Small Craft Advisory 646, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
539 PM AKDT MON AUG 17 2015

.ANALYSIS AND UPPER LEVELS...
CURRENTLY...OVER THE ALASKAN AREA...A LONG WAVE TROUGH EXISTS IN
THE BERING SEA WITH A RIDGE OF HIGH PRESSURE OVER THE EASTERN
GULF OF ALASKA INTO THE PANHANDLE. A SECONDARY WAVE JUST EAST OF
THE KAMCHATKA PENINSULA IS STARTING TO SWING INTO THE TROUGH OVER
THE BERING. A CLOSED LOW IN THE NORTHERN BERING SEA NEAR THE
NORTON SOUND WITH A STRONG VORT MAX IN THE BASE OF THE TROUGH NEAR
THE AKPEN IS POISED TO MOVE EAST TODAY AND TONIGHT. THE TROUGH
OVER THE BERING SEA ALSO HAS A STRONG JET...120KT...ORIENTED FROM
THE BASE OF THE TROUGH NEAR 53N 165W TO THE APEX OF THE RIDGE NEAR
66N 140W.

WITH THE INCREASED ENERGY IN THE UPPER LEVELS...THE SURFACE
REFLECTION FOR WINDS AND PRECIP WERE INCREASED AS WELL. THE
ORIENTATION AND STRENGTH OF THE TROUGH AND RIDGE OVER THE ALASKA
AREA RESULTED IN TAPPING THE MOISTURE FROM THE REMNANTS OF MOLAVE.
NUMEROUS AREAS OF SOUTHCENTRAL ALASKA HAVE EXPERIENCED OVER AN
INCH OF PRECIPITATION IN THE PAST 24 HOURS. DUE TO THE SLOW
PROGRESSION...THE PRECIPITATION RATE SHOULD REMAIN THE SAME
THROUGH TOMORROW MORNING. FURTHER WEST...PRECIPITATION HAS BEEN A
BIT LIGHTER BUT WIDESPREAD.

THE SURFACE LOW WITH ASSOCIATED WITH THE JET IS OVER SOUTHWEST
ALASKA/SHELIKOF STRAIT THIS AFTERNOON. OVER THE GULF AND ADJACENT
WATERS THERE ARE AREAS EXPERIENCING GALE FORCE WINDS. HIGH
PRESSURE BEHIND THE LOW IS BUILDING ACROSS THE SOUTHWEST BERING
SEA.

&&

.MODEL DISCUSSION...
MODELS ARE IN GOOD AGREEMENT THROUGH THE FIRST FEW DAYS OF THE
FORECAST.

AT UPPER LEVELS THE CLOSED LOW OVER THE NORTHERN BERING SEA WITH
ITS ASSOCIATED TROUGH IS ANTICIPATED TO MOVE INLAND TONIGHT AND
SWING THROUGH THE STATE TUESDAY. A MORE ZONAL PATTERN IS EXPECTED
FOR WEDNESDAY OVER THE MAINLAND AS ANOTHER TROUGH BUILDS INTO THE
BERING SEA ON WEDNESDAY AMPLIFYING A RIDGE OVER THE GULF OF
ALASKA. AS THE TROUGH PROGRESSES ACROSS THE BERING WEDNESDAY IS
WHEN THE DIFFERENCES IN NUMERICAL GUIDANCE STARTS TO SHOW. SEE THE
LONG TERM DISCUSSION.

AS THE TROUGH SWINGS EAST TONIGHT THE TROPICAL MOISTURE TAP MOVES
EAST AS WELL. HOWEVER...AS THE JET STREAKS MOVE THROUGH THE AREA
TONIGHT...THERE COULD BE SOME ENHANCED PERIODS OF PRECIPITATION.
IN THE COLDER NORTHWEST FLOW ON THE BACKSIDE OF THE TROUGH...THE
AIR MASS IS UNSTABLE ACROSS SOUTHWEST AND SOUTHCENTRAL ALASKA.
THE QUESTION IS HOW UNSTABLE CAN IT GET WITH CLOUD COVER AND
COOLER AIR. OVER THE BERING AND ALEUTIANS...HIGH PRESSURE BUILDS
IN AT THE SURFACE WITH NORTHWEST FLOW IN THE EASTERN BERING. THIS
COULD PROVIDE SOME SHOWERS ON THE BERING SIDE OF THE ALEUTIAN
CHAIN. AN UPPER LEVEL TROUGH MOVES THROUGH THE BERING TUESDAY AND
OVER THE MAINLAND WEDNESDAY. IT WILL PROVIDE SOME ADDITIONAL LIFT
TO KEEP SHOWER ACTIVITY AROUND WEDNESDAY.

