Flood in Alaska, June 25 to 26, 2013

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
412 PM AKDT TUE JUN 25 2013

.SHORT TERM...THE BIGGEST ITEM TODAY IN THE SHORT TERM IS THE
THUNDERSTORMS THAT WILL BE IMPACTING THE NORTHERN PANHANDLE LATE
THIS AFTERNOON INTO THIS EVENING. EARLY AFTERNOON VISIBLE
SATELLITE IMAGERY SHOWS CLOUDINESS ACROSS MOST OF THE PANHANDLE.
SOUTHERN PANHANDLE ARE UNDER A LAYER OF STRATIFORM CLOUDS AND
PERIODS OF LIGHT RAIN. THIS IS ASSOCIATED WITH A COLD FRONT
BRINGING IN COOLER MARITIME AIR FROM SOUTH TO NORTH, WHICH IS
ATYPICAL. FOR THE REST OF THE PANHANDLE, SUNSHINE IS FILTERING
THROUGH MOSTLY CLOUDY SKY. SCATTERED SHOWERS AND THUNDERSTORMS
DEVELOPED IN NORTH CENTRAL BRITISH COLUMBIA THIS MORNING, AND THEY
WERE HEADING WEST TOWARDS THE PANHANDLE. THESE SHOWERS AND
THUNDERSTORMS WILL BE THE BIG WEATHERMAKER FOR THE NORTHERN
PANHANDLE THIS AFTERNOON.

 SEVERAL FACTORS WILL BE FAVORABLE FOR THUNDERSTORMS TO MAINTAIN
ITSELF AS IT TRACKS ACROSS THE COAST MOUNTAIN. THE FIRST FACTOR:
THERE IS SUFFICIENT WIND SHEAR IN THE LOWER PART OF THE
ATMOSPHERE, AS GFS SUGGESTS ABOUT 40 KT OF WIND SHEAR BETWEEN
SURFACE AND 500 MB. THE WIND SHEAR WILL ENHANCE DOWNSTREAM (NEW
STORMS POPPING UP EAST OF PREEXISTING CONVECTION) DEVELOPMENT.
THE SECOND FACTOR: PRE-EXISTING SEA BREEZES BOUNDARIES WILL
ENHANCE LOW- LEVEL LIFTING THAT WILL MAINTAIN UPDRAFTS THAT FUEL
THUNDERSTORMS. THE THIRD FACTOR: AMPLE MOISTURE IN THE LOWER PART
OF THE ATMOSPHERE. DEW POINTS ARE IN THE MID TO UPPER 50S THIS
AFTERNOON IN THE AREA, WHICH PROVIDE EXTRA ENERGY FOR CONVECTION
EVEN THOUGH TEMPERATURES ARE NOT AS WARM AS YESTERDAY. THERE ARE
ALSO FACTORS THAT WILL DEEM UNFAVORABLE FOR THUNDERSTORM
DEVELOPMENTS. THE BIGGEST FACTOR IS THE PRE-EXISTING CLOUDS AND
COOLER AIR TEMPERATURES TODAY IN CENTRAL AND SOUTHERN PANHANDLE.
WITH THESE FACTORS, CAME UP WITH THUNDERSTORM AREAS THAT COVER THE
COAST MOUNTAIN FROM SKAGWAY/HAINES TO JUNEAU INTO
PETERSBURG/WRANGELL AREA. LYNN CANAL AND PART OF STEPHENS PASSAGE
MAY ALSO BE IMPACTED BY ISOLATED THUNDERSTORMS LATER THIS
AFTERNOON INTO THE EVENING.

 AS OF 2 PM, LIGHTNING STRIKES WERE BEING DETECTED NEAR THE
MENDENHALL ICEFIELD. MORE LIGHTNING STRIKES ARE EXPECTED TO
IMPACT THE NORTHERN AREA THIS AFTERNOON INTO THIS EVENING.

 WIND SHOULD REMAIN PRETTY LIGHT FOR TONIGHT. HIGHER GUSTS TO
30 MPH ARE EXPECTED NEAR SHOWERS. SOUTHERLY WINDS WILL DEVELOP
TOMORROW ACROSS MOST OF THE PANHANDLE AS THE COLD FRONT PUSHES
NORTH ACROSS THE PANHANDLE THROUGH THE SHORT TERM FORECAST
PERIOD. LYNN CANAL WILL SEE SMALL CRAFT ADVISORY WINDS BY
TOMORROW AFTERNOON.

 USES GUIDANCE FROM NAM TO REFRESH THE PRESSURE AND WIND FIELDS.
UTILIZES AN ENSEMBLE OF HIGH RESOLUTION MODEL TO REFRESH THE
PRECIPITATION FIELD. MODEL CONFIDENCE IS BELOW AVERAGE FOR
THUNDERSTORMS THAT WILL BE IMPACTING PART OF THE AREA THIS
EVENING, AS MODELS ARE NOT SKILLFUL IN DEALING WITH CONVECTION
OVER THE COMPLEX TOPOGRAPHY IN SOUTHEAST ALASKA. OTHERWISE, MODEL
CONFIDENCE IS ABOVE AVERAGE FOR OTHER FIELDS.

