Small Craft Advisory 720, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
505 AM AKDT SUN SEP 13 2015

.ANALYSIS AND UPPER LEVELS...THE UPPER LEVEL PATTERN IS EXTREMELY
CONVOLUTED ACROSS THE AREA OF RESPONSIBILITY AND WILL MAKE FOR A
STRANGE PATTERN FOR THE DAYS AHEAD. WORKING FROM WEST TO
EAST...THE SUBTROPICAL JET SURGES FROM SOUTHWEST TO NORTHEAST
ACROSS THE KAMCHATKA PENINSULA INTO THE WESTERN BERING AT ABOUT
150 KT. THIS IS USHERING THE REMNANTS OF FORMER TYPHOON KILO INTO
THE AREA. NEXT UP IS A FINGER OF THE POLAR JET DIVING DOWN FROM
THE BERING STRAIT INTO SOUTHWEST ALASKA AT 100 KT. OVER THE NORTH
PACIFIC IS POSSIBLY THE STRANGEST PART OF THE PATTERN. THE REMNANT
MOISTURE OF WHAT ONCE LONG-AGO WAS HURRICANE JIMENA IS TRYING TO
STREAM NORTHWARD. IT IS NOW A CLOSED LOW WITH ABOUT A 100 KT JET
SUPPORTING IT. ON ITS WEST AND EAST FLANKS ARE TWO AREAS OF
RIDGING WHICH GIVES THE ENTIRE COMPLEX THE LOOK OF A REVERSE OMEGA
BLOCK. FINALLY...THE SUBTROPICAL JET CONTINUES FROM THE WESTERN
HALF OF THE GULF OF AK INTO SOUTHERN CANADA AT ABOUT 120 KT. ALL
THIS TO SAY THAT MUCH OF THE STATE WILL REMAIN UNDER THE SOMEWHAT
UNUSUAL WET NORTHWEST FLOW.

THE COLD FRONT THAT SWEPT THROUGH SOUTHCENTRAL SATURDAY IS NOW
WEAKENING OVER THE COPPER RIVER BASIN. IT BROUGHT ABOUT A HALF
INCH OF RAIN TO WEST ANCHORAGE AND OVER AN INCH TO THE UPSLOPE
REGIONS OF THE SUSITNA VALLEY. WITH SO MUCH LOW LEVEL
MOISTURE...THERE IS PLENTY OF GROUND FOG MOVING IN AND OUT OF THE
AIRPORTS THIS MORNING. A WEAKENING OCCLUSION IS PUSHING A FAIRLY
WIDESPREAD AREA OF LIGHT RAIN THROUGH THE WESTERN HALF OF THE
STATE...BUT THE PRECIPITATION IS NOT EXPECTED TO MAKE IT ACROSS
THE AK RANGE TONIGHT.

&&

.MODEL DISCUSSION...MODELS REMAIN IN GOOD AGREEMENT WITH ALL
FEATURES IN THE SHORT TERM (THROUGH MON MORNING). THIS RESULTS IN
HIGH FORECAST CONFIDENCE DURING THIS PERIOD. FOR SOUTHCENTRAL AND
THE GULF OF AK...A NAM/GFS COMBO WAS PREFERRED. AFTER THAT...THE
MAIN ISSUES THAT EMERGE ARE IN HANDLING WHAT HAPPENS ACROSS THE
BERING UNDER THE STOUT NORTHWESTERLY JET. SYSTEMS SHOULD DEVELOP
AND MOVE QUITE QUICKLY IN THAT FLOW...SO MARGIN FOR ERROR COULD BE
RATHER HIGH...ESPECIALLY REGARDING TIMING OF FEATURES. WHILE THE
AIRMASS IN PLACE ACROSS EASTERN RUSSIA AND THE BERING STRAIT IS
NOT TERRIBLY COLD RIGHT NOW...850 MB TEMPERATURES DO DROP TO NEAR
-8C BY THE MIDDLE OF THE WEEK. WITH SST'S STILL RUNNING NEAR
10C...THIS CONTRAST WILL MAKE THE SCENE RIPE FOR CYCLOGENESIS. THE
WEST FORECAST IS BASED INITIALLY ON A GFS/EC COMBO WITH MORE A
BLEND TO HANDLE THE UNCERTAINTY THAT DEVELOPS AS WE GO THROUGH
NEXT WEEK.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
SOUTHWEST FLOW DEEPENS ACROSS SOUTHCENTRAL THROUGH TODAY SPREADING
WETTING RAINS ACROSS MUCH OF THE REGION TONIGHT. PRECIPITATION
AMOUNTS WILL BE SOMEWHAT HIGHER IN UPSLOPE AREAS ALONG THE
CHUGACH AND TALKEETNA MOUNTAINS WITH SIGNIFICANTLY LESSER
AMOUNTS IN THE COPPER BASIN. LOW LEVEL CONVERGENCE WITH THE
INTERACTION OF THE TURNAGAIN ARM JET COULD PRODUCE SOME LOCALIZED
MODERATE TO HEAVY RAINFALL ACROSS SOUTH AND WEST ANCHORAGE
OVERNIGHT. WINDS ACROSS THE WESTERN GULF WILL BACK AROUND TO A
MORE SOUTHERLY DIRECTION TODAY WITH THE APPROACH OF THE FRONT AND
SPREAD NORTH TO THE COOK INLET REGION/SUSITNA VALLEY THIS
AFTERNOON/TONIGHT. GAP FLOWS WILL ALSO INCREASE ACROSS THE
REGION...MOST NOTABLY TURNAGAIN ARM AND THE COPPER RIVER BASIN.

