Flood in Alaska, September 7 to 9, 2013

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
410 PM AKDT SAT SEP 7 2013

.SHORT TERM...A RELATIVELY UNSETTLED WEATHER PATTERN IN STORE
FOR SOUTHEAST ALASKA OVER THE NEXT 24 HOURS. AFTERNOON SATELLITE
IMAGERY SHOWS A STALLED FRONTAL BOUNDARY OVER THE NORTHERN GULF
COAST EXTENDING SOUTHWARD. A DEVELOPING LOW PRESSURE SYSTEM IS
MOVING NORTH ALONG THE FRONT INTO THE EASTERN GULF. THIS HAS
RESULTED IN MODERATE RAIN OVER YAKUTAT FOR MUCH OF THE MORNING
AND AFTERNOON WITH STEADY LIGHT TO MODERATE RAIN FALLING OVER THE
REMAINDER OF THE NORTHERN PANHANDLE. THE LOW PRESSURE SYSTEM WILL
RAPIDLY MOVE NORTH REACHING THE NORTHEAST GULF OVERNIGHT AT
AROUND 995 MB. THE LOW IS THEN EXPECTED TO PUSH INLAND IN THE
VICINITY OF ICY BAY EARLY SUNDAY MORNING. AS THE LOW MOVES INLAND
AND THE FRONT PUSHES THROUGH THE PANHANDLE A RIDGE WILL RAPIDLY
BUILD IN WITH A STRONG PUSH OF SOUTHERLY WINDS.

 MAIN ISSUES FOR TODAY WERE HEAVY RAINFALL POTENTIAL...WIND AS
THE FRONT MOVES INLAND OVERNIGHT...AND HYDROLOGY CONCERNS.
RAINFALL INTENSITY WILL BE INCREASING THROUGH THE EVENING AND
OVERNIGHT AS THE FRONT MOVES INLAND. RAIN WILL BE HEAVY AT TIMES
OVER MUCH OF THE PANHANDLE WITH THE FRONT EXCEPT ACROSS THE FAR
SOUTH. AM EXPECTING RAINFALL TOTALS TO RANGE FROM 1-4 INCHES
THROUGH SUNDAY AFTERNOON WITH THE HEAVIEST AMOUNTS NORTH OF CROSS
SOUND TO YAKUTAT. RAIN WILL QUICKLY DIMINISH IN INTENSITY SUNDAY
MORNING AS THE FRONT WEAKENS AND MOVES EAST OF THE AREA. PERIODS
OF RAIN WILL STILL PERSIST THROUGH THE AFTERNOON AS ONSHORE FLOW
PERSISTS BEHIND THE FRONT.

 STRONG SURFACE PRESSURE GRADIENT CURRENTLY OVER THE NORTHERN
PANHANDLE WILL CONTINUE GUSTY WINDS THAT HAVE BEEN OBSERVED THIS
AFTERNOON. STRONGER WINDS WILL BECOME MORE WIDESPREAD LATER
TONIGHT IN ASSOCIATION WITH THE FRONT. MODELS ARE INDICATING A
LOW LEVEL JET OF AROUND 50 KTS MOVING OVER THE CENTRAL AND
NORTHERN PANHANDLE LATER THIS EVENING AND OVERNIGHT. HAVE
INCLUDED AT LEAST SMALL CRAFT WINDS OVER THE INNER CHANNELS DUE
TO THIS. GALE FORCE EASTERLY WINDS AHEAD OF THE FRONT IN CROSS
SOUND WILL QUICKLY SHIFT TO POST FRONTAL SOUTHWESTERLIES AND
INCREASE TO 40 KTS WITH HIGHER GUSTS. GALE FORCE WINDS CAN ALSO
BE EXPECTED IN LYNN CANAL WITH THE FRONTAL PASSAGE AND CONTINUING
ON SUNDAY AS THE BUILDING RIDGE CAUSES A 4-5 MB GRADIENT BETWEEN
JUNEAU AND SKAGWAY. HAVE NOT INCLUDED ANY STRONG WIND HEADLINES
IN THIS FORECAST PACKAGE BUT THERE IS THE POTENTIAL FOR A FEW
WIND GUSTS TO REACH 40 MPH ESPECIALLY ACROSS THE NORTHERN
PANHANDLE.

 HYDROLOGY ISSUES ARE A CONCERN WITH THE HEAVY RAINFALL. AT THIS
TIME FLOOD ADVISORIES AND WATCHES ARE IN EFFECT FOR PORTIONS OF
THE NORTHERN PANHANDLE INTO SUNDAY AFTERNOON. REFER TO THE
APPROPRIATE HYDROLOGY PRODUCTS FOR MORE INFORMATION ON EXPECTED
IMPACTS.

 USED THE ECMWF FOR PRESSURE AS IT REMAINS A GOOD COMPROMISE
BETWEEN THE MODELS. CHANGES WERE PRIMARILY LIMITED TO LOCAL
EFFECTS. FORECASTER CONFIDENCE IS ABOVE AVERAGE ON A WET WINDY
PERIOD THROUGH THE NEXT 24 HOURS.

