Gale Warning 275, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
449 PM AKDT TUE MAR 15 2016

.ANALYSIS AND UPPER LEVELS...
UPPER LEVEL PATTERN THIS MORNING SHOWS A LONGWAVE TROUGH LOCATED
OVER THE WEST COAST FOLLOWED BY A BUILDING RIDGE IN THE WESTERN
ALEUTIANS. THE CIRRUS SHIELD FROM THE NEXT MAJOR LOW PRESSURE
SYSTEM IS JUST PEEKING OVER THE TOP OF THE RIDGE AND BEGINNING TO
AFFECT THE WESTERN ALEUTIANS. AT 500 MB A STRONG UPPER LEVEL LOW
IS MOVING SOUTHWARD INTO THE BASE OF THE TROUGH OVER THE WESTERN
COAST OF ALASKA. THIS FEATURE WILL BRING COLD AIR WITH IT AND
TRANSITION THE SOUTHCENTRAL AREA INTO SOUTHWEST FLOW ALOFT...WHICH
WILL BRING AVAILABLE MOISTURE TO FUEL AN UPSLOPE SNOW EVENT FOR
SOUTHCENTRAL AND NORTHEAST PRINCE WILLIAM SOUND. TEMPERATURES THIS
MORNING SHOW A WARM LAYER ON THE ANCHORAGE SOUNDING...BUT ON THE
BETHEL SOUNDING THEY ARE ALL BELOW -10C...INDICATING THE STRENGTH
OF THE COLD AIR MAKING ITS WAY ACROSS THE MAINLAND. THIS WILL
LIKELY MODERATE SOMEWHAT BY THE TIME IT MAKES ITS WAY TO
SOUTHCENTRAL.

&&

.MODEL DISCUSSION...
FOR THE OVERALL PICTURE MODELS ARE IN GOOD ALIGNMENT THIS MORNING.
HOWEVER IN THE FINE SCALE DETAILS FOR THE EASTERN PORTION OF THE
REGION THE MODELS DIFFER SLIGHTLY BUT WITH SIGNIFICANT RESULTS.
THE LOCATION OF THE PRECIPITATION BAND IN SOUTHCENTRAL AND THE
PRECIPITATION TYPE FOR NORTHEAST PRINCE WILLIAM SOUND VARY QUITE A
BIT BETWEEN GFS EC AND GEM REGIONAL. GFS PRODUCES THE LOWEST SNOW
AMOUNTS FOR BOTH LOCATIONS...HOWEVER THIS SCENARIO LOOKS LEAST
LIKELY GIVEN THE POOR RESOLUTION OF THIS MODEL. FOR THE SHORT
TERM A BLEND OF NAM AND GEM REGIONAL WERE CHOSEN FOR THIS
AREA...WITH HIGHEST SNOWFALL AMOUNTS OVER THE ANCHORAGE HILLSIDE
AND IN VALDEZ CITY.

&&

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

THE MAIN CHALLENGE REMAINS THE LOCATION AND INTENSITY OF SNOW OVER
THE SOUTH CENTRAL REGION THROUGH TOMORROW AFTERNOON. THE MODELS
HAVE A SIGNIFICANT SPREAD ON THE AMOUNT OF SNOW POSSIBLE FROM
ANCHORAGE NORTH THROUGH PALMER. THE UPPER LOW AND TROUGH THAT IS
DIGGING SOUTH OVER SOUTHWEST ALASKA IS THE MAIN FORCING MECHANISM
TO THIS EVENT. THIS IS ESTABLISHING A SOUTH SOUTHWEST FLOW UP COOK
INLET WITH THE EXPECTED CONVERGENCE ZONE TO BE ALONG THE
MOUNTAINS. WEAK LOW PRESSURE IS EXPECTED TO BACK INTO PRINCE
WILLIAM SOUND. THE ISSUE REMAINS JUST HOW FAR WEST WILL THIS LOW
MOVE AND THUS HOW MUCH MOISTURE WILL IT FEED INTO THE CONVERGENCE
ZONE. CONFIDENCE IS HIGH THAT THE HIGHER ELEVATIONS OF THE CHUGACH
MOUNTAINS WILL SEE AROUND 6 INCHES OF SNOW OR POSSIBLY MORE BY
TOMORROW AFTERNOON. WE STILL EXPECT SIGNIFICANT ACCUMULATIONS OF
SNOW UP IN THE SUSITNA VALLEY THROUGH TOMORROW
AFTERNOON...ESPECIALLY NORTH OF CHULITNA TOWARD THE PARK. ANOTHER
AREA OF CONCERN WILL BE VALDEZ TO THOMPSON PASS AS THERE IS
POTENTIAL FOR MUCH HEAVIER SNOW THAN CURRENTLY FORECAST. THE
SOUTHWEST FLOW ALOFT WITH GOOD UPPER LIFTING INDICATES A GOOD SNOW
PATTERN FOR THE NORTHERN PRINCE WILLIAM SOUND REGION.

