Gale Warning 87, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
1100 AM AST SUN FEB 18 2007

.SYNOPSIS...
.SHORT RANGE...WHAT MATTERS MOST IS HOW TWO LARGE STREAMS OF
ZONAL FLOW WILL INTERACT AS THEY CONVERGE.  ZONE ONE AT 500 MB
HAS A TROUGH EXTENDING SW FROM OVER DAWSON YUKON TO OVER SEWARD
AND IS ASSOCIATED WITH NW FLOW ALOFT OVER NORTHERN ALASKA
BECOMING ZONAL OVER SOUTHERN YUKON AND THE NORTHERN PANHANDLE.
ZONE TWO HAS A 500 MB SOURCE REGION NEAR THE DATELINE AT
ABOUT 20N AND EXTENDS NE TO A BROAD RIDGE ALOFT OVER THE GULF
OF ALASKA THAT TURNS THE FLOW TO ZONAL.  NOTE THE BROAD
RIDGE OVER THE GULF OF ALASKA IS THE WARM SECTOR RIDGE
PUMPED UP AHEAD AN INTENSIFYING ABOUT 300 NM SSW OF KODIAK
AT THIS TIME.  IT IS THE INTERACTION...OF THIS CYCLOGENETIC
REGION SOUTH OF KODIAK...WITH THE TROUGH ALOFT TO THE NORTH
THAT IS MOST IMPORTANT.  SMALL CHANGES IN MODEL EVALUATIONS OF
HOW THESE FEATURES IN THE TWO STREAMS RESULT IN DISPROPORTIONALLY
LARGE CHANGES IN SUBSEQUENT SOLUTIONS.

THE SHORT RANGE ENSEMBLE FORECAST...OR SREF...HAS DEPICTED
A WIDE SPREAD IN SOLUTIONS FOR HOW THESE TWO BANDS INTERACT
WITH A VARIETY OF ONSHORE TRACKS OF A 990 MB LOW ABOUT 300 NM
SSW OF KODIAK.  FORTUNATELY THE MODEL RUNS HAVE CONVERGED...
PRIMARILY DUE TO THE 12Z GFS TRACK BECOMING MORE LIKE SEVERAL
OTHER MODELS INCLUDING THE NAM.  THE NAM 12Z WILL BE USED AS
PRIMARY GUIDANCE...BUT SOME STATISTICAL SPREAD WILL HAVE TO
BE INCORPORATED TO THE DETERMINISTIC SOLUTION OF THE
INTENSIFYING LOW REACHING PORT ALEXANDER WITH 972 MB
ABOUT 20Z MON.  A WEAKER LOW...PROBABLY ABOUT 978 MB WILL
BE 200 NM SW OF YAKUTAT AT THE SAME TIME.  THE MUCH SLOWER
TRACK OF THE WEAKER LOW IS DUE TO SEVERAL REASONS INCLUDING
THE CYCLONIC INTERACTION WITH THE MAIN WAVE TO THE S...AND THE
COLD OUTFLOW FROM THE ALASKAN INTERIOR ONLY BEING AVAILABLE
AT TOPOGRAPHICALLY FIXED LOCATIONS.

.LONG RANGE...GFS ENSEMBLE MEMBERS DO NOT BEGIN TO SPREAD APART
SIGNIFICANTLY UNTIL 24/18Z. AT THE START OF THIS LONG RANGE
PERIOD...AN UPPER LEVEL ARCTIC OUTBREAK WILL HAVE BEGUN WITH COLD
AIR SPILLING SOUTH ALONG THE ALASKA YUKON BORDER AND INTO THE
NORTHEAST GULF EXTENDING SOUTH DOWN THE PANHANDLE. THE SECOND OF
2 SHORT WAVES REACHES MAX AMPLITUDE AROUND 21/18Z REINFORCING
COLD AIR PENETRATION SOUTHWARD. OPERATIONAL RUN DIGS BOTH THE
UPPER LEVEL TROUGH A LITTLE DEEPER AND IS A LITTLE BIT FASTER
THAN THE ENSEMBLE MEAN. OVER ALASKA AND YUKON...UPPER LEVEL LOWS
MATCH CLOSELY UNTIL 25/12Z AFTER WHICH THE ENSEMBLE MEAN TROUGHS
OUT TO THE SOUTHWEST WHILE THE OPERATIONAL RUN DRIFTS A CLOSED
LOW WEST INTO THE YUKON. FARTHER WEST...THE OPERATIONAL RUN AND
ENSEMBLE MEAN DEVIATE EARLIER AT 24/00Z WITH THE ENSEMBLE MEAN
TROUGHING OUT SOUTH AND WEST OF THE ALEUTIANS WHILE THE
OPERATIONAL RUN DIGS A CLOSED UPPER LEVEL LOW FARTHER SOUTH
BEFORE TURNING IT NE TO TRACK SOUTH OF THE ALEUTIANS AND HEADED
TOWARDS THE PANHANDLE. END RESULT IS AN UPPER LEVEL RIDGE THAT
BUILDS IN THE OPERATIONAL RUN THAT IS UPSTREAM AND OUT OF PHASE
BY NEARLY 90 DEGREES WITH THE ENSEMBLE MEAN. AT THE SURFACE...THE
ENSEMBLE MEAN AND OPERATIONAL MODEL RUN DO NOT BEGIN TO DEVIATE
SIGNIFICANTLY UNTIL 21/12Z WITH THE OPERATIONAL RUN STAMPING THE
LOW NEAR THE DIXON ENTRANCE...FARTHER SOUTH AND SLIGHTLY EAST OF
THE ENSEMBLE MEAN. FARTHER UPSTREAM...THE AFOREMENTIONED
DISPARITY BETWEEN THE ENSEMBLE MEAN AND THE OPERATIONAL RUN
MANIFESTS AS A SIGNIFICANT DIFFERENCE IN LOCATION & TIMING OF THE
NEXT SYSTEM. AT 25/12Z THE OPERATIONAL RUN STAMPS A 996MB SOUTH
AND WEST OF THE 1003MB ENSEMBLE MEAN.

END RESULT IS THAT A WEAK SURFACE LOW IN THE NORTHERN GULF WILL
GET OVERRUN AND MERGE WITH DEVELOPING LOW CURRENTLY SOUTHWEST OF
KODIAK AND COME ASHORE LATE MONDAY NIGHT IN THE VICINITY OF CAPE
DECISION. THIS WILL PRODUCE SNOW SHOWERS IN THE NORTHERN
PANHANDLE AND RAIN IN THE SOUTHERN PANHANDLE. SURFACE
TEMPERATURES WILL BEGIN FALLING AND THE RAIN SNOW LINE WILL THEN
START MOVING SOUTH TO INCLUDE THE ENTIRE PANHANDLE BY TUESDAY
AFTERNOON. OFFSHORE WINDS AND A NORTH SOUTH PRESSURE GRADIENT ARE
EXPECTED TO PRODUCE WINDY TO VERY WINDY CONDITIONS THROUGHOUT THE
PANHANDLE THROUGH FRIDAY. ONSHORE FLOW WILL RETURN TO THE
SOUTHERN ZONES FIRST ON FRIDAY AFTERNOON WITH A BREAK IN THE
WIND. ANY MODERATION TO THE COLD TEMPS LOOKS LIKE IT WILL BE
SMALL THROUGH AT LEAST NEXT WEEKEND AND LIMITED TO THE SOUTHERN
ZONES.

.AJK...WATCHES/WARNINGS/ADVISORIES
PUBLIC...NONE

MARINE...SCA...PKZ036-041>043-051-052
JT/FCF