High Wind in Alaska, April 28, 2015

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
405 PM AKDT MON APR 27 2015

.SHORT TERM...AFTERNOON WV IMAGERY SHOWS A WEAKENING UPPER LOW
POSITIONED OVER THE GULF OF AK. SHARP TROUGH EXTENDS SSE INTO THE
NERN PAC FROM THE GULF LOW...AND IS SHOWING THE INITIAL STAGES OF
A BAROCLINIC LEAF SATELLITE SIGNATURE. THIS FEATURE WILL PROGRESS
TO HAIDA GWAII BY 12Z TUE...AND THEN LIFT NNE ACROSS THE SRN/CNTRL
PANHANDLE THROUGH TUE AFTERNOON. AS THIS TAKES PLACE...STRONG
SURFACE CYCLOGENESIS IS FORECAST TO OCCUR...WITH LOW PRESSURE
BOTTOMING OUT AT AROUND 980 MB BETWEEN 12-18Z TUE.
ECMWF...NAM...AND GEM ARE ALL FAIRLY CLOSE WITH THE MAGNITUDE AND
TRACK OF THE LOW...WHILE THE GFS WAS A SLIGHTLY FASTER OUTLIER.
UPDATED PRESSURE GRIDS REFLECT 12Z ECMWF/NAM SOLUTIONS.

NNELY WINDS WILL BE PREVALENT N OF THE LOW OVER THE CNTRL INSIDE
WATERS THROUGH MIDDAY TUE...OCCASIONALLY REACHING SMALL CRAFT
STRENGTH. MEANWHILE...INTENSE SLY PRESSURE GRADIENT WILL DEVELOP
OVER THE SRN PANHANDLE AND ADJACENT MARINE WATERS...YIELDING STRONG
WINDS FOR AKZ027-028 BY LATE MORNING...AND GALE FORCE WINDS OVER
CLARENCE STRAIT AND MARINE ZONE 41. FUTURE SHIFTS WILL NEED TO RE-
EVALUATE FOR POSSIBLE UPGRADE TO STORM FORCE IN CLARENCE
STRAIT...AND HIGH WIND WARNING FOR THE SRN PANHANDLE LAND ZONES.

AS THE UPPER DISTURBANCE AND ASSOCIATED SURFACE LOW MOVE INTO THE
NRN PANHANDLE BY LATE TUE AFTERNOON...WINDS WILL BECOME SLY OVER
MOST OF THE REGION. SMALL CRAFT WINDS WILL SPREAD NWD TO STEPHENS
PASSAGE AND POSSIBLY LYNN CANAL...THOUGH THE TIMING IS STILL
SOMEWHAT UNCERTAIN. SELY WINDS MAY BECOME GUSTY IN JUNEAU AS
WELL...AND THIS AREA TOO WILL NEED TO BE FURTHER EVALUATED FOR
STRONG WIND POTENTIAL.

PLUME OF PW VALUES NEAR 0.8 INCH ARE FORECAST TO BE ENTRAINED INTO
THE SURFACE CYCLONE AND SPREAD N ACROSS THE SRN PANHANDLE LATE
TONIGHT. THIS MOISTURE COMBINED WITH A DEEP WARM CLOUD LAYER WILL
FAVOR LOCALLY HEAVY RAIN OCCURRING FROM KLAWOCK...KETCHIKAN...AND
HYDER NWD TO PETERSBURG. HOWEVER...SYSTEM MOTION WILL BE
FAST...THUS DURATION OF HEAVY RAIN WILL NOT BE LONG ENOUGH TO
RESULT IN HYDRO ISSUES. AFTER THE LOW PASSES TO THE N...COOLER
MIDLEVEL TEMPERATURES WILL SPREAD E INTO THE SRN PANHANDLE...WHICH
WILL AID IN DESTABILIZING THE ATMOSPHERE AND LEAD TO THE
GENERATION OF MUCAPE VALUES FROM 200-400 J/KG. THE UNSTABLE
ENVIRONMENT COMBINED WITH FORCING FOR ASCENT ASSOCIATED WITH
SEVERAL PASSING UPPER IMPULSES...AS WELL AS TERRAIN RELATED
ASCENT...WILL SUPPORT A SLIGHT CHANCE OF THUNDERSTORMS DURING THE
LATE AFTERNOON.

.LONG TERM...THE TIGHT SOUTHERLY PRESSURE GRADIENTS WILL SPREAD
NORTH INTO LYNN CANAL AS THE PRESSURES OVER THE CANADIAN YUKON
FALL AND THOSE ACROSS THE PANHANDLE RISE. THIS WILL EASILY RESULT
IN GALE FORCE SOUTHERLIES THROUGH LYNN CANAL AND STRONG WIND GUSTS
INTO SKAGWAY. IT MUST BE NOTED THAT MARINE MOS GUIDANCE IS LIFTING
SUSTAINED WINDS TO 45+ KT TUESDAY NIGHT. GIVEN A POST-FRONTAL STRONGLY
CONVECTIVE ENVIRONMENT, GALE SEEMS PROBABLE, AND THESE HIGHER
SUSTAINED WIND SPEEDS ARE FEASIBLE. WENT AHEAD AND INCLUDED SOME
STRONG WIND GUSTS THROUGH JUNEAU TUESDAY EVENING AS WELL, GIVEN
PRESSURE GRADIENTS OF -10 MB BETWEEN JUNEAU AND KETCHIKAN. WINDS
OVER AUKE BAY AND POSSIBLY THE JUNEAU AIRPORT MAY BRIEFLY TURN
SOUTHWEST TOMORROW EVENING. AS STATED EARLIER, TIMING IS NOT
COMPLETELY CERTAIN, AND THINGS COULD OCCUR SLIGHTLY EARLIER AS
WELL. OVER THE CENTRAL AND SOUTHERN PANHANDLE WINDS SHOULD BE
TRENDING DOWN. CONSIDERING THE CONVECTIVE REGIME, THERE WILL
LIKELY BE SOME GUSTY PERIODS ABOVE THE INDICATED SUSTAINS IN THE
ZONES.

