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FXAK67 PAJK 061452
AFDAJK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
552 AM AKST FRI MAR 6 2015

.SHORT TERM...WEAK UPPER DISTURBANCE LOCATED FROM THE SKAGWAY AREA
INTO NRN BC/SRN YUKON WILL BECOME ABSORBED BY WLYS ALOFT THIS
MORNING. MEANWHILE...UPPER PV ANOMALY OVER THE NRN PAC IS BECOMING
DETACHED FROM NRN STREAM FLOW REGIME...AND WILL GENERALLY REMAIN
STATIONARY THROUGH FRI NIGHT. AS THIS TAKES PLACE...ANOTHER
TROUGH WILL BEGIN TO AMPLIFY AS IT MOVES FROM THE BERING SEA INTO
WRN PORTIONS OF INTERIOR AK. SURFACE PRESSURE PATTERN OVER THE
PANHANDLE WILL BECOME DOMINATED BY A RIDGE OF HIGH PRESSURE
BUILDING N DURING THE DAY. THE HIGH COMBINED WITH LOW PRESSURE
OVER NWRN CANADA...AND POSSIBLE LEE-SIDE PRESSURE FALLS DOWNWIND
OF THE CHILKAT MOUNTAINS UNDER WSW FLOW ALOFT...WILL RESULT IN A
LOCALLY TIGHT PRESSURE GRADIENT OVER NRN LYNN CANAL...WITH SLY
WINDS NEAR SMALL CRAFT LEVELS OCCURRING THROUGH THIS AFTERNOON
INTO TONIGHT. CLOSER TO THE HIGH CENTER...WINDS WILL BE WEAKER FOR
CNTRL/SRN AREAS. RESULTANT REDUCTION IN BOUNDARY LAYER MIXING AND
A DRY/SUBSIDENT MIDLEVEL ENVIRONMENT COULD SUPPORT PATCHY FOG
DEVELOPMENT TONIGHT.

OTHERWISE...SATELLITE DERIVED TPW GUIDANCE SHOWS VALUES NEAR 1
INCH EXTENDING NWD FROM THE SUBTROPICS INTO THE NERN PAC. PW
VALUES FROM 0.6-0.8 INCH ARE FORECAST TO SPREAD NWD FROM THE
SUBTROPICAL PLUME DURING FRI NIGHT...AND MOVE INTO THE SRN
PANHANDLE. THE INCREASE IN MOISTURE WILL AID IN THE DEVELOPMENT OF
LIGHT RAIN FOR SRN AREAS TONIGHT. FARTHER N...THE COMBINATION OF
INCREASING LARGE-SCALE ASCENT AHEAD OF THE BERING SEA TROUGH...AND
SLY LOW-LEVEL FLOW EMANATING FROM THE GULF /RESULTING IN OROGRAPHIC
ASCENT INVOF THE ST ELIAS MOUNTAINS/ WILL YIELD POTENTIALLY MODERATE
RAINFALL AMOUNTS W OF THE YAKUTAT AREA/AKZ017. DECIDED TO ASSIGN
SHOWER WORDING FOR THIS AREA OF PRECIPITATION DUE TO RELATIVELY
STEEP LAPSE RATES IN THE 925-700 MB LAYER.

NUDGED PRESSURE GRIDS TOWARD THE NAM/ECMWF/GEM IN ORDER TO
STRENGTHEN THE AREA OF HIGH PRESSURE OVER THE CNTRL/SRN
PANHANDLE. CHANGES TO WINDS WERE MINIMAL...WITH THE EXCEPTION OF
NRN LYNN CANAL AND SKAGWAY...WHERE SPEEDS WERE MANUALLY BUMPED UP.
POP AND QPF WERE REFRESHED WITH AN ENSEMBLE OF COARSE AND HI-RES
GUIDANCE. FEW CHANGES WERE MADE TO TEMPERATURES...WITH VALUES
DURING THE SHORT TERM PERIOD FORECAST TO BE SIMILAR TO WHAT WAS
OBSERVED THU.

.LONG TERM...THE COLDER AIR WE HAVE BEEN ADVERTISING WILL COME IN
TWO BATCHES. BATCH ONE CAN BE TRACED FROM THE BERING SEA TROUGH
ALLUDED TO ABOVE. THIS TROUGH IS CURRENTLY PUSHING EASTWARD FROM
THE WATERS SOUTHWEST OF ST. LAWRENCE ISLAND TO ARRIVE IN THE
NORTHERN GULF LATE SATURDAY NIGHT. WE THINK PRECIPITATION OVER
YAKUTAT WILL REMAIN MAINLY LIQUID THROUGH SATURDAY NIGHT, AS PRE-
FRONTAL WINDS WILL BE MAINLY MARINE IN ORIGIN WITH SOME WEAK WARM
AIR ADVECTION IN PARTICULAR SATURDAY EVENING. HOWEVER BEHIND THE
TROUGH WINDS WILL BACK TO WESTERLY, ALLOWING COLD AIR ADVECTION TO
BEGIN IN EARNEST. THIS WILL SUPPORT A MIX OF SNOW FOR SUNDAY OVER
YAKUTAT WITH LIKELY A COMPLETE CHANGEOVER SUNDAY NIGHT. LIKEWISE,
THE NORTHERN PANHANDLE INCLUDING JUNEAU WILL EXPERIENCE THESE SAME
CHANGES BUT AS MUCH AS 12 TO 18 HOURS LATER WITH GREATER IMPACTS
FROM BATCH TWO.

