Small Craft Advisory 741, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
200 PM AKDT WED SEP 25 2013

.ANALYSIS AND UPPER LEVELS...
A VERTICALLY STACKED LOW OVER THE EASTERN BERING IS WEAKENING AS IT
MOVES TOWARD THE WESTERN GULF TODAY. A SIGNIFICANT SHORTWAVE MOVED
NORTHWARD ALONG THE FRONT LAST EVENING AND RE-INVIGORATED SURFACE
PRESSURE GRADIENTS ACROSS THE REGION...RESULTING IN VERY STRONG GAP
WINDS AND HEAVY RAINFALL ACROSS MUCH OF SOUTHCENTRAL OVERNIGHT AND
THIS MORNING. THE FRONT HAS NOW MOVED THROUGH THE MAT-SU VALLEY AND
IS STALLING AND WEAKENING OVER PRINCE WILLIAM SOUND AND THE NORTH
GULF COAST. SHOWERS ARE BEGINNING TO FILL IN ACROSS COOK INLET AND
THE WESTERN GULF ASSOCIATED WITH THE LOW CENTER ITSELF AND ASSOCIATED
COLD AIR AND INSTABILITY ALOFT. MEANWHILE...GUSTY WEST WINDS AND
SHOWERS BEHIND THE LOW ARE GRADUALLY DIMINISHING OVER THE EASTERN
ALEUTIANS AND ALASKA PENINSULA. FURTHER WEST...AN UNUSUALLY STRONG
JET IS STRENGTHENING ACROSS THE WESTERN BERING...AND WILL BE A KEY
PLAYER IN WEATHER ACROSS THE REGION OVER THE SHORT TERM PERIOD.

.MODEL DISCUSSION...
MODELS ARE IN DECENT AGREEMENT WITH THE DECAYING LOW MOVING INTO THE
GULF. THE NAM HAS BEEN PERSISTENTLY FURTHER NORTH THAN THE GFS...A
SUBTLE DIFFERENCE BUT ONE THAT HAS RAMIFICATIONS FOR THE LONGEVITY OF
THE STRONG WINDS/BARRIER JET CURRENTLY AFFECTING PRINCE WILLIAM
SOUND. THE 12Z ECMWF SHOWS MULTIPLE CIRCULATION CENTERS...WHICH
PROBABLY IS THE MOST LIKELY SCENARIO CONSIDERING THE RAPIDLY
WEAKENING UPPER LEVEL CENTER. THUS WILL OPT FOR A NAM/GFS COMBO THAT
KEEPS A WEAKENED FORM OF THE BARRIER JET THROUGH THE OVERNIGHT
PERIODS. THE NEXT MAJOR LOW WILL BEGIN RAPIDLY INTENSIFYING BY
THURSDAY MORNING OVER THE NORTHERN BERING SEA WITH THE HELP OF AN
ANOMALOUSLY STRONG 215 KT ZONAL JET STREAM. THE MODELS CONTINUE TO
STRUGGLE WITH POSITION AND TIMING OF THIS LOW...AND MOST RECENTLY
HAVE ALL BEGUN TO TREND NORTHWARD WITH THE LOW POSITION BEFORE TAKING
AN ABRUPT SOUTHERLY TURN AND DROPPING INTO THE GULF. THE GFS SEEMS TO
HAVE HANDLED THIS BEST...WITH ADDED SUPPORT FROM THE 12Z ECMWF AND
12Z GEM GLOBAL SOLUTIONS. THE NAM IS THE MOST OBVIOUS OUTLIER...BEING
MUCH TOO FAR SOUTH WITH THE SURFACE LOW POSITION. WHILE MANY OF THE
DETAILS ARE STILL UP IN THE AIR...ONE THING WE CAN SAY CONFIDENTLY IS
THAT THIS WILL BE A VERY FAST MOVING AND POTENT STORM...WITH WINDS
APPROACHING HURRICANE FORCE ACROSS SOME SECTIONS OF THE EASTERN
BERING/ALASKA PENINSULA BEFORE ALL IS SAID AND DONE.

