Small Craft Advisory 703, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
521 AM AKDT TUE SEP 8 2015

.ANALYSIS AND UPPER LEVELS...

OVER SOUTHCENTRAL...A SIMILAR WEATHER REGIME TO THE ONE THAT HAS
PERSISTED FOR THE PAST SEVERAL DAYS REMAINS OVER SOUTHCENTRAL THIS
MORNING. AN AIR MASS FEATURING ABUNDANT MOISTURE WITH WEAK
DISTURBANCES TRAVERSING THROUGH IT REMAINS IN PLACE FOR ONE MORE
DAY TODAY. COLDER AIR MOVING IN ALOFT AND OCCASIONAL BREAKS OF
SUNSHINE HAS MADE THE PRECIPITATION MODE MORE CONVECTIVE. THE 00Z
ANCHORAGE SOUNDING SHOWS 183 J/KG OF CAPE FOR THE CONVECTIVE
PRECIPITATION TO USE. FURTHERMORE...KENAI RADAR SHOWED A
CONVERGENCE ZONE THAT SET UP LAST NIGHT ACROSS WEST ANCHORAGE.
THIS WAS CAUSED BY LIGHT WINDS COMING DOWN TURNAGAIN ARM
CONVERGING WITH THE SOUTHWEST WINDS MOVING UP COOK INLET. THIS
BAND OF RAIN PRODUCED A PRODIGIOUS 1.12 INCHES OF RAIN IN 5 HOURS
TIME AT ANCHORAGE INTERNATIONAL AIRPORT LAST NIGHT. TO PUT THAT
INTO PERSPECTIVE...THAT 1.12 INCHES IN 5 HOURS WAS MORE THAN WAS
RECEIVED AT THE AIRPORT FOR THE ENTIRE MONTH OF AUGUST...0.97
INCHES. ADDITIONAL WAVES OF MOISTURE PRODUCING SCATTERED SHOWER
ACTIVITY ARE MOVING NORTH UP THE LOWER COOK INLET AS WELL.

OVER SOUTHWEST...AN ELONGATED OCCLUDED FRONT IS MOVING NORTH INTO
THE COASTAL KUSKOKWIM DELTA AND BRISTOL BAY AREAS. BETHEL AND KING
SALMON RADAR SHOW LIGHT RAIN MOVING INLAND ASSOCIATED WITH THE
FRONT FROM EGEGIK NORTHWARD PAST TOKSOOK BAY. IR SATELLITE IMAGERY
SHOWS WIDESPREAD STRATUS ALONG WITH INCREASING HIGH CLOUDS
ENVELOPING MUCH OF THE REST OF SOUTHWEST ALASKA AS THE FRONT MOVES
IN.

OVER THE BERING...THINGS ARE BECOMING MUCH MORE ACTIVE AS A
PATTERN CHANGE BEGINS. A POLAR JET STREAK FLOWING FROM NORTHEAST
TO SOUTHWEST IS ADVANCING ACROSS THE NORTHERN BERING. WHILE THIS
IS SLOWLY DEEPENING A LOW CENTERED NEAR THE PRIBILOFS AND
ENHANCING PRECIPITATION ASSOCIATED WITH THE FRONT MOVING INTO THE
COAST OF SOUTHWEST ALASKA...ITS MORE SIGNIFICANT PURPOSE IS TO
PUSH A VERTICALLY STACKED LOW CENTERED NEAR SHEMYA IN THE WESTERN
ALEUTIANS SOUTHEASTWARD INTO A ZONALLY ORIENTED 120 KT SUBTROPICAL
JET STREAK...CAUSING THE JET TO BUCKLE AND BEGIN DIGGING AN UPPER
LEVEL TROUGH. THE CLASH OF THE POLAR JET AND THE SUBTROPICAL JET
IS ALREADY INDUCING SURFACE CYCLOGENESIS SOUTH OF THE CENTRAL
ALEUTIANS. THIS DEVELOPING LOW WILL QUICKLY BECOME THE DOMINANT
PRIMARY WEATHER MAKER FOR SOUTHERN MAINLAND ALASKA FOR THE NEXT
SEVERAL DAYS.

