Small Craft Advisory 737, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
447 AM AKDT MON SEP 21 2015

.ANALYSIS AND UPPER LEVELS...

UPPER LEVEL TROUGH EXTENDS FROM EASTERN RUSSIA TO JUST EAST OF THE
ALCAN BORDER THIS MORNING. ONE LARGE CLOSED LOW COVERS MAINLAND
AK AND THE NORTHERN GULF...AND A SECOND IS AT THE BASE OF THE
TROUGH SOUTH OF THE GULF. A WEAK WAVE IS MOVING ALONG THE WESTERN
FLANK OF THE UPPER TROUGH INTO THE CENTRAL BERING. AN UPPER RIDGE
IS CENTERED OVER THE WESTERN ALEUTIANS AND BERING. AT THE
SURFACE...A BROAD TROUGH IS OVER THE GULF WITH A LOW CENTERED
SOUTH OF PRINCE WILLIAM SOUND...AND A WEAK LOW IS OVER THE NORTH
CENTRAL BERING. THE LATTER LOW IS ATTEMPTING TO CUT THROUGH A
SURFACE HIGH THAT COVERS THE ENTIRE BERING.

HIGH PRESSURE IN THE BERING AND LOW PRESSURE IN THE GULF ARE MAINTAINING
A PRESSURE GRADIENT ACROSS THE EASTERN ALEUTIANS AND AKPEN. WINDS
BLOWING THROUGH CHANNELED TERRAIN OF THE AKPEN ARE REACHING SMALL
CRAFT ADVISORY LEVELS IN DOWNSTREAM MARINE AREAS. OTHERWISE SOME
RAIN AND SNOW SHOWERS ARE MOVING ALONG THE WESTERN FLANK OF THE
UPPER TROUGH OVER SOUTHWEST AK. RAIN SHOWERS AND AN EMBEDDED
THUNDERSTORM ARE MOVING ACROSS PRINCE WILLIAM SOUND...AS THEY
ROTATE AROUND THE LOW TO THE SOUTH.

&&

.MODEL DISCUSSION...

MODELS ARE IN AGREEMENT THAT THE LOW IN THE BERING WILL DISSIPATE
AS IT REACHES THE EASTERN ALEUTIANS BY MIDDAY TODAY. HIGH PRESSURE
WILL THEN RECONSOLIDATE OVER THE BERING AND THE PRESSURE GRADIENT
OVER THE AKPEN WILL STRENGTHEN BRIEFLY THIS AFTERNOON. LOW
PRESSURE IN THE GULF WILL SLOWLY WEAKEN AS IT MOVES TO SOUTHEAST
AK ON TUE. THE NEXT FEATURE WILL BE A FRONT THAT MOVES ACROSS THE
NORTHERN AND CENTRAL BERING BEGINNING TUE AFTERNOON. ONCE AGAIN
THE NAM AND GEM WERE USED TO PROVIDE ADDITIONAL DETAIL...ESPECIALLY
REGARDING THE WIND FIELD THROUGH CHANNELED TERRAIN.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
A BROAD UPPER LEVEL TROUGH WITH WEAK EMBEDDED WAVES CONTINUES TO
PRODUCE A FEW SHOWERS ACROSS SOUTHCENTRAL...PRIMARILY IN THE
PRINCE WILLIAM SOUND REGION AND ALONG THE MOUNTAINS. A PATTERN
CHANGE IS ON THE WAY WITH A STRONG HIGH AMPLITUDE RIDGE OVER THE
WESTERN BERING BEGINNING TO PROGRESS EASTWARD TOWARD MAINLAND
ALASKA. A POLAR JET ON THE LEADING EDGE OF THE RIDGE IS DRIVING
ARCTIC AIR SOUTHWARD ACROSS THE EASTERN BERING AND WESTERN ALASKA.
SOME OF THIS AIR IS BEGINNING TO SPILL ACROSS THE ALASKA PENINSULA
INTO THE WESTERN GULF BEHIND A SHORTWAVE TROUGH. THIS WILL LEAD
TO STRONG GUSTY WINDS ALONG SHELIKOF STRAIT AND PORTIONS OF KODIAK
ISLAND.