.SHORT TERM FORECAST...
SOUTHCENTRAL...AS MENTIONED ABOVE...REMNANT LOW PRESSURE AND COLD AIR
ALOFT WILL LEAD TO A CONTINUATION OF NUMEROUS RAIN SHOWERS TONIGHT
AROUND MUCH OF THE SOUTHERN MAINLAND. THE ONE EXCEPTION WILL BE THE
COPPER RIVER BASIN...WHERE DRIER CONDITIONS WILL PREVAIL. MOST AREAS
WILL DRY OUT SLOWLY DURING THE DAY THURSDAY...THOUGH BREAKS OF
SUNSHINE WILL CONTINUE TO BE FAIRLY HARD TO COME BY AS NUMEROUS WEAK
EMBEDDED SHORTWAVES MOVE THROUGH SOUTHWEST FLOW ALOFT. WITH A
SLIGHTLY MORE NORTHERN TRACK OF THE NEXT STORM AM NOW EXPECTING RAIN
TO SPREAD NORTHWARD THROUGH MUCH OF THE AREA BY FRIDAY MORNING.
HOWEVER...WITH UPPER LEVEL FLOW FROM THE SOUTHEAST EXPECT SIGNIFICANT
DOWNSLOPING WILL BE THE GENERAL PATTERN WHICH WILL CONFINE HEAVY
RAINFALL TO THE FAVORED UPSLOPE AREAS/COASTLINES.

SOUTHWEST...SHOWERS WILL CONTINUE TO DISSIPATE FROM WEST TO EAST
TONIGHT AS THE LOW MOVES INTO THE GULF. AFTER A VERY BRIEF REPRIEVE
THE NEXT STORM WILL SPREAD RAIN AND WIND TO THE COASTLINE THURSDAY
MORNING...WITH RAIN SPREADING INLAND DURING THE DAY. THE LOW IS
EXPECTED TO MOVE TO NUNIVAK ISLAND AND THEN LOOSELY PARALLEL THE
COASTLINE DOWN TOWARD BRISTOL BAY. THE STRONGEST WINDS WILL BE BEHIND
THE LOW CENTER AND ARE CURRENTLY EXPECTED TO STAY JUST OFFSHORE.
HOWEVER...WINDS ALONG THE COASTLINE WILL BE HIGHLY SENSITIVE TO
CHANGES IN THE LOW POSITION.

BERING SEA AND ALEUTIANS...STRONG WINDS OVER THE EASTERN BERING WILL
RAPIDLY DIMINISH THIS EVENING AS HIGH PRESSURE BRIEFLY MOVES INTO THE
REGION. THE NEXT LOW WILL REALLY START TO GET ITS ACT TOGETHER OVER
THE NORTHERN BERING BY EARLY THURSDAY MORNING. AS MENTIONED
ABOVE...THE RAPID STRENGTHENING OF THIS STORM WILL BE AMPLIFIED BY AN
EXCEPTIONALLY STRONG JET STREAM OVER THE NORTHERN BERING SEA. AT THIS
TIME IT LOOKS LIKE THE PRIBILOF ISLANDS WILL GET A GLANCING BLOW FROM
THE STORM AS THE LOW CENTER PIVOTS SOUTHWARD NEAR NUNIVAK ISLAND. THE
FULL STRENGTH OF THE STORM WILL STAY OVER THE OPEN WATER OF THE
EASTERN BERING SEA BEFORE MOVING ONSHORE THE ALASKA PENINSULA NEAR
PORT HEIDEN. WHILE THE CENTRAL PRESSURE OF THE STORM ITSELF WILL BE
FAIRLY STRONG IN THE UPPER 960S. THE MOST SIGNIFICANT CHARACTERISTIC
WILL BE A FORWARD SPEED OF NEARLY 40 MPH. IN ADDITION...A MODEST
AMOUNT OF COLD AIR WILL BE PULLED SOUTHWARD BEHIND THE STORM. ALL
THESE FACTORS WILL WORK TOGETHER TO PACK A POWERFUL
PUNCH...ESPECIALLY TO LOCATIONS IMMEDIATELY WEST OF THE LOW AS IT
CROSSES INTO THE GULF ON FRIDAY. BASED ON CONTINUED MODEL UNCERTAINTY
IN THE FORECAST TRACK...WILL LEAVE THE HIGH WIND WATCH FOR THE ALASKA
PENINSULA LARGELY AS-IS WITH THE AFTERNOON FORECAST. STAY TUNED FOR
UPDATES TO THE FORECAST OVER THE NEXT 36 HOURS.

.LONG TERM FORECAST...
THE UPPER TROUGH OVER THE MAINLAND NOW APPEARS THAT IT WILL PERSIST
AT LEAST THROUGH THE MIDDLE OF NEXT WEEK. THIS WILL KEEP TEMPERATURES
NEAR OR SLIGHTLY-BELOW NORMAL FOR MOST AREAS...ESPECIALLY OVER THE
SOUTHWEST MAINLAND...WITH THE STORM TRACK CONTINUING TO TAKE LOWS
ACROSS THE BERING SEA AND THEN DROPPING SOUTHWARD INTO THE GULF.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...HIGH WIND WATCH 181
         FLOOD ADVISORY 101
MARINE...STORM 160 165 170 172 175
         GALE 150 155 171 176 179 180
FIRE WEATHER...NONE

AMD SEP 13