Small Craft Advisory 891, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1230 PM AKST TUE NOV 12 2013

.ANALYSIS AND UPPER LEVELS...ANOMALOUSLY WARM AND MOIST AIR IS
RIDING UP THE WEST SIDE OF A STRONG HIGH AMPLITUDE RIDGE CENTERED
OVER THE NORTH CENTRAL PACIFIC...AND ACROSS THE BERING SEA INTO
WESTERN ALASKA. GEFS (GLOBAL ENSEMBLE FORECAST SYSTEM) STANDARDIZED
ANOMALY VALUES FOR PRECIPITABLE WATER AND SPECIFIC HUMIDITY WITH
THIS AIR MASS MAX OUT BETWEEN 7.8 AND 9.4...WHICH IS VERY
SIGNIFICANT. WITH COLD AIR IN PLACE OVER THE SOUTHERN MAINLAND
THE FOCUS IS ON ARRIVAL OF THE WARM AIR ALOFT AND THE THREAT OF
FREEZING RAIN PRIOR TO TRANSITIONING TO ALL RAIN. STEADY PRECIP
HAS OVERSPREAD THE KUSKOKWIM DELTA AND IS SPREADING EASTWARD
ACROSS BRISTOL BAY AND THE SOUTHERN KUSKOKWIM VALLEY. BASED ON
THE 12Z SOUNDING FROM SAINT PAUL THERE IS NOT MUCH OF A VERTICAL
SLANT TO THE WARM FRONT...WHICH INDICATES THE TRANSITION FROM
SNOW TO FREEZING RAIN TO RAIN SHOULD BE QUICK. THIS SEEMS TO BE
VERIFYING IN BETHEL WHERE PRECIP HAS REMAINED MOSTLY SNOW WHEN
MODEL THICKNESSES MIGHT SUGGEST THAT THE TRANSITION TO RAIN SHOULD
BE WELL UNDERWAY. BETHEL RECENTLY STARTED TO MIX WITH FREEZING
RAIN WITH A SURFACE TEMPERATURE APPROACHING FREEZING...SO WOULD
EXPECT TEMPERATURES TO RISE ABOVE FREEZING BEFORE TOO LONG WITH
LITTLE OR NO ICE ACCUMULATION FROM FREEZING RAIN. MEANWHILE...CLEAR
AND COLD CONDITIONS DOMINATE OUT AHEAD OF THE WARM FRONT FROM
INTERIOR SOUTHWEST ALASKA AND ACROSS SOUTHCENTRAL ALASKA.

.MODEL DISCUSSION...MODELS ARE GENERALLY IN GOOD AGREEMENT WITH
HANDLING OF WARM FRONT MOVING ACROSS THE SOUTHERN MAINLAND TODAY
AND TONIGHT AND THE COLD FRONT FOLLOWING ON THURSDAY. THE GFS
CONTINUES TO BE SLOWER THAN OTHER SOLUTIONS WITH PROGRESSION OF
THE COLD FRONT...SO PREFER THE FASTER SPEED OF OTHER SOLUTIONS.

MORE SIGNIFICANT DIFFERENCES DEVELOP WITH HANDLING OF NEXT TROUGH
MOVING OUT OF ASIA AND ASSOCIATED SURFACE LOW PRESSURE SYSTEM
TRACKING UP AND OVER THE RIDGE. FOR NOW PREFER TO STICK CLOSE TO
INHERITED FORECAST WHICH MATCHES PRETTY WELL WITH TIMING/PLACEMENT
OF THE ECMWF SOLUTION.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE WARM FRONT WILL FALL APART AS THE UPPER
RIDGE BECOMES POSITIVELY TILTED AND THE WARMEST AIR DIVES SOUTHWARD
TOWARD PACIFIC WHILE THE NORTHERN PORTION OF THE FRONT LIFTS
NORTHEASTWARD TOWARD INTERIOR ALASKA. THIS MAKES FOR A TOUGH
PRECIPITATION FORECAST FOR SOUTHCENTRAL. AS PER LATEST TRENDS IN
MODEL QPF GUIDANCE MOST AREAS WILL STAY DRY AS THE REMNANTS OF THE
WARM FRONT CROSSES INTO SOUTHCENTRAL ALASKA ON WEDNESDAY. THE BEST
CHANCE FOR PRECIPITATION WILL BE FROM THE MATANUSKA VALLEY TO THE
SUSITNA VALLEY. LOW LEVEL FLOW WILL BE OUT OF THE SOUTH...WHICH IS
FAVORABLE FOR WARMING TEMPERATURES...BUT WITH VERY WEAK FLOW AT AND
NEAR THE SURFACE...SURFACE TEMPERATURES WILL BE SLOWER TO WARM.
THUS...A PERIOD OF FREEZING RAIN IS LIKELY AND PLAN TO ISSUE
FREEZING RAIN ADVISORIES FOR THE MAT-SU AREA. FOR ANCHORAGE AND
SOUTHWARD PRECIP LOOKS UNLIKELY DURING THE DAY...BUT TEMPERATURES
WILL MODERATE TO AT OR ABOVE FREEZING WEDNESDAY AFTERNOON...WHICH
CAN STILL CAUSE PROBLEMS DUE TO MELTING SNOW AND ICE. A WEAK UPPER
WAVE TRANSITING THE AREA WEDNESDAY NIGHT LOOKS LIKE IT WILL BRING
AREAS OF RAIN AS FAR SOUTH AS ANCHORAGE...SO THERE MAY BE RAIN
FALLING ON STILL SNOW AND ICE COVERED SURFACES...WHICH ESSENTIALLY
CAUSES SIMILAR IMPACTS TO FREEZING RAIN. WILL ISSUE A SPECIAL WEATHER
STATEMENT TO HIGHLIGHT THE IMPENDING WARM-UP AND LIKELIHOOD OF ICY
CONDITIONS.

