Small Craft Advisory 707, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
515 AM AKDT WED SEP 9 2015

.ANALYSIS AND UPPER LEVELS...

SOUTHCENTRAL... WAVES OF PRECIPITATION CONTINUE TO MOVE NORTHWARD UP
COOK INLET THIS MORNING AS A SERIES OF UPPER LEVEL WAVES TAKE
ADVANTAGE OF THE ATMOSPHERE'S ABUNDANCE OF MOISTURE. THE 00Z
ANCHORAGE SOUNDING SHOWS A NEARLY SATURATED ATMOSPHERE TO THE
TROPOPAUSE NEAR 300 MB...WITH WEAK MID-LEVEL INSTABILITY HELPING
EXPLAIN THE OCCASIONAL HEAVY RAINFALL BEING PRODUCED WITH THESE
BANDS OF RAIN. KENAI RADAR SHOWS A BREAK IN THE RAIN MOVING UP
COOK INLET AS WELL.

SOUTHWEST AND THE BERING...KING SALMON RADAR SHOWS A BAND OF
LIGHT TO OCCASIONALLY MODERATE RAIN MOVING NORTHWESTWARD FROM THE
ALEUTIAN RANGE THROUGH DILLINGHAM AND UP TO THE AHKLUN MOUNTAINS.
THIS FEATURE WILL WEAKEN OVER THE COURSE OF THE MORNING AND PIVOT
WESTWARD. THEN THE MAIN FOCUS BECOMES A CYCLONE NOW UNDERGOING
FAIRLY RAPID CYCLOGENESIS SOUTH OF SAND POINT. OTHER FEATURES
MOVING THROUGH THE AREA INCLUDE A TROUGH IN THE POLAR JET OVER THE
BERING DIGGING SOUTHWARD AND A 120 KT JET STREAK IN THE
SUBTROPICAL JET BUCKLING AROUND THE BASE OF THE TROUGH.

CURRENTLY THE INGREDIENTS IN PLACE FAVORING THE RAPID DEVELOPMENT
OF THE LOW INCLUDE: THE INFUSION OF POLAR AIR OVER THE BERING CLASHING
WITH THE WARM MOIST SUBTROPICAL AIR SOUTH OF THE SUBTROPICAL
JET...THE SURFACE LOW CENTERED IN THE LEFT EXIT REGION OF THE JET
STREAK...AND A VIGOROUS SHORTWAVE IN THE UPPER LEVELS CENTERED
SOUTH OF DUTCH HARBOR. THE SHORTWAVE IS ENHANCING THE BUCKLING OF
THE JET RESULTING IN INCREASED UPPER LEVEL DIVERGENCE OVER THE
SURFACE LOW.

ELSEWHERE IN THE BERING...A COUPLE LOW CIRCULATIONS NEAR THE
PRIBILOFS THAT FORMED FROM THE RAPID SOUTHWARD ADVANCE OF THE
POLAR AIR ARE WEAKENING AS THE FORCING FOCUSES ON THE LOW SOUTH
OF THE ALASKA PENINSULA. THEY HAVE CAUSED A RAPID UPTICK IN THE
WINDS AT SAINT GEORGE OVERNIGHT. FURTHER WEST OVER THE
BERING...WIDESPREAD OPEN CELL CUMULUS ARE MOVING SOUTHWARD AS THE
COLD AIR OVERSPREADS THE WESTERN BERING.

&&

.MODEL DISCUSSION...

UP UNTIL THIS POINT IN THE MODEL EVOLUTION OF THE UPCOMING
STORM...THE GFS HAS BEEN THE MOST CONSISTENT...WITH ITS DEPICTION
BEING THE SLOWEST AND FURTHEST WEST. LARGELY THE OTHER MODELS HAVE
BEEN TRENDING SLOWER AND FURTHER WEST FOLLOWING THE GFS. THE
MODELS LARGELY AGREE ON THE FORECAST TRACK AT THIS POINT...WITH
THE SOLUTION BEING TO TAKE THE FAST DEEPENING CYCLONE INTO THE
ALASKA PENINSULA EAST OF CHIGNIK THIS AFTERNOON...CROSSING THE
ALASKA PENINSULA INTO BRISTOL BAY...THEN SLOWING THE LOW TO A
DRIFT AROUND KING SALMON FOR NEARLY 24 HOURS FROM TONIGHT THROUGH
THE FIRST HALF OF TOMORROW NIGHT WHILE GRADUALLY WEAKENING.

DESPITE THE GENERAL AGREEMENT ON THIS TRACK...SIGNIFICANT
DIFFERENCES ON THE LOCAL SCALE REMAIN. THE 00Z NAM WAS MUCH MORE
DIFFUSE WITH THE RAINFALL AND THE UPPER LEVEL ENERGY ASSOCIATED
WITH THE STORM THAN THE OTHER MODELS. THE GLOBAL MODELS LARGELY
AGREE WITH THE STORM TRACK...STRENGTH...AND MORE CONCENTRATED
NATURE OF THE PRECIPITATION SHIELD WRAPPING AROUND THE LOW
PRESSURE CENTER. HOWEVER...DUE TO THEIR POORER RESOLUTION...A
BLEND WITH THE LOCAL MODELS WAS USED TO BETTER HANDLE THE LOCAL
EFFECTS FROM THE GAP WINDS OVER THE ALASKA PENINSULA TO THE
EXPECTED DEVELOPMENT OF A BARRIER JET ALONG THE NORTH GULF COAST.
THE GLOBAL MODELS WERE FAVORED FOR TIMING...PRECIPITATION
INTENSITY...AND THE LARGE SCALE WIND FIELDS. FORECAST CHANGES
REMAIN FOCUSED ON TWEAKING DEVELOPMENTS ON THE LOCAL SCALE.
CONFIDENCE IS NOW HIGH ON THE GENERAL TRACK OF THE LOW...WHILE
STILL LOWER WITH RESOLVING THE MANY FINER DETAILS.

