Small Craft Advisory 563, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
418 PM AKDT TUE JUL 14 2015

.ANALYSIS AND UPPER LEVELS...

THE DOMINANT FEATURE IN THE UPPER LEVELS IS AN EXPANSIVE TROUGH
WITH TWO LOBES THAT IS BEGINNING THE PROCESS OF RETROGRADING AND
MERGING. THE EASTERN LOBE OF THE TROUGH... CENTERED OVER THE
NORTHERN ALASKA PENINSULA... HAS KEPT THE UPPER LEVELS COLD FOR
THE PAST SEVERAL DAYS WHICH HAS INCREASED THE TOTAL ATMOSPHERIC
INSTABILITY OVER MUCH OF SOUTHERN ALASKA. THIS IN TURN HAS ALLOWED
FOR AFTERNOON SCATTERED THUNDERSTORM DEVELOPMENT AS WEAK
DISTURBANCES PASSED THROUGH WITH TERRAIN INTERACTION.

THE BROAD CYCLONIC FLOW ASSOCIATED WITH THE TROUGH AND A WEAK
SURFACE LOW ALSO CENTERED OVER THE NORTHERN ALASKA PENINSULA IS
CONTINUING TO PUMP GULF MOISTURE ON SOUTHEASTERLY FLOW INTO THE
PRINCE WILLIAM SOUND AND COASTAL KENAI PENINSULA. RESIDUAL CLOUD
COVER IS CROSSING OVER THE KENAI MOUNTAINS INTO THE COOK INLET
REGION. CONVECTIVE DEVELOPMENT HAS ALREADY BEGUN WHERE MORNING
SUNSHINE AND A MOIST ATMOSPHERE HAVE COUPLED WITH TOPOGRAPHIC
ENHANCEMENT BOTH ALONG THE EASTERN SLOPES OF THE KILBUCK
MOUNTAINS AND ALONG THE WRANGELL MOUNTAINS AND THE ALASKA RANGE
IN THE COPPER RIVER BASIN.

&&

.MODEL DISCUSSION...
MODEL AGREEMENT THROUGH 24 HOURS REMAINS PRETTY GOOD. ALL OF THEM
AGREE THAT THE EASTERN LOBE OF THE UPPER LEVEL TROUGH OVER THE
ALASKA PENINSULA WILL WEAKEN AND RETROGRADE WESTWARD TO MERGE WITH
THE WESTERN LOBE SOUTH OF THE CENTRAL ALEUTIANS. MEANWHILE THE
WESTERN LOBE WILL AMPLIFY AND DIG SOUTHWARD INTO A POSITIVELY
TILTED TROUGH. THAT WILL RAPIDLY INTENSIFY A SOUTHWEST TO
NORTHEAST ORIENTED JET STREAK ALONG THE ALASKA PENINSULA DURING
THE DAY THURSDAY. THIS GENERAL SCENARIO IS AGREED UPON BY ALL OF
THE MODELS...BUT DIFFERENCES IN THE DETAILS EMERGE QUICKLY IN THIS
PATTERN. THE NAM IS SLOWER THAN THE GLOBAL MODELS (ECMWF/GFS) WITH
BRINGING THE PRECIPITATION SHIELD INTO SOUTHCENTRAL...THUS THE
GLOBAL MODELS WERE FAVORED.

&&

.FIRE WEATHER...
THE THERMAL TROUGH IS POSITIONED OVER THE UPPER KUSKOKWIM
VALLEY EXTENDING TOWARDS THE LOWER KUSKOKWIM VALLEY. THIS
FEATURE WILL PRODUCE ISOLATED WET THUNDERSTORMS THIS EVENING. BY
WEDNESDAY AFTERNOON/EVENING HOURS THERE WILL BE SOME ISOLATED WET
THUNDERSTORMS OVER THE AHKLUN AND KUSKOKWIM MOUNTAINS AS THE
THERMAL TROUGH MOVES FARTHER NORTH. BY LATE WEDNESDAY EVENING A
WEATHER FRONT IN THE EASTERN BERING SEA WILL ENTER THE REGION.
THIS WILL PUT A DAMPER ON ANY WET THUNDERSTORM ACTIVITY DUE TO THE
ONSHORE FLOW AND MARINE LAYER MOISTURE MOVING INTO THE INTERIOR.

