Gale Warning 870, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
414 PM AKST FRI DEC 4 2015

.ANALYSIS...

A NEGATIVELY TILTED TROUGH WITH AN EMBEDDED CLOSED LOW IS LOCATED
SOUTH OF KODIAK ISLAND IN THE NORTHERN PACIFIC. THE UPPER LEVEL
JET WRAPPING AROUND THE BASE OF THE LOW IS SUPPORTING THE
DEVELOPMENT OF ADDITIONAL SURFACE LOWS ALONG A FRONTAL BOUNDARY
ASSOCIATED WITH THIS FEATURE. A PLUME OF MOISTURE VISIBLE ON
SATELLITE IMAGERY IS BEING TRANSPORTED NORTHWESTWARD AHEAD OF THE
COMPLEX LOW TO JUST SOUTH OF THE ALASKA PENINSULA AND INTO THE
SOUTHWESTERN GULF. OVER THE SOUTHCENTRAL MAINLAND...AN UPPER-
LEVEL WAVE THAT BROUGHT BANDED SNOW SHOWERS TO THE AREA OVERNIGHT
HAS MOVED INTO THE COPPER RIVER BASIN THIS AFTERNOON. OFFSHORE
NORTHEASTERLY SURFACE FLOW OVER THE SOUTHWEST MAINLAND IS KEEPING
THE AREA MAINLY CLEAR AND DRY... WITH THE EXCEPTION BEING SOME LOW
CLOUDS JUST WEST OF THE ALEUTIAN RANGE ASSOCIATED WITH AN UPPER
LEVEL DISTURBANCE MOVING NORTHEASTWARD. FURTHER WEST...GUSTY
NORTHERLY FLOW PREVAILS OVER THE BERING SEA AND THE ALEUTIANS.
A 180 KT ZONAL JET STREAK MOVING INTO THE BASE OF THE LONGWAVE
TROUGH AND DIFFLUENT UPPER-LEVEL FLOW SOUTH OF THE WESTERN
ALEUTIANS WILL SET THE STAGE FOR ADDITIONAL CYCLONE DEVELOPMENT IN
THE NORTHERN PACIFIC LATER THIS EVENING.

.MODEL DISCUSSION...
MODELS TODAY ARE IN GOOD AGREEMENT ON THE OVERALL PATTERN BUT
DIFFER IN THE SMALL SCALE FEATURES...WHICH PROVIDES A FORECAST
CHALLENGE SINCE MOST OF THE WEATHER IN THE SHORT TERM IS BEING
DRIVEN BY THESE SMALLER FEATURES. THE MAIN DIFFERENCES ARE IN THE
TIMING OF THE SHORTWAVE MOVING THROUGH SOUTHCENTRAL AND THE TROUGH
MOVING THROUGH THE BERING. GFS SEEMS TO BE AN OUTLIER FOR BOTH
FEATURES AND SO THE EC/NAM SOLUTION WAS PREFERRED. ALL FORECAST
RUNS HAVE BEEN VARYING IN RUN TO RUN CONSISTENCY WITH TIMING AND
LOW PLACEMENT...SO FEW CHANGES WERE MADE TO THE FORECAST DUE TO
LOW CONFIDENCE IN THE MODELS. THIS SHOULD MAINLY AFFECT FEATURE
TIMING AS THE OVERALL PATTERN IS FAIRLY BENIGN AND THE MODELS ARE
IN GOOD AGREEMENT IN FEATURE STRENGTH.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
A WEAK UPPER LEVEL SHORT-WAVE WILL LIFT NORTHWARD ACROSS
SOUTHWEST ALASKA TONIGHT...WITH THE COOK INLET TO SUSITNA VALLEY
CORRIDOR SITTING ON THE EASTERN PERIPHERY OF THE WAVE. WITH WEAK
VERTICAL MOTION AND A SLIGHTLY MORE STABLE ENVIRONMENT THAN WITH
THE PRIOR WAVE EXPECT NO MORE THAN ISOLATED TO SCATTERED
SHOWERS...MAINLY IN THE UPSLOPE AREAS OF THE NORTHERN SUSITNA
VALLEY. IN THE WAKE OF THIS WAVE SOUTHCENTRAL WILL SETTLE IN
UNDERNEATH AN UPPER LEVEL RIDGE. THIS WILL PROVIDE DRY AND
SEASONABLY COOL CONDITIONS.

