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FXAK67 PAJK 060019
AFDAJK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
319 PM AKST THU MAR 5 2015

.SHORT TERM...SATELLITE IMAGERY THIS AFTERNOON SHOWING A BROAD
MID LEVEL TROUGH OVER THE CENTRAL AK GULF EXTENDING TO THE
SOUTHEAST WITH A CLOSED LOW CROSSING OVER DIXON ENTRANCE. THIS
SOUTHERN LOW HAS BEEN MOVING IN AMPLE MOISTURE TO THE SOUTH
ENHANCING RAIN FALL RATES. UNDER THE TROUGH SOME WEAK CYCLONIC
CIRCULATIONS CAN BE SEEN IN THE GULF ROTATING TOWARD THE
NORTHEAST. REMNANTS OF THE FRONT THAT WAS TRACKING OVER THE
PANHANDLE OVERNIGHT ARE CROSSING OVER THE SOUTHERN PANHANDLE EARLY THIS
EVENING. BEHIND THE FRONT COLDER AIR MOVING IN ALOFT HAS
TRANSITIONED PRECIP FROM STRATIFORM TO CONVECTIVE. THE TROUGH WILL
MOVE TO THE EAST WITH HIGH PRESSURE RIDGE BUILDING OVER THE
PANHANDLE TONIGHT. PRECIP WILL DIMINISH FROM NORTHWEST TO
SOUTHEAST INITIALLY WITH A SECOND WAVE MOVING IN FROM THE WEST
THIS EVENING AND SWINGING UP TO THE NORTHEAST. CURRENTLY THERE ARE
CLEAR BREAKS OVER THE FAR NORTHERN PANHANDLE BUT WITH THE NEXT
WAVE SHOULD SEE SOME MORE CLOUD COVER AND SHOWER ACTIVITY. A
LEADING SHORT WAVE AHEAD OF A STRONG LOW OVER THE INTERIOR WILL
MOVE IN FROM THE WEST FRIDAY WITH INCREASING PRECIP FOR THE
YAKUTAT REGION.

THE NORTHERN AND EASTERN PORTIONS OF THE JUNEAU AREA RAIN SHOWERS
BEGAN TO MIX WITH THEN TRANSITION TO SNOW. THIS PROBABLY DUE TO THE
MORE INTENSE SHOWERS MIXING DOWN COLDER AIR THAT MOVED IN ALOFT OR
AREA OF SLANTWISE CONVECTION DEVELOPING ALONG THE COASTAL
MOUNTAINS. AS OF NOW THERE HAS ONLY BEEN A TRACE OF SNOW
ACCUMULATION. WHILE THERE ARE SNOW SHOWERS IN JUNEAU THERE ARE ONLY RAIN
SHOWERS TO THE NORTH IN GUSTAVUS AND HAINES, WITH RAIN TO THE
SOUTH IN ANGOON. HAVE LOWERED SNOW LEVELS FOR LOCATIONS NORTH OF
ICY STRAIT LATE TONIGHT TO THE SURFACE THINKING THAT THE SHOWERS
TONIGHT WILL STILL MIX IN SOME SNOW. NO ACCUMULATION IS EXPECTED
EXCEPT FOR LOCATIONS ABOVE 600 FT. THE EXCEPTION IS HYDER WHERE
SNOW HAS LASTED THROUGH THE DAY WITH UP TO 1 INCH OF ACCUMULATION.
IN ADDITION AS SHOWER INTENSITY DECREASES THINK THE REDUCED
DOWNWARD FORCING WILL HAVE PRECIP SWITCH BACK TO MORE RAIN.

WINDS WILL BE LIGHT DUE TO THE WEAK GRADIENT OVER THE AK GULF AND
PANHANDLE TONIGHT AND EARLY FRIDAY. AS THE HIGH PRESSURE RIDGE
BUILDS OVER THE PANHANDLE THE SOUTHERLY GRADIENT WILL STRENGTHEN
WITH SOUTHERLY WINDS, ESPECIALLY OVER THE NORTHERN INNER
CHANNELS, INCREASING THROUGH THE DAY. SURFACE TEMPERATURES
OVERNIGHT HAS BEEN TRICKIER TO DETERMINE. FOR LOCATIONS WITH THE
HEAVIER SHOWERS BRINGING IN COLD AIR, TEMPS THIS AFTERNOON HAVE
DROPPED TO THE LOW 30S, WHILE AREAS WHERE SHOWERS ARE MORE
SPORADIC TEMPS ROSE TO THE UPPER 30S TO LOW 40S. STILL WIDE SPREAD
ON MOS GUIDANCE TONIGHT BUT FOR NOW HAVE SURFACE LOWS IN THE LOW
TO MID 30S FOR NORTHERN HALF OF THE PANHANDLE AND UPPER 30S TO THE
SOUTH. WENT WITH MORE PERSISTENCE FOR FRIDAY DAYTIME HIGHS AS SOME
BREAKS IN PRECIP WILL ALLOW SOME INCREASED WARMING BUT DO HAVE
COLDER AIR ALOFT TO OFFSET THIS IF IT MIXES DOWN.

