Small Craft Advisory 761, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
153 PM AKDT WED OCT 1 2014

.ANALYSIS AND UPPER LEVELS...
THE UPPER LEVEL PATTERN IS INTERESTING TODAY WITH A RIDGE OVERT HE
BERING SEA AND A LOW CENTERED IN THE ARCTIC WHICH EXTENDS INTO
WESTERN CANADA. THIS IS PRODUCING ZONAL WEST TO NORTHWESTERLY
FLOW ALOFT OVER THE MAINLAND PART OF THE STATE. A CUT-OFF LOW IS
CENTERED WELL SOUTH OF THE ALASKA PENINSULA WITH THE JET STREAM
RUNNING AROUND IT TO THE SOUTH AND EAST AND THEN CLIPPING THE
GULF OF ALASKA AS IT TRACKS OVER SOUTHEAST ALASKA.
THIS CUT-OFF LOW WITH JET SUPPORT IS THE REASON THE SURFACE LOW
NEAR IT WILL STRENGTHEN AND THEN TRACK INTO THE GULF OF ALASKA.
THE MAINLAND OF THE STATE REMAINS UNDER A STRONG RIDGE OF HIGH
PRESSURE AT THE SURFACE.

&&

.MODEL DISCUSSION...
MODELS ARE HONING IN ON A COMMON SOLUTION FOR THE TRACK OF THE LOW
THAT IS CURRENTLY SOUTH OF THE ALASKA PENINSULA WHICH WILL BE
STRENGTHENING AND MOVING INTO THE GULF OF ALASKA TOMORROW AND THEN
TO THE NORTH GULF COAST FRIDAY. THE GFS IS A LITTLE FASTER IN ITS
PROGRESSION THROUGH THURSDAY NIGHT THAN THE ECMWF, NAM AND GEM
WHICH ARE ALL THREE CLUSTERED CLOSE TOGETHER. BY FRIDAY NIGHT THEY
ARE ALL IN AGREEMENT IN PUSHING THE UPPER LEVEL ENERGY NORTHEAST
INTO THE YUKON AND SPLITTING THE SURFACE LOW INTO TWO SEPARATE
LOWS. ONE FOLLOWING THE UPPER LEVEL LOW INTO THE YUKON AND THE
OTHER ALONG THE NORTH GULF COAST. THE NORTH GULF COAST LOW WILL
BECOME THE MAIN ONE AS COLD AIR FROM THE INTERIOR OF THE STATE
FILTERS INTO IT.
THERE ARE SOME SUBTLE DIFFERENCES IN THE MODELS IN TERMS OF
TEMPERATURES AND MOISTURE WHICH WILL BE IMPORTANT IN DETERMINING
PRECIPITATION TYPE IN THE COPPER RIVER BASIN AS WELL AS HAS FAR
WEST THE PRECIPITATION WILL MOVE.
THE NEXT CHALLENGE FOR THE
MODELS IN THE GULF OF ALASKA REGION WILL BE THIS WEEKEND AS
ANOTHER LOW DEVELOPS TO THE SOUTH OF THE ALASKA PENINSULA AND
TAKES A SIMILAR TRACK AS THE ONE MOVING INTO THE AREA THE NEXT TWO
DAYS. MODELS ALL HAVE THIS HAPPENING BUT THE TIMING AND STRENGTH
OF THIS LOW VARY GREATLY. THE RIDGE OF HIGH PRESSURE OVER THE
BERING SEA WILL PERSIST LARGELY UNCHANGED INTO THE WEEKEND.

&&

.SHORT TERM FORECAST...
SOUTHCENTRAL ALASKA...
SUNNY SKIES WILL START TO GIVE WAY TO A FRONT APPROACHING FROM
THE SOUTH. THIS WILL START TONIGHT IN KODIAK AND THEN TOMORROW
NIGHT ALONG THE NORTH GULF COAST. BY THURSDAY NIGHT THE SURFACE
LOW WILL BE MOVING TOWARD THE GULF COAST WITH A COLD UPPER LEVEL
TROUGH DESCENDING OVER SOUTHCENTRAL FROM THE NORTH. THE RESULT
WILL BE TO BRING PRECIPITATION ALONG THE NORTH GULF COAST AND INTO
THE COPPER RIVER BASIN. PRECIPITATION TYPE WILL BE RAIN ALONG THE
COAST BUT THE BIG QUESTION WILL BE HOW MUCH COLD AIR WILL BE ABLE
TO MAKE IT INTO THE COPPER RIVER BASIN AND AT WHAT ELEVATION THE
SNOW WILL TURN TO RAIN. AT THIS TIME IT LOOKS LIKELY THAT THOMPSON
PASS, CHICKALOON PASS, MACLAREN PASS AND ISABEL PASS WILL BE COLD
ENOUGH TO SEE ACCUMULATING SNOWFALL FRIDAY AND FRIDAY NIGHT. THE
QUESTION IS WHETHER THIS WILL MAKE IT ALL THE WAY DOWN TO THE
GLENNALLEN AREA AND THIS IS WHERE THE SUBTLE DIFFERENCES IN THE
MODELS COMES INTO PLAY. TEMPERATURES ARE CLOSE TO WHAT THEY SHOULD
BE FOR SNOW BUT EARLY SEASON SITUATIONS OFTEN FAVOR RAIN. IT IS
POSSIBLE THAT THERE IS COLD RAIN IN THE LOWER PARTS OF THE COPPER
RIVER BASIN AND THE PASSES WILL SEE SNOWFALL. THE COLD AIR WILL
BUILD IN BETTER THIS WEEKEND OVER SOUTHCENTRAL ALASKA. THEREFORE
IF A SECONDARY LOW MOVES OVER THE COPPER RIVER BASIN SATURDAY INTO
SUNDAY THERE IS A BETTER CHANCE OF SNOW OVER THE ENTIRE AREA.
THERE IS A CHANCE...A CHANCE...THAT THE MOISTURE FROM THE COPPER
RIVER WILL MOVE FAR ENOUGH WESTWARD BY SATURDAY, IN CONJUNCTION
WITH THE COLDER AIR, TO BRING SOME SNOW PRETTY FAR DOWN THE
MOUNTAINS AROUND ANCHORAGE AND THE MAT-SU VALLEY.

