Small Craft Advisory 904, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
400 PM AKST WED DEC 12 2012

.ANALYSIS AND UPPER LEVELS...
IN THE UPPER-LVLS...AN UPPER RIDGE AXIS IN THE NORTHEAST PACIFIC
OCEAN HAS FULLY RETREATED SOUTHEAST OF THE GULF OF AK INTO BRITISH
COLUMBIA. IN IT'S WAKE A BROAD BUT SOMEWHAT FLAT TROUGH ENCOMPASSES
THE REMAINDER OF THE AOR FROM SOUTHCENTRAL ALASKA BACK THROUGH THE
BERING SEA. MOIST SOUTHWEST MID-LVL FLOW IS VEERING TO A MORE
WESTERLY COMPONENT.

AT THE SURFACE...A FRONTAL ZONE IS DRAPED ACROSS CENTRAL INTERIOR
ALASKA WESTWARD INTO THE CENTRAL TO WESTERN BERING SEA. THERE ARE
VARIOUS WAVES OF LOW PRESSURE ALONG THE FRONTAL AXIS. ONE LOW OVER
THE FAR UPPER KUSKOKWIM VALLEY IS DRIFTING EAST AND FILLING AS IT
SQUEEZES OUT SOME LIGHT SNOWS OVER THE SW MAINLAND. ANOTHER LOW IS
CENTERED AROUND 250 MILES WEST OF THE PRIBILOF ISLANDS AND DRIFTING
EAST...THOUGH SOME GALES REMAIN OVER MANY MARINE AREAS. ALSO OF NOTE
IS A DEVELOPING LOW 350 MILES SOUTH OF ATKA MOVING NORTHEAST.

FURTHER EAST...A TRIPLE POINT LOW FORMED IN THE NORTHWEST GULF EARLY
THIS MORNING ALONG A FRONTAL BOUNDARY AND IS MOVING EAST OF CAPE
SUCKLING THIS EVENING. AS SUCH...THE HEAVIEST AREAS OF SNOW HAVE
LARGELY ENDED AS THE SUPPORTING ENERGY AND LIFT ARE MOVING
AWAY...ALTHOUGH SOME RESIDUAL WARM AIR ALOFT JUST ABOVE THE SURFACE
STILL SUPPORTS SOME POCKETS OF FREEZING RAIN IN SOUTHCENTRAL AK. COLD
ADVECTION IS FILLING IN BEHIND THE GULF TRIPLE POINT HOWEVER...AND
COUPLED WITH MODEST PRESSURE RISES ARE SUPPORTING STRENGTHENING
WESTERLY GALES ACROSS GULF OFF-SHORE MARINE AREAS AND ADJACENT
COASTAL WATERS.

.MODEL DISCUSSION...
MODEL AGREEMENT IS BELOW AVERAGE...BUT THE COMPLEX PATTERN IS HANDLED
SATISFACTORILY ALL THINGS CONSIDERED. THE LOW WEST OF THE PRIBILOFS
MOVES TO AROUND 100 MILES EAST OF SAINT PAUL THU AFTERNOON...THEN
TRACKS TO BRISTOL BAY'S WATERS LATE THU NIGHT BEFORE BECOMING
ABSORBED BY THE SYSTEM THAT IS DEVELOPING SOUTH OF ATKA. THAT PACIFIC
SYSTEM WILL QUICKLY ADVANCE TO THE EASTERN ALEUTIANS LATE TONIGHT...
BRUSH KODIAK ISLAND THU EVENING...AND REACH PEAK INTENSITY IN THE 970
MB RANGE IN THE GENERAL VICINITY OF MIDDLETON ISLAND FRI AFTERNOON.
SYSTEM SPEED RATHER FAST INITIALLY AS IT RIDES ALONG ZONAL 120 KT
UPPER-JET...BUT SLOWS TO A CRAWL AS IT WORKS INTO THE GULF WHILE THE
UPPER-FLOW AMPLIFIES. THE ECMWF AND NAM SURFACE POSITIONS ARE CLOSEST
IN THEIR DEPICTIONS OF THIS SYSTEM AS IT SLIPS FROM THE PACIFIC TO
THE GULF...WITH THE GFS AND UKMET SOLUTIONS LAGGING CONSIDERABLY
FURTHER SOUTH. THE 18Z GFS TRENDED THE TRACK NORTH A BIT TOWARDS THE
ECMWF/NAM. HAVE INCORPORATED A BLEND OF NAM AND ECMWF FOR THE
SOUTHCENTRAL TO GULF FORECAST PACKAGES. MODELS DO HAVE SOME AGREEMENT
BY THE WEEKEND SHOWING AN AMPLIFYING TROUGH AND COLDER AIR FROM THE
ARCTIC BLEEDING SOUTHWARD TO SOUTHERN PORTIONS OF THE MAINLAND.

