Small Craft Advisory 34, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
417 PM AKST MON JAN 12 2015

.ANALYSIS AND UPPER LEVELS...

A DEEP LONGWAVE TROUGH CONTINUES ACROSS THE ALASKA REGION...WITH
THE GULF AND MAINLAND ALASKA ON THE "WARM"/EASTERLY SIDE OF THIS
FEATURE. AS SUCH...YET ANOTHER STORM AND SURGE OF WARM AIR IS
MOVING NORTHWARD THROUGH THE GULF TODAY BRINGING ANOTHER ROUND OF
RAIN AND UNSEASONABLY HIGH FREEZING LEVELS TO KODIAK AND MUCH OF
THE REST OF SOUTHERN ALASKA. THE BULK OF THE PRECIP IS ASSOCIATED
WITH THE WEATHER FRONT THAT IS PUSHING TOWARD THE KENAI PENINSULA
AND NORTH GULF COAST. GUSTY NORTH/NORTHEAST WINDS AHEAD OF THE
FRONT HAVE ALLOWED TEMPERATURES TO SURGE WELL INTO THE 40S AT MOST
COASTAL LOCATIONS...EVEN REACHING 50 AROUND KACHEMAK BAY.
RELATIVELY COLD AIR REMAINS TRAPPED AT THE LOWEST ELEVATIONS OF
INTERIOR ALASKA...WHICH WILL BE PROBLEMATIC AND REPRESENTS THE
MAIN FORECAST CHALLENGE DURING THE NEXT 24 HOURS.

OUT WEST...GUSTY OFFSHORE WINDS ACROSS THE SOUTHWEST MAINLAND ARE
KEEPING TEMPERATURES FAIRLY STEADY AT MOST LOCATIONS. BELOW-
FREEZING AIR AT THE SURFACE IS CONFINED TO THE LOWER KUSKO VALLEY
AND THE DELTA...WHILE MOST OF BRISTOL BAY IS WELL ABOVE FREEZING.

WAY OUT WEST...A VERTICALLY STACKED AND WEAKENING LOW PRESSURE
CENTER IS NEARLY STATIONARY SOUTH OF SHEMYA...BUT CONTINUES TO
SPREAD RAIN SHOWER ACTIVITY AND GUSTY WINDS TO MUCH OF THE WESTERN
BERING.

&&

.MODEL DISCUSSION...

THE MODELS ARE ALL IN RELATIVELY GOOD SYNOPTIC AGREEMENT
TODAY...THOUGH THE NAM DOES SEEM A TOUCH SLOW (1-2 HOURS) BRINGING
FRONTAL PRECIPITATION NORTHWARD THROUGH THE GULF THIS AFTERNOON
AND EVENING. THE BIGGEST CHALLENGE AROUND THE AREA IS THE THREAT
OF FREEZING RAIN AROUND VARIOUS PORTIONS OF THE MAINLAND. FOR THE
MOST PART THE BELOW-FREEZING AIR IS EXPECTED TO REMAIN ENTRENCHED ACROSS
PORTIONS OF ANCHORAGE...THE SUSITNA VALLEY...AND THE COPPER RIVER
BASIN. IN THESE AREAS THE BIGGEST QUESTION IS HOW MUCH
PRECIPITATION WILL FALL...BECAUSE ANY PRECIP THAT DOES FALL WILL
BE IN THE FORM OF FREEZING RAIN. THE ANSWER TO THAT QUESTION
DEPENDS IN LARGE PART UPON AN UPPER LEVEL SHORTWAVE THAT MOVES
OVER SOUTHCENTRAL SOMETIME AROUND RUSH HOUR/6AM TOMORROW
(TUESDAY) MORNING. IF THIS FEATURE IS STRONG ENOUGH IT WILL LIKELY
PRODUCE SOME PRECIP IN AREAS USUALLY PROTECTED BY DOWNSLOPING. THE
12Z NAM HAD BEEN THE STRONGEST WITH THIS FEATURE...BUT WITH THE
18Z RUN HAS SINCE BACKED OFF TO A WEAKER SOLUTION FAVORED BY THE
GLOBAL MODELS (GFS/GEM/ECMWF). CONFIDENCE IS BELOW AVERAGE TODAY.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...