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)... AN UPPER
LEVEL TROUGH CONTINUES TO DIG SOUTHEASTWARD ACROSS SOUTHWEST
ALASKA WITH CLOUDS AND SCATTERED SNOW SHOWERS OUT AHEAD OF IT AND
CLEARING SKIES AND GUSTY NORTHERLY WINDS BEHIND IT. CLOUDS AND A
FEW SHOWERS WILL LINGER ALONG THE ALASKA RANGE/ALEUTIAN
RANGE/ALASKA PENINSULA THROUGH EARLY WEDNESDAY. OTHERWISE DRY AND
FRIGID CONDITIONS WILL SETTLE IN FOR THE REMAINDER OF THE WEEK.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE LAST VESTIGES OF AN UPPER TROUGH ARE EXITING ACROSS THE
EASTERN ALEUTIANS AND ALASKA PENINSULA INTO THE NORTH PACIFIC.
COLD ADVECTION BEHIND IT IS PRODUCING WIDESPREAD STRATO-CUMULUS
CLOUDS ACROSS MOST OF THE CENTRAL AND EASTERN BERING AND
ALEUTIANS. SHOWERS ARE PRIMARILY CONFINED TO THE EASTERN BERING
WHERE ADVECTION IS STRONGEST. AN AMPLIFYING RIDGE WILL BUILD IN
FROM THE WEST TONIGHT THROUGH WEDNESDAY BRINGING AN END TO THE
GUSTY WINDS AND SHOWERS.

THE MAIN FOCUS FARTHER WEST IS A BOMBING LOW OVER THE NORTHWEST
PACIFIC. THIS IS ON TRACK AS PER PREVIOUS FORECASTS WITH A STORM
FORCE FRONT SET TO AFFECT THE WESTERN ALEUTIANS LATE TONIGHT
THROUGH WEDNESDAY AND THEN QUICKLY ELONGATE AND WEAKEN ALONG THE
WEST SIDE OF THE UPPER RIDGE WEDNESDAY NIGHT AND THURSDAY. THE
FRONTAL SYSTEM WILL BE OCCLUDING AS IT REACHES THE ALEUTIANS...WITH
MARGINALLY WARMER AIR MOVING IN BEHIND THE OCCLUSION. PRECIP WILL
START AS SNOW...BUT THERE WILL BE JUST ENOUGH WARM AIR ADVECTION
TO AT LEAST MIX WITH RAIN. IN ANY CASE...SNOW ACCUMULATION WILL BE
MINOR AS TEMPERATURES QUICKLY WARM TO FREEZING OR ABOVE.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THE MAINLAND TROUGH WILL CONTINUE TO SLOWLY LIFT NORTH AND
EASTWARD THROUGH THE END OF THE WORK WEEK...WITH AN INVERTED
SURFACE TROUGH/FRONT OVER THE EASTERN GULF HELPING TO KEEP
PRECIPITATION CONFINED TO EASTERN PRINCE WILLIAM SOUND AND THE
COPPER RIVER BASIN BY FRIDAY. THE UPPER TROUGH WILL CONTINUE TO BE
THE DOMINANT FEATURE DURING THE WEEKEND AS IT IS REINFORCED BY
SHORTWAVE ENERGY DROPPING OUT OF THE ARCTIC AND MOVES OVER THE
CENTRAL INTERIOR AS A DEEP CLOSED LOW. THIS WILL BRING A
CONTINUATION OF COOL AND DRY CONDITIONS FOR THE SOUTHWEST
MAINLAND...WITH PRECIPITATION CHANCES GRADUALLY RETURNING TO
SOUTHCENTRAL AK AS ANOTHER DISTURBANCE MOVES INTO THE GULF FROM
THE SOUTHEAST.

PRECIPITATION OVER THE BERING WILL INITIALLY BE CONFINED TO THE
WESTERN AND CENTRAL ALEUTIANS WITH HIGH AMPLITUDE RIDGING IN PLACE
OVER THE EASTERN BERING...HOWEVER THE CHANCE FOR PRECIPITATION
WILL SPREAD EASTWARD OVER THE WEEKEND AS THE RIDGE WEAKENS AND
ANOTHER UPPER LOW SLIDES EASTWARD ALONG THE CHAIN.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...HEAVY FREEZING SPRAY...160 179 180 185
 		STORM WARNING...177 178 GALE WARNING...175 176 185.

&&

SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHCENTRAL ALASKA...SA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SB
LONG TERM...CB