FURTHER WEST IN THE BERING...A LOW FORMS 400 MILES NORTH OF ATTU ON
THU AND TRACKS INTO EASTERN SIBERIA ON FRI. A FRONT IS DRAGGED
THROUGH THE BERING...BUT DISAGREE ON IT'S ORIENTATION AND INTENSITY.
THE ECMWF IS STRONGER WITH THE FRONT AND EVEN DEVELOPS A LOW ALONG IT
THAT TRACKS FROM THE WESTERN BERING FRI MORNING TO NEAR THE PRIBILOFS
EARLY SAT MORNING. HEAVE LEANED A BIT MORE ON THE GFS...WHICH SHOWS A
WEAKER AND MORE DIFFUSE SYSTEM IN THE ESTABLISHED UPPER ZONAL
FLOW...BUT ALSO ACKNOWLEDGE LOWER CONFIDENCE IN POSITIONS/DEVELOPMENT
BECAUSE OF THIS AFOREMENTIONED FLOW.

.SHORT TERM FORECAST...
SOUTHCENTRAL...
AS PREVIOUSLY MENTIONED...THE HEAVY PRECIPITATION SHIELD HAS SHIFTED
EAST...THOUGH CONTINUED WEAK CYCLONIC FLOW ALOFT TO STILL SUPPORT
SOME PERIODS OF LIGHT SNOW THROUGH TONIGHT. WITH A MORE WEST FLOW
ALOFT DEVELOPING...THE FOCUS OF RESIDUAL LIGHT PRECIP SHOULD SHIFT TO
THE MOUNTAINS. THE APPROACH OF THE GULF SYSTEM LATE THU INTO FRI
BRINGS ANOTHER ROUND OF SNOW TO MAINLAND SOUTHCENTRAL. CURRENT TRACK
KEEPS LION'S SHARE OF QPF OVER THE PRINCE WILLIAM SOUND AREA...THOUGH
A SHIFT FURTHER NORTH TO WEST COULD ALLOW MORE PRECIPITATION TO SPILL
OVER THE CHUGACH/KENAI RANGES. FOR KODIAK ISLAND...ABOVE AVERAGE
TEMPERATURES TODAY WILL REVERT BACK AS COLD-ADVECTION FILTERS BEHIND
THE LOW TRACKING EAST OF CAPE SUCKLING. THIS SHOULD ENABLE THU
PRECIPITATION TO FALL AS SNOW...THOUGH SOME CHANGEOVER TO RAIN IS
POSSIBLE AS SOME WARMER AIR SHOULD ACCOMPANY THE NEXT PACIFIC LOW.

SOUTHWEST...
LINGERING PRECIPITATION IS LARGELY TRANSITIONING BACK TO SNOW AS THE
VERTICAL PROFILE COOLS WITH LOWERING THICKNESSES...BUT COVERAGE IS
BECOMING MORE SHOWERY IN NATURE IN RATHER WEAK CYCLONIC FLOW ALOFT. A
QUICK SHOT OF RAIN FOR THE AKPEN WITH THE DEVELOPING PACIFIC SYSTEM
MOVING THROUGH QUICKLY ON THU...WHICH MAY BRUSH SOUTHERN REACHES OF
BRISTOL BAY WITH SOME SNOW. SOME WEAK UPPER-RIDGING BUILDS IN FRI AND
COUPLED WITH OFF-SHORE FLOW SHOULD ALLOW EVEN LESS COVERAGE OF PRECIP
AND DECREASED CLOUDINESS.

BERING SEA/ALEUTIANS...
PROXIMITY OF UPPER-JET TO THE ALEUTIAN CHAIN WILL SUPPORT WAVES OF
VARYING INTENSITY ROTATING THROUGH THE FLOW AND YIELD PERIODS OF
LIGHT PRECIPITATION. PRECIP TO BE ACCOMPANIED BY BRISK WESTERLY GALES
INTO FRI.

.LONG TERM FORECAST...
FORECAST CHALLENGES EARLY WEEKEND SURROUND THE LINGERING LOW IN THE
GULF. IN IT'S WAKE...MODELS SUGGEST A COOLER/DRIER AIRMASS AND SOME
OFF-SHORE FLOW FOR THE SOUTHERN MAINLAND. ACROSS THE
BERING/CHAIN...STRONG JET LOOKS TO KEEP THINGS ACTIVE WITH VARIOUS
STORMS THROUGH MID NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...WINTER WEATHER ADVISORY...101 111 141 (ENDING THIS EVENING)
MARINE...GALE...120 130 132 138 150 155 160 165 170 172 175 176 179
                180 185 351 352 411 412 413 414
         HEAVY FREEZING SPRAY...127 160 185 414
FIRE WEATHER...NONE

MALOY