Small Craft Advisory 68, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
610 AM AKST FRI JAN 21 2011

.SHORT TERM...A COMPACT LOW WITH A VERY WELL DEFINED BAROCLINIC
CLOUD SHIELD LIES OVER THE EASTERN GULF AND PANHANDLE THIS
MORNING. THE LOW HAS BEEN INTENSIFYING AS IT APPROACHED THE GULF
BUT WILL TRACK INLAND AND BEGIN TO WEAKEN BEFORE IT POSES TOO
MUCH OF A WIND THREAT TO SOUTHEAST ALASKA. THE ASSOCIATED WEATHER
FRONT HAS BEEN IMPACTING THE PANHANDLE ALL NIGHT WITH WIDESPREAD
LIGHT PRECIP. RADAR HAS BEEN PAINTING LARGE AREAS OF 40 TO 50 DBZ
REFLECTIVITY HOWEVER THE HEAVIEST PRECIP APPEARS TO HAVE REMAINED
OFFSHORE THUS FAR. WARM LOW LEVEL AIR IN ALL LOCATIONS EXCEPT
HAINES HAVE RESULTED IN RAIN RATHER THAN SNOW.

THE MAIN FORECAST CHALLENGE TONIGHT HAS BEEN HAINES...WHICH HAS
REMAINED WELL BELOW FREEZING WITH A NORTHWEST WIND COMING DOWN THE
CHILKAT RIVER VALLEY FROM CANADA. WARM LOW LEVEL AIR ALOFT HAS
RESULTED IN PERIODS OF LIGHT FREEZING RAIN OVERNIGHT IN THE HAINES
ZONE. AT TIMES WHEN WIND SPEEDS HAVE PICKED UP AND MIXED THE COLD
AIR UP...THE FREEZING RAIN HAS CHANGED TO SNOW. UPPER AIR MODEL
GUIDANCE SUGGESTS THIS CONDITION WILL PERSIST THROUGH FRIDAY
NIGHT. HOWEVER...RISING SURFACE TEMPS EARLY THIS MORNING AND A
PRESSURE GRADIENT SHIFT LATER TODAY SHOULD CAUSE WINDS TO SHIFT
AROUND TO THE SOUTHEAST LATE THIS AFTERNOON. ONCE THAT
HAPPENS...ENOUGH VERTICAL MIXING SHOULD OCCUR OVER THE CHILKAT
RIVER VALLEY TO ELIMINATE THE THE THREAT OF ANY MORE FREEZING
RAIN. WITH THAT IN MIND...HAVE LET FREEZING RAIN ADVISORY FOR ZONE
18 EXPIRE AT 5 AM THIS MORNING. PRECIP TONIGHT IS EXPECTED TO
PROVIDE ENOUGH EVAPORATIVE COOLING IN THIS REGION TO DECREASE THE
TEMPERATURES ALOFT ENOUGH TO CHANGE THE RAIN TO SNOW AND SO HAVE
INCLUDED 2 TO 3 INCHES OF SNOW ALONG THE HAINES HIGHWAY.

ELSEWHERE...A LOW LEVEL JET MAXIMA BETWEEN 60 AND 65 KTS
ASSOCIATED WITH THE LOW APPROACHING THE OUTER COAST AND THOROUGH
VERTICAL MIXING OF THE LOWER LAYERS OF THE ATMOSPHERE WILL BE
ENOUGH TO PRODUCE A SHORT LIVED WIND ADVISORY FOR ZONES 27 AND 28
THIS MORNING. ALSO...ONCE THE SOUTHERLY PRESSURE SETS UP OVER THE
FAR NORTHERN INNER CHANNELS...SMALL CRAFT CONDITIONS OVER LYNN
CANAL WILL SET UP AND WINDY CONDITIONS WILL PREVAIL OVER ZONE 18.
PRESSURE GRADIENT GUIDANCE SUGGESTS THIS MAY TURN INTO A WIND
ADVISORY AS WELL BUT DO NOT HAVE ENOUGH CONFIDENCE IN THOSE
NUMBERS SO HAVE DECIDED TO LIMIT THE GUSTS TO 35 MPH.

FOR TONIGHT...ANOTHER FRONT WILL APPROACH THE PANHANDLE FROM THE
SOUTHWEST. QPF VALUES ASSOCIATED WITH THIS FRONT WILL RESULT IN
PERIODS OF HEAVY RAIN OVER MOST OF THE NORTHERN HALF OF THE
PANHANDLE AND THE EASTERN PORTIONS OF ZONE 26.

USED A BLEND OF NAM AND EC FOR PRESSURE AND WINDS. BLENDED IN GFS
FOR POP. FORECAST CONFIDENCE IS AVERAGE.

.LONG TERM...WE ARE TO REMAIN WET AND UNSEASONABLY WARM.

OUR GEN PATN IS TO CHANGE LITTLE WELL INTO NEXT WEEK: ONE POLAR
FRONT ZONE WAVE AFTER ANOTHER MOVING TO THE NE ACRS OUR AOR AND
NO ACCESS TO ARCTIC AIR. CONFIDENCE IS ABOVE AVERAGE, WITH THE
ONLY SGFNT ISSUES BEING ASSOCD WITH TIMING OF EACH FNTL WV, AND
THE MODEL CHOICES FOR THIS ARE DISCUSSED BELOW.

LONG WV 500H TROUGH WILL CONTINUE TO EXTEND S FM OVR THE CHUKCHI SEA
TO OVR PADK, AND A BROAD LONG WAVE RIDGE ALOFT WILL PERSIST
NW FM OVR THE KSEA VCNTY TO AT LEAST BANKS ISLAND. THIS TROUGH-TO
-RIDGE PAIRING IS ON A LARGE SCALE AND MATTERS FOR A COUPLE OF
REASONS: FIRST THE STRONGER POLAR FRONT ZONE WAVES WILL TEND TO
REMAIN W OF OUR AOR AND THE LARGE SCALE ANTICYCLONIC FLOW OVER US
SUPPRESSES CYCLOGENEIS ON THE POLAR FRONT ZONE WAVES. SECOND, THE RIDGE
IS TOO FAR DOWNSTREAM AND TOO BROAD TO SUPPORT AN ARCTIC AMS
ANYWHERE NEAR OUR AOR.

MODELS:  THE NAM AND ECMWF HAVE A GOOD CONSENSUS AND HAVE INITIALIZED
WELL.  THE FORECASTS FOLLOW THIS GUIDANCE CLOSELY THROUGH 12Z MON.
THE GENERAL CONSENSUS OVER A WIDER RANGE OF MODELS IS USED THRFTR,
WITH THE NE TRACK OF POLAR FRONT ZONE WAVES INTO THE PNHDL, AND THE
LACK OF ARCTIC AMS, ARE WELL REPRESENTED.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WIND ADVISORY FROM 9 AM THIS MORNING TO 3 PM AKST THIS AFTERNOON
     FOR AKZ027-028.
MARINE...GALE WARNING FOR PKZ036-041-042.
         SMALL CRAFT ADVISORY FOR PKZ011>013-021-022-031>035-043-051-052.

&&

FRITSCH/JBT