Winter Storm in Alaska, March 23 to 24, 2010

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
508 AM AKDT TUE MAR 23 2010

.SHORT TERM...OVERRUNNING PRECIP EVENT IS WELL UNDERWAY ACROSS
THE PANHANDLE THIS MORNING AND THE 12Z SATELLITE SHOWS A LARGE
PLUME OF MID LEVEL MOISTURE ADVECTING OUT OF THE NORTHEAST
PACIFIC AND ACROSS THE PANHANDLE. AN OCCLUDED FRONT NOW IN THE
EASTERN GULF WILL GO ALOFT AND ADVECT THROUGH THE PANHANDLE BY
THIS AFTERNOON. A WEAK TRIPLE POINT LOW IN THE NORTHEASTERN GULF
THIS MORNING WILL MOVE NORTHWEST AND FILL INTO THE AFTERNOON BUT
NOT BEFORE ENHANCING THE BARRIER FLOW ACROSS THE NORTHERN GULF
WITH GALE FORCE WINDS. ARE SEEING MODERATE RAINFALL AMOUNTS
ACROSS THE SOUTHERN PANHANDLE WITH STRONG PVA AT MID LEVELS.
IMPRESSIVE ISENTROPIC UPGLIDE WILL CONTINUE THROUGH THE DAY WITH
SOUTHERLY 850 MB WINDS NEAR 40 KNOTS. OVERRUNNING IS ALSO
OCCURRING ACROSS THE CENTRAL AND NORTHERN PANHANDLE ATTM...WITH
LIGHT TO MODERATE SNOWFALL ALONG THE ICY STRAIT CORRIDOR AND
POINTS NORTHWARD. A 1040 MB SURFACE HIGH IN NORTHWEST CANADA
CONTINUES TO SUPPLY COLD AIR INTO THE NORTHERN PANHANDLE AND
COMBINED WITH THE LIGHT SURFACE WINDS ACROSS THE ICY STRAIT
CORRIDOR WILL CONTINUE TO SEE ACCUMULATING SNOWFALL THROUGH THE
EARLY MORNING. AS BOUNDARY LAYER WINDS INCREASE OUT OF THE
SOUTHEAST BY LATE THIS MORNING THE RAIN/SNOW LINE WILL RETREAT TO
THE NORTHERN PANHANDLE. THE MAIN FORECAST PROBLEM THIS MORNING IS
TIMING THE CHANGEOVER TO RAIN ACROSS THE ICY STRAIT
CORRIDOR...ESPECIALLY IN THE TAFS AND THE AMOUNT OF SNOWFALL THAT
WILL OCCUR TONIGHT ACROSS AKZ019. USED A BLEND OF THE 00Z
ECMWF/NAM THIS MORNING AS THE 00Z ECMWF BEST INITIALIZED THE MID
LEVEL MOISTURE AND VORTICITY FIELD ACROSS THE GULF AND PANHANDLE.
THE SURFACE PRESSURE GRADIENT ALONG THE COAST MOUNTAINS WAS ALSO
MUCH BETTER CAPTURED IN THE 00Z NAM/ECMWF KEEPING THE MAJORITY OF
THE STRONG PRESSURE GRADIENT IN THE YUKON WHILE THE 00Z GFS
BRINGS TOO STRONG OF A NORTHERLY SURFACE PRESSURE GRADIENT OVER
SKAGWAY. FORECAST CONFIDENCE IS BELOW AVERAGE THROUGH THE SHORT
TERM PERIOD DUE TO COMPLICATIONS IN PRECIP TYPE.

