Small Craft Advisory 291, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
429 PM AKDT THU APR 2 2015

.ANALYSIS AND UPPER LEVELS...

UPPER LEVEL RIDGES ARE OVER THE BERING AND NORTH AMERICAN WEST
COAST THIS AFTERNOON...WITH A LARGE CUT-OFF LOW COVERING THE GULF OF
AK AND A TROUGH MOVING EAST OVER KAMCHATKA. A WAVE OVER NORTHWEST
AK IS SLIDING SE. THE RIDGE HAS BEEN BUILDING OVER THE BERING
DURING THE DAY...A PRECURSOR TO AN UPCOMING PATTERN CHANGE. AT
THE SURFACE...A WEATHER FRONT IS DRAPED ALONG THE ENTIRE GULF
COAST...ASSOCIATED WITH A LOW WELL SOUTH OF THE GULF. THIS FRONT
CONTINUES TO BRING RAIN TO CORDOVA AND SOME GUSTY OUTFLOW WINDS TO
THE SEWARD AND WHITTIER AREAS. A RIDGE OF HIGH PRESSURE IS OVER
THE EASTERN BERING....WITH A RAIN PRODUCING FRONT ALONG THE
WESTERN FLANK.

&&

.MODEL DISCUSSION...

MODELS ARE IN EXCELLENT AGREEMENT THAT A PATTERN CHANGE IS
UNDERWAY. AS THE KAMCHATKA TROUGH MOVES EAST IT WILL CAUSE THE
BERING RIDGE TO BUILD...WHICH IN TURN WILL SLOWLY CAUSE THE CUT-
OFF LOW OVER THE BERING TO EXIT SOUTHEAST ALONG WITH ITS
ASSOCIATED SURFACE LOW. THIS WILL PULL THE GULF FRONT AWAY FROM THE
MAINLAND ON FRI.

ALSO ON FRI THE KAMCHATKA TROUGH CLOSES INTO AN UPPER LOW THAT
THEN MOVES OVER THE KUSKOKWIM DELTA ON SAT...VERTICALLY STACKED
OVER A SURFACE LOW THAT MOVES FROM THE WESTERN ALEUTIANS TO THE
DELTA IN THE SAME PERIOD. A TRIPLE POINT LOW FROM THIS FEATURE
DEVELOPS EAST OF THE AKPEN ON SAT.

BY SAT AFTERNOON THE UPPER CONFIGURATION WILL CONSIST OF A NEW
TROUGH OVER KAMCHATKA...A BROAD RIDGE FROM THE WESTERN BERING TO
THE EASTERN PACIFIC WITH A DECAYING LOW OVER SW AK...AND THE
FORMER GULF LOW APPROACHING THE PACIFIC NORTHWEST.

WHILE THIS IS HAPPENING...THE NEXT KAMCHATKA LOW AND ASSOCIATED
FRONT APPROACH THE WESTERN ALEUTIANS SAT AFTERNOON.

THE FORECAST UTILIZES THE CONSISTENCY OF THE GFS/EC SOLUTION
THROUGH SAT.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (TONIGHT THROUGH SAT NIGHT)...

THE MULTI-CENTERED AREA OF LOW PRESSURE IN THE SOUTHERN GULF
CONTINUES TO PUSH SOUTH INTO THE NORTH PACIFIC. THIS HAS LED TO A
STEADY DECREASE IN RAINFALL ACROSS THE NORTHERN GULF COAST AS
DRIER AIR ASSOCIATED WITH A MID/UPPER LEVEL RIDGE CLASHES WITH THE
DEPARTING UPPER LEVEL LOW IN THE SOUTHERN GULF OF ALASKA. AS THE
UPPER LOW SLIDES EASTWARD...THE UPPER LEVEL RIDGE WILL BEGIN TO
SITUATE ITSELF ACROSS SOUTHCENTRAL ALASKA AND BRING CLEARING SKIES
STARTING TONIGHT. THIS WILL ALLOW MUCH OF SOUTHCENTRAL ALASKA TO
BE DRENCHED IN SUNSHINE FRIDAY AND SATURDAY BEFORE THE NEXT AREA
OF LOW PRESSURE AND ASSOCIATED FRONT MOVE INTO THE WESTERN GULF AND
BRINGS RAIN CHANCES BACK TO KODIAK ISLAND AND INCREASING CLOUDS
TO THE REST OF SOUTHCENTRAL ALASKA.