AT THE SURFACE...A LOW NEAR SHELIKOF STRAIT WILL SWING THROUGH
THE PRINCE WILLIAM SOUND AREA TONIGHT AND INTO THE YUKON TUESDAY
AFTERNOON. AS THE LOW PASSES AND WITH NORTHWEST FLOW...SOME STRONG
GAP WINDS WILL DEVELOP ON THE WEST SIDE OF THE GULF OF ALASKA AND
THEN INTO PRINCE WILLIAM SOUND LATER TUESDAY INTO WEDNESDAY AS
THE SURFACE LOW GETS ENHANCED BY THE SHORT WAVE TROUGH PASSING
THROUGH THE AREA. BY THURSDAY...A LOW DEVELOPING ALONG THE JET
FROM 50N 180 TO THE RIDGE APEX NEAR 61N 160W WILL BRING RAIN
ACTIVITY TO THE CENTRAL AND WESTERN ALEUTIANS. THE UPPER TROUGH
OVER THE WESTERN BERING AT THIS TIME WILL ALSO BRING A LOW
SKIRTING THROUGH THE NORTHERN BERING SEA.

&&

.FIRE WEATHER...
WETTING RAINS WILL CONTINUE OVER SOUTHCENTRAL THROUGH TONIGHT INTO
TOMORROW MORNING. RAIN ACTIVITY WILL DECREASE FROM WEST TO EAST
ACROSS SOUTHWEST AND SOUTHCENTRAL ALASKA TOMORROW.
HOWEVER...HUMIDITY LEVELS WILL BE REMAIN RATHER HIGH THROUGH
THURSDAY. SOME GAP WINDS ALONG THE COAST OF THE GULF OF ALASKA AND
THE AKPEN COULD POSE THREATS TO ANY FIRE ACTIVITY CURRENT OR
PLANNED.

&&

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

RAINFALL ACROSS INTERIOR PORTIONS OF SOUTHCENTRAL ALASKA HAS
TAPERED OFF TO SHOWERS THIS AFTERNOON BUT IS EXPECTED TO
REDEVELOP INTO STEADY LIGHT TO MODERATE RAINFALL DURING THE
EVENING HOURS. THE NORTHERN GULF COAST CONTINUES TO SEE A DELUGE
AS A POTENT UPPER LEVEL JET CONTINUES THE INFLUX OF MOISTURE FROM
THE SUBTROPICS. THE UPPER LEVEL TROUGH WILL SLIDE ACROSS
SOUTHCENTRAL ALASKA OVERNIGHT AND WITH FAVORABLE JET DYNAMICS
SHOULD DEVELOP A DEFORMATION ZONE OUT AHEAD OF THE UPPER LEVEL
TROUGH. THIS WILL AID IN WRAPPING PRECIPITATION AROUND THE SFC LOW
AS IT PUSHES FROM THE KENAI PENINSULA...INTO PRICE WILLIAM SOUND
AND ULTIMATELY INTO EASTERN ALASKA/CANADA. THE MODELS HAVE VARIED
ON THE STRENGTH AND TIMING OF THIS FEATURE AS THE 18Z SYNOPTIC
RUNS HAVE THE DEFORMATION ZONE MOVING QUICKER EASTWARD...WHICH
WOULD MAKE SENSE BASED ON THE STRENGTH OF THE UPPER LEVEL JET.
THIS WILL LIKELY LEAD TO INTERIOR LOCATIONS ACROSS SOUTHCENTRAL
ALASKA SEEING AROUND ONE HALF TO TWO-THIRDS OF AN INCH OF
ADDITIONAL RAINFALL. IN THE MEANTIME...THE NORTHERN GULF COAST
WILL CONTINUE TO BE IMPACTED BY THE FIREHOUSE (SUBTROPICAL
MOISTURE) AND SEE AN ADDITIONAL 1 TO 3 INCHES...WITH THE HIGHEST
AMOUNTS FURTHER EAST TOWARD YAKUTAT.