MEANWHILE...KODIAK ISLAND AND THE GULF OF ALASKA CAN EXPECT MORE
ACTIVE WEATHER AS A SERIES OF DEEP LOWS TRANSIT THE NORTHEAST
PACIFIC. THE GULF WILL BE ON THE NORTHERN PERIPHERY OF THE MAIN
LOW CIRCULATION...WITH UPPER LEVEL WAVES AND SURFACE FRONTAL
BOUNDARIES TRACKING WESTWARD ACROSS THE GULF WHILE DIMINISHING
SIGNIFICANTLY IN INTENSITY. THE AIR MASS SITTING OVER KODIAK
ISLAND IS COLD ENOUGH TO SUPPORT SNOW...BUT WARMER LOW LEVEL AIR
WILL ADVECT IN FROM THE EAST ON SATURDAY. THEREFORE...THE BULK OF
PRECIP IN THE FORECAST WILL FALL AS RAIN.

AS HIGH PRESSURE ALOFT BUILDS OVER SOUTHCENTRAL THROUGH THIS
WEEKEND...RADIATIONAL COOLING WILL ALLOW FOR TEMPERATURES TO
PLUNGE BELOW ZERO AT NIGHT IN PARTS OF THE SOUTHERN INTERIOR.
AN OFFSHORE PRESSURE GRADIENT WILL DEVELOP AND ALLOW FOR OUTFLOW
WINDS TO DEVELOP OUT OF THE GAPS OF THE CHUGACH RANGE. WITHOUT ANY
UPPER LEVEL FORCING...WIND SPEEDS WILL BE ENTIRELY DEPENDENT ON
THE LOW-LEVEL PRESSURE AND THERMAL GRADIENTS...AND THUS WILL NOT
BE PARTICULARLY STRONG THROUGH THE WEEKEND.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...

THE WEATHER OVER SOUTHWEST ALASKA WILL BE ON THE QUIET SIDE
THROUGH THE WEEKEND WITH PRECIPITATION CHANCES NOT EXPECTED UNTIL
SUNDAY MORNING/AFTERNOON ACROSS COASTAL LOCATIONS. A WEAK
SHORTWAVE WILL PUSH ACROSS THE NORTHERN BRISTOL BAY AREA AND
KUSKOKWIM DELTA THIS EVENING AND OVERNIGHT WHICH WILL ALLOW THE
850 MB FLOW TO SHIFT TO A WEST/SOUTHWEST ORIENTATION. THIS
ONSHORE FLOW ALOFT WILL BE ENOUGH TO TRAP SOME MOISTURE BELOW THE
ROBUST SURFACE INVERSION SEEN ON THE PAKN AND PABE RAOBS BUT THE
SURFACE LAYER WILL REMAIN PRETTY WELL MIXED. THIS WILL ALLOW A LOW
STRATUS LAYER TO DEVELOP ACROSS SOME INLAND LOCATIONS INCLUDING
BETHEL...KING SALMON AND DILLINGHAM. THIS STRATUS DECK SHOULD NOT
BE TRULY SOLID SO WHILE ONE LOCATION MAY SEE LOW CLOUDS...A
LOCATION NOT TOO FAR AWAY MIGHT SEE SIGNIFICANTLY LESS CLOUDS.
THESE CLOUDS WILL MAKE OVERNIGHT TEMPERATURE FORECASTS A BIT
DIFFICULT BUT THE OVERALL TREND CONTINUES TO SHOW BELOW NORMAL
TEMPERATURES WITH A TOUCH OF A WARM-UP TOWARD THE END OF THE
WEEKEND.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...