MODELS IN GENERAL AGREEMENT FOR SYNOPTIC SCALE PATTERN. HAD
BEST AGREEMENT BETWEEN THE 00Z ECMWF AND 12Z NAM FOR THE MESOSCALE
CIRCULATIONS AND USED THESE TO UPDATE GRIDS. OVERALL CHANGES TO
WIND AND PRESSURE WERE MINOR. WITH THE CONVECTIVE ENVIRONMENT
MODEL MAY HAVE BEEN ON THE HIGH SIDE WITH POPS, SO USED A BLEND OF
A WETTER SREF AND DRIER NAM. SWITCHED OVER TO CONVECTIVE WORDING
DUE TO THE COLDER AIR ALOFT. FORECAST CONFIDENCE IS AVERAGE.

.LONG TERM...UPPER LEVEL MODEL AGREEMENT VERY GOOD THROUGH MONDAY
MORNING, AT WHICH TIME GFS BECOME A LITTLE FASTER AT ADVANCING AN
UPPER TROUGH ACROSS THE EXTREME NORTHERN GULF AND INTO THE
PANHANDLE. DIFFERENCES REMAIN THEREAFTER, WITH GFS MAINTAINING A
MINOR UPPER RIDGE BETWEEN TWO UPPER LOWS SOUTH OF THE GULF WHILE
ECMWF DIGS A DEEPER TROUGH UPSTREAM WHILE THE WEAKER UPPER LOW
FARTHER DOWNSTREAM GETS EJECTED INTO SOUTHERN BRITISH COLUMBIA.
GFS THEN BECOME MORE PROGRESSIVE WITH A DEEP UPPER TROUGH OVER THE
PANHANDLE THROUGH THE WEEKEND WHILE ECMWF RETROGRADES SOME UPPER
ENERGY TO BUILD A DEEPER AND MORE BROAD UPPER LOW SOUTH OF THE
ALEUTIANS WITH UPPER RIDGING OVER THE PANHANDLE.

AT THE SURFACE, A DEEP LOW TRACKING EAST FROM RUSSIA THAT HAS BEEN
THE FOCUS OF CONSIDERABLE ATTENTION FOR SEVERAL SHIFTS NOW REMAINS
IN THE FORECAST PICTURE, ALTHOUGH ALL MODELS ARE NOW TRACKING THIS
SYSTEM ACROSS ALASKA FARTHER NORTH THAN IN PREVIOUS RUNS. WHILE
THE SHIFT IN THE TRACK TO THE NORTH IS SUBSTANTIAL, A SIGNIFICANT
AMOUNT OF PRECIPITATION IS STILL FORECAST TO PASS OVER THE
PANHANDLE FROM NORTHWEST TO SOUTHEAST AS THE FRONT MOVES THROUGH.
ECMWF CONTINUES TO BE THE MOST AGGRESSIVE WITH PRECIP AMOUNTS AND
AREAL COVERAGE BEGINNING SATURDAY MORNING IN YAKUTAT AND FINALLY
CLEARING THE AREA TO THE SOUTH ON MONDAY MORNING. COINCIDING WITH
A MORE NORTHERLY TRACK OF THE SURFACE FEATURE, LOWER LEVEL COLD
AIR SWEEPING IN FROM THE NORTHWEST ON THE BACK SIDE OF THE LOW
DOES NOT MAKE IS AS FAR SOUTH IN CURRENT MODEL RUNS. 850MB AIR
TEMPS STILL DOWN TO -10C AND ARE EXPECTED TO STILL BE SUFFICIENT
FOR ACCUMULATING SNOW SUNDAY NIGHT THROUGH TUESDAY AFTERNOON FOR
MANY PANHANDLE LOCATIONS FIRST AS THE FRONT MOVES THROUGH AND THEN
IN POST FRONTAL SHOWER ACTIVITY. COLD AIR REMAINS IN PLACE AFTER
THAT, BUT PRECIP MOVES OFF TO THE SOUTHEAST. OVERNIGHT LOWS
MONDAY, TUESDAY, AND WEDNESDAY NIGHT WILL BE SIGNIFICANTLY COLDER
THAN WHAT WE HAVE BEEN SEEING RECENTLY WITH WIDESPREAD TEMPS
GETTING DOWN TO THE LOW TO MID 20S.

USED A BLEND OF NAM AND ECMWF THROUGH SUNDAY NIGHT FOR UPDATES TO
PRESSURE AND WIND. NUDGED TEMPS - MOSTLY DOWN - PER GFS AND ECMWF
MOD GUIDANCE. POP AND QPF PRIMARILY FROM ECMWF WITH SOME HELP FROM
BOTH GEM AND GFS. OVERALL FORECAST CONFIDENCE REMAINS AVERAGE.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.

&&

$$

PRB/FRITSCH

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