AFTER THE STORM...THERE IS GENERAL MODEL AGREEMENT IN A PERSISTENT
UPPER LEVEL TROUGH REMAINING ACROSS MUCH OF SOUTHERN MAINLAND
ALASKA.

&&

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

MODEL DIFFERENCES IN STORM TRACK AND TIMING WILL MAKE FOR A
CHALLENGING WIND FORECAST OVER THE NEXT 24 TO 36 HRS. AS A NORTH
PACIFIC LOW DEVELOPS AND HEADS TOWARD THE AK PEN AND ITS ASSOCIATED
OCCLUDED FRONT MOVES TOWARD THE NORTH GULF COAST EXPECT GALES
ALONG AND AHEAD OF THE FRONTAL ZONE. A E-SE BARRIER JET DEVELOPS
ALONG THE NORTH GULF COAST AHEAD OF THE FRONT TONIGHT AND WILL
LIKELY HAVE STORM FORCE GUSTS...BEFORE DIMINISHING THU MORNING.
ELSEWHERE WINDS WILL BE FAIRLY BRISK...ESPECIALLY THROUGH THE
FAVORED GAPS (TURNAGAIN ARM/PORTAGE VALLEY/COPPER RIVER) THROUGH
THU.

RAIN ACROSS INTERIOR SOUTHCENTRAL WILL TAPER OFF TO SHOWERS THIS
AFTERNOON AS THE REMNANTS OF A FRONTAL BOUNDARY WEAKEN AND THE
850 MB FLOW BECOMES MORE SOUTHEASTERLY. RAIN WILL CONTINUE ALONG
THE COASTS TODAY AND SHOULD INCREASE IN INTENSITY TONIGHT AS THE
SURFACE OCCLUDED FRONT MOVES TO THE NORTH GULF COAST. RAIN AND
SHOWERS WILL SPREAD INTO INTERIOR SOUTHCENTRAL ON THU AS THE
OCCLUDED FRONT MOVES INLAND THE UPPER FLOW BECOMES MORE SOUTHERLY.

&&

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

A STRONG LOW PRESSURE SYSTEM MOVING IN FROM THE NORTH PACIFIC WILL
TRACK TOWARD THE ALASKA PENINSULA TODAY...THEN SLOWLY MOVE NORTH
ALONG THE SPINE OF THE ALASKA AND ALEUTIAN RANGES THROUGH
THURSDAY. RAIN WILL BE HEAVY AT TIMES ALONG THE ALASKA PENINSULA
THROUGH BRISTOL BAY TODAY...WITH RAPIDLY INCREASING EASTERLY WINDS
OVER THOSE AREAS. MORE SHOWERY CONDITIONS WILL REMAIN OVER THE
KUSKOKWIM DELTA WHERE NORTHERLY FLOW WILL REMAIN PERSISTENT.

&&

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

BROAD NORTHERLY FLOW WILL REMAIN ACROSS THE WESTERN AND CENTRAL
BERING SEA AS COLDER AIR AND SHOWERS POUR IN FROM THE BERING
STRAIT. THE OTHER BIG STORY WILL BE THE CONTINUED DEVELOPMENT OF A
VIGOROUS LOW CURRENTLY SOUTH OF SAND POINT...WHICH WILL TRACK
NORTHWARD TO THE ALASKA PENINSULA BY TONIGHT. THIS WILL BRING A
SWATH OF VERY HEAVY RAIN AND ISOLATED THUNDERSTORMS TO THE WESTERN
ALASKA PENINSULA AND THE MARINE COASTAL WATERS SOUTH OF IT. STRONG
GALE FORCE WINDS WILL ALSO IMPACT THESE AREAS BEFORE THE LOW MOVES
EAST AND WEAKENS THURSDAY NIGHT.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
TROPICAL SYSTEMS KILO AND JIMENA ARE BIG QUESTION MARKS FOR THIS
FORECAST. TYPHOON KILO WILL GO EXTRA-TROPICAL AND JOIN THE ARCTIC
JET LATER THIS WEEK...THEN WORK ITS WAY INTO THE WESTERN BERING
SEA BY THE END OF THE WEEKEND...THEN EASTWARD INTO MAINLAND
ALASKA EARLY NEXT WEEK. NUMERICAL MODELS ARE STRUGGLING WITH THE
DETAILS ON THIS. IT IS ALSO POSSIBLE THAT MOISTURE FROM TROPICAL
STORM JIMENA WILL JOIN THE "PARTY" AS THE LEADING FRONT REACHES
WESTERN ALASKA...PROBABLY LATE MONDAY OR SOMETIME EARLY TUESDAY.
BEST BET IS TO USE THE ENSEMBLE FORECASTS. WPC PREFERS THE ECMWF
ENSEMBLE AND WENT VERY STRONGLY IN THAT DIRECTION. MODELS SHOULD
CATCH ON OVER THE NEXT DAY OR TWO...AND THEN MORE DETAIL CAN BE
ADDED TO THE FORECAST.

MEANWHILE...EARLIER IN THE FORECAST THE UPPER LOW OVER NORTHERN
ALASKA WILL SLIDE GRADUALLY NORTHWARD OVER THE WEEKEND. THIS WILL
ALLOW THE FLOW ALOFT TO FLATTEN (BECOME MORE WESTERLY AND LESS
CYCLONIC). THE RESULT WILL BE A BRIEF BREAK FROM THE WET/WINDY
WEATHER IN THE SHORT TERM PORTION OF THE FORECAST.

&&

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

&&

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