Gale Warning 637, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
1235 PM AKDT WED SEP 16 2009

.ANALYSIS AND UPPER LEVELS...A STRONG GULF LOW IS NOW NEARLY
STATIONARY SOUTHEAST OF KODIAK ISLAND. IT HAS BECOME VERTICALLY
STACKED AND WILL WEAKEN FROM HERE ON OUT. THE OCCLUDED FRONT IS
HEADING WESTWARD ACROSS SOUTHWEST ALASKA PRODUCING A FAIRLY NARROW
BAND OF RAIN RIGHT ALONG IT. OTHERWISE...THE BULK OF RAIN IS HUNG
UP ALONG THE GULF COAST WITH STRONG EASTERLIES PRODUCING THE USUAL
DOWNSLOPE DRYING FROM THE WESTERN KENAI TO THE SUSITNA VALLEY. A
SHORT-WAVE TROUGH EXTENDING SOUTHEAST FROM THE LOW CENTER IS
ROTATING NORTHWARD TOWARD THE GULF...WITH A TROUGH REFLECTED AT
THE SURFACE AS WELL...WHICH WILL BRING ANOTHER ROUND OF WIND AND
RAIN AS IT ROTATES ACROSS THE GULF AND SOUTHCENTRAL.

MEANWHILE...OUT WEST THE LOW THAT TRACKED EASTWARD ALONG THE
ALEUTIANS THE PAST COUPLE DAYS IS NOW DROPPING SOUTHEASTWARD
INTO THE PACIFIC AS THE GULF LOW ROTATES NORTHWESTWARD OVER
TOP OF IT. IN ITS WAKE IS BROAD NORTHERLY FLOW ACROSS THE BERING
SEA WITH RIDGING UPSTREAM OVER EASTERN SIBERIA AND THE FAR
WESTERN BERING BLOCKING THE WESTERLIES FROM BRINGING IN ANY
NEW STORMS.

.MODEL DISCUSSION...MODELS INITIALIZED WELL...BUT DIFFERENCES
QUICKLY EVOLVE WITH HANDLING OF THE GULF LOW AS IT CONTINUES
WESTWARD AND WEAKENS. BY THURSDAY A BROAD TROUGH WILL BE CENTERED
ALONG THE SOUTHWEST COAST ACROSS THE AKPEN TO SOUTH OF KODIAK
ISLAND. MODELS INDICATE WEAK SURFACE LOWS IN DIFFERENT LOCATIONS
ALONG THIS TROUGH AXIS...BUT THIS SHOULD HAVE LITTLE IMPACT ON
THE OPERATIONAL FORECAST. THERE ARE ALSO DIFFERENCES IN THE TRACK
OF A SURFACE LOW ACROSS THE EASTERN GULF THURSDAY NIGHT AND
FRIDAY...BUT MODELS HAVE CONVERGED ON THIS BEING A FAIRLY WEAK
LOW WITH A VERY WEAK PRESSURE GRADIENT ACROSS THE GULF WATERS...SO
WILL PICK THE GFS FOR NOW SINCE ANY CHANGES IN PLACEMENT WILL NOT
HAVE MUCH IMPACT ON THE WIND FORECAST. LASTLY IN THE SHORT-TERM...AS
THE TROUGH AXIS BEGINS TO ROTATE MODELS INDICATE AT A SURFACE LOW
OVER THE KUSKOKWIM DELTA ROTATING SOUTHWARD ACROSS THE BERING AND
STRENGTHENING AS COLDER AIR FILTERS INTO THE BACK SIDE OF IT. THE
GFS IS THE DEEPEST WITH THIS LOW AND AM NOT READY TO BUY INTO THAT
YET...BUT THERE IS GOOD RUN-TO-RUN CONSISTENCY SO WILL BEGIN TO
TREND TOWARD THIS SOLUTION.