&&

.MODEL DISCUSSION...

THE MODELS ALL CONTINUE TO AGREE ON PRECIPITATION DEVELOPMENT
TODAY ACROSS MUCH OF SOUTHCENTRAL ALASKA AS AN UPPER LEVEL SHORTWAVE
AND POSITIVE VORTICITY ADVECTION ASSOCIATED WITH THE APPROACH OF
THE LEADING EDGE OF THE SUBTROPICAL JET STREAK MOVES NORTHEAST
ALONG THE WESTERN GULF COAST. THUS...FORECAST CONFIDENCE FOR THIS
FIRST EVENT IS HIGH.

THEN ALL EYES TURN TOWARDS THE ALASKA PENINSULA AND THE
DEVELOPMENT OF THE NEXT CYCLONE. SINCE YESTERDAYS MODEL RUNS...
MOST OF THE MODELS HAVE SHIFTED TOWARDS YESTERDAYS GFS
RUN...KEEPING THE LARGE STORM MOVING MORE SLOWLY AND STAYING
FURTHER WEST ALONG THE ALASKA PENINSULA. HOWEVER...THAT IS WHERE
THE SIMILARITIES IN THE MODELS END. LARGE DIFFERENCES IN THE
MODELS QUICKLY ARISE IN DETERMINING HOW LONG THE CENTER OF THE LOW
LINGERS NEAR THE SOUTHERN COAST OF THE ALASKA PENINSULA...WHERE
THE LOW MOVES AFTER THAT...AND THE TIMING OF THE RESULTING
PRECIPITATION. THE 06Z RUN OF THE GFS MOVES THE CENTER OF THE LOW
ACROSS THE ALASKA PENINSULA ALONG COASTAL BRISTOL BAY THURSDAY
MORNING...WHEREAS THE EC AND NAM BOTH DEPICT A MORE TYPICAL TRACK
OF THE LOW THROUGH SHELIKOF STRAIT AND INLAND UP COOK INLET. IF
THAT SCENARIO VERIFIES...THEN THE FLOW ALOFT WILL BE SOUTHEAST TO
EAST ACROSS THE CHUGACH AND KENAI MOUNTAINS...RESULTING IN SIGNIFICANT
DOWNSLOPING. FORECAST CONFIDENCE IS HIGH REGARDING THE LOW
DEVELOPMENT THROUGH WEDNESDAY AFTERNOON...THEN LOWERS
SIGNIFICANTLY BEYOND THAT.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2...TUE AND
WED)...AFTER A RELATIVELY NICE LABOR DAY...RAIN AND INCREASING
WINDS WILL HIGHLIGHT THE FORECAST FOR THE NEXT SEVERAL DAYS. WHILE THE
HEAVY SHOWERS OVER WEST ANCHORAGE OVERNIGHT WILL CONTINUE TO
DISSIPATE...THE ATMOSPHERE WILL REMAIN QUITE MOIST TODAY.
SOUTHERLY FLOW WILL TURN JUST SLIGHTLY MORE SOUTHEASTERLY THIS
MORNING ALLOW FOR A BRIEF BREAK WITH SOME CROSS-BARRIER FLOW. BUT
BY TONIGHT ANOTHER WEAK UPPER LEVEL WAVE WILL TRACK UP THE COOK
INLET AND SHOULD BRING ALL AREAS ANOTHER ROUND OF LIGHT RAIN.
THESE "PULSES" IN THE PATTERN WILL LEAD TO BRIEF PERIODS OF
GUSTIER WINDS THROUGH THE PORTAGE VALLEY AND TURNAGAIN ARM.