THE SOUTHCENTRAL ALASKA REGION WILL HAVE ISOLATED WET THUNDERSTORMS
OVER THE NORTHWESTERN COPPER RIVER BASIN AND THE TALKEETNA MOUNTAINS
THIS EVENING. THIS CONVECTIVE ACTIVITY WILL COINCIDE WITH AN UPPER
LEVEL DISTURBANCE MOVING THROUGH THE REGION THIS EVENING. BY
WEDNESDAY THE REGION WILL CONTINUE WITH ONSHORE FLOW WITH SOME
MOISTURE MAKING INTO THE INTERIOR. BY THURSDAY A WEATHER FRONT
WILL APPROACH THE NORTHERN GULF COASTLINE. THIS WILL BRING
WIDESPREAD WETTING RAINS THROUGH THE REGION.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE SHOWERY PATTERN IS ON THE DECLINE THIS AFTERNOON AS THE
RESPONSIBLE TROUGH IS WEAKENING IN THE GULF OF ALASKA. ISOLATED TO
SCATTERED SHOWERS WILL CONTINUE THROUGH THE EVENING HOURS TONIGHT
WITH A FEW MORE BREAKS IN THE CLOUDS THIS EVENING. THERE WILL
STILL BE LINGERING SHOWERS TOMORROW AFTERNOON IN MOST AREAS BUT
THEY WILL BE CONFINED FIRST TO HIGHER TERRAIN...MOVING OFF OF THE
MOUNTAINS IN THE AFTERNOON AND EVENING HOURS.

IN THE EVENING WEDNESDAY A FRONT WILL ENTER THE SOUTHERN GULF OF
ALASKA WHICH WILL CHANGE THE PATTERN OVERHEAD. RAIN WILL
OVERSPREAD MOST AREAS FROM SOUTH TO NORTH ON THURSDAY. GAP WINDS
IN FAVORED AREAS WILL ALSO INCREASE TO VALUES NOT SEEN IN A WHILE
ACROSS THE AREAS...ESPECIALLY TURNAGAIN ARM AND THE COPPER RIVER
VALLEY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (TONIGHT THROUGH THURSDAY)...
A BIT MORE SUNSHINE TODAY THAN PREVIOUS DAYS HAS HELPED TO
INITIATE ISOLATED THUNDERSTORMS ONCE AGAIN ACROSS SOUTHWEST
ALASKA. THE AIRMASS IS SLOWLY LOSING INSTABILITY AS TEMPERATURES
ALOFT WARM AND LAPSE RATES DECREASE. THE WEAKENING UPPER LOW
SOUTH OF KING SALMON IS CURRENTLY ACTING AS A FOCUS FOR DEEP
CONVECTION...BUT THIS FEATURE WILL CONTINUE TO WEAKEN THROUGH
TOMORROW. IN ORDER FOR ANY THUNDERSTORMS TO REDEVELOP TOMORROW
THERE WILL LIKELY NEED TO BE SOME SORT OF DYNAMIC FORCING
MECHANISM AS PURE THERMODYNAMIC INSTABILITY ALONE WILL NOT BE
SUFFICIENT. THE REMNANTS OF THE AFOREMENTIONED UPPER LOW WILL BE
DRIFTING NORTHWARD TOMORROW AFTERNOON AND MAY BE JUST STRONG
ENOUGH TO PROVIDE THIS LIFT...THUS HAVE OPTED TO INCLUDE ONE MORE
DAY OF THUNDERSTORMS BEFORE THE STRONG LOW COMING UP FROM THE
BERING SEA BRINGS A ROUND OF WIDESPREAD RAIN AND GUSTY WINDS AS WE
HEAD INTO THE LATTER HALF OF THE WEEK. INTERIOR PORTIONS OF THE
BRISTOL BAY ZONE WILL SEE THE STRONGEST WINDS FROM THE APPROACHING
FRONT...AND WILL LIKELY SEE GUSTS JUMP INTO THE 40-50 MPH RANGE.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (TONIGHT THROUGH
THURSDAY)...
A WEATHER FRONT ASSOCIATED WITH A LOW IN THE NORTH PACIFIC IS
ABOUT TO IMPACT THE CENTRAL ALEUTIANS THIS EVENING BEFORE
SPREADING EASTWARD DURING THE DAY TOMORROW. THE LOW CENTER ITSELF
IS HOVERING AROUND 990 MB...BUT WINDS WILL STRUGGLE TO REACH
BEYOND THE SMALL CRAFT (25-30 KT) RANGE AS THE LOW BECOMES
VERTICALLY STACKED AND GRINDS UP INTO THE EASTERN BERING BY
THURSDAY AFTERNOON. AS AN UNSEASONABLY STRONG SOUTHWEST JET STREAK
MOVES INTO THE ALASKA DOMAIN ON THURSDAY MOST OF THE HEAVY
RAINFALL WILL IMPACT THE SOUTHERN MAINLAND...WITH ONLY THE EASTERN
ALASKA PENINSULA SEEING A SUSTAINED PERIOD OF MODERATE RAIN
HEADING INTO THE LATTER HALF OF THE WEEK.