UNCERTAINTY REMAINS HIGH IN THE EXTENDED FORECAST PERIODS WITH LOW
MOVING INTO THE GULF THIS WEEKEND AND HANDLING OF TYPHOON CHO-WAN
AS IT BECOMES EXTRA-TROPICAL AND MOVES TO THE NORTHWEST PACIFIC
SUNDAY INTO MONDAY TIME FRAME. THE EC/GFS HAVE ACTUALLY GONE FROM
RADICALLY DIFFERENT SOLUTIONS TO FAIRLY CLOSE SOLUTIONS FOR
CHO-WAN...BUT EXPERIENCE SAYS THIS SOLUTIONS WILL CHANGE AS WE GET
CLOSER. LARGER MODEL DIFFERENCES REMAIN WITH THE LOW MOVING INTO
THE GULF...ALTHOUGH THE EUROPEAN HAS BEEN THE MORE CONSISTENT MODEL.
BASED ON THE LOW CONFIDENCE WILL LEAVE ALL EXTENDED FORECASTS AS IS
FOR NOW.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...TROUGH SWINGING ACROSS GULF TONIGHT AND
SOUTHCENTRAL TOMORROW MORNING WILL BE VERY SIMILAR TO THE OCCLUDED
FRONT THAT CROSSED LAST NIGHT INTO THIS MORNING...WITH GALES ACROSS
MUCH OF THE GULF AND MODERATE TO HEAVY RAIN ALONG THE GULF COAST
WHILE DOWNSLOPING KEEPS OTHER AREAS MOSTLY DRY. AFTER THIS TROUGH
MOVES THROUGH A SURFACE RIDGE WILL QUICKLY BUILD ALONG THE THE
NORTH GULF COAST. COMBINATION OF A TIGHT PRESSURE GRADIENT AND
PRESSURE RISES WILL PRODUCE STRONG WINDS FROM CORDOVA TO TURNAGAIN
ARM. THIS WONT LAST TOO LONG AS THE RIDGE MOVES INLAND AND WEAKENS
THURSDAY NIGHT. RAIN WILL ALSO GREATLY DIMINISH BEHIND THE TROUGH.
WEAK DISORGANIZED FLOW WILL PERSIST OVER THE REGION THROUGH SATURDAY
WITH NO MAJOR FEATURES TO SPEAK OF. THEREFORE...EXPECT MOSTLY DRY
CONDITIONS WITH A MIX OF SUN AND CLOUDS.

SOUTHWEST ALASKA...WEAK UPPER WAVES EMBEDDED WITHIN THE CIRCULATION
AROUND THE GULF LOW WILL CONTINUE TO PRODUCE A FEW SHOWERS ACROSS
SOUTHWEST ALASKA TONIGHT. THE MORE SIGNIFICANT UPPER TROUGH HEADING
ACROSS THE GULF TONIGHT WILL MOVE ACROSS SOUTHWEST ALASKA
FRIDAY...LIKELY PRODUCING A BAND OF RAIN SIMILAR TO THE ONE BEING
OBSERVED TODAY. AS THE TROUGH AXIS CONTINUES TO ROTATE INTO THE
BERING THURSDAY NIGHT AND FRIDAY SOUTHWEST WILL DRY OUT AND REMAIN
MOSTLY DRY INTO SATURDAY UNDER WEAK DISORGANIZED FLOW.

BERING SEA/ALEUTIANS...WITH A LOW TO THE EAST AND RIDGE TO THE WEST
BROAD NORTHERLY FLOW WILL DOMINATE THE BERING SEA AND ALEUTIANS FOR
THE NEXT FEW DAYS. AN UPPER SHORT-WAVE WILL DROP DOWN THE EAST SIDE
OF THE SIBERIAN RIDGE WEDNESDAY NIGHT AND THURSDAY...ADVECTING
COLDER AIR SOUTHWARD...WHICH WILL HELP STRENGTHEN A LOW MOVING
SOUTHWESTWARD OUT OF THE KUSKOKWIM DELTA...WHICH WILL TIGHTEN UP THE
PRESSURE GRADIENT ACROSS THE CENTRAL BERING. THERE IS UNCERTAINTY
IN HOW STRONG THIS SURFACE LOW WILL BECOME. FOR NOW...WILL STICK
WITH SMALL CRAFT NORTHERLIES...BUT GALE FORCE IS A POSSIBILITY. AN
AREA OF PRECIPITATION WILL ACCOMPANY THIS TROUGH AND SURFACE LOW.

.LONG TERM FORECAST...AS MENTIONED IN THE MODEL DISCUSSION
UNCERTAINTY REMAINS HIGH FOR THE EXTENDED PERIOD. WILL CONTINUE
TO MONITOR POTENTIAL FOR A STRONG STORM IN THE GULF LATER THIS
WEEKEND AND POTENTIAL FOR TYPHOON CHO-WAN TO AFFECT THE ALEUTIANS
BY EARLY NEXT WEEK (AFTER TRANSITIONING TO EXTRA-TROPICAL OF COURSE).

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...120 125 130 132 138 140.
FIRE WEATHER...NONE.

SEB SEP 09