BY WEDNESDAY AFTERNOON...AN IMPRESSIVE SYSTEM SOUTH OF THE WESTERN
ALASKA PENINSULA WILL TRACK OVER KODIAK. WHILE MODELS HAVE BEEN
STRUGGLING WITH THE EXACT TRACK OF THE SURFACE LOW...THE TREND HAS
BEEN TO BRING IT FURTHER WEST AND NEARLY DIRECTLY THROUGH COOK
INLET. THIS SYSTEM SHOULD FEATURE SOME CLASSIC "FALL" SIGNATURES
WITH A ROBUST BARRIER JET ALONG THE COAST...MODERATE DOWNSLOPING
WINDS ACROSS COOK INLET...AND EVENTUALLY RAIN FOR ALL AREAS. THERE
IS AN OUTSIDE CHANCE FOR SOME THUNDERSTORMS ACROSS THE SOUTHERN
GULF NEAR KODIAK WEDNESDAY AFTERNOON AND EVENING...BUT THE BEST
POTENTIAL FOR CONVECTION LOOKS TO REMAIN FURTHER WEST. SOUTHEAST
WINDS WITH THE SYSTEM SHOULD FAVOR GUSTY CONDITIONS THROUGH
TURNAGAIN ARM AND THE PORTAGE VALLEY...BUT SHOULD KEEP ANCHORAGE
AND THE MAT VALLEY OUT OF THE HEAVIER PRECIPITATION INITIALLY. BY
THURSDAY...THE UPPER LEVEL CENTER CUTS RIGHT THROUGH COOK INLET
AND WILL BRING YET ANOTHER ROUND OF RAIN TO THE REGION.

&&

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

A TROUGH OF LOW PRESSURE AND ASSOCIATED FRONTAL ZONE OVER THE
EASTERN BERING WILL CONTINUE TO SPREAD MOISTURE AND PRECIPITATION
OVER SOUTHWEST TODAY THROUGH TONIGHT. A VIGOROUS LOW WILL THEN
RAPIDLY DEEPEN ACROSS THE NORTH PACIFIC THEN MOVE TO THE ALASKA
PENINSULA WEDNESDAY AND WEDNESDAY NIGHT...BRINGING MORE RAIN TO
MOST OF SOUTHWEST. GUSTY EAST TO NORTHEAST WINDS WILL ALSO IMPACT
THE BRISTOL BAY REGION AS THIS LOW MOVES INLAND.

&&

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

A BROAD LOW OVER THE CENTRAL BERING WILL MERGE WITH AN ARCTIC
TROUGH TODAY...WITH INCREASING NORTHERLY WINDS SPREADING OVER MOST
OF THE WESTERN AND CENTRAL BERING SEA AND ALEUTIANS. THE BIG STORY
THEN TURNS TO A POWERFUL LOW EXPECTED TO MOVE TOWARD THE ALASKA
PENINSULA TUESDAY NIGHT INTO WEDNESDAY. THIS WILL MOSTLY IMPACT
THE EASTERN ALEUTIANS EAST OF UNALASKA. GIVEN THE RAPID
INTENSIFICATION OF THIS LOW AIDED BY A SUBTROPICAL WARM FRONT AND
STRONG JET STREAK...THUNDERSTORMS ARE POSSIBLE AS THE FRONT MOVES
OVER THE ALASKA PENINSULA. STRONG AND GUSTY NORTHERLY WINDS WILL
DEVELOP ON THE BACKSIDE OF THIS LOW...STRONGEST OVER THE FAR
WESTERN ALASKA PENINSULA.

&&

.MARINE...

