Flood in Alaska, August 22 to 23, 2011

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
226 PM AKDT MON AUG 22 2011

.SHORT TERM...BROAD LOW WILL SLOWLY WEAKEN OVER THE GULF THROUGH
TUE. A COUPLE OF TROFS WILL MOVE NE THRU THE AREA THROUGH TUE
MORNING. A LOW WILL DEVELOP S OF THE GULF AND HEAD NE TO JUST S
OF THE FAR SERN GULF BY LATE TUE. USED A BLEND OF 12Z
GFS/UKMET/ECMWF FOR THIS PERIOD.

 MAIN ISSUE IS THE ONGOING FLOOD SITUATION ON MENDENHALL LAKE AND
RIVER. WATER LEVELS ARE STILL RISING ON BOTH AND ARE NOW NEARING
MODERATE FLOOD LEVEL. IT IS NOT CERTAIN WHETHER THE RUNOFF FROM
JUST THE RAINFALL OR POSSIBLY A SECONDARY RELEASE FROM A GLACIAL
LAKE IS CONTRIBUTING TO THE RISE. AFTER COORD WITH RFC...HAVE
ISSUED A FLOOD WARNING FOR BOTH THE LAKE AND RIVER THRU 20Z WED.

 OTHERWISE BANDS OF PRECIP WILL CONTINUE TO MOVE ONSHORE THRU TUE
AS THE TROFS MOVE IN. ALSO...STEADIER RAIN WILL LIKELY MOVE NE
INTO THE SE GULF AND SRN PANHANDLE BY LATE TUE. LOOKS LIKE
ANOTHER 1 INCH OR SO OF RAIN ON AVERAGE ACROSS THE PANHANDLE OVER
THE NEXT 24 HRS. WINDS WILL GENERALLY REMAIN BELOW SCA EXCEPT FOR
CLARENCE STRAIT...WHERE SE WINDS AROUND 25 KT ARE EXPECTED
ESPECIALLY LATER TONIGHT AND TUE. NRN LYNN CANAL WILL BE CLOSE TO
SCA LEVELS AS WELL...BUT FOR NOW WILL KEEP THEM JUST UNDER. SEAS
HAVE DROPPED BELOW 8 FT FOR NOW BUT AS LOW MOVES NE TOWARD THE SE
GULF...WILL SEE SEAS PICK UP TO AROUND 8 FT IN THE SE GULF
TUE...SO WILL HAVE SCA FOR SEAS THERE.

 &&

 .LONG TERM...TUESDAY NIGHT THROUGH MONDAY. EXTENDED RANGE MODEL
CONSISTENCY AND AGREEMENT REMAINS RATHER GOOD THROUGH THE
WEEKEND. THE CURRENT WET WEATHER WILL CONTINUE INTO THE FIRST
PART OF THE EXTENDED PERIOD WITH A WEAK LOW MOVING INTO THE
SOUTHERN PANHANDLE ON WED. HYDROLOGY ISSUES MAY BECOME A CONCERN
FOR THE SOUTH AS THIS SMALL SYSTEM BRINGS MORE RAIN TO AN ALREADY
SOAKED SOUTHERN PANHANDLE. SREF PROBABILITIES ARE SUGGESTING A
GOOD POSSIBILITY OF QPF AMOUNTS GREATER THAN .25 INCHES IN
6 HOURS DURING TUES NIGHT AND A CHANCE OF AMOUNTS GREATER THEN
.50 INCHES IN 6 HOURS. WHILE THE SOUTH GETS EVEN WETTER THE NORTH
WILL START DRYING OUT AS THE EASTERLY LOW LEVEL FLOW TO THE NORTH
OF THE SYSTEM WILL HELP START TO SHUT OFF THE PERSISTENT RAINS.
THE COMPLETE DISSIPATION OF THE LOW IN THE NORTHERN GULF WILL
ALSO HELP AS WELL.

 BY WED NIGHT, MOST OF THE PANHANDLE WILL BE DRYING OUT AS A
RIDGE BUILDS OVER THE EASTERN GULF AND PANHANDLE. THIS DRY PERIOD
WILL LAST INTO THURS BEFORE THE NEXT SYSTEM STARTS KNOCKING ON
OUR DOOR THURS EVENING. MODEL CONSENSUS IS RATHER GOOD WITH THE
LONG RANGE MODELS ALL AGREEING ON A 980 MB LOW ENTERING THE GULF
SOUTH OF KODIAK EARLY THURS THEN WEAKENING AND CURVING INTO THE
NORTHERN GULF JUST SE OF PRINCE WILLIAM SOUND BY LATE FRI NIGHT.
THE ASSOCIATED FRONT WILL SLAM INTO THE PANHANDLE EARLY FRI WITH
RAIN AND GALE FORCE WINDS FOR THE OUTSIDE WATERS AND SOME INSIDE
WATERS.

 BEHIND THIS SYSTEM IS YET ANOTHER LOW THAT WILL FOLLOW ALMOST
THE SAME PATH FOR SUN. STILL SOME MODEL DIFFERENCES ON THIS ONE
AS THE EC IS FASTER THAN THE GFS ON WHEN THE FRONT GETS TO THE
PANHANDLE. IT IS ALSO LOOKING LIKE THIS SYSTEM WILL BE MORE OF A
RAIN MAKER THAN THE ONE ON FRIDAY DUE TO A BETTER TROPICAL
CONNECTION AND A FRONT THAT WILL HOLD TOGETHER AS IT COMES OVER
THE AREA.

 GIVEN MODEL AGREEMENT DECIDED TO GO WITH A BLEND OF GFS AND EC
WITH A BIT OF THE UK EARLY ON. ADDED IN THE HPC GUIDANCE FOR
FRIDAY ONWARD TO EVEN OUT THE MODEL DIFFERENCES TOWARD THE LATE
WEEKEND. FORECAST CONFIDENCE IS AVERAGE TO ABOVE AVERAGE THROUGH
THE EARLY WEEKEND THEN DROPS TO AVERAGE AFTER THAT POINT.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...AREAL FLOOD WARNING UNTIL NOON AKDT WEDNESDAY FOR AKZ025.
MARINE...SMALL CRAFT ADVISORY FOR PKZ036.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ041-042.

&&

RWT/EAL