Small Craft Advisory 671, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
459 AM AKDT WED AUG 26 2015

.ANALYSIS AND UPPER LEVELS...

IR SATELLITE AND RADAR IMAGERY SHOW AN EXTENSIVE AREA OF CLOUDS
AND PRECIPITATION MOVING INTO WESTERN ALASKA THIS MORNING. WHILE
MOST OF THE FORCING FOR THE HEAVIEST PRECIPITATION IS STAYING
NORTH OF SOUTHWEST ALASKA...RADAR IMAGERY FROM BETHEL AND KING
SALMON SHOW BANDS OF SHOWERS MOVING EAST ACROSS THE KUSKOKWIM
VALLEY AND DELTA...WITH THE HEAVIEST PRECIPITATION IN THE DELTA.
OVER SOUTHCENTRAL... SCATTERED SHOWERS ARE DEVELOPING FROM THE
SUSITNA VALLEY SOUTH THROUGH ANCHORAGE. BIOLOGICAL CONTAMINATION
OF THE RADAR ECHOES IS MAKING RAIN DETERMINATION MUCH MORE
DIFFICULT...ESPECIALLY SINCE THE RAINFALL ACTIVITY OVER SOUTHCENTRAL
IS SO LIGHT. A DEVELOPING LEESIDE TROUGH OVER THE WESTERN GULF
WILL BECOME A BIG PLAYER IN THE FUTURE WEATHER AS IT INTENSIFIES
OVER THE COMING DAYS.

MEANWHILE OUT WEST...A STRONG VERTICALLY STACKED HIGH CENTERED
SOUTH OF THE EASTERN ALEUTIANS REMAINS STATIONARY. IT HAS BEEN
RESPONSIBLE FOR PUMPING VERY WARM AND MOIST SUBTROPICAL AIR IN THE
MIDDLE AND UPPER LEVELS NORTHWARD ACROSS THE CENTRAL AND WESTERN
BERING. THIS IS HELPING TO MOVE A STRONG 150 KT JET STREAK
NORTHWARD AS THE UPPER RIDGE FLATTENS OUT OVER THE NORTHERN
BERING. AT THE SURFACE... A WEAK SURFACE LOW IS RAPIDLY MOVING
EAST ACROSS THE BERING STRAIT. THE COMBINATION OF THE JET STREAK
AND THE SURFACE LOW ARE ADVECTING MOISTURE INTO SOUTHWEST ALASKA.

&&

.MODEL DISCUSSION...