THE ATMOSPHERE HAS ALREADY BEGUN TO STABILIZE AS THE SFC AND UPPER
LEVEL LOW DEPART TO THE SOUTH WHICH SHOULD HELP LIMIT AFTERNOON
CONVECTION ACROSS SOUTHCENTRAL ALASKA. HOWEVER...SCATTERED RAIN
AND SNOW SHOWERS WILL BE POSSIBLE ALONG THE TALKEETNA MOUNTAINS
AND THE ALASKA RANGE THROUGH SATURDAY AS THE UPPER LEVEL TROUGH
STRUGGLES TO MAKE PROGRESS EASTWARD IN THAT VICINITY. SFC PRESSURE
IS EXPECTED TO RISE QUICKLY BEHIND THE DEPARTING LOW IN THE GULF
TONIGHT AND INTO FRIDAY. THIS SHOULD LOW FOR A PUSH OF ISALLOBARIC
FLOW THROUGH GAPS (WHITTIER...THOMPSON PASS...SEWARD) DURING THIS
TIME FRAME. SEA BREEZES LOOKS TO BE HELD IN CHECK AGAIN ON FRIDAY
WITH TEMPERATURES NEAR 50 DEGREES EXPECTED IN BOTH SEWARD AND VALDEZ.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THE UPPER LEVEL RIDGE BUILDING OVER THE BERING WILL CONTINUE TO
AMPLIFY...TURNING THE LOW LEVEL FLOW OVER SOUTHWEST ALASKA MORE
NORTHERLY TONIGHT AND NORTHEAST FRIDAY AS THE UPPER RIDGE AXIS
REACHES THE WEST COAST. THIS SHIFT IN THE FLOW TO A MORE OFFSHORE
DIRECTION SHOULD FINALLY END THE LIGHT SNOW ALONG THE KUSKOKWIM
DELTA COAST THIS EVENING AND MORE RAPIDLY CLEAR OUT CLOUDS FOR A
MOSTLY SUNNY DAY FRIDAY. THE NEXT POTENTIAL IMPACT IS THE FRONTAL
SYSTEM APPROACHING THE WEST COAST LATE FRIDAY NIGHT AND PUSHING
ONSHORE SATURDAY. WITH COLD AIR IN PLACE AND OFFSHORE FLOW AT THE
SURFACE...PRECIPITATION TYPE...MOST LIKELY SNOW BUT ALSO POSSIBLY
MIXED WITH FREEZING RAIN...WILL BE A CONCERN PARTICULARLY FOR THE
KUSKOKWIM DELTA AREA.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE NEXT LOW WILL CROSS INTO THE BERING FRIDAY AND THEN DEEPEN AS
IT CONTINUES NORTHEAST FRIDAY AND FRIDAY NIGHT BRINGING AN AREA
OF GALES WITH IT AS IT CROSSES THE BERING. THE NEXT AND
POTENTIALLY STRONGER SYSTEM WILL REACH THE WESTERN ALEUTIANS
SATURDAY AFTERNOON PROPELLED BY A RATHER STRONG 100 KT WESTERLY
500 MB JET.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7...SUN THROUGH THU)...THIS
TIME FRAME WILL FEATURE A FAIRLY ACTIVE ZONAL PATTERN. SYSTEM
AFTER SYSTEM WILL MOVE FROM THE WESTERN BERING...CROSS THE
BERING...AND THEN IMPACT THE WESTERN HALF OF THE MAINLAND WITH
SOME LEFTOVER ENERGY CROSSING THE AKPEN INTO THE GULF OF AK.
MODELS ARE IN SURPRISINGLY GOOD AGREEMENT WITH THIS INCREASINGLY
ACTIVE PATTERN WITH THE GLOBAL MODELS OUT-PERFORMING THE NAM AT
THIS TIME.

THINGS WILL SLOWLY DRY OUT ACROSS THE WEST EARLY SUNDAY AS THE
FIRST FRONTAL BOUNDARY FALLS APART AND MOVES EAST. THIS COULD
POSSIBLY BRING A FEW RAIN AND SNOW SHOWERS ACROSS PARTS OF SOUTH
CENTRAL...BUT THE BEST CHANCE FOR PRECIPITATION WILL BE THE
EASTERN COPPER RIVER BASIN AND THE COASTAL AREAS. ANOTHER
OCCLUDING WARM FRONT WILL BE CLOSE ON ITS HEELS IN THE BERING AND
SHOULD REACH THE WEST COAST BY SUNDAY NIGHT. THIS WILL BE A VERY
BROAD SYSTEM WHICH SHOULD ALLOW FOR SOME REASONABLY COLD AIR TO BE
PULLED DOWN ON ITS WESTERN FLANK. 850MB TEMPS WILL FALL TO -10 TO
-15 C BY LATE MONDAY. THIS THERMAL GRADIENT WILL HELP TO CREATE
SOME STORM FORCE WINDS WHICH SHOULD BE ABLE TO MIX DOWN TO THE
SURFACE IN THE COLD AIR ADVECTION. THIS SYSTEM WILL BRING STEADY
MIXED PRECIPITATION TO MUCH OF THE BRISTOL BAY AND YK DELTA AREAS
ALL DAY MONDAY.

HOW THIS ENERGY TRANSLATES ACROSS THE ALASKA AND ALEUTIAN RANGES
INTO SOUTH CENTRAL IS WHERE WE START TO SEE SOME MODEL
DIFFERENCES. THE EC SHOWS THAT COLDER AIR ON THE BACKSIDE OF THE
LOW HELPING TO DIG THE UPPER LEVEL TROUGH FURTHER WEST AND SOUTH.
THIS LEADS TO THE LOW CENTER MAKE A RATHER BENIGN CROSSING INTO
THE GULF OF AK AND THEN TRACKING SOUTHEAST INTO THE N PAC.
MEANWHILE...THE 18Z GFS AND 12Z GEM GLOBAL KEEP THE SURFACE LOW
MUCH FURTHER NORTH NEAR NUNIVAK ISLAND MONDAY MORNING. IF THIS
SCENARIO WERE TO PAN OUT...THERE WOULD BE A MUCH BETTER CHANCE OF
STEADY PRECIPITATION TRACKING UP COOK INLET UNDER SOUTH-SOUTHWEST
FLOW ON TUE. EVEN SO...IT DOES LOOK LIKE MOST OF THE
PRECIPITATION WOULD BE RAIN FOR MAJOR POPULATION CENTERS.

TUE-WED YET ANOTHER FRONT IS ROLLING INTO THE PICTURE FROM THE
WESTERN BERING. THIS ONE WILL ALSO MAKE A FAST TREK ACROSS THE
BERING AND WILL LIKELY HAVE SOME FORM OF A TRIPLE POINT LOW
DEVELOP NEAR OR JUST SOUTH OF KODIAK BY THURSDAY MORNING. SO AFTER
A QUICK RESPITE FROM THE PRECIPITATION FOR THE WEST ON
WED...ANOTHER ROUND OF MOISTURE HITS THE COAST BY THU.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 155 165 170 171 172 173 174 175 176 177 178 179 185
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...MC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...MO