.LONG TERM...THINGS CONTINUE TO CALM DOWN SUNDAY EVENING...AND FOR
THE LYNN CANAL REGION, THEY BEGIN TO DIMINISH. AT THIS STAGE, WE
THINK THERE WILL BE ONE LAST GASP FOR THE STRONG WINDS OVER
SKAGWAY EARLY SUNDAY EVENING AS THE RIDGE TIPS FORWARD, AND WHILE THE
GRADIENTS WILL REMAIN TIGHT IN THE NORTH BETWEEN THE YUKON LEE
SIDE LOW AND THE BUILDING RIDGE OVER THE EASTERN GULF THROUGH
SUNDAY NIGHT, THE GALES WILL CALM TO A MUCH MORE MANAGEABLE 20 KT
BY EARLY MONDAY MORNING. EVEN WITH COLDER AIR ALOFT CONTINUING TO
ADVECT INTO THE NORTHERN PANHANDLE. ON THE OTHER SIDE OF THE
SPECTRUM THE SOUTHERN PANHANDLE MAY SEE SOME BREAKS IN THE CLOUDS
FOR SUNDAY NIGHT AS 500 MB PRESSURE HEIGHTS RISE HIGHEST HERE AND
DRIER AIR IS ADVECTED IN AT THE MID LEVELS. WENT SAFE WITH 26 AND
27, BUT DID CALL THE SOUTHERN INNER CHANNELS PARTY CLOUDY. THE
OTHER CONCERN EARLY MONDAY MORNING WILL BE THE FOG CONDENSING WITH
SO MUCH GROUND SATURATION. AT THIS POINT, BLANKETED ALL AREAS
BELOW 300 FEET WITH PATCHY FOG WITH THE EXCEPTION OF
HAINES/SKAGWAY/NORTHERN LYNN AS WINDS WILL KEEP THE ATMOSPHERE
FROM DECOUPLING FOR FOG FORMATION. IN THE NORTH THIS WILL MAINLY
STEM FROM HIGH LEVELS OF GROUND SATURATION...IN THE SOUTH IT WILL
BE FROM COOLING TEMPERATURES AND CLEARING SKIES.

RIDGE SETTLES OVER THE PANHANDLE MONDAY INTO TUESDAY AHEAD OF THE
NEXT SYSTEM MOVING INTO THE WESTERN GULF LATE MONDAY...THIS
EFFECTIVELY SWITCHES MARINE WATER DIRECTIONS OVER THE EASTERN GULF
FROM SOUTHWEST TO SOUTHEAST YET AGAIN. GFS, GEM, AND NAM ARE ADDING A
LITTLE MORE GUSTO TO THE FRONT MOVING FROM THE SOUTHEAST MONDAY
NIGHT INTO TUESDAY NIGHT AND THUS HAVE INCREASED THE WINDS ACROSS
THE OUTSIDE WATERS TO SMALL CRAFT LEVELS. SOME MOUNTING EVIDENCE
OF HEAVY RAIN COMING INTO YAKUTAT TUESDAY AFTERNOON AND TUESDAY
NIGHT, BUT AT THIS POINT, STILL DON'T LOOK AS IMPRESSIVE AS THE
SYSTEM WE ARE CURRENTLY DEALING WITH...THUS LEFT IT OUT. HOWEVER,
THE 18Z GFS IS COMING IN WETTER FOR THE MID-WEEK FRONTAL
PASSAGE...ESPECIALLY FOR YAKUTAT.

A RIDGE BUILDS BEHIND THE MID-WEEK FRONT, BUT BY THE WEEKEND, A
NEW SYSTEM LIKELY ARRIVES WITH MORE WIND AND WIND FOR SOUTHEAST
ALASKA. THE ACTIVE...PROGRESSIVE PATTERN CONTINUES.

USED GFS AS A GUIDANCE FOR CHANGES FOR MONDAY NIGHT INTO TUESDAY
NIGHT...BUT GEM AND NAM AND TO SOME DEGREE ECMWF WERE ALL COMING
IN LINE WITH THE FRONT MOVING IN. ADDED A LITTLE NEW RUN OF THE
SREF TO HANDLE POP FIELDS THROUGH MONDAY AND BLENDED WITH EC, GFS,
AND GEM FOR MONDAY NIGHT INTO TUESDAY. CONFIDENCE IN FORECAST GOOD
WITH SYNOPTIC PATTERN, BUT FALLS WITH DETAILS...ESPECIALLY LATE IN
THE WEEK.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...AREAL FLOOD ADVISORY FROM 10 PM THIS EVENING TO 4 PM AKDT SUNDAY
     FOR AKZ025.
         FLOOD WATCH FROM 10 PM AKDT THIS EVENING THROUGH SUNDAY
     AFTERNOON FOR AKZ019.
         AREAL FLOOD ADVISORY UNTIL 4 PM AKDT SUNDAY FOR AKZ017-018.
MARINE...GALE WARNING FOR PKZ012-013-022-042-043-051.
         SMALL CRAFT ADVISORY FOR PKZ011-021-031>036-041-052-053.

&&

TPS/JWA