THE JET STREAM CONTINUES TO RESIDE ACROSS THE NORTH PACIFIC WHICH
WILL KEEP THE MAJORITY OF STORM SYSTEMS SOUTH OF THE ALEUTIAN
ISLANDS THROUGH THE BEGINNING OF NEXT WEEK. MODELS ARE HAVING A
DIFFICULT TRYING TO RESOLVE SEVERAL LARGE FRAGMENTS OF UPPER LEVEL
ENERGY SOUTH OF THE ALEUTIAN ISLANDS THIS WEEKEND DUE TO COMPLEX
NATURE OF THE UPPER LEVEL PATTERN. IT LOOKS LIKE A VERY COMPLEX
AND ELONGATED AREA OF LOW PRESSURE WITH MULTIPLE LOW CENTERS WILL
PASS SOUTH OF THE ALEUTIANS THROUGH THE WEEKEND AS THEY AMPLIFY
THE UPPER LEVEL TROUGH OVER THE BERING SEA. THE BERING SEA SHOULD
REMAIN SOMEWHAT UNSTABLE AS COLDER AIR FROM SIBERIA FILTERS IN
ACROSS THE REGION. WENT AHEAD AND UPPED POPS ACROSS LARGE PORTIONS
OF THE REGION TO LIKELY THROUGH THE DURATION OF THE SHORT-TERM AS
THIS COLD AIR ADVECTION STEEPENS MID-LEVEL LAPSE RATES AND HELPS
SPARK ADDITIONAL SNOW AND RAIN SHOWERS.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7...MON THROUGH
FRI)...UNCERTAINTY REMAINS IN THE EXTENDED FORECAST AS MODELS
CONTINUE TO TRY TO "SOLVE" THE RIDDLE. THE FAST 170KT ZONAL JET
WILL REMAIN MORE OR LESS SOUTH OF OUR AREA OF RESPONSIBILITY. BUT
A FEW "KINKS" IN THE JET WILL MAKE FOR ACTIVE WEATHER FROM TIME TO
TIME. AS A ROBUST PIECE OF ENERGY FROM RUSSIA ROUNDS THE BASE OF
OUR LONGWAVE TROUGH...IT WILL CAUSE THE JET TO SURGE NORTH INTO
THE GULF OF ALASKA MON INTO TUE. THIS WILL LEAD TO THE
DEVELOPMENT OF A FAIRLY STRONG LOW PRESSURE SYSTEM IN THE GULF.
HOWEVER...THE STRENGTH AND LOCATION OF THIS LOW ARE STILL QUITE
UNCLEAR. MOST OF THE MODELS...ESPECIALLY THE GFS...HAVE MADE LARGE
JUMPS TO THE EAST TOWARDS THE PANHANDLE WITH TODAY'S RUNS. THEY
HAVE ALL GENERALLY TRENDED SLIGHTLY WEAKER AS WELL. TO GIVE YOU
AN IDEA OF THE MODEL DISPARITY...BY TUE MORNING THE ECMWF STAMPS
THE LOW AT 945MB WHILE THE GFS IS AT 960 MB. THE TWO CENTERS ARE
500 MILES APART. THE GFS THEN CARRIES THE LOW TOWARD SE AK BEFORE
SWINGING BACK TO THE NORTHWEST THROUGH THE GULF ON WED. THE EC
CARRIES RIGHT ALONG THE EASTERN PERIPHERY OF KODIAK ISLAND
TRACKING ALMOST DUE NORTH. FORECAST CONFIDENCE REMAINS LOW WITH
THIS FEATURE...BUT IT DOES LOOK LIKE A WET AND WINDY WEEK FOR THE
GULF.

A WEAK FINGER OF THE POLAR JET WILL ALSO CONTRIBUTE TO WEATHER
NEXT WEEK. IT WILL DIVE OFF THE KAMCHATKA PENINSULA INTO THE
WESTERN BERING FROM TIME TO TIME. THIS WILL ONLY SERVE TO ENHANCE
THE COLD AIR SHOWERS AND WINDS OVER MUCH OF THE BERING. MODELS ARE
HINTING AT THE POSSIBLE DEVELOPMENT OF A FEW MESOSCALE POLAR LOWS UNDER
THIS JET ACTIVITY...BUT IT IS MUCH TOO EARLY TO PINPOINT THE
DETAILS.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES 138.
         HEAVY FREEZING SPRAY 140 160 180 181 185
&&

SYNOPSIS AND MODEL DISCUSSION...LF/RD
SOUTHCENTRAL ALASKA...CC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC
LONG TERM...MO