Small Craft Advisory 681, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
257 PM AKDT SUN SEP 7 2014

.ANALYSIS AND UPPER LEVELS...
AN UPPER LEVEL TROUGH OVER THE COPPER RIVER BASIN LIFTS SLOWLY
NORTH AND WEAKENS AS A LOW AMPLITUDE RIDGE OVER THE WESTERN
GULF/BRISTOL BAY MOVES OVER THE CENTRAL GULF MON AND THEN TO THE
EASTERN GULF TUE. THE DIFFUSE LOW COMPLEX OVER THE SOUTHERN BERING
SEA LIFTS NORTH AND EAST AS AN OPEN TROUGH AS A DEVELOPING CENTER
DROPS SOUTH OUT OF EASTERN RUSSIA TUE. THE END RESULT WILL BE
BOARD SOUTHERLY FLOW SETTING UP ACROSS THE GULF AND MAINLAND TUE.

OUTFLOW WINDS WERE EVIDENT ACROSS THE NORTH GULF COAST THIS
MORNING. A WEAK OCCLUDED FRONT EXTENDING FROM COASTAL SW ALASKA TO
SOUTH OF KODIAK WAS DRIFTING SLOWLY NORTH. PRECIPITATION WAS
FAIRLY SPARSE AS WHAT LITTLE MOISTURE THIS SYSTEM HAS WAS INTERACTING
WITH VERY DRY UPSTREAM CONDITIONS. A SECONDARY FRONTAL SYSTEM ON
ITS HEALS ACROSS THE AK PEN IS THE MORE POTENT FEATURE. GUSTY
SOUTHERLY WINDS WERE EVIDENT FROM THE EASTERN ALEUTIANS TO THE AK
PEN.

&&

.MODEL DISCUSSION...
MODELS THROUGH THE SHORT TERM ARE IN RELATIVELY GOOD AGREEMENT
WITH THE SYNOPTIC FEATURES AND QPF PLACEMENT. AFTERNOON PACKAGES
WILL BE BASED IN PART ON A BLEND OF THE NAM/EC.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...
OUTFLOW WINDS ACROSS THE NORTH GULF WILL CONTINUE TO DIMINISH THIS
AFTERNOON AS PRESSURE GRADIENTS RELAX. WEAK WARM ADVECTION AND
INCREASED CLOUD COVER WITH THE WEAKENING OCCLUDED FRONT TONIGHT
WILL HELP TO MODERATE OVERNIGHT TEMPS ACROSS THE REGION. A
SECONDARY FRONT BEHIND THIS FEATURE WILL BRING INCREASED CHANCES
OF RAIN TO THE GULF AND THE SOUTHERN MAINLAND ON MON. THE FRONT
WEAKENS ACROSS SOUTHCENTRAL MON NIGHT AS A SECONDARY LOW DEVELOPS
ALONG THE TRAILING FRONTAL TROUGH OVER THE SOUTHERN GULF. THIS
FEATURE THEN MOVES TO THE NORTHERN GULF ON TUE AND BRINGS THE
POTENTIAL FOR WIDESPREAD RAINS ACROSS MUCH OF SOUTHCENTRAL...ESPECIALLY
ALONG THE NORTH GULF COAST. THE POTENTIAL FOR RAIN CONTINUES
ACROSS MUCH OF SOUTHCENTRAL THROUGH WED AS THE BERING SEA TROUGH
MOVES EAST AND IMPACTS THE AREA.

.SOUTHWEST ALASKA...
LIGHT RAIN...ASSOCIATED WITH THE SECONDARY FRONT APPROACHING SW
ALASKA...WILL SPREAD ACROSS THE AREA THROUGH TONIGHT. UNSETTLED
WEATHER WITH PERIODS OF RAIN AND/OR SHOWERS WITH POST FRONTAL
TROUGHS WILL CONTINUE INTO WED.

BERING SEA/ALEUTIANS...
ANY RESIDUAL GALES ACROSS THE MARINE ZONES SHOULD TREND TO SMALL
CRAFT LEVELS OVERNIGHT TONIGHT TO MON AS THE BERING LOW CONTINUES
TO FILL AND THE PRESSURE GRADIENT RELAXES SOME. THERE WILL STILL
BE PERIODS OF RAIN AS LOBES OF ENERGY ROTATE AROUND THE PARENT
UPPER-LOW IN THE BERING SEA.

&&

.LONG TERM FORECAST (THU THROUGH MON)...THE PARENT LOW WILL REMAIN
OVER THE WESTERN BERING TO START THE EXTENDED PORTION OF THE
FORECAST. AS THIS LOW STARTS TO INTERACT WITH A 120 KT SUB-
TROPICAL JET...MOST MODELS INDICATE SOME FORM OF A WEAK WAVE
MOVING THROUGH THE EASTERN BERING ON THU. THIS WILL HELP TO SHIFT
THE PARENT LOW CENTER A BIT FARTHER EAST AND SET THE STAGE FOR A
STRONGER SYSTEM. THE REMNANTS OF TROPICAL STORM FENGSHEN WILL
TRACK QUICKLY TOWARDS THE WESTERN TO CENTRAL ALEUTIANS. THE MODELS
ARE NOT HANDLING THE INTERACTION BETWEEN THE PARENT LOW AND
FENGSHEN WELL AT ALL. THE ECMWF CONTINUES TO FLIP-FLOP BETWEEN
FROM A LOW SOMEWHERE IN THE RANGE OF 970MB TO ITS MOST RECENT RUN
WITH A 945MB LOW. MEANWHILE...THE GFS HAS NOT BEEN SHOWING THE LOW
PHASING BUT RATHER JUST "PIN-WHEELING" THROUGH THE PATTERN. THE
18Z GFS RUN DID SHOW A 5-10MB DROP IN SURFACE PRESSURE THOUGH AND
ITS ENSEMBLE MEAN IS STRONGER YET. WITH THIS MUCH
UNCERTAINTY...WE OPTED TO NOT MAKE HUGE CHANGES TO THE FORECAST
AND JUST LEAN IT TOWARDS THE STRONGER EC MODEL AS IT TENDS TO
HANDLE THESE TROPICAL INTERACTIONS BETTER. THE GFS IS TYPICALLY A
BIT SLOWER TO CATCH ON TO BIGGER AMPLIFICATIONS IN THE SYNOPTIC
PATTERN...BUT IT IS THAT TIME OF YEAR. THIS AMPLIFICATION WILL
MEAN BETTER CHANCES FOR HEAVY PRECIPITATION SPREADING FROM WEST TO
EAST ACROSS THE ALEUTIANS TOWARDS THE COAST. IT WILL ALSO NUDGE
WINDS FROM THE SMALL CRAFT LEVEL TOWARDS GALE (IF THE EC WERE A
PERFECT- PROG...THIS WOULD BE OUR FIRST STORM FORCE LOW) LEVEL
LATE THU ACROSS MUCH OF THE BERING. DOWNSTREAM OVER THE
MAINLAND...THIS WOULD CREATE A SHARP RIDGE THAT COULD CREATE A
BREAK IN THE WET WEATHER FOR MANY PLACES ON THU AND EVEN FRI
MORNING. AFTER THAT THOUGH...A STRONG FRONT LOOKS TO USHER IN
HEAVY PRECIPITATION AND STRONGER WINDS FOR THE WEEKEND.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES 170 173 174 176
FIRE WEATHER...NONE.

&&

RMC/MO SEP 14