Small Craft Advisory 709, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
137 PM AKDT SUN SEP 15 2013

.ANALYSIS AND UPPER LEVELS...UPPER LEVEL FLOW IS GENERALLY
PROGRESSIVE ACROSS THE NORTHERN HEMISPHERE...ALTHOUGH AS INDIVIDUAL
SYSTEMS DEEPEN THE FLOW BECOMES MORE AMPLIFIED AND PROGRESSION
TEMPORARILY SLOWS. TAKING A LOOK AT WATER VAPOR IMAGERY OVER
ALASKA...A LARGE UPPER LOW SPINNING OVER THE GULF OF ALASKA IS
SLOWLY SHIFTING EASTWARD TOWARD SOUTHEAST ALASKA. A FEW WAVES
ROTATING AROUND THE NORTH SIDE OF THE LOW ARE BRINGING A FEW SHOWERS
TO THE EASTERN COPPER BASIN AND SOUTHEASTERN PRINCE WILLIAM SOUND.
THERE ARE MULTIPLE SURFACE LOWS WITHIN THE LARGER CYCLONIC
CIRCULATION OVER THE GULF...BUT THE LARGER IMPACT IS THE PRESSURE
GRADIENT BETWEEN THE GULF LOW AND HIGH PRESSURE OVER THE SOUTHERN
HALF OF THE ALASKA MAINLAND. THIS IS PRODUCING GAP WINDS ALONG
MUCH OF THE GULF COAST AND ALASKA PENINSULA.

MEANWHILE A WEAK VERTICALLY STACKED LOW OVER THE NORTHWEST BERING
IS SLOWLY TRACKING EASTWARD WITH AREAS OF RAIN SPREADING EASTWARD
ACROSS THE CENTRAL AND EASTERN BERING SEA. A STRENGTHENING JET
STREAK ON THE BACK SIDE OF THIS LOW IS POISED TO INITIATE
AMPLIFICATION OF THE UPPER TROUGH. WEAK RIDGING DOMINATES THE
REMAINDER OF THE FORECAST AREA RESULTING IN GENERALLY BENIGN
CONDITIONS.

.MODEL DISCUSSION...THERE ARE MINOR DIFFERENCES IN LOW POSITIONS
BUT ALL MODELS DEPICT THE SAME GENERAL SYNOPTIC EVOLUTION OF THE
PATTERN OVER THE NEXT FEW DAYS. ONE NOTABLE DIFFERENCE IS THE
HANDLING OF THE GULF LOW AS IT LIFTS NORTHWARD ACROSS SOUTHCENTRAL
ON MONDAY NIGHT AND TUESDAY. THERE WILL BE SOME WESTWARD PROGRESSION
OF THE TROUGH AND ASSOCIATED RAIN...BUT MODELS DIFFER ON THIS DETAIL.
THERE SEEMS TO BE A SLIGHT TREND TOWARD A FURTHER WEST SOLUTION
WHICH WOULD BRING RAIN BACK INTO ANCHORAGE AND THE MAT-SU BY LATE
TUESDAY. WILL STICK WITH CHANCE POPS FOR NOW AND WAIT FOR ONE MORE
MODEL CYCLE.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE COMBINATION OF JUST A COUPLE KNOTS OF
WINDS AND A FEW LOCALIZED CLOUDS ALLOWED TEMPERATURES TO REMAIN
ABOVE FREEZING IN EAST ANCHORAGE THIS MORNING. A CHANGE IN THE
ORIENTATION OF THE PRESSURE GRADIENT IS FAVORABLE FOR WINDS TO
GO CALM TONIGHT AND FOR TEMPERATURES TO BOTTOM OUT AT OR BELOW
FREEZING. THUS...A FREEZE WARNING REMAINS IN PLACE TONIGHT FOR
EAST ANCHORAGE AND LOW-LYING AREAS OF THE MATANUSKA VALLEY.

AS THE GULF LOW COMPLEX MOVES TO THE EASTERN GULF OFFSHORE FLOW
WILL STRENGTHEN OVER SOUTHCENTRAL. THIS WILL INDUCE A POSITIVE
FEEDBACK AS PRESSURE THEN FALLS OVER THE NORTHERN GULF IN THE LEE
OF THE COASTAL RANGE AND WINDS STRENGTHEN FURTHER. THERE IS NOT
ENOUGH UPPER SUPPORT FOR WINDY CONDITIONS EVERYWHERE. MOST OF
THE WIND WILL BE CONFINED TO GAPS ALONG THE COAST...INCLUDING THE
SEWARD...WHITTIER...AND VALDEZ AREAS. THE PRESSURE GRADIENT WILL
WEAKEN DURING THE DAY MONDAY AS THE LOW COMPLEX BEGINS TO LIFT
NORTHWARD AND PRESSURE STARTS TO FALL OVER INTERIOR SOUTHCENTRAL.
MEANWHILE...STRONG WINDS WILL PERSIST OVER THE WESTERN GULF AND
THROUGH GAPS ALONG THE ALASKA PENINSULA AS THE SYSTEM CURRENTLY
OVER THE NORTWEST BERING DIVES SOUTHEASTWARD INTO THE NORTHEAST
PACIFIC AND RE-INFORCING HIGH PRESSURE BUILDS IN BEHIND IT
MAINTAINING A TIGHT PRESSURE GRADIENT.

