Area Forecast Discussion
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FXAK67 PAJK 191319
AFDAJK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
519 AM AKDT SUN APR 19 2015

.SHORT TERM...LARGE UPPER TROUGH AND ASSOCIATED SURFACE LOW
PRESSURE WAS SITUATED OVER THE ERN BERING SEA AND ERN ALEUTIANS
EARLY THIS MORNING. FRONTAL BOUNDARY EXTENDED EWD FROM THE SURFACE
LOW ACROSS THE NRN GULF...AND THEN CURVED SWD ACROSS THE CNTRL
GULF. SRN FRONTAL SEGMENT WILL SLOWLY PROGRESS TOWARD THE COAST OF
SERN AK BY SUN NIGHT AS AN UPPER-LEVEL SHORTWAVE TROUGH SHIFTS E
ACROSS THE CNTRL GULF. IN ADDITION...MODEL GUIDANCE SHOWS A SUBTLE
UPPER IMPULSE MOVING N IN ADVANCE OF THE CNTRL GULF
SHORTWAVE...AND IS FORECAST TO AID IN THE DEVELOPMENT OF A MODEST
SURFACE WAVE/LOW PRESSURE CENTER SSW OF SITKA DURING SUN NIGHT.

GALE FORCE WINDS CURRENTLY OCCURRING ALONG THE FRONT THIS MORNING
WILL WEAKEN BY MIDDAY...BUT SMALL CRAFT WINDS WILL CONTINUE INTO
PART OF THE NIGHT OVER OUTSIDE MARINE ZONES 41 AND 42 IN RESPONSE
TO PREVIOUSLY MENTIONED WEAK SURFACE CYCLOGENESIS. OVER THE INSIDE
WATERS...SURFACE ISOBAR ORIENTATION IS GENERALLY PARALLEL TO N-S
CHANNELS. HOWEVER...BELIEVE SLY SPEEDS WILL BE ENHANCED SOMEWHAT
BY 30 KT 925 MB JET. THUS...MAINTAINED 20 KT WINDS FROM CLARENCE
STRAIT TO STEPHENS PASSAGE AND LYNN CANAL FOR MUCH OF THE SHORT
TERM PERIOD.

MEANWHILE...SATELLITE DERIVED PW GUIDANCE SHOWS VALUES NEAR 1 INCH
/CLOSE TO 3 STANDARD DEVIATIONS ABOVE CLIMATOLOGY/ SPREADING NWD
TOWARD THE CNTRL/SRN COAST OF SERN AK THIS MORNING. THIS MOISTURE
WILL SPREAD INLAND ACROSS THE SOUTHERN TWO THIRDS OF THE CWA BY
TONIGHT. IN ADDITION...A WARM PRE-FRONTAL BOUNDARY LAYER AIRMASS
WILL AID IN RAISING FREEZING LEVELS TO AROUND 3000 FT AMSL OVER
THE NRN PANHANDLE...AND 7000 FT AMSL OVER THE SRN PANHANDLE.
THE COMBINATION OF RELATIVELY RICH MOISTURE AND A DEEP WARM CLOUD
LAYER WILL SUPPORT THE POTENTIAL FOR ROBUST STRATIFORM RAINFALL.

THE FINAL INGREDIENT FOR HEAVY RAIN WILL BE A PLUME OF STRONG
700-500 MB UVV/S FORCED BY PREVIOUSLY MENTIONED UPPER
IMPULSE...WHICH WILL REACH THE SITKA AREA BY 03-06Z TONIGHT...AND
THEN THE GUSTAVUS TO JUNEAU AREA TOWARD 06-12Z. AS A
RESULT...RAINFALL RATES IN EXCESS OF 0.5 IN/HR WILL BE
POSSIBLE...YIELDING RAINFALL TOTALS FROM 1-2 INCHES BY 15Z MON.
FARTHER W...A DEEP LAYER OF COLD AIR WILL SPREAD ACROSS THE CNTRL
GULF AND AKZ017 TONIGHT. THIS WILL DESTABILIZE THE ATMOSPHERE AND
SUPPORT A TRANSITION TO A CONVECTIVE SHOWER REGIME.

INHERITED PRESSURE GRIDS WERE NUDGED TOWARD THE 00Z NAM AND
ECMWF...WHICH GAVE THE WEAK SURFACE LOW FORMING TONIGHT OVER THE
ERN GULF MORE DEFINITION. POP AND QPF GRIDS WERE UPDATED WITH A
BLEND OF THE ECMWF/GFS/NAM.

.LONG TERM...THE HEAVY RAIN OVER THE PANHANDLE SUNDAY NIGHT WILL
RELENT MONDAY FOR TWO REASONS. FIRST A SHORTWAVE CURRENTLY
ROUNDING THE BASE OF THE TROUGH AROUND 45 N 160 W WILL RIDE AHEAD
OF THE NORTHWARD MOVING WAVE OVER THE CENTRAL GULF, AND DIFFUSE
THE ENERGY EARLIER CONCENTRATED ACROSS THE NORTHEAST GULF TO AREAS
WELL TO THE SOUTH. IN ADDITION, THIS SHORTWAVE BYPASSING SOUTHEAST
ALASKA WILL SHUNT MUCH OF THE FEEDING MOISTURE EASTWARD RATHER
THAN NORTHWARD. FINALLY A WEAK BUT DRYING RIDGE IN THE LOW AND MID
LEVELS WILL HELP THIN THE DEEP MOISTURE SITUATED OVER THE
PANHANDLE FROM MONDAY NIGHT. THUS QPF AMOUNTS FOR MONDAY WILL
TOTAL LESS THAN A HALF INCH FOR ALL BUT MISTY FJORDS.