A SHORTWAVE BEHIND THE STRONG SYSTEM LIFTING NORTH THROUGH THE
PANHANDLE TUESDAY WILL EXTEND A SLIGHT THUNDERSTORM THREAT FOR
SOUTHEASTERN GULF AND SOUTHERN PUBLIC ZONES TUESDAY NIGHT. WHILE
NAM STILL INDICATING THE POTENTIAL OF THUNDERSTORMS INTO
WEDNESDAY, HELD OFF ON MENTIONING AT THIS POINT, AS ECMWF AND GFS
BEGIN TO REDUCE INSTABILITY WITH THE BEGINNINGS OF SOME WARMER AIR
ADVECTION ALOFT FROM THE WEST.

FROM WEDNESDAY NIGHT BEYOND, THE MODELS ARE RECOGNIZING A COUPLE
OF SHORTWAVES SKIRTING EASTWARD ACROSS THE GULF OF ALASKA WITHIN A
ZONAL TRACK. ONE SHOULD ARRIVE THE EASTERN GULF ON WEDNESDAY NIGHT
WITH THE SECOND THURSDAY NIGHT/FRIDAY. AS CURRENTLY, MOST OF THE MODELS
ARE INDICATING THE MOST STABLE AIR REMAINING TO THE SOUTH, WE HAVE
LEFT SOUTHEAST WITHIN A CONVECTIVE REGIME ALL WEEK. THE FORECAST
READS AS IF THERE WILL BE NO BREAKS BETWEEN SYSTEMS, BUT
TRUTHFULLY, SHOWERS WILL BECOME SCATTERED AT TIMES BETWEEN
SYSTEMS, BUT UNCERTAINTY OVER TIMING IS STILL GREAT ENOUGH TO
DEMAND GUESSING GAMES SO A WHITEWASH IS NECESSARY.

WHILE WE ARE ON THE SUBJECT OF SHOWERS, WE SHOULD TAKE A QUICK
ASIDE TO CLARIFY WHAT WE MEAN BETWEEN ISOLATED SHOWERS, SCATTERED
SHOWERS, NUMEROUS SHOWERS, AND JUST SHOWERS. EACH DESCRIPTOR IS
ASSOCIATED WITH A PERCENT OF MEASURABLE PRECIPITATION, OR "POP"
FORECAST. ISOLATED SUGGESTS A 10 TO 20 PERCENT CHANCE OF
MEASURABLE PRECIPITATION RECEIVED AT ANY PARTICULAR SPOT, SAY,
WHERE YOU ARE. LIKEWISE, SCATTERED REFERS TO 30 TO 50 PERCENT,
AND NUMEROUS 60 TO 70 PERCENT. CONTRARY TO HOW IT MAY READ,
"NUMEROUS" SHOWERS GENERALLY INDICATES THAT SHOWER COVERAGE WILL
BE SUCH THAT THE CHANCES OF GETTING WET ARE A LITTLE LESS THAN
"SHOWERS" WITHOUT A DESCRIPTOR. SHOWERS WITHOUT A DESCRIPTOR
SUGGESTS A CONFIDENCE THAT MOST EVERYONE WILL GET WET.

WITH A WET WEAK, WE COOLED TEMPERATURES A BIT FOR CLOUDY AND
SHOWERY CONDITIONS, PERHAPS TWO OR THREE DEGREES BELOW NORMAL.
THESE SYSTEMS ARE FAIRLY WEAK AND THE GENERALLY ZONAL FLOW DOES
NOT SUGGEST GREAT QUANTITIES OF RAINFALL.

AFTER THE FRIDAY WAVE SLIDES EAST INTO CANADA, RIDGING WILL
ATTEMPT TO SET UP AHEAD OF A SHARPER TROUGH THAT DEVELOPS INTO AN
UPPER LOW AS IT ARRIVES IN THE SOUTHWEST GULF. THIS SYSTEM LOOKS
TO APPROACH THE PANHANDLE AND THEN DROP SOUTHEAST FOR EARLY NEXT
WEEK. AS THE TOURISTS FLOCK TO SOUTHEAST ALASKA COME EARLY MAY, IT
IS POSSIBLE FOR A BRIEF SPAN OF DRY WEATHER FOR THEIR STAY, AS
WELL AS A DRY BEGINNING OF THE WEEK FOR THOSE OF US WHO CALL OUR
CORNER OF THE GREAT LAND HOME.

USED NAM/EC FOR TUE NIGHT/WED. THEN BLENDED TO GEM, GFS, NAM, AND
EC TO ACCOUNT FOR MODEL SPREAD WITH WAVES COMING IN WED NIGHT
THROUGH FRI. WPC FROM LATE FRIDAY ONWARD. FORECAST CONFIDENCE GOOD
TUESDAY NIGHT/WEDNESDAY, AND FALLS TO AVERAGE WEDNESDAY NIGHT AND
BEYOND.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND FROM TUESDAY MORNING THROUGH TUESDAY AFTERNOON FOR
     AKZ027.
         STRONG WIND FROM TUESDAY MORNING THROUGH TUESDAY AFTERNOON FOR
     AKZ028.
MARINE...GALE WARNING FOR PKZ036-041.
         SMALL CRAFT ADVISORY FOR PKZ012-013-031>035-042-043-051-052.

&&

GARNER/ADKINS

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