Small Craft Advisory 989, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
1255 PM AKST FRI DEC 12 2008

.ANALYSIS AND UPPER LEVELS...THE UPPER LEVEL FLOW ACROSS THE REGION
IS SPLIT THIS MORNING. THE NORTHERN STREAM JET ROUNDS THE BOTTOM OF
A LONG-WAVE TROUGH SITTING OVER RUSSIA THEN HEADS NORTHEASTWARD
ACROSS THE WESTERN BERING SEA OVER TOP OF A MAINLAND ALASKA RIDGE
INTO THE ARCTIC. THE SOUTHERN STREAM EXTENDS ACROSS THE WEST PACIFIC
BETWEEN 30N AND 40N...HEADS SOUTH AROUND A CUT-OFF LOW NEAR 37N
170W...NORTH TO THE EASTERN ALEUTIANS AND THEN EAST OVER TOP OF A
STRONG EAST PACIFIC RIDGE. A SHORT-WAVE EXTENDING FROM SIBERIA
SOUTHEASTWARD ACROSS THE EASTERN BERING IS TRACKING
NORTHEASTWARD...WITH MOST OF THE ENERGY STARTING TO LIFT TOWARD
NORTHERN ALASKA DUE TO RIDGE OUT AHEAD OF IT. A SURFACE FRONT
BENEATH THIS TROUGH IS QUICKLY WEAKENING AS IT PUSHES INLAND OVER
SOUTHWEST ALASKA. A STRONG SURFACE HIGH OVER INTERIOR ALASKA
COMBINED WITH A TROUGH OVER THE NORTHERN GULF IS PRODUCING THE
USUAL GUSTY WINDS THROUGH GAPS ALONG THE NORTH GULF COAST. AREAS
OF FOG HAVE FORMED INLAND WHERE THE PRESSURE GRADIENT IS WEAKER.
FINALLY...A STRONG SURFACE LOW EAST OF KAMCHATKA IS TRACKING QUICKLY
NORTH UNDERNEATH THE UPPER JET. A COLD FRONT EXTENDS FROM THIS LOW
SOUTHWARD ACROSS THE WESTERN BERING AND ALEUTIANS...RESULTING IN
WIDESPREAD GALE FORCE SOUTHERLIES ACROSS THE REGION.

.MODEL DISCUSSION...GOOD MODEL AGREEMENT AND CONSISTENCY THROUGH
SATURDAY NIGHT LEADS TO HIGH FORECAST CONFIDENCE. BEYOND THAT
DIFFERENCES BEGIN TO SHOW UP OUT WEST...INITIALLY WITH SYSTEMS
TRACKING JUST EAST OF KAMCHATKA AND THEN OVER THE ENTIRE NORTH
PACIFIC/BERING REGION. DIFFERENCES DONT INITIALLY HAVE MUCH IMPACT
ON WEATHER OVER FORECAST AREA SO WILL USE LATEST GUIDANCE THROUGH
MONDAY WITH NO CHANGES AFTER THAT POINT.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THERE WILL BE A SLIGHT WEAKENING OF PRESSURE
GRADIENT ALONG THE NORTH GULF COAST THIS AFTERNOON AS A LOW DOWN
NEAR WASHINGTON STATE HEADS INLAND AND PRESSURE RISES OVER THE GULF.
HOWEVER...THAT WILL BE SHORT-LIVED. THE UPPER TROUGH OVER SOUTHWEST
ALASKA THIS AFTERNOON WILL CONTINUE TO RAPIDLY WEAKEN TONIGHT AS IT
IS ABSORBED INTO RIDGE BUILDING BEHIND IT RIDGE AHEAD OF IT WITH ALL
FLOW BECOMING ANTICYCLONIC BY LATE TONIGHT. THUS...SOUTHCENTRAL WILL
SEE NO MORE THAN SOME MID TO HIGH CLOUDS AS THE REMNANTS OF THE
SURFACE FRONT MOVE THROUGH THIS AFTERNOON AND TONIGHT. THIS CLOUD
COVER SHOULD HELP DISSIPATE SOME OF THE FOG IN PLACE OVER NORTHERN
COOK INLET AND THE SUSITNA VALLEY. SKIES WILL GRADUALLY CLEAR AGAIN
AFTER MIDNIGHT SO FOG WILL LIKELY REFORM. MEANWHILE...THE BUILDING
RIDGE ALOFT WILL STRENGTHEN THE SURFACE HIGH OVER INTERIOR ALASKA
AND TIGHTEN UP THE GRADIENT ALONG THE NORTH GULF COAST ONCE AGAIN.
AN UPPER JET STREAK WILL MOVE DOWN THE EAST SIDE OF THE UPPER RIDGE
AND COMBINE WITH COLD ADVECTION TO ENHANCE SURFACE WINDS ALONG THE
NORTH GULF COAST. THE GRADIENT WILL WEAKEN SATURDAY AND SUNDAY AS
THE SURFACE HIGH BUILDS DOWN OVER THE GULF.

SOUTHWEST ALASKA...THE SURFACE FRONT OVER SOUTHWEST ALASKA WILL
QUICKLY FALL APART THIS AFTERNOON AND EVENING BRINGING AN END TO
THE SNOWFALL. CLEARING SKIES BEHIND THE FRONT WILL LEAD TO FOG
FORMATION TONIGHT...AND AREAS OF FOG WILL LIKELY PERSIST THROUGH
THE WEEKEND AS HIGH PRESSURE BUILDS IN. THE UPPER RIDGE WILL SLOWLY
SLIDE EASTWARD ALLOWING ANOTHER BERING SEA FRONT TO REACH THE
SOUTHWEST COAST BY MONDAY.

BERING SEA/ALEUTIANS...THE WESTERN BERING WILL CONTINUE TO BE
IMPACTED BY SYSTEMS TRACKING NORTHEASTWARD FROM THE NORTHWEST
PACIFIC TO THE ARCTIC THROUGH THE NEXT FEW DAYS. MEANWHILE...THE
CLOSED LOW CURRENTLY OVER 1000 MILES SOUTH OF THE ALEUTIANS WILL
SLOWLY TRACK NORTHWARD UP THE WEST SIDE OF THE BUILDING RIDGE TO
THE EAST...PUSHING A FRONT NORTHWARD ACROSS THE EASTERN ALEUTIANS
AND INTO THE BERING SEA SATURDAY AND SUNDAY. VERY WARM TROPICAL AIR
WILL ACCOMPANY THIS FRONT AND QUICKLY CHANGE PRECIPITATION TO RAIN.

.LONG TERM FORECAST...A BLOCKING RIDGE WILL REMAIN IN PLACE FROM
THE EAST PACIFIC TO MAINLAND ALASKA THROUGH MUCH OF NEXT WEEK...SO
ACTIVE WEATHER WILL REMAIN OUT WEST. MEDIUM RANGE MODELS INDICATE
THIS RIDGE MAY RETROGRADE INTO THE BERING AT THE END OF NEXT
WEEK...BUT IT IS TOO EARLY TO HAVE ANY CONFIDENCE IN THAT SOLUTION.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...121 155 165 170 171 172 175 176 180 185.
FIRE WEATHER...NONE.

SEB DEC 08