Small Craft Advisory 320, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
523 AM AKDT SUN APR 12 2015

.ANALYSIS AND UPPER LEVELS...
BROAD CYCLONIC UPPER LOW FLOW CENTERED OVER THE EASTERN BERING
SEA AND THE NORTHERN GULF CONTINUE SHOWER ACTIVITY OVER MUCH OF
THE AOR. COLD AIR ASSOCIATED WITH THE UPPER TROUGH ARE REFLECTED
IN UPPER AIR SOUNDINGS THIS MORNING. 850 MB TEMPS ACROSS THE
ANCHORAGE BOWL AND NORTH GULF WERE -5C TO -6C AND -11C TO -13C
OVER SOUTHWEST ALASKA/EASTERN BERING SEA. AS A RESULT MOST SHOWER
ACTIVITY HAS BEEN IN THE FORM OF SNOW WITH SOME MIXED TYPES OVER
THE GULF AND COASTS. COLD ADVECTION ACROSS THE EASTERN BERING SEA
PRODUCED STRONG NORTHERLY WIND FROM THE CENTRAL ALEUTIANS TO OVER
BRISTOL BAY. COLD AIR SQUEEZING THROUGH THE ALEUTIAN RANGE
PRODUCED GALE FORCE WESTERLY GAP FLOWS OVER THE WESTERN GULF.

&&

.MODEL DISCUSSION (THROUGH MONDAY)...
MODELS ARE ALL PAINTING THE SAME BASIC PICTURE OVER THE NEXT
COUPLE OF DAYS. ALL HAVE SIMILAR TIMING OF THE UPPER WAVE MOVING
SOUTH ACROSS THE EASTERN BERING TONIGHT...THE UPPER WAVE MOVING
NORTH ACROSS SOUTHCENTRAL SUN NIGHT...AND THE DEVELOPING SURFACE
LOW MOVING SOUTH OF THE CHAIN SUN/MON.

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

THE MAIN FORECAST CHALLENGE TODAY WILL REVOLVE AROUND A SLOW
MOVING TROUGH AXIS TRACKING FROM THE GULF INTO SOUTHCENTRAL TODAY
AND TONIGHT. CONVECTIVE SHOWERS IN THE NORTHERN GULF WILL ORGANIZE
ALONG A FRONT AS THE TROUGH AXIS LIFTS NORTH...WITH SNOW SHOWERS
IMPACTING COASTAL LOCATIONS. THE BEST CHANCES FOR HEAVIER SNOW
SHOWERS INLAND WILL BE ALONG THE KENAI PENINSULA AND TURNAGAIN
ARM...ALTHOUGH SHOWERS WILL BE POSSIBLE ACROSS ALL INLAND
LOCATIONS BY AFTERNOON...AIDED BY DIURNAL HEATING. A ROGUE
LIGHTNING STRIKE CAN NOT BE RULED OUT OVER THE KENAI PENINSULA
THIS AFTERNOON. OTHERWISE...PRESSURE RISES BEHIND THE UPPER TROUGH
WILL SUPPORT THE DEVELOPMENT OF SOUTHEAST WINDS THROUGH FAVORED
GAPS...INCLUDING TURNAGAIN ARM AND INTO PORTIONS OF ANCHORAGE BY
THE EVENING.

