Small Craft Advisory 302, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
430 PM AKDT FRI APR 5 2013

.MODEL DISCUSSION...
FORECAST MODELS CONTINUE TO AGREE SYNOPTICALLY AT LEAST THROUGH
FRIDAY. THE UPPER LOW CURRENTLY OVER WESTERN ALASKA TO WOBBLE AROUND
WESTERN ALASKA THROUGH FRIDAY. MOST GUIDANCE WANT TO THEN START
PUSHING THE LOW TO THE EAST OVER THE WEEKEND. HOWEVER...THE ECMWF
DOES DROP A CLOSED LOW INTO WESTERN ALASKA FRIDAY TO KEEP THE UPPER
LOW KEEPING TO LOW OVER THE AREA THROUGH THE WEEKEND. IN THE
MEANTIME...THE ECMWF SOLUTION WOULD HOLD OFF A LOW THAT IS EXPECTED
TO MOVE INTO THE WESTERN BERING FRIDAY INTO SUNDAY. THE PREFERRED
SOLUTION AT THIS POINT IS TO ANTICIPATE THE LOW IN THE GULF MOVING
OFF TO THE EAST WITH A RIDGE PROGRESSING SLOWLY EASTWARD IN THE
BERING. AT THE SURFACE...A LOW OVER THE KENAI PENINSULA TO SLIP NORTH
TOMORROW. A LOW IN THE NORTH PACIFIC TO ROLL INTO THE GULF MONDAY
WITH A SECONDARY FEATURE COMING INTO PLAY TUESDAY. THE AREA OF LOW
PRESSURE TO WEAKEN OVER THE WEEK BUT REMAIN. OVER THE BERING...HIGH
PRESSURE OVER SIBERIA TO MOVE OVER THE WESTERN BERING MONDAY AND
SLOWLY SLIDE EAST THROUGH THE WEEK.

.ANALYSIS AND UPPER LEVELS...
AT THE SURFACE...A LOW OVER THE KENAI PENINSULA IS A FOCUSING
MECHANISM FOR SNOWFALL OVER SOUTH CENTRAL ALASKA TODAY. THE HIGH
PRESSURE OVER SIBERIA COMBINED WITH LOW PRESSURE IN THE NORTH PACIFIC
IS CREATING A STRONG PRESSURE GRADIENT OVER THE BERING AND
SOUTHWEST ALASKA WITH MODERATE NORTHEAST WINDS ACROSS THE REGION. A
WEAK SURFACE TROUGH ASSOCIATED WITH AN UPPER LOW NEAR SHEMYA EXTENDS
FROM UNALASKA BACK TO THE KAMCHATKA PENINSULA WITH RAIN AND SNOW
SHOWERS.
AT UPPER LEVELS...THE ARCTIC UPPER LOW IS IN WESTERN ALASKA WITH HIGH
PRESSURE OVER THE BERING AND ANOTHER UPPER LOW NEAR SHEMYA. THE JET
STREAM WRAPPING AROUND THE MAIN UPPER LOW IN MAINLAND ALASKA IS NOT
THAT STRONG AT AROUND 40-50 KT. A STRONG COLD FRONT AT MID LEVELS IS
PRESENT AROUND THE LOW PUSHING THROUGH THE EASTERN BERING AND ACROSS
SOUTH CENTRAL ALASKA.

