Small Craft Advisory 582, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
522 AM AKDT THU OCT 11 2012

.SHORT TERM...A FRONTAL TROUGH MOVING NORTHWARD INTO THE SE GULF
WILL BE THE MAIN FEATURE TO AFFECT THE WEATHER THROUGH THURSDAY.
THE CLOUD SHIELD AHEAD OF THE TROUGH IS CAUSING OVERCAST SKIES
AREA WIDE THIS MORNING...HOWEVER AS THE SYSTEM DRAWS NEAR THE
NORTHERN ZONES WILL SEE MID LEVEL FLOW TURN OFFSHORE WITH MORE
BREAKS IN THE CLOUD COVER DEVELOPING INTO THE AFTERNOON. SOUTHERN
ZONES WILL SEE CONDITIONS DETERIORATE THROUGH THE DAY AS RAIN
MOVES INTO THE AREA. EXPECT RAIN LIKELY OVER SOUTHERN BARANOF
ISLAND BY THE AFTERNOON...THE MORE PERSISTENT RAIN WILL BE
DELAYED A BIT LONGER FOR THE SITKA AREA SINCE THERE WILL BE AN
EAST DOWNSLOPE WIND ALL DAY THERE.

 HIGH PRESSURE OVER THE YUKON AND THE LOWER PRESSURE IN THE SE
GULF WILL CONTINUE THE NORTHERLY WINDS THROUGH THE INSIDE WATERS
TODAY. ELDRED ROCK IN NORTHERN LYNN CANAL HAS HAD NORTHERLY GUSTS
TO GALE FORCE THIS MORNING...THE GRADIENT IS PROGGED TO SLACKEN
SLIGHTLY THIS AFTERNOON BUT REMAINING SMALL CRAFT LEVEL. IF MORE
BREAKS DEVELOP IN THE CLOUD COVER AS WITH YESTERDAY...THE WINDS
SHOULD EASE FURTHER DUE TO THE SEA BREEZE COUNTER ACTING THE
PRESSURE GRADIENT. THERE IS CAA WITH THIS NORTHERLY FLOW AS WELL.
THIS HAS BEEN THE MAIN FORECAST CHALLENGE. WITH THIS TYPE OF SET
UP IN THE WINTER TIME THE MUCH COLDER INTERIOR AIR FLOWING OUT OF
CANADA CAUSES STRONG WINDS OUT OF INTERIOR PASSES.
HOWEVER...SINCE THERE IS NOT A SNOW PACK YET THE WINDS ARE NOT
BEING FORCED OUT OF THE PASSES. INSTEAD THE AIR IS WARMING AS IN
FLOWS DOWN THE PASSES AND COUNTERING THE EXPECTED COLD AIR
ADVECTION. MODELS ARE NOT PICKING UP ON THIS. INSTEAD THEY HAVE
BEEN FORECASTING TEMPERATURES IN THE MID TO UPPER 20S FOR THE FAR
NORTHER BORDER AREAS. ALSO COUNTERING THESE LOW TEMPERATURES IS
THE AMOUNT OF CLOUD COVER. MODELS HAVE HAD CLEAR SKIES
OVERNIGHT...WHICH WOULD NORMALLY ALLOW FOR GOOD RADIATIONAL
COOLING. BUT WITH THE CLOUD SHIELD AHEAD OF THE TROUGH MOVING IN
THIS NOT POSSIBLE...ESPECIALLY WITH ANY MIXING. THEREFORE HAVE
KEPT MAX TEMPERATURES SIMILAR TO WHERE THEY WERE YESTERDAY...OR A
COUPLE DEGREES COOLER DO TO MORE CLOUD COVER TODAY. AND HAVE
INCREASED LOW TEMPERATURES FOR TONIGHT SEVERAL DEGREES...TO THE
MID TO UPPER 30S FOR YAKUTAT/HAINES/SKAGWAY AND TO AROUND 40F
ELSEWHERE. WHITE PASS WILL AGAIN BE THE COLDEST LOCATION WITH
TEMPERATURES JUST BELOW FREEZING. FROST IS POSSIBLE IN WIND
SHELTERED VALLEY LOCATIONS AWAY FROM MARINE AREAS...SUCH AS THE
MENDENHALL VALLEY AND OUT THE ROAD IN HAINES.