AS MENTIONED IN THE MODEL DISCUSSION...THE MAIN CHALLENGE IS AGAIN
FREEZING RAIN POTENTIAL ACROSS MUCH OF THE AREA. CONFIDENCE IS
HIGH THAT THE THOMPSON PASS AREA WILL REMAIN BELOW FREEZING...AND
WITH WARM AIR SURGING IN ALOFT...FREEZING RAIN IS LIKELY TO
DEVELOP DURING THE OVERNIGHT HOURS TONIGHT. A SIMILAR STORY WILL
UNFOLD OVER THE COPPER RIVER BASIN...WHERE THE FACT THAT THE
FRONTAL STRUCTURE IS WEAKENING OVERNIGHT WILL LIKELY LEAD TO AN
INCREASE IN THE AMOUNT OF PRECIPITATION "SPILLING OVER" THE
CHUGACH MOUNTAINS AND PERHAPS TRAVELING ALL THE WAY NORTHWARD
THROUGH THE BASIN. AS A RESULT...A FREEZING RAIN ADVISORY HAS BEEN
ISSUED FOR BOTH OF THESE LOCATIONS FOR TONIGHT AND PERSISTING INTO
TOMORROW.

THE EVOLUTION ACROSS UPPER COOK INLET IS MORE PROBLEMATIC...AS THE
STRENGTH OF THE UPPER LEVEL FORCING IS QUESTIONABLE AS TO WHETHER
IT WILL BE STRONG ENOUGH TO OVERCOME THE TYPICAL DOWNSLOPING AND PRODUCE
FREEZING RAIN ACROSS PORTIONS OF ANCHORAGE TO THE WESTERN
MATANUSKA VALLEY AND SOUTHERN SUSITNA VALLEY (SEE MODEL DISCUSSION
ABOVE). WE WILL BE WATCHING THE NEXT FEW MODEL RUNS CLOSELY FOR
CHANGES TO THE STRENGTH OF THIS FEATURE. THE MOST LIKELY SCENARIO
AT THIS TIME IS THAT SOME PRECIP WILL FALL...BUT SHOULD NOT HAVE
THE WIDESPREAD IMPACTS TO WARRANT AN ADVISORY.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...