MONDAY WILL BE DRIER AND LESS ACTIVE AS A WEAK OFFSHORE PRESSURE
GRADIENT COMBINES WITH WEAK UPPER RIDGING ALOFT...KEEPING MOST
SHOWER ACTIVITY OVER THE MOUNTAINS. IN OTHER WORDS...MOST INLAND
LOCATIONS WILL EXPERIENCE A RATHER PLEASANT SPRING DAY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2...SUN AND
MON)...AS SNOW SHOWERS DIE DOWN ACROSS THE LOWER KUSKOKWIM VALLEY
AND THE DELTA TODAY...AN UPPER LEVEL DISTURBANCE WILL BRING SOME
MORE STEADY SNOWFALL TO LOCATIONS FROM ILIAMNA TO KING SALMON. THE
UPPER LEVEL LOW CURRENTLY CROSSING THE ALASKA PENINSULA WILL LINK
UP WITH THE FRONT OVER THE NORTHERN GULF. THE UPPER LEVELS WILL
THEN START TO TAKE ON A NEGATIVE TILT AND CREATE SOME LOWER LEVEL
CONVERGENCE ON THE LEEWARD SIDE OF THE RANGES. MODELS ARE SHOWING
A WEAK SURFACE LOW DEVELOPING JUST AFTER MIDNIGHT TONIGHT INTO
SUNDAY MORNING. THIS SHOULD MAKE FOR A RELATIVELY NARROW CORRIDOR
OF ACCUMULATION SNOW...WITH THE HIGHEST AMOUNTS NEAR THE ALEUTIAN
RANGE NEAR ILIAMNA. WEAK MID LEVEL RIDGING WILL MOVE QUICKLY INTO
THE AREA ON MONDAY AND BRING AN END TO ANY LINGERING
PRECIPITATION. IT WILL ALSO LIKELY BRING SOME PARTIALLY CLEARING
SKIES AND SOME OF THE WARMEST TEMPERATURES WE HAVE SEEN OVER THE
PAST FEW DAYS.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2...SUN AND
MON)...NORTHWEST FLOW CONTINUES TO PLAGUE AREAS ACROSS THE
BERING...ESPECIALLY ON THE BERING SIDE OF THE CHAIN. THIS IS
MAKING FOR RELENTLESS SNOW SHOWERS WHICH ARE OCCASIONALLY HEAVY
ENOUGH TO BRING VISIBILITY DOWN BELOW A MILE. AS WE GO THROUGH
SUNDAY...WE SHOULD SEE THE PATTERN FINALLY BEGIN TO SHIFT AS A
FAIRLY POTENT LOW APPROACHES ATKA AND ADAK FROM THE WEST
SOUTHWEST. IN FRONT OF THIS LOW...WEAK RIDGING SHOULD CAUSE WINDS
TO SHIFT AROUND TO THE SOUTHEAST AND BRING CALMER CONDITIONS THAN
WE HAVE SEEN THE PAST FEW DAYS. BUT THEN THE FOCUS SHIFTS TO THAT
LOW AS IT SKIRTS BY JUST SOUTH OF THE CHAIN. WITH COLD AIR IN
PLACE AND ABUNDANT MOISTURE COMING UP WITH THE FRONT...SNOW WILL
START OVER THE CENTRAL ALEUTIANS LATE THIS EVENING AND SPREAD
EAST REACHING DUTCH HARBOR BY MONDAY AFTERNOON. ACCOMPANYING THE
SNOW WILL BE STRONG NORTHEAST TO EAST WINDS. THE COMBINATION WILL
LEAD TO DRIVING SNOW REDUCING VISIBILITIES TO ONE HALF MILE AT
TIMES. RIGHT NOW IT LOOKS AS IF THE LOW WILL STAY JUST FAR ENOUGH
SOUTH TO KEEP THE WORST CONDITIONS SOUTH OF THE CHAIN.

&&

.LONG TERM FORECAST (MONDAY NIGHT THROUGH SATURDAY)...

THE MEAN UPPER LEVEL TROUGH/LOW POSITION WILL BE SITUATED ACROSS
THE NORTHERN AND EASTERN BERING INCLUDING THE SOUTHWESTERN
MAINLAND AND WESTERN GULF AND EXTENDING SOUTH INTO THE NORTH
PACIFIC THROUGH MID-WEEK. AS NUMEROUS WEAK DISTURBANCES ROTATE
THROUGH...A STRONGER LOW WILL MOVE ACROSS THE NORTH PACIFIC
GRAZING THE ALEUTIANS AND ALASKA PENINSULA INTO TUESDAY...AND
THEN HELP FEED THE BASE OF THE TROUGH OVER THE WESTERN GULF
WEDNESDAY INTO THURSDAY. THE SURFACE LOW WILL WEAKEN SIGNIFICANTLY
BEFORE MOVING INLAND LATE WEDNESDAY THROUGH LATE THURSDAY.

A FOLLOWING LOW WILL MOVE ACROSS THE WESTERN AND CENTRAL ALEUTIANS
WEDNESDAY AND THURSDAY AND THEN REACH MATURATION OVER THE EASTERN
BERING BY FRIDAY. AS THE LOW MEANDERS INLAND OVER SOUTHWEST ALASKA
NEXT WEEKEND SEVERAL DISTURBANCES WILL ROTATE INTO SOUTHERN
ALASKA AND KEEP THE WEATHER QUITE ACTIVE FOR APRIL.

THERE ARE SOME IMPORTANT ASPECTS TO NOTE WITH THIS PATTERN. THE
FIRST IS THAT THE TRACK OF LOW PRESSURE SYSTEMS WILL CONTINUE TO
KEEP COLD AIR CLOSE TO SOUTHERN ALASKA. THE NEXT IS THAT MODELS
SEEM TO BE IN AGREEMENT THAT THE INDIVIDUAL DISTURBANCES ROTATING
AROUND THE UPPER LOW(S) WILL BE FAIRLY STRONG AND BE ACCOMPANIED
BY ACTIVE JET STREAKS. WHAT THIS ALL MEANS IS THAT WE CAN NOT RULE
OUT THE POTENTIAL FOR A SIGNIFICANT SNOW/RAIN EVENT FOR SOUTHERN
ALASKA IN THE LATE-WEEK TO EARLY WEEKEND PERIOD.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES 155 170 172-178
         HEAVY FREEZING SPRAY 180
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...CC