THERE WILL BE A SLIGHT CHANCE FOR A THUNDERSTORM OR TWO TUESDAY
AFTERNOON AS COLD AIR AT 850 MB AND 500 MB DROPS SOUTH OVER THE
INTERIOR OF ALASKA. WHILE SFC BASED HEATING WILL BE SIGNIFICANTLY
LIMITED BY CLOUD COVER AND RAINFALL...THE MID-LEVEL LAPSE RATES
MAY BE JUST ENOUGH TO SUPPORT A FEW ELEVATED THUNDERSTORMS.
CONFIDENCE AT THIS TIME IS LOW BUT THE CHANCE FOR THUNDERSTORMS
DOES EXIST. OUTFLOW WINDS WILL BEGIN TO BE FELT TUESDAY NIGHT
HEADING INTO WEDNESDAY AS DECENT PRESSURE RISES BEHIND THE LOW
MOVING ACROSS THE AREA ARE OBSERVED...AS WELL AS COLDER AIR
BEGINNING TO FILTER IN ACROSS THE INTERIOR. THE WINDS WILL
GENERALLY BE 20 MPH OR LESS EXCEPT NEAR WHITTIER WHERE GUSTS UP
TO 35-40 MPH WILL BE POSSIBLE ON WEDNESDAY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
SHOWERS WILL GRADUALLY DIMINISH IN COVERAGE TONIGHT FROM WEST TO
EAST AS THE UPPER LEVEL TROUGH AXIS PUSHES TO THE EAST. THIS WILL
YIELD A PERIOD OF SCATTERED TO ISOLATED SHOWERS THROUGH WEDNESDAY
AS FAST ZONAL FLOW IS ESTABLISHED OVER THE REGION...WITH ANOTHER
DISTURBANCE POTENTIALLY HELPING TO BRIEFLY ENHANCE SHOWER
ACTIVITY AS IT MOVES OVERHEAD TUESDAY NIGHT. PERSISTENT ONSHORE
FLOW WILL PROMOTE LOW CLOUDS AND FOG ACROSS THE KUSKOKWIM DELTA
AND PORTIONS OF BRISTOL BAY THROUGH WEDNESDAY.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
SHOWER ACTIVITY WILL GENERALLY REMAIN LIMITED TO THE ALASKA
PENINSULA AND EASTERN ALEUTIANS THROUGH TUESDAY AS NORTHWEST FLOW
SETS UP OVER THE EASTERN BERING IN THE WAKE OF THE DEPARTING LOW.
OTHERWISE...WIDESPREAD STRATUS AND PATCHY FOG WILL DOMINATE THE
BERING THROUGH WEDNESDAY AS WEAK HIGH PRESSURE BUILDS OVER THE
ALEUTIAN CHAIN. ANOTHER TROUGH WILL BEGIN TO DIG OVER THE WESTERN
BERING BY WEDNESDAY...BRINGING A CHANCE OF RAIN TO THE WESTERN
ALEUTIANS.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THE PATTERN ALOFT STARTS OFF PROGRESSIVE WITH A SHORTWAVE TROUGH
EXITING THE MAINLAND INTO THE GULF ON WEDNESDAY AFTERNOON. THIS
WILL STRENGTHEN AN OUTFLOW WIND PATTERN ACROSS THE NORTH GULF
COAST WITH DECREASING CLOUDS AND PRECIPITATION. THE WINDS WILL BE
SHORT LIVED AS A TROUGH FROM THE BERING SEA WILL DROP SOUTH AND
TAP SOME BAROCLINIC ENERGY OVER THE NORTHERN PACIFIC RESULTING IN
A STRONGER DEVELOPING SURFACE LOW PRESSURE SYSTEM MOVING INTO THE
GULF OF ALASKA. IT IS STILL A LITTLE SOON FOR SPECIFICS BUT GIVEN
THE ANTECEDENT CONDITIONS THE SCENARIO SEEMS MORE THAN PLAUSIBLE
WITH THE BAROCLINIC ZONE ALREADY IN PLACE FROM THE NORTH PACIFIC
TO THE GULF OF ALASKA. THE PATTERN THEN TRENDS TO A HIGHER
AMPLITUDE AS SEVERAL UPPER LEVEL TROUGHS SHAPE THE PATTERN...ONE
NEAR KAMCHATKA WHICH WILL PUMP UP AN UPPER LEVEL RIDGE OVER THE
BERING SEA AND ANOTHER DROPPING IN FROM THE CHUKCHI...EVENTUALLY
SETTING UP SHOP IN THE GULF.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES...119 120 131.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JN
SOUTHCENTRAL ALASKA...MC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...ML