A STRONG FRONT IS MOVING RAPIDLY NORTHWARD THROUGH THE GULF OF
ALASKA AND WILL MOVE OVER SOUTH CENTRAL ALASKA TONIGHT. THIS FRONT
IS BRINGING WARM AIR WITH IT AND MOST OF THE PRECIPITATION THAT
FALLS WILL BE RAIN. TEMPERATURES ALOFT WERE ALREADY ABOVE
FREEZING IN ANCHORAGE FROM THE MORNING BALLOON LAUNCH AND THIS
SERVES AS AN INDICATION OF THE WARM AIR ALOFT OVER MUCH OF THE
AREA. THERE WILL BE SOME AREAS OF FREEZING RAIN AS THE
PRECIPITATION MOVES IN BUT IT IS NOT EXPECTED TO BE WIDESPREAD AND
AREAS TO THE LEE OF THE MOUNTAINS SUCH AS ANCHORAGE WILL MISS MOST
OF THE PRECIPITATION DUE TO DOWNSLOPING.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
THE INCOMING FRONTAL SYSTEM CURRENTLY STRETCHED FROM KODIAK ISLAND
ACROSS THE ALASKA PENINSULA AND INTO BRISTOL BAY WILL CONTINUE TO
LIFT NORTH INTO SOUTHWEST ALASKA THIS EVENING. AS THE PARENT LOW
APPROACHES THE ALASKA PENINSULA FROM THE SOUTH...A SECONDARY
SURFACE LOW WILL SPIN UP JUST TO THE NORTH OF THE WESTERN TIP OF
THE PENINSULA TONIGHT. THIS NEW LOW WILL THEN TRACK OFF TO THE
WEST HELPING TO SHEAR WEAKEN AND SLOW THE NORTHWARD PROGRESS THE
FRONTAL SYSTEM AS IT LIFTS INTO SOUTHWEST ALASKA. BASED THIS AND
THE DOWNSLOPING EASTERLY FLOW ALOFT AND DRY NORTH TO NORTHEAST
OFFSHORE SURFACE WINDS MOST OF THE PRECIPITATION SHOULD REMAIN IN
THE BRISTOL BAY ZONE 161 AREA WHERE TEMPERATURES ARE ALREADY WARMER
WITH ONLY LOW CHANCES FURTHER NORTH. FREEZING RAIN WILL STILL BE A
CONCERN ALONG SOUTHWEST COASTAL AREAS OF 155 BUT SHOULD REMAIN
SPOTTY WITH LOW QPF AMOUNTS EXPECTED.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE SURFACE LOW SOUTH OF SHEMYA WILL CONTINUE TO GRADUALLY WEAKEN
AS IT DRIFTS SOUTH THROUGH TUESDAY. THE NEXT LOW WILL APPROACH THE
EASTERN ALEUTIANS FROM THE SOUTH ON WEDNESDAY BRINGING ADDITIONAL
EASTERLY GALES NORTH WITH IT. IN THE BERING...EAST TO NORTHEAST
WINDS WILL CONTINUE THEN STRENGTHEN A BIT AS THE NEW LOW THAT
BREAKS OF FROM THE LARGE WESTERN GULF LOW TRACKS OFF TO THE WEST
AROUND 55N AND TIGHTENS THE SURFACE PRESSURE GRADIENT OVER THE
NORTHERN GULF.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THERE REMAINS GOOD MODEL AGREEMENT THAT A NEARLY STATIONARY AND
BROAD UPPER LEVEL TROUGH THAT WILL SETUP ALONG THE ALEUTIANS ON
WEDNESDAY...BRINGING CONTINUED STORMY WEATHER THE SOUTHERN
MAINLAND AND ALEUTIANS THROUGH THE WORK WEEK. THE FIRST OF THESE
SYSTEMS WILL ENTER THE GULF ON WEDNESDAY WITH AT LEAST GALE FORCE
WINDS AND WIDESPREAD RAIN THROUGHOUT THE GULF COAST. INTERIOR
SOUTHCENTRAL WILL LIKELY REMAIN WARM ENOUGH ALOFT FOR
PRECIPITATION TO FALL MAINLY AS RAIN THROUGH THE END OF THIS
WEEK...AND WITH SURFACE TEMPERATURES HOVERING AROUND FREEZING THE
POTENTIAL FOR AREAS OF FREEZING RAIN WILL ALSO CONTINUE.

BY THIS WEEKEND...THE MODELS CONTINUE TO BE IN GOOD AGREEMENT THAT
AN ARCTIC TROUGH WILL DIVE SOUTHWARD THROUGH THE BERING STRAIT AND
INTO THE BERING SEA...WHICH IS CAUSING SOME DISAGREEMENT WITH THE
MODELS AS IT INTERACTS WITH THE BROAD TROUGH OVER THE ALEUTIANS.
BECAUSE OF THIS THE FORECAST STARTING THIS WEEKEND CONTINUES TO
HEAVILY USE WPC GUIDANCE...WHICH USED AN ENSEMBLE MEAN BLEND TO
HANDLE THE MODEL DISCREPANCIES. WHILE THIS LOW WILL DRAG DOWN
SOME COLD SIBERIAN AIR INTO THE BERING...THERE IS DECENT AGREEMENT
THAT THIS WILL BECOME A PERSISTENT FEATURE THROUGH THE START OF
NEXT WEEK. WITH A STATIONARY TROUGH OVER WITH BERING WITH CONTINUED
RIDGING OVER THE YUKON...THERE DOES NOT APPEAR TO AN END THE THE
WARM...MOIST...AND DEEP SOUTHERLY FLOW CURRENTLY DOMINATING THE
MAINLAND FOR AT LEAST THE NEXT WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...FREEZING RAIN ADVISORIES 131 AND 141.
MARINE...STORMS 130 131 138.
GALES 119 120 132 160 178 180 181 185 351 352 411 412 413 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AD
SOUTHCENTRAL ALASKA...MC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...DEK