 FOR TODAY WILL CONTINUE TO SEE AN IMPRESSIVE OVERRUNNING PRECIP
EVENT WITH SOUTH TO SOUTHWEST WINDS FROM 500 MB TO 850 MB NEAR
45 KNOTS. WILL CONTINUE TO SEE MODERATE SNOWFALL ACROSS THE ICY
STRAIT CORRIDOR THIS MORNING UNTIL THE WINDS INCREASE OUT OF THE
SOUTH ALONG STEPHENS PASSAGE AND CHATHAM STRAIT. THE HEAVIEST
SNOWFALL AMOUNTS THIS MORNING WILL BE NEAR HOONAH WITH 2 TO
4 INCHES OF SNOWFALL AS EASTERLY UPSLOPE FLOW IS INCREASING
SNOWFALL RATES. THE STRONG NORTHERLY SURFACE PRESSURE GRADIENT
WHILE BRINGING LOCALLY WINDY CONDITIONS ALONG LYNN CANAL AND NEAR
SKAGWAY WILL ALSO LIMIT PRECIP AMOUNTS THROUGH THIS AFTERNOON FOR
AKZ018 AND AKZ019. FOR THIS REASON BELIEVE PRECIP AMOUNTS IN THE
00Z MODELS TO BE OVERDONE FOR THE HAINES AREA THROUGH MOST OF THE
DAY SO ONLY ISSUED A WWY FOR THEM. HOWEVER...IF 850 MB WINDS
BECOME MORE SOUTHEASTERLY THEN NOT ONLY WILL HAINES SEE
OVERRUNNING AT MID LEVELS BUT ALSO IN LOW LEVELS ACROSS THE
ARCTIC FRONT AND SO COULD SEE LOCALLY HEAVY SNOWFALL AMOUNTS.
WILL HAVE TO MONITOR THE 850 MB WIND FLOW CLOSELY THROUGH THE DAY
FOR SNOWFALL AMOUNTS IN AKZ019. ELSEWHERE ACROSS THE PANHANDLE
WILL SEE LOCALLY BREEZY CONDITIONS ALONG THE INNER CHANNELS AND
SOUTHERN PANHANDLE WITH THE 850 MB LLJ NEAR 40 KNOTS. WILL
CONTINUE TO SEE MODERATE RAINFALL...ESPECIALLY NEAR KETCHIKAN
THROUGH THE DAY AS PLENTY OF MOISTURE WILL CONTINUE TO ADVECT
OVER THE AREA ALONG THE MID LEVEL BAROCLINIC BOUNDARY. THERE
REMAINS A STRONG NORTHERLY SURFACE PRESSURE NEAR YAKUTAT THIS
MORNING AND THAT WILL LIMIT PRECIP AMOUNTS THERE AND PROVIDE
LOCALLY WINDY CONDITIONS OUT OF INTERIOR PASSES. WILL SEE ONLY
MINOR SNOW ACCUMULATIONS NEAR YAKUTAT WITH THE BREEZY CONDITIONS
CONTINUING THROUGH THE DAY AND THE WIND TRAJECTORIES REMAINING
EAST-SOUTHEASTERLY THROUGH 800 MB.

 FOR TONIGHT AM CONCERNED AKZ019 COULD RECEIVE HEAVY
SNOWFALL...ESPECIALLY NEAR HAND OFF WITH LONG TERM AS THE
BOUNDARY LAYER WINDS ALONG LYNN CANAL AND THE 850 MB WINDS WILL
BECOME SOUTHEASTERLY. THIS WILL ALLOW STRONG ISENTROPIC LIFT
ACROSS THE ARCTIC BOUNDARY AND COMBINED WITH THE OVERRUNNING AT
MID LEVELS COULD LEAD TO LOCALLY HEAVY SNOWFALL. WILL NEED TO
MONITOR MODEL TRENDS THROUGH THE DAY. OTHERWISE THE REST OF THE
PANHANDLE WILL SEE LOCALLY HEAVY RAINFALL AT TIMES WITH SOUTHERLY
850 MB WINDS UPWARDS OF 40 KNOTS AND STRONG ISENTROPIC LIFT. WILL
SEE A DEVELOPING SURFACE LOW NOW NEAR 45N 143W MOVE INTO THE
EASTERN GULF BY LATE TONIGHT AND THAT WILL BRIEFLY TURN THE MID
LEVEL FLOW MORE EASTERLY AND DIMINISH THE PRECIP RATES. HOWEVER
THE SURFACE LOW WILL RAPIDLY MOVE INTO THE NORTHERN GULF BY EARLY
WEDNESDAY MORNING AND ALLOW THE MID LEVEL FLOW TO ONCE AGAIN
BECOME SOUTHERLY AND INCREASE PRECIP RATES...ESPECIALLY NEAR
KETCHIKAN. WILL CONTINUE TO SEE LOCALLY BREEZY CONDITIONS OUT OF
THE SOUTHEAST ALONG THE OUTER COAST TONIGHT WITH GUSTS TO NEAR
30 KNOTS IN ASSOCIATED WATERS. ACROSS THE NORTHERN PANHANDLE WILL
CONTINUE TO SEE STRONG NORTHERLY WINDS AS THE 1040 MB SURFACE
HIGH IN NORTHWEST CANADA WILL SLOWLY ADVECT EASTWARD INTO EARLY
WEDNESDAY MORNING SLOWLY RELAXING THE SURFACE PRESSURE GRADIENT.
DRIVING WILL BE HAZARDOUS ALONG THE KLONDIKE HIGHWAY WITH BLOWING
SNOW AND LOCALLY WINDY CONDITIONS TONIGHT.