THE SECOND BATCH OF COLD AIR, ORIGINATING OVER NORTHEAST SIBERIA AT
THIS TIME WILL FOLLOW A SHORTWAVE ON A SIMILAR TRACK AS THE FIRST,
SWINGING DOWN INTO THE BERING SEA SATURDAY EVENING ON ITS WAY TO
THE NORTHERN GULF BY MONDAY MORNING. THIS WILL REINFORCE THE COLD
AIR MASS THAT ARRIVED WITH THE FIRST TROUGH. THE THEORY IS THAT AS
THESE SHORT-WAVES DROP SOUTHWEST FROM THE LEE OF THE ALASKA RANGE
INTO THE WESTERN GULF, THEY WILL HELP SPIN UP A SURFACE FEATURE
THAT WILL ENHANCE PRECIPITATION ON ITS FORWARD SIDE. THE FIRST
FEATURE WILL BE FED BY A LONGER PACIFIC FETCH AND HENCE WILL BE
MORE JUICY THAN THE SECOND WHICH HAS BEEN STARVED FROM MOISTURE BY
THE FIRST SYSTEM. NEVERTHELESS, THE PATTERN IS SUCH THAT
CONFIDENCE IS MODERATE THAT SNOW WILL FALL OVER THE NORTHERN
PANHANDLE AHEAD OF THIS SYSTEM AS IT PROGRESSES EASTWARD. BUT QPF
IS NOT SO IMPRESSIVE SO AT THIS POINT, WE ARE TALKING
ACCUMULATIONS OF POTENTIALLY FROM 2 TO 4 INCHES MONDAY THROUGH
MONDAY NIGHT FOR HAINES/JUNEAU. BUT A LOT OF THIS DEPENDS ON
TEMPERATURES. MODEL GUIDANCE SUGGESTING COOLER TEMPERATURES FOR
THIS AREA MONDAY, HOWEVER, MOS NOT SO MUCH. DID COOL THE NORTHERN
PANHANDLE UNDER THIS SYSTEM BY A COUPLE OF DEGREES FOR MONDAY
WHICH HELPED TO ENHANCE SNOWFALL FROM PREVIOUS FORECAST. BUT AT
THIS POINT, THIS EVENT LOOKS SUB-ADVISORY AND ONLY SIGNIFICANT
BECAUSE OF OUR LACK OF SNOWFALL THIS WINTER.

CURRENT FORECAST HAS COLD AIR STICKING AROUND SOUTHEAST WITH
ZONAL FLOW FROM THE BERING SEA KEEPING TEMPERATURES BELOW NORMAL.
HOWEVER, UNCERTAINTY IS RISING ABOUT A DEEPENING LONGWAVE TROUGH
MIDWEEK OVER THE NORTH PACIFIC PERHAPS KICKING A CUT-OFF
LOW NORTHEASTWARD TO NUDGE THE ARCTIC FRONT AWAY FROM THE
PANHANDLE. THUS MENTION OF SNOWFALL IN JEOPARDY FROM WEDNESDAY
INTO THURSDAY FOR MUCH OF THE PANHANDLE AS SOUTHERLY FLOW BEGINS
TO TAKE ROOT. NO CHANGES AT THIS TIME AS WE WILL WAIT AND SEE HOW
SUCCESSIVE RUNS HANDLE THIS CHANGE.

WINDSPEEDS WERE RAISED INTO THE WEEKEND FOR SOME OF THE NORTHERN
PANHANDLE NORTH-SOUTH PASSAGES AS WELL AS JUNEAU AND SKAGWAY TO
ACCOUNT FOR INCREASING GRADIENTS ASSOCIATED WITH STRONGER LOW
PRESSURE CROSSING INTO CANADA AND A BUILDING RIDGE SUNDAY MORNING
AHEAD OF MONDAY`S SYSTEM.

USED A BLEND OF ECWMF AND GFS TO BETTER HANDLE SECOND SYSTEM IN
THE GULF AND NUDGED TO ECMWF TO HANDLE THE POTENTIAL OF A WARMER
DISTURBANCE MOVING NORTH INTO THE SOUTHEAST GULF TUESDAY/TUESDAY
NIGHT. OTHERWISE ONLY SMALL TWEAKS. TEMPERATURES MAY NEED TO BE
WARMED MID-WEEK ONWARD. FORECAST CONFIDENCE GOOD THROUGH
MONDAY...FALLING RAPIDLY AFTERWARDS.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ012.

&&

$$

GARNER/ADKINS

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