Gale Warning 99, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
514 PM AKST THU FEB 5 2009

.SHORT TERM...A LOW CURRENTLY 600 NM S OF COLD BAY IS TRACKING
NE AND IS THE CENTER OF A LARGE-SCALE CYCLOGENETIC SYSTEM TO
ENTER THE GULF OF ALASKA FRI AS IT BRINGS A BROAD BAND OF
MODERATE INTENSITY PRECIPITATION TO SE AK.  A LOW AND OCCLUSION
ALREADY IN THE WRN GLFALSK WILL BEGIN WEAKENING TONIGHT DUE  TO
THE STRONG WARM SECTOR RIDGE THAT IS ASSOCIATED WITH THE
LARGE-SCALE CYCLOGENETIC SYSTEM.

 A MARITIME MODIFIED ARCTIC AMS IN THE PANHANDLE WILL MODIFY
AWAY TO WARMER MARITIME IN A S TO N SEQUENCE TONIGHT THROUGH  FRI
NIGHT. THE RESULT OF THIS WARMING IS THAT THE PRECIPITATION
ASSOCIATED WITH THE STRONG CYCLOGENTIC SYSTEM WILL CHANGE TO ALL
RAIN UP TO FREDERICK SOUND FRI, AND CONTINUE TO WARM NORTHWARD
FRI NIGHT AND SAT.

 NOTE THE WINTER STORM WATCH IN EFFECT MEANS ONLY A 60 TO 70
PERCENT CONFIDENCE THAT THE WINTER STORM WILL OCCUR.  THE
CONFIDENCE IS LIMITED PRIMARILY BY THE CONCERN THE WARM UP FROM
THE SOUTH COULD OCCUR RAPIDLY ENOUGH TO LIMIT THE  SNOW FALL TO
BELOW 6 INCHES IN 12 HOURS.  THE THERMAL THRESHOLDS FOR A SNOW TO
RAIN TRANSITION ARE OVER THE NRN HALF OF THE PANHANDLE FRI NIGHT
INCLUDING 1290M 1000-850 MB THICKNESS, -6C 850 MB TEMP, AND 528
DAM THICKNESS FOR THE 1000-500 MB. NAM USED AS PRIMARY GUIDANCE.

 ABOUT THAT WEAKER SYSTEM THAT IS ALREADY IN THE GULF:  A 996 MB
LOW IN THE VICINITY OF KODIAK HAS AN OCCLUSION EXTDG SE FM SEWARD
TO A TRIPLE POINT ABOUT 200 NM SE OF KODIAK.  THE LOW WILL
DISSIPATE WHILE TRACKING NE, AND THE OCCLUSION WILL WEAKEN AS IT
TRACKS NE TO EXTEND S FM YAKUTAT 12Z FRI.

.LONG TERM...WINTER STORM POTENTIAL FOR ZONES 17...19...20...21
AND 25 FOR FRIDAY AFTERNOON THROUGH SATURDAY MORNING THE PRIMARY
FOCUS IN THE LONG TERM. 05 12Z NAM12 WAS USED AS PRIMARY GUIDANCE
THROUGH SUNDAY. MINIMAL CHANGES WERE MADE TO THE PRESSURE AND
WIND GRIDS FROM DAY 4 TO DAY 8 AS THE MODELS STILL HAVE A LARGE
SPREAD ON SOLUTIONS PAST DAY 6 AND WOULD HAVE INTRODUCED LARGE
CONTINUITY DIFFERENCES INTO THE GRIDS.

THE 12Z AND 18Z GFS AND 12Z NAM12 ARE IN GOOD AGREEMENT ON TIMING
AND TRACK OF THE 990MB LOW INTO THE NORTH CENTRAL GULF. THE 12Z EC
IS A BIT SLOWER AND FURTHER WEST AND WAS NOT THE PREFERRED
SOLUTION. NAM12 BRINGS NEARLY AN INCH AND A QUARTER OF LIQUID
EQUIVALENT TO PAJN WHILE GFS BRINGS OVER ONE AND A HALF INCHES OF
LIQUID EQUIVALENT FROM FROM 18Z ON THE 6TH TO 18Z ON THE 7TH.
THERE IS ABUNDANT MOISTURE WITH THIS SYSTEM. THE CENTRAL AND
SOUTHERN ARE PROGGED TO RECEIVE OVER AN INCH OF RAIN WITH THIS EVENT.

FOR ZONES 17...19...20...21 25 THE QUESTION IS HOW LONG WILL THE
COLD AIR STAY IN PLACE BEFORE MIXING WITH RAIN. SNOW MODEL AND
MODEL THICKNESS INDICATE AROUND 12Z OR 15Z FOR JUNEAU AND BETWEEN
9Z AND 12Z FOR HOONAH AND AROUND 9Z FOR PAYA. MODEL THICKNESS
VALUES DO NOT SUPPORT A CHANGE OVER IN ZONE 19. SNOW TOTALS COULD
BE SIGNIFICANTLY HIGHER IF THE RAIN MIX HAPPENS LATER. AT THE VERY
LEAST THE SNOW THAT FALLS WILL BE VERY WET AND HEAVY AND ON THE
ORDER OF 9 OR 10 TO 1.

AS THE LOW APPROACHES THERE IS STILL CONCERN FOR ZONES 23...27 AND
28 FOR STRONG WINDS. 12Z NAM12 BRINGS 65KTS AT 850 OVER THESE
ZONES. THIS SHOULD BE CLOSELY MONITORED FOR THE DEVELOPMENT OF
ADVISORY OR EVEN WARNING LEVEL WINDS IF THE LOW MOVES ON A MORE
EASTERLY TRACK.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER STORM WATCH FROM FRIDAY AFTERNOON THROUGH LATE FRIDAY
     NIGHT FOR AKZ017-019>021-025.
MARINE...SMALL CRAFT ADVISORY FOR PKZ022-033>036-041>043-051-052.

&&

JBT/CCC