHWEST OVER DUTCH HARBOR...WHICH REMAINS
COMPLETELY OUT OF PHASE WITH THE MORE RELIABLE GLOBAL MODELS SUCH AS
THE ECMWF...GFS...GFS ENSEMBLE MEAN...AND THE UKMET. THE CANADIAN GEM
STILL SUPPORTS A STRONGER FRONT...BUT IT HAS BACKED OFF SIGNIFICANTLY
FROM PREVIOUS RUNS. AS A RESULT...THE NAM WILL BE UTILIZED THROUGH
THE SHORT TERM TO DEPICT THE STRONG GAP WINDS...WITH THE GFS AND
ECMWF REMAINING THE MODELS OF CHOICE HEADING INTO THE WEEKEND. THE
NAM WAS COMPLETELY DISCARDED BY LATE WEEK.

.SHORT TERM FORECAST...

SOUTHCENTRAL...
THE BIG STORY WILL BE PLUMMETING TEMPERATURES AS POLAR AIR CONTINUES
TO MAKE ITS WAY INTO SOUTHCENTRAL...AND THEN INTO THE GULF OF ALASKA.
TEMPERATURES ARE ALREADY FALLING REGION WIDE...AND WILL LIKELY BOTTOM
OUT WELL BELOW AVERAGE BY THURSDAY MORNING. MODERATE GAP WINDS
THROUGH VALDEZ...THE MATANUSKA VALLEY...AND OTHER MOUNTAIN
PASSES/TERRAIN WILL PEAK TODAY THEN SLOWLY DIMINISH DURING THE
OVERNIGHT AS THE PRESSURE GRADIENT WEAKENS. PREDOMINANTLY DRY AND
FAIR CONDITIONS WILL LAST THROUGH THURSDAY BEFORE A WEAK DISTURBANCE
SUPPORTS SNOW SHOWER DEVELOPMENT ACROSS PRINCE WILLIAM SOUND AND THE
NORTH GULF COAST LATE THURSDAY INTO FRIDAY. A MUCH MORE SUBSTANTIAL
SURGE OF MOISTURE WILL BEGIN IMPACTING PORTIONS OF SOUTHCENTRAL LATE
FRIDAY INTO SATURDAY (SEE EXTENDED DISCUSSION).

SOUTHWEST ALASKA...
A DOMINANT NORTHEASTERLY FLOW WILL PERSIST OVER THE SOUTHWEST
MAINLAND THROUGHOUT THE WEEK...BRINGING IN COLDER AND DRIER
CONDITIONS TO THE AREA FROM THE INTERIOR. WIND CHILLS OF 15 TO 30
BELOW ZERO WILL DEVELOP BY TONIGHT ACROSS THE WINDIEST LOCATIONS
ALONG THE KUSKOKWIM DELTA.

ALEUTIANS/BERING SEA...
THE WEAK SURFACE LOW OVER THE WESTERN BERING WILL BECOME OVERRUN BY
HIGH PRESSURE BUILDING OVER THE NORTHERN BERING AND AN APPROACHING
NORTH PACIFIC LOW...THEN DISSIPATE BY WEDNESDAY EVENING. AS THE
SURFACE RIDGE CONTINUES TO BUILD OVER THE BERING THURSDAY
AFTERNOON...COLDER TEMPERATURES AND STRONGER NORTHEASTERLY WINDS ARE
EXPECTED TO PERSIST OVER THE AREA. ISOLATED TO SCATTERED RAIN AND
SNOW SHOWERS WILL FOLLOW AS COLD AIR OVER THE WARM WATERS CREATES
REGIONS OF INSTABILITY.

.LONG TERM FORECAST...

LATE FRIDAY INTO SATURDAY...A MODERATELY STRONG NORTH PACIFIC LOW
WILL TRACK INTO THE GULF OF ALASKA...AND WILL BEGIN SPREADING
WIDESPREAD PRECIPITATION ACROSS THE NORTHERN GULF COAST...THEN INTO
INLAND SOUTHCENTRAL FROM COOK INLET EASTWARD. ALL PRECIPITATION WILL
BEGIN AS SNOW...THEN TRANSITION TO WET SNOW AND RAIN ALONG THE
COAST...MAINLY FROM THE KENAI PENINSULA EASTWARD DURING THE DAY
SATURDAY. THE DETAILS OF THE TRACK OF THE LOW AND THE STRENGTH OF THE
WARM FRONT STILL REMAIN UNCERTAIN AT THIS TIME...WHICH WILL HAVE AN
IMPACT ON HOW FAR THE WARM AIR INVADES THE INTERIOR. A FEW WEAK
SOLUTIONS SLIDE THE FRONT SO FAR EAST THAT MUCH OF SOUTHCENTRAL WEST
OF PRINCE WILLIAM SOUND REMAIN LARGELY DRY AS NORTHEAST WINDS WOULD
TEND TO DRY THINGS OUT. THE FORECAST WILL CONTINUE TO TAKE A MIDDLE
GROUND APPROACH...WHICH CALLS FOR WIDESPREAD PRECIP ALONG THE NORTH
GULF COAST INTO THE COPPER BASIN. AREAS FARTHER WEST...INCLUDING
ANCHORAGE...WILL REMAIN ON THE EDGE WHERE SMALL TRACK DIFFERENCES CAN
YIELD LARGE DIFFERENCES IN PRECIP INTENSITY AND DURATION.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE 127 128 130 132 138 139 180.
         HEAVY FREEZING SPRAY 121 126 127 128 129 130 138 139 140 141

                              160 180 181 185.
FIRE WEATHER...NONE.

AHSENMACHER/PEPE DEC 13