SOUTHWEST ALASKA...
SOUTHWEST ALASKA IS UNDER THE STRONG RIDGE AND WILL REMAIN MOSTLY
CLEAR FOR THE NEXT FEW DAYS. THERE MAY BE A FEW CLOUDS TRACKING
ALONG THE NORTHERN PART OF THE AREA OVERNIGHT FROM A SHORT WAVE
MOVING THROUGH THE ZONAL FLOW OVER THE NORTHERN PART OF THE STATE.
MORE CLOUDS MAY ALSO SNEAK INTO THE AREA NEAR THE ALASKA RANGE BY
FRIDAY DUE TO AN UPPER LEVEL TROUGH DESCENDING OVER SOUTH CENTRAL
ALASKA AT THAT TIME.

BERING SEA/ALEUTIANS...
HIGH PRESSURE OVER THE CENTRAL BERING SEA WILL PERSIST INTO THE
WEEKEND. THERE IS A FRONT THAT IS BRUSHING UP TO THE EASTERN
ALEUTIANS AND ALASKA PENINSULA TODAY WHICH IS BRINGING SOME GUSTY
WINDS AND A CHANCE OF RAIN TO THE AREA THROUGH TONIGHT. THIS WILL
MOVE OUT OF THE AREA AS THE LOW THAT IS SOUTH OF THE AREA MOVES
INTO THE GULF OF ALASKA TOMORROW. WINDS WILL REMAIN SOMEWHAT GUSTY
BUT NOT AS STRONG AS TODAY AND THEY WILL ALSO SHIFT FROM MORE OF
AN EAST TO NORTHWEST DIRECTION TO THE NORTHWEST.
THE FAR WESTERN ALEUTIANS ARE ON THE OTHER SIDE OF THE RIDGE AND
WILL BE IMPACTED BY A FRONT FROM A LOW NEAR KAMCHATKA THOUGH
TOMORROW. THIS WILL BRING IN SOME GUSTY SOUTHEAST WINDS AND A
CHANCE OF RAIN.

&&

.LONG TERM FORECAST (SUN OCT 5 - WED OCT 8)...

THE EXTENDED FORECAST BECOMES INCREASINGLY COMPLEX AND CHALLENGING
AS A COLD UPPER LOW FORMS ACROSS SOUTHERN ALASKA...AND A SERIES OF
NORTH PACIFIC LOWS MOVES INTO THE GULF. THE FIRST OF THESE WILL BE
TRACKING THROUGH THE COPPER BASIN AT THE BEGINNING OF THE
WEEKEND...WITH ANOTHER LOW MOVING IN SOMETIME SUNDAY AFTERNOON
THROUGH SUNDAY NIGHT. THE TRACK REMAINS HIGHLY UNCERTAIN...WITH
THE ECMWF TAKING A MORE EASTERLY TRACK...AND THE GFS AND CANADIAN
GDPS FAVORING A WESTWARD TRACK INTO THE COPPER BASIN. THE IMPACT
WOULD BE THE POTENTIAL FOR ACCUMULATING SNOW SPREADING TO THE
WESTERN COPPER BASIN IN THE LATTER SOLUTIONS...WITH LITTLE TO NONE
IN THE FORMER SOLUTION. ONE CERTAINTY IS THE COLDEST AIR OF THE
SEASON WILL BE ADVECTING INTO SOUTHERN ALASKA. HIGH PRESSURE WILL
BUILD WEST TO EAST...WITH OVERNIGHT LOWS PLUMMETING AS SKIES CLEAR
IN THE WAKE OF THE UPPER LOW. UNCERTAINTY GROWS BY MIDWEEK AS A
SERIES OF STRONG LOWS ASSOCIATED WITH A POWERFUL NORTH PACIFIC JET
STREAM SLOWLY ERODE THE BLOCKING PATTERN OVER THE BERING AND TRACK
EAST.

-AHSENMACHER

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 119 120 130 131 132 138 150 155 172 174.
FIRE WEATHER...NONE.

&&

OCT 14