Small Craft Advisory 182, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
251 PM AKST TUE FEB 22 2011

.SHORT TERM...DEEPENING ARCTIC AIRMASS AS WELL AS INCREASING N
WINDS AT AND ABOVE THE BOUNDARY LAYER WILL RESULT IN AN
INTENSIFIED OUTFLOW TONIGHT. MSLP PRESSURE RISING ABOUT 12 MB
OVERNIGHT IN THE YUKON AND THIS WILL RESULT IN A BROAD PRESSURE
DOME OF ABOUT 1054 MB CENTERED ABT 100 MI NW OF CYXY 12Z WED.
OVERALL CONFIDENCE IN THIS SHORT RANGE FORECAST PACKAGE ABOVE
AVERAGE. THE MODEL CONSENSUS IS GOOD: THE CHANGES FROM MODEL RUN
TO RUN ARE SMALL AND WHILE TRANSIENT FEATURES WILL OCCUR IN
THE OUTFLOW THROUGH COMPLEX TERRAIN, THE UPPER LEVEL SUPPORTING
FEATURES ARE LARGE, SLOW MOVING AND CONTRIBUTE TO THE MODELS
GETTING A LOCK.

NOTE: THAT THE WIND ADVISORIES IN THE NORTHERN PNHDL HAVE ONE
EXCEPTION: THE SKAGWAY ZONE IS RECEIVING A WINTER WEATHER ADVISORY
AS, BESIDES THE WIND ADVISORY, WIND CHILLS SHOULD REACH 30 F BELOW
ZERO AT HIGHER ELEVATIONS INCLUDING WHITE PASS.

ONLY SMALL CHANGES TO FORECAST PACKAGE FOR TONIGHT AND WEDNESDAY.
CHANGES WERE MADE TO SOME FIELDS SUCH AS TEMPERATURES AND WIND
CHILLS, BUT THESE ARE UPDATES FOR DETAILS ABOUT COMPLEX TERRAIN
ISSUES THAT TEND TO BE IDENTIFIED AS WE GET CLOSER TO THE EVENTS.

LONG-WAVE RIDGE ALOFT OVER A PACD TO PAOT LINE IS TILTING E, GAINING
AMPLITUDE, AND SHOULD EXTEND NE OVR A PACD TO PAFA LINE BY 12Z
TNGHT. A TROUGH ALOFT THAT EXTENDS WWD OVR A CWDL TO PAMD LINE
IS A SLOWER TO PROPAGATE THAN THE UPSTREAM LARGE-SCALE RIDGE. THE
RESULT IS AN INCREASE IN N WINDS ALOFT. MOST OF THE DEPTH OF THE
TROPOSPHERE ALL THE WAY DOWN TO THE LOWER BOUNDARY LAYER HAS N
FLOW. THE MODEL CONSENSUS FOR THIS UPPER LEVEL FLOW SCENARIO IS
GOOD.

THE MODEL CONSENSUS IS GOOD ESPECIALLY FOR THE OFFSHORE
CONDITIONS. THE PRIMARY GUIDANCE IS THE NAM, ALONG WITH LOCAL
MODELS THAT USE THE NAM FOR NESTING (LARGER CONTEXT).

&&

.LONG TERM...THE AREA OF HIGH PRESSURE OVER THE GULF AND SOUTH
CENTRAL ALASKA WILL BE FORCED TO THE SOUTH AND EAST AS A FAMILY OF
LOWS MOVE NORTHEAST ACROSS THE WESTERN BERING SEA ACROSS THE
ARCTIC TO BANKS ISLAND IN NORTHWEST CANADA. AS THE UPPER LEVEL
RIDGE IS FORCED SOUTH JET STREAM WILL AT LEAST ONE SYSTEM ACROSS
THE NORTHERN GULF AT THE END OF THE WEEK BEFORE THE RIDGE REBUILDS
OVER THE WESTERN HALF OF THE STATE AND DETOURS ALL THE SYSTEMS
THROUGH THE BERING TO THE ARCTIC REGIONS.

THE LOW WILL FORM IN THE NORTH GULF AS A COLD FRONT DRAGS ACROSS
THE STATE FRIDAY EVENING. THEN SOUTH AND EAST THROUGH THE
PANHANDLE BY SUNDAY MORNING. EXPECT THE PRECIP TO START AS SNOW,
AND THEN WILL LIKE MIX WITH/CHANGE TO RAIN ALONG THE COASTAL AREAS
OF THE NORTHEAST GULF COAST THE COAST OF CHICAGOF AND BARANOF
ISLANDS AS WELL AS PRINCE OF WALES. INNER CHANNEL AREAS WILL VERY
LIKE STAY COOL ENOUGH FOR SNOW OR WET SNOW. MODELS SHOW ANOTHER
BURST OF COLD TEMPS ON THE HEALS OF THE FRONT SO SHOWERS
TRANSITIONING TO SNOW BEFORE SHUTTING OFF THROUGH SUNDAY. MOISTURE
LOOKS LIMITED WITH SYSTEM SO AT PRESENT DO NOT THINK THERE WILL BE
MORE THAN A FEW INCHES OF ACCUMULATIONS EXPECTED BUT THIS WILL BE
REFINED IN FUTURE FORECASTS.

THE INVERTED TROUGH WILL REFORM EARLY NEXT WEEK WITH THE HIGH
REBUILDING OVER THE WESTERN GULF AND INTERIOR OF ALASKA AND NW
CANADA. THE UPPER LEVEL PATTERN GOES BACK TO AN OMEGA BLOCK WHICH
MEANS A FEW MORE DAYS OF THE DRY WEATHER SHOULD BE CONTINUING
THROUGH THE EARLY PORTION OF NEXT WEEK.

CONFIDENCE IS BETTER THAN AVERAGE AND LOWERING TO AVERAGE INTO THE
WEEKEND.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WIND ADVISORY FROM 3 AM TO NOON AKST WEDNESDAY FOR AKZ019-020-
     025.
         WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO NOON AKST
     WEDNESDAY FOR AKZ018.
         WIND ADVISORY UNTIL 6 AM AKST WEDNESDAY FOR AKZ017.
MARINE...HEAVY FREEZING SPRAY WARNING FOR PKZ011>013-031.
         HEAVY FREEZING SPRAY WARNING FOR PKZ032-034-035.
         GALE WARNING FOR PKZ011>013-022-031-041-043-051.
         SMALL CRAFT ADVISORY FOR PKZ021-032>036-042-052.

&&

JBT/BEZENEK