Storm Warning 60, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
1112 AM AKST MON DEC 22 2008

.SHORT TERM...MAP ANALYSIS: SURFACE RIDGE OVER NORTHWESTERN
CANADA INTO THE ALASKAN INTERIOR STILL PERSISTING BUT IS A LITTLE
WEAKER THAN YESTERDAY. A VERY WEAK TROUGH OVER THE NORTH CENTRAL
GULF HAS SOME CLOUDS OVER THE NORTHERN GULF ONSHORE FROM YAKUTAT
WESTWARD. A WARM FRONTAL BOUNDARY EXTENDS SOUTH FROM THE KODIAK
ISLAND FROM A LOW JUST NORTH OF THE ISLAND WHILE THE COLD FRONT
LIES ALONG THE ALEUTIAN CHAIN MONDAY MORNING.

FORECAST REASONING: AGAIN THE MODELS SHY ON THE STRENGTH OF THE
HIGH OVER THE YUKON AND BRITISH COLUMBIA REGION. SO PREFERRED TO
USE THE GEM REGIONAL MODEL BUT WAS TEMPERING IT WITH A THE
NAM...ALTHOUGH THE GFS WAS CLOSE TOO.

FORECAST ISSUE THROUGH THE SHORT TERM WILL BE THE START OF THE
SNOW EVENT ON TUESDAY FOR THE FORECAST AREA ALONG WITH EXPECTED
AMOUNTS. A SHORT WAVE ABOUT THE UPPER LOW IN THE NORTHERN BERING
SEA WILL HELP TO GENERATE A NEW LOW MOVING MOVING INTO THE
NORTHERN GULF SOUTH OF THE KENAI TUESDAY MORNING. ASSOCIATED
FRONTAL FEATURE WILL SPREAD SNOW INTO THE NORTH AND CENTRAL
PANHANDLE REGION BY TUESDAY AFTERNOON. WITH ALL OF THE COLD
TEMPERATURES AROUND IT SHOULD BE SNOW...ALTHOUGH OUTER COASTAL
AREAS FROM ABOUT SITKA SOUTHWARD MAY BE AT OR JUST ABOVE FREEZING.
THERE IT COULD BE SLUSH OR MORE LIKELY SNOW I THINK. AT THE
PRESENT EXPECT THE BEST ACCUMULATION AREA WILL OVER NORTHERN
CHICHAGOF ISLAND. FOR THE MOMENT LEANING TOWARDS ISSUING AN
ADVISORY FOR THE SNOW IN ZONES 21 AND 22 (HOONAH AND
PELICAN/ELFIN COVE AREA) STARTING TUESDAY. WILL GO WITH A HIGHER
THAN 10:1 RATIO FOR SNOW BASED ON THE TEMPERATURES IN THE AREA.

CURRENT ADVISORY FOR WINDS/WIND CHILL IN THE SKAGWAY AREA WILL
LIKELY BE CANCELLED BY THIS AFTERNOON PACKAGE ISSUANCE. OVERALL
AVERAGE TO ABOVE AVERAGE ON THE FORECAST PACKAGE. THE TRICKY SNOW
AMOUNT ISSUE LOWERS CONFIDENCE A LITTLE.

&&

.LONG TERM...PATTERN SHIFT IS UNDERWAY AS THE BLOCKING UPPER RIDGE
KICKS OUT EAST. SATELLITE IMAGERY SHOWS THE SHORTWAVE ALONG 150W
OR SO POISED AS THE KICKER. TWO FORECAST CHALLENGES: 1) DEPTH AND
TRACK OF LOW SURELY TO FORM IN THE EASTERN GULF TUE NIGHT AND 2)
HOW MUCH SNOW WILL FALL AS THE WEAKENING FRONT MOVES ONSHORE
SUPPORTED ALOFT BY A MODERATE STRENGTH SHORTWAVE? OUR ANSWER WAS
TO USE A BLEND OF MODELS TO PREDICT HOW MUCH WATER THEN CONVERT
ALL TO SNOW EXCEPT FOR BRIEF RAIN IN ZONE 27 TUESDAY THEN INTO
LONGTERM. WE USED AROUND 15:1 FOR MOST AREAS WITH MODERATE TO HIGH
CONFIDENCE INTO WEDNESDAY. MODIFIED THE WIND FIELDS TO MATCH
ANCHORAGE AND USED A WEAKENED AK-GFS-40 BY SUBTRACTING A FEW
MILLIBARS THEN TRANSITION INTO THE HPC PRESSURE AND WIND GRIDS.
DID NOT GO STORMS AGAIN FOR TUE NIGHT. AGAIN, MODERATE CONFIDENCE
THAT A TRANSITORY UPPER RIDGE WILL MOVE THROUGH W TO E ON THURSDAY
THEN LOW CONFIDENCE AFTER THAT.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WIND ADVISORY UNTIL NOON AKST TODAY FOR AKZ018.
MARINE...SMALL CRAFT ADVISORY FOR PKZ011>013-021-035.

&&

BEZENEK/JCC