A POWERFUL LOW WILL RAPIDLY INTENSIFY ACROSS THE FAR EASTERN
MARINE ZONES OF 150 AND 155 TUESDAY NIGHT AND WEDNESDAY. WHAT IS
KNOWN IS A STRONG SUBTROPICAL WARM FRONT WILL INTERACT WITH A
STRONG JET STREAK...ALLOWING FOR POTENTIAL DEVELOPMENT OF
THUNDERSTORMS NEAR THE LOW CENTER. WHAT IS LESS CERTAIN IS THE
EXACT TRACK OF THIS LOW...AND THE STRENGTH AS IT MOVES OVER THE
COASTAL MARINE WATERS. SOME STRONGER MODEL SOLUTIONS WOULD RESULT
IN NEAR STORM FORCE WIND GUSTS NEAR THE ALASKA PENINSULA COAST AT
THE PEAK OF THE EVENT...WHILE WEAKER SOLUTIONS KEEP THE STRONGEST
NORTH TO NORTHWEST WINDS OUTSIDE OF THE COASTAL MARINE ZONES. WITH
THAT SAID...THERE IS HIGH CONFIDENCE IN AN INTENSE LOW MOVING
THROUGH THIS AREA WITH AREAS OF SUSTAINED GALE FORCE WINDS...AND
MODERATE CONFIDENCE IN THE DEVELOPMENT OF THUNDERSTORMS.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THE STRONG LOW SPINNING UP TO THE SOUTH OF THE ALASKA PENINSULA ON
WEDNESDAY CONTINUES TO BE THE MAJOR WEATHER FEATURE WHICH WILL
IMPACT THE MIDTERM FORECAST. IN COMPARISON TO YESTERDAY...MODELS
HAVE BROUGHT THE TRACK OF THE SURFACE LOW FURTHER TO THE WEST WITH
THE LOW TRACKING THROUGH THE WESTERN GULF SUCH THAT IT BRUSHES
OVER THE EASTERN COAST OF KODIAK ISLAND WEDNESDAY NIGHT BEFORE
CONTINUING NORTH TOWARDS THE KENAI PENINSULA THURSDAY AND TO THE
NORTHERN GULF THURSDAY NIGHT. EXPECTING TO SEE BARRIER JET WINDS
IN THE NORTHERN GULF...EAST WINDS IN PRINCE WILLIAM SOUND AND EAST
TO NORTHEAST WINDS THROUGH THE BARRENS INCREASE TO GALES WEDNESDAY
NIGHT AS THE INITIAL FRONTAL SYSTEM LIFTS NORTH ACROSS THE GULF
AHEAD OF THE LOW. SOUTHERLY GALES WILL ALSO SPREAD NORTH ACROSS
THE GULF ON THURSDAY TO THE EAST OF AND FOLLOWING THE LOW NORTH.
IN ADDITION ALSO EXPECTING STRONG GAP WINDS THROUGH PASSAGE CANAL
AND TURNAGAIN ARM WEDNESDAY NIGHT AND THURSDAY BEFORE BEGINNING TO
TAPER OFF THURSDAY NIGHT.

A COLD UPPER LEVEL LOW CENTERED OVER NORTHERN ALASKA AND SEVERAL
SHORTWAVE TROUGHS SWINGING IN FROM THE WEST AND NORTHWEST WILL
KEEP CONDITIONS COOL WITH SCATTERED SHOWERS AROUND THROUGH THE
WEEKEND. THE NEXT MAJOR LOW PRESSURE SYSTEM WILL APPROACH THE
WESTERN ALEUTIANS AROUND SATURDAY. MODEL UNCERTAINTY REMAINS
RATHER HIGH WITH REGARDS TO THE TIMING...TRACK AND STRENGTH OF
THIS FEATURE AS WELL AS THE SURFACE RIDGE AMPLIFYING OUT AHEAD OF
IT OVER THE ALEUTIANS AND BERING. HOWEVER IT IS ONE THAT DEFINITELY
BEARS WATCHING AS IT WILL CONTAIN QUITE A BIT OF REMNANT TROPICAL
MOISTURE. FOR GULF THE NEXT LOW WILL TRACK UP FROM THE SOUTH
SOMETIME AROUND SUNDAY OR MONDAY AND WILL ALSO LIKELY BE A RATHER
WARM AND MOIST SYSTEM

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES 155. MARINE WEATHER STATEMENT 150 155.
FIRE WEATHER...NONE.

&&

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