AS THE GULF LOW COMPLEX LIFTS NORTHWARD PRECIP WILL MOVE INTO THE
EASTERN COPPER BASIN AND PRINCE WILLIAM SOUND AREAS MONDAY NIGHT
AND SPREAD WESTWARD TUESDAY. CLEAR SKIES AND NORTHERLY FLOW OUT
AHEAD OF THIS SYSTEM WILL ALLOW FREEZING LEVELS TO DROP TO 3000
FEET OR LOWER. THUS...EXPECT SNOW TO FALL NEAR EUREKA ON THE
RICHARDSON HIGHWAY (ABOUT 3300 FEET ELEVATION). SURFACE TEMPERATURES
ARE STILL QUITE WARM AND PRECIP IS EXPECTED TO BE ON THE LIGHT
SIDE...SO WOULD NOT EXPECT MUCH IF ANY SNOW TO STICK TO THE ROADWAY
AT THIS ELEVATION. AS MENTIONED ABOVE THERE IS UNCERTAINTY IN HOW
FAR WEST THE PRECIP WILL SPREAD ON TUESDAY. FOR NOW EXPECT THE BULK
OF PRECIP TO REMAIN EAST OF ANCHORAGE AND THE MAT-SU...BUT THERE IS
A CHANCE THAT RAIN COULD MOVE INTO THE REGION BY TUESDAY AFTERNOON.
AN UPPER LEVEL TROUGH MOVING IN FROM THE WEST ON WEDNESDAY LOOKS
LIKE A HIGHER PROBABILITY OF RAIN FOR THESE AREAS.

SOUTHWEST ALASKA...PRECIP ASSOCIATED WITH THE NORTHWEST LOW AND
TROUGH WILL MOVE ONSHORE ALONG THE KUSKOKWIM DELTA LATER TODAY.
HOWEVER...THE JET STREAK MENTIONED ABOVE WILL CAUSE THE TROUGH
TO RAPIDLY DIG SOUTHEASTWARD TO THE NORTHEAST PACIFIC TONIGHT
AND TUESDAY...WHICH WILL KEEP RAIN CONFINED TO THE SOUTHWEST
ALASKA COAST. THINGS WILL DRY OUT TUESDAY AS THIS TROUGH EXITS
TO THE SOUTH. ANOTHER TROUGH WILL THEN DROP OUT OF THE ARCTIC
TUESDAY NIGHT AND WEDNESDAY...BUT IT DOES NOT LOOK STRONG ENOUGH
TO PRODUCE ANY PRECIP. IT WILL BRING A SHOT OF COLDER AIR AND
RESULT IN A DOWNWARD TREND IN TEMPERATURES...WITH MINIMUM
TEMPERATURES AT OR BELOW FREEZING EXPECTED FOR INLAND VALLEYS.

BERING SEA/ALEUTIANS...THE WEAK SYSTEM OVER THE NORTHWEST BERING
AND ANOTHER WEAK SYSTEM WEST OF THE ALEUTIANS WILL QUICKLY TRANSIT
THE REGION TONIGHT AND MONDAY WITH HIGH PRESSURE BUILDING RIGHT
BACK IN. THE NEXT MAJOR SYSTEM IS A TYPHOON CURRENTLY AFFECTING
JAPAN. IT WILL BE PICKED UP BY A TROUGH EXITING ASIA AND TRACK
NORTHWARD TO THE SEA OF OKHOTSK. THE WESTERN BERING AND ALEUTIANS
WILL BE ON THE PERIPHERY OF THIS SYSTEM AND WILL ONLY SEE A GLANCING
BLOW AS THE FRONTAL SYSTEM PROGRESSES EASTWARD AND WEAKENS.

.LONG TERM FORECAST...UPPER LEVEL FLOW WILL REMAIN PROGRESSIVE FOR
THE DAYS 4 THROUGH 7 TIME PERIOD WITH A STRONG SUB-TROPICAL JET
SETTING UP ALONG THE ALEUTIANS. THIS WILL SUPPORT ACTIVE WEATHER
ACROSS THE ALASKA REGION.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...FREEZE ADVISORY 101 111.
MARINE...GALE WARNING...130 176.
FIRE WEATHER...NONE.

SEB SEP 13