THE EC HAS COME INTO EXCELLENT AGREEMENT WITH THE AMERICAN MODELS
CONCERNING BOTH THE TIMING AND INTENSITY OF THE PRECIPITATION
ASSOCIATED WITH THE PASSAGE OF BOTH THE SHORTWAVE AND THE SOUTHWARD
MOVEMENT OF THE JET STREAK THROUGH THURSDAY. THE MOST NOTEWORTHY
MODEL DIFFERENCE CONCERNS THE DEVELOPMENT OF A SURFACE LOW OVER
THE NORTHERN GULF ON THURSDAY AS A SHORTWAVE IN THE POLAR JET DIVES
SOUTH. THE SHORTWAVE WILL INITIATE A MERGER OF THE POLAR AND
SUBTROPICAL JETS. THE GAP WINDS WILL INCREASE IN RESPONSE TO THE
RAPIDLY TIGHTENING PRESSURE GRADIENT BETWEEN THE GULF LOW AND THE
SUBTROPICAL HIGH ANCHORED SOUTH OF THE EASTERN ALEUTIANS. THE EC IS
NOT AS AGGRESSIVE AS THE AMERICAN MODELS AT DEVELOPING THE
SURFACE LOW. THUS THE EC WAS ELIMINATED IN FAVOR OF THE
GFS...WHICH BETTER DEPICTS MORE JET ENERGY ROUNDING THE BASE OF
THE DIGGING TROUGH OVER THE EASTERN GULF THU NIGHT.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
A WEATHER SYSTEM WILL QUICKLY WORK ITS WAY ACROSS SOUTH CENTRAL
ALASKA TODAY IN THE FAST WESTERLY FLOW ALOFT. THERE IS PLENTY OF
MOISTURE ASSOCIATED WITH THIS SYSTEM AND QPF TOTALS SHOULD BE
DECENTLY HIGH. A BRIEF BREAK IS EXPECTED TONIGHT BEFORE THE NEXT
SYSTEM ARRIVES ON THURSDAY. THE FLOW ALOFT WILL BECOME
NORTHWESTERLY AS THE ALEUTIAN RIDGE RETROGRADES. THIS NEXT SYSTEM
WILL NOT BE AS MOISTURE LADEN AS THE WEDNESDAY ONE. HOWEVER...THIS
SYSTEM DOES HAVE A STRONG VORTICITY MAXIMUM. IT WILL ALSO HAVE A
FAIRLY STRONG COLD FRONT DUE TO THE NORTHWESTERLY FLOW. STRONG
WINDS WILL DEVELOP LATE THURSDAY AS THE SURFACE LOW DROPS INTO THE
GULF AND INTENSIFIES. GAP FLOW WINDS WILL BE ESPECIALLY STRONG FOR
THIS TIME OF YEAR. 850 MB TEMPERATURES WILL DROP BELOW ZERO OVER
MOST OF SOUTH CENTRAL BY LATE THURSDAY NIGHT. IT IS POSSIBLE THAT
SOME OF THE HIGHER ELEVATIONS (ABOVE 2500 FEET) COULD SEE A
DUSTING OF SNOW AT THAT TIME...THE ATMOSPHERE DOES DRY OUT FAIRLY
QUICKLY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
WIDESPREAD STRATIFORM RAIN WILL TAPER OFF TO SCATTERED SHOWERS
FROM WEST TO EAST THROUGH THIS AFTERNOON AS THE FIRST IN A SERIES
OF WAVES EMBEDDED IN MOISTURE RICH ZONAL FLOW EXTENDING FROM THE
BERING SEA TRACKS INTO THE INTERIOR. ANOTHER PAIR OF DISTURBANCES
WILL THEN MOVE OVERHEAD IN RAPID SUCCESSION LATE TONIGHT INTO
THURSDAY MORNING AND PROVIDE ANOTHER QUICK SHOT OF ENHANCED
RAINFALL BEFORE DIVING SOUTHEAST INTO THE GULF. THE SECOND OF
THESE WAVES WILL USHER IN A DRYING AND COOLING TREND AS NORTHWEST
FLOW TAKES HOLD HEADING INTO THE END OF THE WEEK.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
PRECIPITATION WILL GRADUALLY DIMINISH IN COVERAGE ACROSS THE
BERING THROUGH THURSDAY NIGHT AS THE PATTERN SLOWLY AMPLIFIES AND
THE NORTH PACIFIC RIDGE RETROGRADES WESTWARD. THIS WILL KEEP MOST
OF THE RAINFALL CENTERED OVER THE WESTERN TIP OF THE ALEUTIANS AS
DEEP SUBTROPICAL MOISTURE CONTINUES TO STREAM NORTHWARD ON THE
WESTERN PERIPHERY OF THE RIDGE. A FEW SHOWERS WILL ALSO DEVELOP
ALONG THE ALASKA PENINSULA IN BRISK NORTHWEST FLOW ACROSS THE
EASTERN BERING. OTHERWISE...WIDESPREAD STRATUS AND FOG WILL
CONTINUE TO IMPACT MUCH OF THE REGION THROUGH THE END OF THE WEEK
BENEATH THE STRONG HIGH.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...

BEGINNING THE EXTENDED PERIOD ON SATURDAY...ANOTHER VIGOROUS
SHORTWAVE TROUGH MOVES INTO SOUTHERN ALASKA LATE SATURDAY AFTERNOON
INTO SUNDAY. THERE IS ENOUGH DISCREPANCY WITH HOW THE LOW DEVELOPS
AND THE EXACT TRACK...WHICH WILL DRAMATICALLY INFLUENCE THE
PRECIPITATION DISTRIBUTION AND STRENGTH OF SURFACE WINDS IN THE
GULF. THE ECMWF AND GFS DIFFER ON THESE DETAILS...WITH THE ECMWF
CURRENTLY SUGGESTING A FARTHER SOUTH TRACK THROUGH THE CENTRAL
GULF...AND THE GFS FAVORING A NORTHERN GULF SOLUTION. BOTH OF
THESE SOLUTIONS HINT AT A STRONG GALE FORCE LOW TRACKING THROUGH
THE GULF...BUT THE CURRENT FORECAST REMAINS MUCH MORE CONSERVATIVE
GIVEN UNCERTAINTY. EITHER WAY... THIS SYSTEM WILL CLEAR THROUGH BY
SUNDAY EVENING WITH DRIER WEATHER ON MONDAY. THEREAFTER MUCH
UNCERTAINTY WITH THE REMNANTS OF FORMER TYPHOON ATSANI CLOUDS THE
FORECAST FOR NEXT WEEK...WITH A MORE GENERAL WPC ENSEMBLE APPROACH
FAVORING FLAT RIDGING MOVING EAST ACROSS THE BERING.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...JA