AS THE RIDGE NEARS SOUTHCENTRAL TONIGHT INTO TUESDAY FLOW WILL
BECOME MORE NORTHERLY AND ARCTIC AIR WILL BEGIN TO FILTER DOWN
FROM INTERIOR ALASKA LEADING TO STRENGTHENING WINDS AREA-WIDE.
WINDS WILL PEAK MOST PLACES TUESDAY NIGHT INTO WEDNESDAY AS THE
POLAR JET CROSSES SOUTHCENTRAL. UPPER LEVEL FORCING IS LOOKING
MORE IMPRESSIVE THAN IT DID IN PRIOR DAYS...WITH A SHORTWAVE
TROUGH AND JET STREAK DROPPING DOWN FROM NORTHERN ALASKA WITH
ASSOCIATED INCREASING DOWNWARD MOTION. THEREFORE...HAVE TRENDED
STRONGER WITH WINDS...ESPECIALLY THROUGH COASTAL GAPS SUCH AS
VALDEZ.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THE LINGERING COLD UPPER TROUGH OVERHEAD WILL BE THE FOCUS FOR
ANY PRECIPITATION OVER THE NEXT COUPLE DAYS. FORCING FOR SYNOPTIC ASCENT
IS RATHER MEAGER SO THE BEST CHANCE FOR SHOWERS WILL BE DAYTIME
INSTABILITY OVER THE TERRAIN. EXPECT SCATTERED SNOW AND RAIN
SHOWERS OVER MAINLY THE KUSKOKWIM MOUNTAINS AND ALASKA RANGE WITH
ISOLATED SHOWERS ELSEWHERE. THE KUSKOKWIM DELTA COAST WILL SEE
ISOLATED TO SCATTERED SHOWERS WHILE THE NORTHERLY FLOW CONTINUES. CLOUDS
WILL SLOWLY DECREASE TODAY AND TOMORROW AS NORTHERLY WINDS BEGIN
TO LIGHTEN UP AS WELL. BY LATE TUESDAY THE COLD AIR ALOFT BEGINS
TO MODERATE AS THE UPPER TROUGH EXITS TO THE EAST AND A WEAK FRONT
APPROACHES FROM THE WEST.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
A SHARP DISTURBANCE PROPAGATING THROUGH THE NORTHWESTERLY FLOW
OVER THE LOW LEVEL BAROCLINIC ZONE WILL BE THE MAIN PLAYER IN
ACTIVE WEATHER EARLY TODAY. THIS DISTURBANCE WILL BRING RAIN TO
THE PRIBILOFS EARLY FOLLOWED BY THE EASTERN ALEUTIANS AND ALASKA
PENINSULA THIS MORNING. THINGS GET QUIETER ON TUESDAY BUT COLD
NORTHERLY FLOW OVER CLIMATOLOGICALLY WARM SEA SURFACE TEMPERATURES
WILL PROMOTE ISOLATED TO SCATTERED SHOWERS OVER THE EASTERN BERING
SEA. HIGH PRESSURE WILL SIT OVER THE WESTERN ALEUTIANS THROUGH THE
SHORT TERM WHILE A FRONT AND UPPER DISTURBANCE WILL TRAVERSE THE
NORTHERN BERING SEA THROUGH LATE TUESDAY BRINGING STEADY RAIN.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THE MODELS REMAIN IN VERY GOOD AGREEMENT THAT RIDGING FROM THE
CENTRAL BERING SEA INTO THE MAINLAND ON TUESDAY WILL CONTINUE
INTO THURSDAY. TO THE SOUTH OF THIS RIDGE...A WEAK LOW PRESSURE
SYSTEM WILL REMAIN OVER THE SOUTHERN GULF...PROVIDING ENOUGH OF A
GRADIENT FOR WEAK OFFSHORE FLOW TO THE SOUTH MAINLAND BUT WITH
OTHERWISE MOSTLY CLEAR AND COOL CONDITIONS. ON THURSDAY...MODEL
AGREEMENT BEGINS TO BREAK DOWN WITH THE TRACK OF A FRONTAL SYSTEM
THAT LOOKS TO APPROACH THE WEST COAST FROM SIBERIA...ESPECIALLY
WITH REGARDS TO THE SPEED OF THIS SYSTEM. IT DOES NOW APPEAR
LIKELY THAT THE FRONT WILL BRING RAIN TO THE SOUTHWEST MAINLAND
ON THURSDAY...HOWEVER MODEL AGREEMENT DIMINISHES SIGNIFICANTLY ON
FRIDAY WITH BOTH HOW FAST AND HOW DEEP THE TROUGH WILL DIG INTO
THE MAINLAND. THE GFS HAS BEEN CONSISTENTLY FASTER AND FURTHER
NORTH...KEEPING MUCH OF THE PRECIPITATION NORTH OF THE AOR.
MEANWHILE THE EC HAS BEEN MUCH SLOWER AND DEEPER WITH THE
TROUGH...BRINGING WIDESPREAD RAIN TO SOUTHCENTRAL BY NEXT
WEEKEND. THE GFS HAS SINCE TRENDED SLOWER WITH THIS SYSTEM...HOWEVER
THERE STILL REMAINS SIGNIFICANT UNCERTAINTY WITH HOW FAR SOUTH
PRECIPITATION WILL DEVELOP. AS A RESULT...THE PRECIPITATION
FORECAST AROUND SOUTHCENTRAL WILL REMAIN CHANCE TO SLIGHT CHANCE
FOR FRIDAY/SATURDAY...BUT FURTHER MODEL TRENDS WILL CONTINUE TO BE
MONITORED TO SEE IF THIS NEEDS TO BE CHANGED.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ML
LONG TERM...DEK