.LONG TERM...OVERALL THE REX BLOCK PATTERN HOLDS THROUGH THE
WEEKEND WITH A LOW OVER THE NORTHEAST PACIFIC AND A HIGH OVER THE
YUKON AND THE ALASKA INTERIOR. THE LOW WILL USHER A FRONT NORTH
INTO THE PANHANDLE ON WEDNESDAY BUT WILL STALL ACROSS THE NORTHERN
PANHANDLE WEDNESDAY NIGHT AS IT RUNS INTO THE HIGH TO THE NORTH AND
STEADILY WEAKEN THROUGH THURSDAY. THIS FRONT WILL SPREAD RAIN TO
THE NORTH AND JUNEAU AND MUCH OF THE ICY STRAIT WEDNESDAY NIGHT
INTO THURSDAY. BY THURSDAY NIGHT RAIN CHANCES WILL STILL LURK AS
THE LOW IS TO THE SOUTH, BUT THE LOW BEGINS TO SLOWLY DRIFT TO THE
SOUTH...SHORTWAVES TO BE WEAKER AS THEY DRIFT INTO THE PANHANDLE.
THIS OVERALL PATTERN HAS NOT CHANGED FROM THE PREVIOUS FORECAST.

DID NOT CHANGE THE GALE ON THURSDAY AFTERNOON FOR NORTHERN LYNN
BUT GUIDANCE SEEMS TO BE FALLING AWAY FROM THAT. STILL EXPECT GOOD
WIND SOMEWHERE ACROSS THE MOUNTAINS THERE AS WARM TEMPERATURES AND
THERMAL LOW AT THE SURFACE TO THE NORTH WILL CONTRAST WITH THE
COOLER TEMPERATURES OVER SOUTHEAST. BUT THAT WILL BE EXAMINED
MORE THOROUGHLY TONIGHT.

CHANGES WERE MADE THIS WEAKEN...AS THE LOW ALOFT TO THE SOUTH
MOVES FARTHER SOUTH, FLOW WILL BECOME MORE EAST, AND THE HIGH TO
THE NORTH WILL SINK TO THE SOUTH. THIS WILL EFFECTIVELY DRY THE
PANHANDLE FROM THE CHANCES OF RAIN AND EVEN CLEAR OUT SOME SKIES
FOR THE WEEKEND. TEMPERATURES MAINLY IN THE 70S OVER THE INTERIOR
NORTH LOOK ON TRACK. BEYOND THE WEEKEND, THE REX BLOCK BEGINS TO
FALL APART AS THE LOW IN THE BERING DIGS AND PROMOTES A MORE
PROGRESSIVE PATTERN. THIS SHOULD TRANSLATE INTO MORE OF AN ONSHORE
PATTERN FOR SOUTHEAST ALASKA. WITH THIS IN MIND, DID COOL
TEMPERATURES A BIT INTO THE 60S.

CHANGES MADE WERE MINOR. DID BLEND A LITTLE TO THE NAM WEDNESDAY
NIGHT, BUT MADE NO ADDITIONS THE REST OF THE WEEK AS OVERALL
PATTERN LOOKED SOLID. HOWEVER, DID USE SOME ECMWF AND BLENDED WITH
WPC THIS WEEKEND TO BRING IN A LITTLE OF THE DEVELOPING THERMAL
TROUGH. CONFIDENCE IN OVERALL PATTERN FAIRLY GOOD.

.HYDROLOGY...WITH THE WARM TEMPERATURES YESTERDAY AFTERNOON IN
THE HIGHER ELEVATIONS AWAY FROM LYNN CANAL, THE TAIYA RIVER
CERTAINLY RESPONDED...RISING ABOVE MINOR FLOOD STAGE OF 16.9 FT TO
16.9FT EARLY THIS MORNING BUT REMAINING HIGH NEAR 16.6 FT AS OF
130 PM THIS AFTERNOON. LATE WORD AS OF 3PM THE TAIYA RIVER HAS
RISEN FAST TO 16.9 FT. WITH THUNDERSTORMS AND SHOWERS APPROACHING,
FELT CONFIDENT ENOUGH TO ISSUE A WARNING. THE TAIYA WILL EXCEED
MODERATE FLOOD STAGE OF 17.0 FT SOON WITH KNEE TO THIGH DEEP
WATER. WE ARE EXPECTING A CREST OF 17.2 FT THIS EVENING. WATER
LEVELS LIKE LAST NIGHT WITH THE SHOWERS AND THUNDERSTORMS WILL
LIKELY REMAIN ABOVE MODERATE FLOOD STAGE INTO WEDNESDAY MORNING.
RIGHT NOW, WE ARE WATCHING THE CHILKAT RIVER AS WELL FOR WATERS
RISING ON THURSDAY MORNING, BUT AT THIS POINT WATERS...WHILE
BANKFULL ARE NOT EXPECTED TO RISE BEYOND MINOR FLOOD STAGE.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...AREAL FLOOD WARNING UNTIL 10 AM AKDT WEDNESDAY FOR AKZ018.
MARINE...SMALL CRAFT ADVISORY FOR PKZ012-013.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ041.

&&

RCL/JWA