Small Craft Advisory 640, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
400 AM AKDT SAT AUG 24 2013

.ANALYSIS AND UPPER LEVELS...

THE STRONG PAIR OF POTENTIAL VORTICITY ANOMALIES WHICH TRACKED OVER
SOUTHCENTRAL YESTERDAY HAVE NOW MERGED WITH A LARGE POLAR VORTEX
CENTERED EAST OF THE BEAUFORT SEA...FORMING A LARGE UPPER LEVEL LOW
OVER SOUTHERN ALASKA. ANOTHER POTENTIAL VORTICITY ANOMALY AND
ASSOCIATED 100 KT JET MAXIMA IN NORTHWEST FLOW CURRENTLY OVER
SOUTHWEST ALASKA WILL MERGE WITH THE AFOREMENTIONED TROUGH...FORMING
A STRONGER CLOSED LOW IN THE GULF OF ALASKA THIS WEEKEND. FARTHER
WEST...AN UPPER RIDGE AXIS REMAINS OVER THE CENTRAL AND EASTERN
BERING SEA WITH A DEEPENING SUBTROPICAL WAVE DISTURBANCE ALONG ITS
WESTERN PERIPHERY NEAR 45N 170E.

AT THE SURFACE...A LOW IN THE NORTHERN GULF OF ALASKA WILL TRACK
SOUTHEAST THROUGH TODAY...WITH WEAK HIGH PRESSURE OVER THE INTERIOR
BUILDING OVER THE WEEKEND AS MASS SUBSIDENCE INCREASES. THIS WILL
SETUP A CLASSIC FALL LIKE OFFSHORE FLOW PATTERN. FARTHER WEST...A
GALE FORCE SUBTROPICAL LOW OVER THE WESTERN BERING WILL TRACK
NORTH...PUSHING A WARM FRONTAL BOUNDARY THROUGH THE BERING THROUGH
SATURDAY.

.MODEL DISCUSSION...

THE NUMERICAL GUIDANCE IS ALL IN RELATIVELY STRONG AGREEMENT WITH THE
GENERAL UPPER AIR PATTERN INTO EARLY NEXT WEEK...WITH THE BIGGEST
DISCREPANCY ASSOCIATED WITH A SUBTROPICAL WAVE DISTURBANCE TRACKING
ACROSS THE SOUTHERN BERING SEA EARLY NEXT WEEK. THE ECMWF HAS
CONSISTENTLY BEEN MUCH MORE AGGRESSIVE PHASING THAT FEATURE WITH A
LARGER UPPER LOW OVER EASTERN SIBERIA...THEN TRACKING A WAVE
DISTURBANCE INTO SOUTHERN ALASKA TUESDAY (SEE EXTENDED DISCUSSION).
IN THE SHORT TERM...THE NAM WAS FAVORED OVER SOUTHERN ALASKA DUE TO
ITS INCREASED RESOLUTION AND SUPERIOR HANDLING OF CHANNELED GAP WINDS
WHILE THE GFS AND ECMWF WERE FAVORED OVER THE BERING.

.SHORT TERM FORECAST...

SOUTHCENTRAL...
ISOLATED TO SCATTERED SHOWERS CONTINUE ACROSS THE COPPER RIVER BASIN
THIS MORNING. THESE WILL CONTINUE THROUGH THE AFTERNOON...BUT
DIMINISH IN INTENSITY AS THE DAY PROGRESSES. A FEW POCKETS OF LOCALLY
DENSE FOG THIS MORNING WILL RAPIDLY BURN OFF BY MID MORNING.
ELSEWHERE...A FEW ISOLATED SHOWERS ALONG THE TALKEETNA RANGE...ALASKA
RANGE...AND HIGHER CHUGACH ARE POSSIBLE THIS AFTERNOON...BUT STRONG
DRYING AND SUBSIDENCE WILL LIKELY KEEP ALL OF SOUTHCENTRAL OUTSIDE OF
THE COPPER RIVER BASIN DRY. OFFSHORE FLOW WILL INCREASE THIS
AFTERNOON AND INTO THE OVERNIGHT AS HIGH PRESSURE BUILDS OVER THE
INTERIOR...WITH ENHANCED NORTH TO WEST CHANNELED GAP WINDS THROUGH
FAVORED TERRAIN. GENERALLY DRY AND QUIESCENT WEATHER WILL CONTINUE
THROUGH MONDAY.

SOUTHWEST ALASKA...
DRY AND QUIESCENT WEATHER WILL DOMINATE THE REGION TODAY AS HIGH
PRESSURE BUILDS AND CLOUDS DIMINISH. A FRONT WILL APPROACH THE
WESTERN COAST THIS EVENING AND SPREAD INLAND THROUGH SUNDAY BEFORE
FALLING APART SUNDAY NIGHT. EXPECT RAIN ALONG THE COAST WITH MORE
SHOWERY CONDITIONS FARTHER INLAND.

BERING SEA/ALEUTIANS/ALASKA PENINSULA...
A SUBTROPICAL LOW OVER THE WESTERN BERING SEA WILL TRACK NORTH...WITH
A WARM FRONT TRACKING ACROSS THE ENTIRE BERING BY LATE SATURDAY. RAIN
AND INCREASING WEST TO SOUTHWEST WINDS WILL IMPACT THE REGION...WITH
AREAS OF FOG LIKELY REMAINING ENTRENCHED OVER MOST OF THE AREA
THROUGH THE SHORT TERM.

.LONG TERM FORECAST...

TUESDAY AND BEYOND...THERE IS SOME DISCREPANCY AMONG THE GLOBAL
GUIDANCE WITH RESPECT TO A SUBTROPICAL WAVE DISTURBANCE TRACKING
ACROSS THE SOUTHERN BERING SEA...AND ITS INTERACTION WITH AN UPPER
LOW OVER EASTERN SIBERIA. THE ECMWF IS MUCH MORE AGGRESSIVE WITH BOTH
THE SUBTROPICAL LOW AND THE DEGREE OF PHASING...WITH WET WEATHER
ENVELOPING MUCH OF SOUTHCENTRAL TUESDAY AND WEDNESDAY. THE GFS AND
ITS ENSEMBLE ARE NOT NEARLY AS BULLISH...WITH THE NET RESULT BEING
DRIER WEATHER LASTING THROUGH WEDNESDAY. FOR THE TIME BEING...THE
ECMWF IS FAVORED AS IT TENDS TO INITIALIZE SUBTROPICAL WAVES A BIT
BETTER THAN THE GFS. THEREAFTER...THE UPPER LOW WILL LIKELY REMAIN
THROUGH THURSDAY BEFORE SLOWLY TRACKING EASTWARD...WITH SHORT LIVED
HIGH PRESSURE BUILDING OVER SOUTHERN ALASKA LATE FRIDAY AND SATURDAY.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE 185 413.
FIRE WEATHER...NONE.

AHSENMACHER AUG 13