&&

.LONG TERM...A WEAKENING UPPER LEVEL SHORTWAVE TROUGH IN THE
EASTERN GULF WILL CONTINUE PUSHING AN UPPER LEVEL RIDGE EAST OF
THE COAST MOUNTAINS AWAY FROM THE FORECAST DOMAIN THROUGH EARLY
THURSDAY MORNING. ANOTHER WEAK SHORTWAVE ENTERING THE CENTRAL GULF
THURSDAY WILL NOT ORGANIZE WELL DUE TO A WEAK UPPER RIDGE BUILDING
ACROSS THE EASTERN GULF THROUGH LATE THURSDAY NIGHT OR EARLY
FRIDAY MORNING. A STRONG UPPER LOW IN THE NORTHEAST PACIFIC WILL
ENTER THE SOUTHERN GULF ON FRIDAY AND THE WEAK RIDGE OVER THE
PANHANDLE WILL MOVE EAST AS THE UPPER LOW MOVES INTO THE NORTH
CENTRAL GULF BY SATURDAY AFTERNOON. THROUGH EARLY FRIDAY
MORNING...MODELS ARE IN GENERAL AGREEMENT AND COMBINATION OF THE
ECMWF AND NAM ARE BLENDED INTO THE INHERITED GRIDS. MODELS BEGIN
DIVERTING SIGNIFICANTLY TOWARDS THE WEEKEND WITH THE INCOMING
OCCLUSION ASSOCIATED WITH THE STRONG LOW IN THE SOUTHERN GULF.

 THE SURFACE HIGH IN THE NORTHWESTERN CANADA WILL MOVE EAST AND
THE NORTHERLY PRESSURE GRADIENT WILL RELAX ACROSS THE NORTHERN
INNER CHANNELS. AS THE SURFACE LOW IN THE NORTHERN GULF
WEAKENS...THE ASSOCIATED FRONTAL BOUNDARY WILL WEAKEN OVER THE
NORTHERN PANHANDLE WEDNESDAY EVENING. EXCEPT THE HYDER ZONE...A
BROAD BUT PRETTY WEAK HIGH PRESSURE WILL BRIEFLY DOMINATE THE
FORECAST DOMAIN PRIOR TO THE NEXT STRONG FRONT ARRIVING ON FRIDAY
WITH A CHANCE POP. THE PRE-FRONTAL BOUNDARY IS EXPECTED TO ARRIVE
EASTERN GULF EARLY FRIDAY MORNING AND GENERATE GALE FORCE WINDS
AND ROUGH SEAS ACROSS THE OUTSIDE WATERS. AS THE FRONT MOVES INTO
THE AREA LATE FRIDAY...LIKELY TO CATEGORICAL POPS WILL RETURN TO
THE FORECAST DOMAIN THEN CONTINUE THROUGH EARLY NEXT WEEK. WITH
THE INCOMING FRONT...POSSIBLE ADVISORY LEVEL WIND GUSTS ARE
EXPECTED ALONG THE COASTAL SECTIONS FRIDAY NIGHT THROUGH SATURDAY
NIGHT. WARMING TREND WILL BEGIN ACROSS THE AREA BEHIND THE FIRST
FRONT AND THIS WARNING TREND WILL CONTINUE WITH THE NEXT FRONT.
EXPECT PRECIPITATION TYPE MOSTLY RAIN EXCEPT THE NORTHERN
PANHANDLE AS MIXING WITH SNOW.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER WEATHER ADVISORY FROM NOON TODAY TO 6 AM AKDT WEDNESDAY
     FOR AKZ019.
         WIND ADVISORY UNTIL 6 PM AKDT THIS EVENING FOR AKZ018.
MARINE...GALE WARNING FOR PKZ012-022-043-052.
         SMALL CRAFT ADVISORY FOR PKZ011-013-021-033>036-041-042-051.

&&

PSS/AHN