Small Craft Advisory 76, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
546 AM AKST TUE JAN 20 2015

.SHORT TERM...VERY ACTIVE WEATHER PATTERN CONTINUES THIS MORNING
WITH INTENSIFYING AND CLOSING OFF UPPER LOW OVER THE CENTRAL
GULF. SURFACE ANALYSES SHOW A 990 LOW SOUTHWEST OF YAKUTAT WITH AN
ACTIVE TRAILING WARM FRONT NOW ENTERING THE PANHANDLE. THE LOW IS
NOW BEGINNING TO FILL BUT THE WARM ADVECTION IS IMPRESSIVE. HAVE A
NUMBER OF HIGHLIGHTS CURRENTLY....ALL THE OFFSHORE WATERS ARE IN
GALES ALONG WITH THE SOUTHERN INNER CHANNELS. GRADIENT ALONG THE
COAST IS VERY STRONG SO WE HAVE STRONG WIND S/SE GUSTS MENTIONED
FOR ALL OF THE SOUTH AND JUNEAU AND THE COASTAL NORTHERN AREAS,
EXCEPTING YAKUTAT. FIGURING WINDS FOR LYNN CANAL IS A BIT OF A
CHALLENGE TONIGHT AS GRADIENTS ARE WEAK AND REVERSING...BUT WINDS
SHOULD COME AROUND TO SOUTHERLY THERE OVER THE NEXT FEW HOURS AND
PICK UP. ALSO WINTER STORM WARNING FOR THE KLONDIKE HIGHWAY AND A
SNOW ADVISORY TODAY FOR THE HAINES HIGHWAY NEAR THE BORDER STILL
LOOK RIGHT ON THE MARK WITH NEW GUIDANCE. MODELS PUMPING OUT 11 TO
17" THROUGH WED FOR THE KLONDIKE AND 3-5" TODAY NEAR HAINES
CUSTOMS AS WARM MARITIME AIR RUNS UP AND OVER THE COLD INTERIOR
AIR MASS...LOOKS LIKE THE EVENT SHOULD REMAIN ALL OR NEARLY
ENTIRELY SNOW FOR WHITE PASS.

THEN...A NEW LOW PRESSURE SYSTEM ENTERS THE PICTURE BY 15Z WED JUST
SOUTHWEST OF 310 IN RESPONSE TO YET ANOTHER INTENSIFYING WAVE
ALOFT WITH A SECOND ROUND OF WIND ENHANCEMENT AND RAINFALL
GETTING ITS ACT TOGETHER. WATER VAPOR LOOP ALREADY SHOWS
IMPRESSIVE JET DYNAMICS AND NORTHWARD MOISTURE TRANSPORT WITH
THIS NEW SYSTEM WELL SOUTH OF THE GULF CURRENTLY. 850MB PROGS SHOW
THE EC WINDS GETTING TO 60+ KTS IN THE KETCHIKAN AREA DURING THE
03-15Z PERIOD...LEADING TO WIND SHEAR ISSUES FOR THE AVIATION
FORECASTS THERE. INHERITED PRESSURE GRIDS WERE RIGHT ON THE MARK
WITH THE NEW MODEL GUIDANCE OVERALL FOR BOTH SYSTEMS...WINDS
REQUIRED SOME TWEAKING ON MOSTLY INNER CHANNEL SPEEDS,
SPECIFICALLY FROM 06Z ON IN CHATHAM, STEPHENS, CROSS AND SUMNER
AND A BUMP UP OF WINDS AT 12-15Z WED. ON THE SOUTHERN PORTION OF
310. MEANWHILE LIMITED CLARENCE TO 40 KTS DURING THE PERIOD UP TO
15Z BASED ON ACCOMPANYING GRADIENTS. HOWEVER CLARENCE DOES NOT
LOSE GALE-FORCE WINDS BETWEEN THE TWO LOWS AS THE SECOND ONE JUST
SERVES TO INCREASE THE PREEXISTING SOUTHEASTERLY FLOW THERE.
TWEAKED THE SNOW LEVELS BASED ON CURRENT OBS AND THE EC/NAM PROGS
FOR PARTIAL THICKNESSES...BRINGING THE HAINES AREA DOWN TO ABOUT
900 FEET TONIGHT AND HYDER DOWN TO 1500 TONIGHT BUT OUTSIDE THE
ROADS AND HIGHER TERRAIN ONLY RAIN WILL OCCUR. QPF TOTALS THROUGH
36 HRS SHOW JUNEAU GETTING UP INTO THE 1" CATEGORY WHILE THE SNOWY
AREAS TO THE NORTH GET TO THE 2" RANGE AND THE YAKUTAT VICINITY AT
AND EXCEEDING 3". HYDROLOGICALLY SPEAKING THINGS COULD GET
INTERESTING FROM WEDNESDAY ONWARD FOR THE HEAVY RAIN SCENARIO...WE
ARE FORTUNATE NOT TO HAVE A LOT OF PREEXISTING SNOW PACK WITH THIS
PATTERN OR ELSE WE WOULD LIKELY ALREADY HAVE FLOODING PROBLEMS.