THE WARM-UP WILL NOT BE LONG LASTING WITH A COLD FRONT EXPECTED TO
QUICKLY MOVE THROUGH DURING THE DAY THURSDAY AND CAUSE EVERYTHING
TO REFREEZE BY THURSDAY NIGHT. EXPECT GAP WINDS TO RE-DEVELOP ALONG
THE GULF COAST IN THE WAKE OF THE COLD FRONT.

SOUTHWEST ALASKA...AS INDICATED IN ANALYSIS DISCUSSION ABOVE
GENERALLY EXPECT A QUICK TRANSITION FROM SNOW TO RAIN ACROSS
SOUTHWEST ALASKA. HOWEVER...COLD AIR IN THE INTERIOR VALLEYS OF
THE SOUTHERN KUSKOKWIM VALLEY IS HARDER TO SCOUR OUT...SO EXPECT
A LONGER PERIOD OF FREEZING RAIN THERE WITH SOME MINOR ACCUMULATION
OF ICE. PRECIP WILL COME TO EN END AS A COLD FRONT BRINGS A RETURN
TO COLDER AND DRIER CONDITIONS WEDNESDAY NIGHT INTO THURSDAY.
ANOTHER WARM FRONT WILL MOVE OVER TOP OF THE BERING RIDGE THURSDAY
LIKELY ARRIVING IN SOUTHWEST ALASKA THURSDAY NIGHT AND FRIDAY.
THIS WILL RESULT IN ANOTHER WARM-UP WITH GOOD POTENTIAL FOR
FREEZING RAIN ONCE AGAIN.

BERING SEA/ALEUTIANS...SYSTEMS WILL CONTINUE TO TRACK UP AND OVER
THE RIDGE...MAINTAINING VERY WARM AND JUICY CONDITIONS ACROSS THE
BERING SEA AND ALEUTIANS THROUGH THE END OF THE WEEK.

.LONG TERM FORECAST...MEDIUM RANGE MODELS INDICATE A SHIFT IN THE
PATTERN WITH BREAK DOWN OF THE NORTH-PACIFIC RIDGE. THERE IS QUITE
A SPREAD IN SOLUTIONS ALONG WITH FAIRLY LARGE RUN-TO-RUN
INCONSISTENCIES. HOWEVER...THERE IS CONSISTENCY IN DEPICTION OF
GENERALLY COLDER AIR SETTLING IN ACROSS THE EASTERN BERING AND
MAINLAND ALONG WITH DEVELOPMENT OF A STRONG LOW IN THE GULF EARLY
NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...WINTER WEATHER ADVISORY 152.
MARINE...GALE WARNING...177 178 181 185.
FIRE WEATHER...NONE.

SEB NOV 13

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE ANCHORAGE AK
115 PM AKST TUE NOV 12 2013

AKZ101-111-121-125-131-135-141-145-131200-

...WARMER TEMPERATURES WILL RETURN TO SOUTHCENTRAL ALASKA WEDNESDAY
INTO THURSDAY...

VERY WARM AIR IS CURRENTLY MAKING ITS WAY ACROSS THE BERING SEA
AND INTO WESTERN ALASKA. SOME OF THIS WARMER AIR WILL GRADUALLY
MOVE INTO SOUTHCENTRAL ALASKA WEDNESDAY AFTERNOON THROUGH WEDNESDAY
NIGHT CAUSING TEMPERATURES TO EVENTUALLY RISE TO FREEZING OR ABOVE
FOR MUCH OF SOUTHCENTRAL ALASKA.

AT THIS POINT IN TIME THERE IS A LOT OF UNCERTAINTY IN THE
PRECIPITATION FORECAST. THE BEST CHANCE FOR PRECIPITATION WILL
INITIALLY BE ACROSS THE MATANUSKA AND SUSITNA VALLEYS WHERE SNOW
IS EXPECTED TO DEVELOP BY WEDNESDAY AFTERNOON AND CHANGE TO FREEZING
RAIN AS TEMPERATURES ALOFT RISE ABOVE FREEZING. THE THREAT OF
FREEZING RAIN WILL PERSIST THROUGH WEDNESDAY NIGHT BEFORE COLDER
AIR WORKS ITS WAY BACK INTO THE REGION ON THURSDAY.

