Small Craft Advisory 553, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
115 PM AKDT FRI JUL 19 2013

.ANALYSIS AND UPPER LEVELS...THE SOUTHERN TIER OF THE FORECAST
AREAS FROM THE ALEUTIANS TO THE GULF OF ALASKA IS DOMINATED BY
STABLE HIGH AMPLITUDE WAVES WITH A BLOCKING HIGH CENTERED SOUTH OF
THE ALASKA PENINSULA...A CLOSED LOW SOUTH OF THE WESTERN ALEUTIANS...
AND ANOTHER RIDGE OVER THE NORTHWEST PACIFIC. A SURFACE LOW SPINNING
SOUTH OF THE ALEUTIANS HAS SPAWNED OFF A FRONTAL SYSTEM WHICH IS
HEADING NORTHWARD ACROSS THE ALEUTIANS INTO THE SOUTHERN BERING...
BRINGING WITH IT LOTS OF WARM AIR AND MOISTURE. SOME OF THIS IS
ROUNDING THE TOP OF THE RIDGE AND HEADING FOR SOUTHWEST ALASKA.
ASIDE FROM PRODUCING AREAS OF RAIN THE BERING AND ALEUTIANS
CONTINUE TO BE PLAGUED BY EXTENSIVE DENSE FOG.

MEANWHILE...IN THE NORTHERN STREAM FLOW A POSITIVELY TILTED TROUGH
CENTERED OVER THE NORTHERN BERING SEA IS HEADED EASTWARD TOWARD
WESTERN ALASKA. A STRONG UPPER JET DROPS SOUTHEASTWARD OUT OF
SIBERIA AND THEN EASTWARD ACROSS THE SOUTHERN ALASKA MAINLAND.
WAVES EMBEDDED IN THE FLOW OUT AHEAD OF THIS ARE PRODUCING SHOWERS...
PRIMARILY INLAND. AT THE SURFACE THERE IS A TIGHT PRESSURE GRADIENT
BETWEEN THE EASTERN BERING HIGH AND A TROUGH ALONG THE NORTH GULF
COAST WHICH IS PRODUCING STRONG WINDS ALONG THE ALASKA PENINSULA
AND INTO THE WESTERN GULF.

.MODEL DISCUSSION...THE NAM IS LAGGING A BIT WITH A VORT-MAX MOVING
THROUGH THE BASE OF THE NORTHERN BERING TROUGH. THIS RESULTS IN A
SIGNIFICANTLY SLOWER PROGRESSION OF THE TROUGH COMPARED WITH OTHER
MODELS. THE NAM IS ALSO AN OUTLIER WITH TRIPLE POINT LOCATION OVER
THE CENTRAL BERING AND DEPICTION OF SURFACE RIDGE OVER THE EASTERN
BERING. OTHERWISE...ALL OTHER MODELS ARE FAIRLY CLOSE WITH HANDLING
OF SYNOPTIC SCALE FEATURES.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE UPPER TROUGH OVER THE NORTHERN BERING
POSES THE MAIN FORECAST CHALLENGE. AT THE MOMENT THE TROUGH IS
POSITIVELY TILTED AND RAIN OUT AHEAD OF IT IS FAIRLY DISORGANIZED.
THE TROUGH WILL BECOME NEUTRALLY TILTED TONIGHT...BUT THEN WILL
SPLIT INTO TWO AS THE MAIN VORT-MAX MOVING THROUGH THE TROUGH
TRACKS RAPIDLY EAST-SOUTHEAST ACROSS SOUTHCENTRAL ON SATURDAY.
THIS MAKES IT VERY DIFFICULT TO PINPOINT WHEN AND WHERE RAIN WILL
FALL. BOTTOM LINE THOUGH...THIS WILL BE A MINOR RAIN EVENT IN AREAL
COVERAGE...DURATION...AND QPF. WILL TRY AND CONVEY THIS MESSAGE
IN THE TEXT PRODUCTS. IN THE WAKE OF THIS TROUGH A PIECE OF THE
NORTHWEST PACIFIC RIDGE WILL BUILD EASTWARD ACROSS THE BERING AND
CONNECT UP WITH THE PACIFIC RIDGE AND THEN PROGRESS EASTWARD INTO
MAINLAND ALASKA EARLY NEXT WEEK. WEAK UPPER WAVES TRANSITING THE
EASTERN PERIPHERY OF THE RIDGE WILL MAINTAIN SOME CLOUDS AND A
FEW DIURNAL SHOWERS ACROSS THE REGION THROUGH THE WEEKEND. A TREND
TOWARD WARMER AND SUNNIER CONDITIONS WILL THEN BEGIN ON MONDAY.

SOUTHWEST ALASKA...THE BERING TROUGH WILL MAINLY AFFECT THE
KUSKOKWIM DELTA AND KUSKOKWIM VALLEY TONIGHT WITH A BRIEF PERIOD
OF RAIN. BRISTOL BAY WILL BE ON THE SOUTHERN PERIPHERY OF THE
SHORT-WAVE AND CAN EXPECT JUST A FEW SHOWERS. THE WARM FRONT BACK
OVER THE SOUTH-CENTRAL BERING SEA WILL MOVE OVER TOP OF THE RIDGE
TOWARD BRISTOL BAY TONIGHT AND SATURDAY...BUT IT LOOKS LIKE THE
BULK OF RAIN WITH THIS SHOULD BE CONFINED TO THE ALASKA PENINSULA
WITH DRIER CONDITIONS AS YOU HEAD NORTHWARD FROM BRISTOL BAY. A
RIDGE WILL BUILD IN FROM THE WEST SUNDAY AND MONDAY WHICH WILL
PRODUCE WARM AND DRY CONDITIONS INLAND. WITH THE SURFACE HIGH
REMAINING TO THE WEST ONSHORE FLOW WILL BRING STRATUS AND FOG
ONSHORE...KEEPING COASTAL AREAS SIGNIFICANTLY COOLER.

BERING SEA/ALEUTIANS...THE LOW SOUTH OF THE ALEUTIANS WILL CONTINUE
TO PRODUCE PERIODS OF RAIN ALONG THE ALEUTIANS AND SOUTHERN BERING
SEA. WARM ADVECTION UP THE WEST SIDE OF THE RIDGE WILL ALSO MAINTAIN
WIDESPREAD DENSE FOG ACROSS THE REGION.

.LONG TERM FORECAST...FOR THE FIRST HALF OF NEXT WEEK THERE IS
ABOVE AVERAGE CONFIDENCE WITH HIGH AMPLITUDE RIDGE HOLDING STRONG
OVER THE EASTERN BERING/WESTERN MAINLAND. THIS WILL RESULT IN
MOSTLY DRY CONDITIONS FROM THE EASTERN BERING SEA TO SOUTHCENTRAL
ALASKA WITH WARM TEMPERATURES INLAND. MODELS QUICKLY DIVERGE BY
THE END OF NEXT WEEK IN DEPICTION OF AN UPPER LEVEL WAVE DROPPING
OUT OF THE ARCTIC ACROSS MAINLAND ALASKA SO FORECAST CONFIDENCE
DROPS SIGNIFICANTLY.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING 130.
FIRE WEATHER...NONE.

SEB JUL 13