ON MON...RAIN ACROSS MUCH OF THE REGION WILL GIVE WAY
TO SHOWERS AS MINOR WAVES EMBEDDED IN THE BROAD CYCLONIC UPPER
FLOW PROPAGATE WESTWARD. GAP FLOWS AND WINDS ACROSS COOK INLET
AND THE MAINLAND SHOULD DIMINISH...WITH WINDS ALONG THE COPPER
RIVER THE SLOWEST TO WEAKEN. WINDS ACROSS THE WESTERN GULF WILL
REMAIN RELATIVELY STRONG AND SHIFT TO A MORE WESTERLY DIRECTION.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
A RAPIDLY ADVANCING FRONT WILL OVERSPREAD SOUTHWEST ALASKA WITH
WIDESPREAD RAIN FROM WEST TO EAST TODAY. AS THE WARM FRONT
PASSES...A WARM SECTOR (WITH PLENTIFUL TROPICAL MOISTURE) WITH
STRONG WINDS JUST ABOVE THE SURFACE WILL PUSH TO THE COAST AND
HIGHER ELEVATIONS INLAND. THE TRAILING COLD FRONT WILL PUSH
SOUTHEAST ACROSS THE SOUTHWEST MAINLAND LATE TONIGHT INTO MONDAY
AND WILL BRING SHOWERY CONDITIONS TO THE REGION...WITH UPSLOPE
RAIN CONTINUING ALONG THE WESTERN SLOPES OF THE KUSKOKWIM...ALASKA...
AND ALEUTIAN RANGES THROUGH MONDAY NIGHT.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
WIDESPREAD RAIN AND SOUTHWEST GALE FORCE WINDS ACROSS THE CENTRAL
BERING WILL MOVE EAST TO BRISTOL BAY AND THE ALASKA PENINSULA
COAST THIS EVENING AND OVERNIGHT. BEHIND THE TRAILING COLD
FRONT...WINDS WILL MODERATE WITH JUST SCATTERED SHOWERS WITH ANY
WEAK DISTURBANCES THAT MOVE WEST TO EAST ACROSS THE BERING IN FAST
ZONAL FLOW THROUGH MONDAY NIGHT.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
AN ARCTIC TROUGH OVER THE NORTH MAINLAND EARLY NEXT WEEK WILL
SLOWLY DESCEND TO THE SOUTHERN MAINLAND BY MID WEEK. THIS WILL
DIRECT SHORTWAVES FROM SIBERIA THROUGH THE NORTHERN STREAM AND
INTO THE EASTERN BERING/ALEUTIANS...CREATING A SHOWERY REGIME
UNDER PERSISTENT COLD AIR ADVECTION AND NORTHWESTERLY FLOW. OVER
THE MAINLAND...THE ARCTIC TROUGH WILL REMAIN OVERHEAD THROUGH THE
END OF NEXT WEEK...PRODUCING ENOUGH FORCING FOR AT LEAST A CHANCE
OF RAIN WITH LIGHT WINDS. AFTERWARDS THE MODELS STRUGGLE WITH THE
EXACT PLACEMENT OF THE TROUGH...BUT ARE IN GENERAL AGREEMENT WITH
A TROUGH AXIS PERSISTING OVER THE MAINLAND INTO NEXT WEEK. THIS
MEANS THAT COOL AND SHOWERY WEATHER WILL CONTINUE AT LEAST THROUGH
NEXT WEEKEND.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES 138 150 155 160 165 170 179-181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CC
LONG TERM...DEK/DS