THE ANCHORAGE AREA LOOKS LIKE IT WILL STAY DRY ON WEDNESDAY BUT
RAIN LOOKS LIKELY BY WEDNESDAY EVENING. BY THIS POINT IN TIME
SURFACE TEMPERATURES MAY BE ABOVE FREEZING...BUT RAIN FALLING ON
SNOW AND ICE CAN STILL CAUSE IMPACTS SIMILAR TO FREEZING RAIN.

A COLD FRONT WILL MOVE SWIFTLY ACROSS SOUTHCENTRAL ALASKA ON
THURSDAY BRINGING AN END TO PRECIPITATION AND A RETURN TO FREEZING
TEMPERATURES.

IMPACTS...WARMER TEMPERATURES...ALONG WITH PERIODS OF RAIN OR
FREEZING RAIN IN SOME AREAS WILL LEAD SLIPPERY CONDITIONS ON AREA
ROADWAYS AND WALKWAYS. THE WORST CONDITIONS ARE LIKELY FROM
ANCHORAGE NORTHWARD TO THE MATANUSKA AND SUSITNA VALLEYS. AREAS
FURTHER SOUTH WILL SEE THE WARMEST TEMPERATURES AND MOST SIGNIFICANT
MELTING. WHERE THE GROUND REMAINS FROZEN...ANY RUNOFF WILL NOT SOAK
INTO THE GROUND.  THIS WILL ONLY COMPOUND THE PROBLEM OF SLIPPERY
CONDITIONS IN THOSE AREAS.  AS TEMPERATURES DROP THURSDAY AND
THURSDAY NIGHT WHATEVER IS LEFT ON THE GROUND WILL REFREEZE.

FOR THE LATEST FORECASTS ON THIS IMPENDING WARM-UP PLEASE VISIT
WWW.WEATHER.GOV/ANCHORAGE.

SEB

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE ANCHORAGE AK
130 PM AKST TUE NOV 12 2013

AKZ101-111-121-125-131-135-141-145-131200-

...WARMER TEMPERATURES WILL RETURN TO SOUTHCENTRAL ALASKA WEDNESDAY
INTO THURSDAY...

VERY WARM AIR IS CURRENTLY MAKING ITS WAY ACROSS THE BERING SEA
AND INTO WESTERN ALASKA. SOME OF THIS WARMER AIR WILL GRADUALLY
MOVE INTO SOUTHCENTRAL ALASKA WEDNESDAY AFTERNOON THROUGH WEDNESDAY
NIGHT CAUSING TEMPERATURES TO EVENTUALLY RISE TO FREEZING OR ABOVE
FOR MUCH OF SOUTHCENTRAL ALASKA.

AT THIS POINT IN TIME THERE IS A LOT OF UNCERTAINTY IN THE
PRECIPITATION FORECAST. THE BEST CHANCE FOR PRECIPITATION WILL
INITIALLY BE ACROSS THE MATANUSKA AND SUSITNA VALLEYS WHERE SNOW
IS EXPECTED TO DEVELOP BY WEDNESDAY AFTERNOON AND CHANGE TO FREEZING
RAIN AS TEMPERATURES ALOFT RISE ABOVE FREEZING. THE THREAT OF
FREEZING RAIN WILL PERSIST THROUGH WEDNESDAY NIGHT BEFORE COLDER
AIR WORKS ITS WAY BACK INTO THE REGION ON THURSDAY.

THE ANCHORAGE AREA LOOKS LIKE IT WILL STAY DRY ON WEDNESDAY BUT
RAIN LOOKS LIKELY BY WEDNESDAY EVENING. BY THIS POINT IN TIME
SURFACE TEMPERATURES MAY BE ABOVE FREEZING...BUT RAIN FALLING ON
SNOW AND ICE CAN STILL CAUSE IMPACTS SIMILAR TO FREEZING RAIN.

A COLD FRONT WILL MOVE SWIFTLY ACROSS SOUTHCENTRAL ALASKA ON
THURSDAY BRINGING AN END TO PRECIPITATION AND A RETURN TO FREEZING
TEMPERATURES.

IMPACTS...WARMER TEMPERATURES...ALONG WITH PERIODS OF RAIN OR
FREEZING RAIN IN SOME AREAS WILL LEAD TO SLIPPERY CONDITIONS ON
AREA ROADWAYS AND WALKWAYS. THE WORST CONDITIONS ARE LIKELY FROM
ANCHORAGE NORTHWARD TO THE MATANUSKA AND SUSITNA VALLEYS. AREAS
FURTHER SOUTH WILL SEE THE WARMEST TEMPERATURES AND MOST SIGNIFICANT
MELTING. WHERE THE GROUND REMAINS FROZEN...ANY RUNOFF WILL NOT SOAK
INTO THE GROUND. THIS WILL ONLY COMPOUND THE PROBLEM OF SLIPPERY
CONDITIONS IN THOSE AREAS. AS TEMPERATURES DROP THURSDAY AND
THURSDAY NIGHT WHATEVER IS LEFT ON THE GROUND WILL REFREEZE.

FOR THE LATEST FORECASTS ON THIS IMPENDING WARM-UP PLEASE VISIT
WWW.WEATHER.GOV/ANCHORAGE.

SEB