MEANWHILE...FURTHER WEST...MARINE FOG/STRATUS WILL REMAIN
WIDESPREAD ACROSS THE WESTERN BERING...LIKELY SEEING ONLY BRIEF
RELIEF AS THE LOW MOVING INTO THE EASTERN BERING SEA HAS LIMITED
INFLUENCE MIXING THE LOWER ATMOSPHERE IN THE FAR WESTERN REACHES
OF THE AREA. LIGHT WINDS WILL SLOWLY BECOME STRONGER AND NORTHERLY
THROUGH THURSDAY...THOUGH CONDITIONS ARE EXPECTED TO REMAIN DRY.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
A LOW PRESSURE SYSTEM TRACKING THROUGH THE EASTERN ALEUTIANS
EARLY THURSDAY WILL QUICKLY SHIFT NORTHWARD INTO THE EASTERN
BERING BY THE AFTERNOON...AS A STRONG JET STREAK SETS UP OVER
THE AKPEN. MODELS HAVE CONTINUED TO TREND TOWARD A MUCH LESS
AGGRESSIVE PHASING OF THE UPPER LEVEL ENERGY COMING OFF EASTERN
RUSSIA WITH THE UPPER LOW TRACKING INTO THE WESTERN COAST OF
ALASKA BY THE END OF THE WEEK. MODERATE TO HEAVY RAINFALL
ASSOCIATED WITH THIS SYSTEM WILL BEGIN TO BLANKET THE SOUTHERN
MAINLAND...WITH SOME AREAS ALONG THE KENAI/CHUGACH MOUNTAINS
EXPERIENCING SOME DOWNSLOPING DUE TO STRONGER CROSS BARRIER FLOW
ON THURSDAY. HOWEVER...WITH MODELS NOW SHOWING THE WAVE OVER THE
SOUTHWESTERN COAST ON THURSDAY SHIFTING SLIGHTLY TO THE
EAST...BROAD SOUTHWEST FLOW WILL QUICKLY BUILD OVER THE AREA. AS A
RESULT...DOWNSLOPING WILL QUICKLY FADE AND STEADIER RAINFALL WILL
BUILD OVER THE KENAI/CHUGACH MOUNTAINS BY FRIDAY. IT IS WORTH
NOTING THAT RUN TO RUN MODEL CONSISTENCY HAS NOT BEEN GOOD OVER
THE PAST FEW DAYS. THERE HAS BEEN IMPROVEMENT THOUGH WITH THE
MODEL RUNS TODAY...SO THERE IS NOW MODERATE CONFIDENCE IN THIS
SOLUTION.

MODELS THEN COME INTO FAIR AGREEMENT WITH THE PARENT TROUGH
SHIFTING NORTHWARD OVER THE NORTHERN INTERIOR BY SATURDAY
AFTERNOON...GIVING WAY TO AN AMPLIFYING RIDGE BUILDING OVER THE
EASTERN BERING AND MUCH OF THE MAINLAND BY TUESDAY MORNING.
HOWEVER...THE FINER DETAILS SUCH AS STRENGTH AND POSITIONING STILL
NEED TO BE WORKED OUT...AS WELL AS HOW EACH MODEL IS HANDLING THE
WEAKER WAVES CIRCULATING AROUND THE TOP OF THE RIDGE DIFFERENTLY.
THUS THE FORECAST LEANED HEAVIER ON THE WPC ENSEMBLE APPROACH.

&&

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

&&