.LONG TERM...HEAVY PRECIPITATION THAT ARRIVES TODAY WILL REMAIN
INTO THE EXTENDED RANGE FORECAST PERIOD. LOW LEVEL WARM AIR
ADVECTION WILL PUT AN END TO THE SNOWFALL EVERYWHERE EXCEPT
POSSIBLY NEAR WHITE PASS, BUT HEAVY RAIN WILL CONTINUE THROUGH
LATE WEDNESDAY EVENING WITH A DIMINISHING TREND TO THE SOUTH AND
EAST ALONG THE WESTERN SLOPES OF THE COAST MOUNTAINS. THE BREAK
WILL BE SHORT LIVED, AS THE NEXT SHORT WAVE WILL BRING HEAVY RAIN
TO YAKUTAT ON THURSDAY AFTERNOON, THEN FOR THE REMAINDER OF THE
PANHANDLE FROM FRIDAY MORNING TILL SATURDAY MORNING. HIGH
CONFIDENCE THAT FRIDAY WILL BE A RAIN EVENT FOR ALL PANHANDLE
LOCATIONS, INCLUDING WHITE PASS. WIND WILL REMAIN AN ISSUE AND
WITH ALL THE EXCESS RAIN BEGINNING TODAY, THE RISK OF MUDSLIDES
AND UPROOTED TREES WILL REMAIN HIGH FOR THE REMAINDER OF THE WEEK.

UPDATES TONIGHT FOCUSED ON PRECIPITATION AND WINDS ON FRIDAY. USED
ECMWF FOR PRESSURE AND WINDS. POP AND QPF FROM A GEM/ECMWF BLEND.
OVERALL CONFIDENCE IS AVERAGE TRENDING ABOVE AVERAGE FOR ANOTHER
ROUND OF HEAVY PRECIPITATION.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND UNTIL NOON AKST TODAY FOR AKZ023-026-027.
         STRONG WIND FROM THIS EVENING THROUGH LATE TONIGHT FOR AKZ027.
         STRONG WIND UNTIL 9 AM AKST THIS MORNING FOR AKZ022-024.
         STRONG WIND UNTIL NOON AKST TODAY FOR AKZ021-025.
         STRONG WIND UNTIL 3 PM AKST THIS AFTERNOON FOR AKZ028.
         WINTER WEATHER ADVISORY UNTIL 6 PM AKST THIS EVENING FOR AKZ019.
         WINTER STORM WARNING UNTIL 5 AM AKST WEDNESDAY FOR AKZ018.
MARINE...GALE WARNING FOR PKZ022-031-033>036-041>043-051-052.
         SMALL CRAFT ADVISORY FOR PKZ011-013-021-032-053.

&&

WESLEY/FRITSCH

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