TO THE NORTHWEST IN YAKUTAT, ONSHORE FLOW COUPLED WITH COOLER
TEMPERATURES ALOFT WILL KEEP SHOWERS IN THE FORECAST. AS THE
FRONT SLOWLY MAKES PROGRESS THROUGH THE PANHANDLE, SOME
INSTABILITY MAY DEVELOP IN THE LOW LEVELS AS COOLER AIR SLIDES
EAST. HOWEVER, SINCE THE UPPER LEVELS WILL REMAIN RELATIVELY
WARM, AT 500 MB FOR EXAMPLE, WE KEPT THINGS STRATIFORM FOR MONDAY.
BY MONDAY NIGHT, THE TEMPERATURES AT 500 MB DROP ENOUGH TO SUGGEST
A MORE DEEPLY UNSTABLE AIR MASS AND STIR US TO WONDER ABOUT THE
POSSIBILITY OF CONVECTION THAT COULD SUPPORT LIGHTNING OVER THE
NORTHERN GULF. HELD OFF ON MENTIONING THUNDERSTORMS AT THIS TIME,
AS THE SUPPORTING CHARACTER OF THE SHORT-WAVE ITSELF DOES NOT
IMPRESS US WITH GRAVITAS. BUT EVEN WITHOUT THE SHORT-WAVE,
CONDITIONS COULD BE TOO RIPE AND THE PROXIMITY TO FORCED ASCENT
IN THE FORM OF THE ST. ELIAS MOUNTAINS MAY JUST PROVIDE THE
SPARK.

THE RIDGING EARLY IN THE WEEK THAT THAT MAY YIELD SOME SUNSHINE
STILL HAS THIS POTENTIAL. HOWEVER, THE ECMWF HAS NOW ADOPTED A
SLIGHTLY LESS FAVORABLE ORIENTATION FOR THIS RESULT. THE IDEA OF A
DIRTY RIDGE WHERE ONSHORE FLOW IN THE LOW TO MID LEVELS SPREADS
MOISTURE AND CLOUDINESS ONSHORE NEEDS TO BE PRESERVED. THE NEWEST RUN
OF THE ECMWF APPEARS TO BE LEANING MORE TOWARDS A WESTERLY
COMPONENT TO THESE WINDS, RATHER THAN THE SLIGHTLY MORE NORTHERLY
COMPONENT OF THE GFS AND NAM, AND EVEN THE GEM. IN ADDITION, THE
ECMWF ADVANCES THE RIDGE AXIS QUICKER TUESDAY EVENING. LARGELY
BROAD-BRUSHED THE NORTHERN PANHANDLE WITH MAINLY CLOUDY SKIES
TUESDAY. HOWEVER, THIS COULD BE A SUNNIER KIND OF MOSTLY CLOUDY.
SCATTERED SHOWERS WILL DIMINISH THROUGH LATE TUESDAY AS THE FLOW
BEGINS TO VEER OFFSHORE WITH A MAJOR SYSTEM REACHING THE CENTRAL
PANHANDLE BY WEDNESDAY.

WITH THE GFS AND ECWMF IN MODERATE AGREEMENT, WE RAISED WINDSPEEDS
TO GALE FORCE OVER 310 AND SOME OF THE OUTSIDE WATERS AHEAD OF THE
WEDNESDAY SYSTEM. AS THE LATITUDE OF THE SYSTEM WILL BE EQUAL TO
THAT OF DIXON ENTRANCE, PRECIPITATION OVER THE PANHANDLE WILL BE
LIMITED TO CHANCE POPS ACROSS THE COAST AND THE SOUTH. BUT AS THIS
SYSTEM EDGES NORTHWARD INTO THURSDAY, WHETHER A WHOLE LOW AS THE
ECMWF SUGGESTS OR A STRONG TRIPLE POINT FEATURE AS THE GFS HINTS
AT, WE HAVE NUDGED THE MOISTURE A LITTLE FARTHER NORTH SOONER
THAN PREVIOUS.

TEMPERATURES DURING THE NIGHTTIME AND SNOW LEVELS WILL FALL
BEHIND THE FRONT TO NEAR FREEZING/SEA LEVEL ACROSS THE FAR
NORTHERN ZONES INTO MID-WEEK. A SLOW BUT STEADY REBOUND WILL BEGIN
TO OCCUR WEDNESDAY NIGHT INTO THE WEEKEND WITH SOME WARM AIR
ADVECTION ASSOCIATED WITH THE MID-WEEK GULF SYSTEM.

USED NAM AND ECMWF FOR UPDATES MONDAY/MONDAY NIGHT. STRONGLY
NUDGED TO ECMWF AND GFS TO ADD STRENGTH TO THE WEDNESDAY SYSTEM.
EXTENDED FORECAST REMAINS WELL INTACT. COOLED TEMPERATURES A TAD
IN THE MID-RANGE WHILE WARMING THINGS A TAD TOWARD THE WEEKEND.
FORECAST CONFIDENCE DECENT.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING FOR PKZ042-043-051-052.
         SMALL CRAFT ADVISORY FOR PKZ021-022-033>036-041-053.

&&

$$

GARNER/ADKINS

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