 THE OFFSHORE FLOW AHEAD OF THE TROUGH WILL KEEP THE RAIN SOUTH
OF A JUNEAU TO TENAKEE LINE TONIGHT. BUT WILL LIFT NORTH
THEREAFTER. A FEW LOCATIONS OVER THE SOUTH...SUCH AS
KETCHIKAN...WILL SEE MODERATE TO HEAVY RAIN DEVELOPING TONIGHT.
CLARENCE STRAIT IN ADDITION TO THE OUTSIDE WATERS WILL SEE SELY
WINDS INCREASING TO SMALL CRAFT LEVEL WITH THE FRONT TONIGHT. THE
REMAINING INSIDE WATERS WILL CONTINUE TO SEE WINDS OF 20-25KT OUT
OF THE NORTH AHEAD OF THE FRONT TONIGHT.

 THE 00Z RUN OF THE ECMWF INITIALIZED THE PRESSURE PATTERN VERY
WELL THIS MORNING AND WAS GIVEN MORE WEIGHT FOR THE PRESSURE
GRADIENTS. FOR OTHER FIELDS AN ENSEMBLE APPROACH WAS TAKEN...WITH
SIGNIFICANT ADJUSTMENTS MADE AS DISCUSSED ABOVE WHERE MODELS WERE
LACKING AND FOR LOCAL EFFECTS. FORECASTER CONFIDENCE IS AVERAGE.

&&

.LONG TERM...MODELS ARE FAIRLY PERSISTENT WITH AN UPPER LEVEL
SHORTWAVE TROUGH AND EMBEDDED CLOSED UPPER LOW ROTATING OVER THE
GULF THROUGH THE MIDDLE OF NEXT WEEK. AT THE SURFACE...MODELS ARE
IN GENERAL AGREEMENT THROUGH LATE SATURDAY WITH A COMPLEX LOW
PRESSURE SYSTEM ACROSS THE WESTERN GULF FRIDAY CONSOLIDATING TO A
BETTER DEFINED LOW ACROSS THE CENTRAL GULF. THE TRANSITION INTO A
WET PATTERN WILL BEGIN OVER THE SOUTH CENTRAL AND SOUTHERN
PANHANDLE ON FRIDAY WITH INCREASING CLOUDS AND POPS OVER THE
NORTHERN PANHANDLE. REFRESHED THE INHERITED FORECAST PACKAGE WITH
THE NAM AND ECMWF FOR THEIR MAKING OF GOOD CONSENSUS.

 FRONTAL PASSAGES ASSOCIATED WITH THE LOW WILL GENERATE SMALL
CRAFT WINDS ACROSS THE MOST INNER CHANNELS AND SCA WINDS/ROUGH
SEAS WILL PREVAIL ACROSS THE OUTER COASTAL WATERS THROUGH THE
WEEKEND WITH A MIN GALE FORCE WINDS OVER THE CENTRAL OUTSIDE
WATERS ON SATURDAY. LIKELY TO CATEGORICAL POPS WILL CONTINUE OVER
THE WEEKEND WITH RAINFALL POSSIBLY HEAVY AT TIMES OVER THE SOUTHERN
AREAS.

 ASSOCIATED WITH THE CLOSED UPPER LOW ROTATING OVER THE
GULF...THE SURFACE LOW WILL BECOME NEAR STATIONARY ACROSS THE
CENTRAL GULF THROUGH EARLY NEXT WEEK. THE EXACT LOCATION OF THE
LOW IN THE CENTRAL GULF IS UNCERTAIN AND THE MODEL DIFFERENCES
BECOME LARGER AS THE LOW BEGINS DRIFTING NORTHEASTWARD ACROSS THE
NORTHEASTERN GULF COAST THROUGH THE MIDDLE OF NEXT WEEK. LEANED
OVER THE HPC ENSEMBLE APPROACH FROM SUNDAY THROUGH THE MIDDLE OF
NEXT WEEK. GENERALLY WET WEATHER PATTERN WILL CONTINUE WITH
SLIGHTLY IMPROVING WEATHER ON LATE WEDNESDAY.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ011>013-022-031-033>036-041>043-051.

&&

HANNON/AHN