.SHORT TERM FORECAST...
FOR SOUTH CENTRAL ALASKA...
COLDER AIR HAS FILTERED INTO THE AREA COMBINED WITH A SOMEWHAT
CONVERGENT FLOW ALONG THE KENAI MOUNTAINS HAS PRODUCED A BAND OF SNOW
AND THIS ACTIVITY IS ANTICIPATED TO CONTINUE. IN ADDITION...SOME COLD
ADVECTION WITH A BIT OF OVERRUNNING MOISTURE AND VORTICITY ADVECTION
TO BE AN ADDED TO THE LIFTING MECHANISM ELEMENTS OVER SOUTH CENTRAL
ALASKA. THE NEGATIVE VORTICITY ADVECTION AND DRY AIR ASSOCIATED WITH
THE UPPER LOW CENTER TO DROP OVER THE AREA TOMORROW AFTERNOON ENDING
THE SNOW EVENT FOR ANCHORAGE AND THE MAT-SU BY EARLY EVENING. IN THE
MEANTIME...COLD ADVECTION COMBINED WITH A FAVORABLE PRESSURE GRADIENT
TONIGHT INTO TOMORROW WILL BRING SOME STRONG WINDS TO THE BARREN
ISLAND REGION. THIS GRADIENT TO REMAIN THROUGH MUCH OF THE WEEK BUT
BE UP AND DOWN OF GALE CRITERIA. THE LOW IN THE NORTH PACIFIC IS
ANTICIPATED TO PUSH INTO THE GULF ON MONDAY AND POSSIBLY SETTLE INTO
THE SOUND AREA. ANOTHER PIECE TO ROLL INTO THIS PUZZLE TUESDAY.
DEPENDING ON THE EXACT POSITIONS OF THESE TWO PIECES...WEST OF THE
CHUGACH COULD GET A LOT OF ADDITIONAL SNOW OR LITTLE SNOW. THE FLOW
ALOFT REMAINS SOUTHERLY SO MOISTURE FROM THE GULF TO REMAIN THROUGH
MOST OF THE WEEK. AND WITH THE COLD AIR ALOFT...THE AIR MASS WILL
REMAIN CONDITIONALLY UNSTABLE.

FOR SOUTHWEST ALASKA...COLD AND CLEAR CONDITIONS MAINLY TO PERSIST
THROUGH EARLY NEXT WEEK. THE GRADIENT CURRENTLY PRODUCING MODERATE
NORTH TO NORTHEAST WINDS WILL LESSEN OVER THE MAINLAND TONIGHT AS
THE UPPER LOW MOVES OVERHEAD AND THE HIGH PRESSURE OVER THE SIBERIA
SETTLES INTO THE WESTERN BERING. NEAR 20 DEGREES BELOW NORMAL TEMPS
TO REMAIN THROUGH MID WEEK WITH LOWS IN THE 10 TO 20 BELOW RANGE. THE
MOVEMENT OF THE HIGH PRESSURE CENTER WILL CHANGE THE PRESSURE
GRADIENT TO PRODUCE A MORE NORTHERLY FLOW SOME INSTABILITY SHOWERS
DEVELOPING ALONG THE AKPEN STARTING TUESDAY.

FOR ALEUTIANS...THE UPPER LOW NEAR SHEMYA TO REMAIN IN THE AREA
THROUGH TONIGHT WITH SHOWERS AS WELL. THE PRESSURE GRADIENT TO LESSEN
OVER THE AREA MONDAY AS THE CENTER OF THE HIGH PRESSURE MOVES OVER
THE BERING. AS THE HIGH SETTLES OVER THE AREA AND NORTHERLY FLOW WILL
BRING MORE INSTABILITY SHOWERS.

.LONG TERM FORECAST...
AN AREA OF LOW PRESSURE TO REMAIN OVER THE GULF THROUGH FRIDAY WITH
SHOWERS ENCIRCLING THE AREA. THE LOW WILL MOVE EAST OVER NEXT
WEEKEND. THE HIGH PRESSURE OVER THE BERING WILL SLOWLY SLIDE EAST
THROUGH THE REMAINDER OF THE WEEK. A LOW PRESSURE SYSTEM WITH AN
ASSOCIATED FRONT TO MOVE INTO THE WESTERN BERING FRIDAY. ANOTHER LOW
TO FOLLOW THROUGH THIS AREA SATURDAY INTO SUNDAY AND HEAD TOWARD THE
KAMCHATKA PENINSULA.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...WINTER WEATHER ADVISORY ZONE 101 AND 111.
MARINE...HEAVY FREEZING SPRAY 130 140 150 160 165 179 411 412 413 414
         GALE WARNING 130 155 170 172 175 176 411 413 414.
FIRE WEATHER...NONE.

NELSON APR 13