Small Craft Advisory 919, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1235 PM AKST TUE NOV 19 2013

.ANALYSIS AND UPPER LEVELS...A FAIRLY COMPLEX PATTERN IS IN PLACE
AT THE MOMENT WITH MOVEMENT TOWARD ANOTHER CHANGE IN THE LARGE SCALE
PATTERN. AN UPPER LOW IS DROPPING SOUTHWARD ACROSS MAINLAND ALASKA
WITH A SHORT-WAVE OVER THE NORTHERN BERING COMING INTO PHASE. AREAS
OF LIGHT SNOW AR FALLING OVER SOUTHWEST ALASKA OUT AHEAD OF THIS
SHORT-WAVE. THE AIR MASS ACCOMPANYING THESE FEATURES IS NOT MUCH
COLDER THAN WHAT IS ALREADY IN PLACE OVER MAINLAND ALASKA...SO IT IS
MORE OF A REINFORCEMENT OF ARCTIC AIR. A SEPARATE UPSTREAM SHORT-WAVE
OVER EASTERN RUSSIA IS DIGGING SOUTHEASTWARD DOWN THE EAST SIDE OF A
RIDGE CENTERED OVER THE FAR WESTERN BERING. THE INTERACTION OF THIS
SHORT-WAVE WITH THE TROUGH OVER MAINLAND ALASKA IS ONE OF THE KEYS
TO THE PATTERN CHANGE. THE OTHER KEY IS A STRENGTHENING SYSTEM JUST
SOUTH OF THE ALEUTIANS. A LONG FETCH OF TROPICAL MOISTURE IS FEEDING
INTO THIS SYSTEM. TO THIS POINT IN TIME THAT MOISTURE IS BEING
BLOCKED FROM MOVING TOWARD MAINLAND ALASKA...BUT THAT WILL ALL
CHANGE OVER THE NEXT COUPLE DAYS.

.MODEL DISCUSSION...CONSIDERING THE COMPLEX INTERACTION OF ARCTIC
SHORT-WAVES WITH THE SYSTEM OVER THE NORTH PACIFIC...MODELS SEEM
TO ALL SHOW THE SAME GENERAL IDEA. THERE ARE CERTAINLY SOME KEY
DIFFERENCES INCLUDING TRACK OF THE PRIMARY LOW AS IT MOVES TO
MAINLAND ALASKA AND ON WHETHER A TRIPLE POINT LOW WILL DEVELOP
OVER THE GULF. HOWEVER...THERE IS ENOUGH COMMON CHARACTERISTICS
AMONG THE SOLUTIONS TO BEGIN TO HONE IN ON SOME OF THE LIKELY
IMPACTS. AS FAR AS A FAVORED SOLUTION FOR NOW PREFER A FARTHER
WEST LOW TRACK CLOSER TO THE UPPER LEVEL CENTER OVER THE BERING.
THE ECMWF SEEMS TO PROVIDE A NICE MIDDLE GROUND. WILL IGNORE THE
MORE AGGRESSIVE GFS/NAM SOLUTION WITH TRIPLE POINT DEVELOPMENT. NO
MATTER WHAT HAPPENS OVER THE GULF EXPECT WIDESPREAD GALES WITH
GOOD POTENTIAL FOR STORMS ALONG THE NORTH GULF COAST AS A FRONT
(OR TRIPLE POINT) APPROACHES ON FRIDAY. THERE IS A CONSISTENT TREND
TOWARD EVERYTHING MOVING THROUGH FASTER...SO THE LATEST FORECASTS
WILL REFLECT THAT.

.SHORT TERM FORECAST...AS THE SHORT-WAVE OVER EASTERN RUSSIA REACHES
THE CENTRAL BERING SEA IT WILL INTERACT WITH THE TROUGH DROPPING
SOUTHWARD ACROSS THE MAINLAND AND LEAD TO DEVELOPMENT OF AN UPPER
LOW CENTER ON WEDNESDAY. THIS WILL CAUSE THE REMAINDER OF THE TROUGH
TO STALL OVER THE SOUTHERN MAINLAND AND THEN MOVE BACK NORTHWARD
WEDNESDAY NIGHT AS SOUTHERLY FLOW STRENGTHENS ON THE EAST SIDE OF
THE DEEPENING UPPER CENTER. THE MORE SIGNIFICANT IMPACT OF THIS
EVOLVING PATTERN IS THE DEEPENING LOW WILL EVENTUALLY TAP INTO THE
TROPICAL MOISTURE OVER THE NORTH PACIFIC AND FORCE IT NORTHWARD
TOWARD MAINLAND ALASKA.

THE SURFACE LOW TRACK AND DEPTH WILL BE THE KEY TO DETERMINING
PLACEMENT OF THIS TROPICAL AIR MASS AND EXTENT OF WARM AIR MOVING
TO MAINLAND ALASKA. BASED ON THE LATEST CLUSTER OF MODEL SOLUTIONS
THE BULK OF WARM AIR WILL CROSS THE GULF AND MOVE INTO SOUTHCENTRAL
THURSDAY NIGHT INTO FRIDAY. WITH SUCH A COLD AIR MASS IN PLACE OUT
AHEAD OF THIS SYSTEM EXPECT PRECIPITATION TO START AS ALL SNOW. LOW
LEVEL FLOW WILL REMAIN OUT OF THE NORTH AND GAP WINDS WILL STILL BE
BLOWING WITH A PERSISTENTLY TIGHT PRESSURE GRADIENT. THUS...WOULD
EXPECT AREAS OF BLOWING SNOW. AS THE WARM AIR OVERRIDES THE COLD
AIR AT THE SURFACE THERE WILL BE GOOD POTENTIAL FOR FREEZING
RAIN...ESPECIALLY OVER INTERIOR AREAS OF SOUTHCENTRAL. WITH ABUNDANT
MOISTURE AND THE EXPECTATION OF MODERATE TO HEAVY PRECIPITATION ALONG
THE COAST SNOW WILL LIKELY HANG ON UNTIL TEMPERATURES ARE AROUND
FREEZING...SO BY THE TIME PRECIPITATION CHANGES TO RAIN SURFACE
TEMPERATURES SHOULD BE ABOVE FREEZING AND FREEZING RAIN WILL NOT BE
A PROBLEM. WHERE PRECIPITATION STAYS ALL SNOW...PRIMARILY BACK OVER
SOUTHWEST ALASKA...THERE IS POTENTIAL FOR REACHING SNOW ADVISORY
CRITERIA. THE FAST MOVEMENT OF THE SYSTEM IS THE ONE THING THAT
MIGHT LIMIT AMOUNTS. THERE IS TOO MUCH UNCERTAINTY AND IT IS TOO
EARLY TO ISSUE ANY WATCHES WARNINGS OR ADVISORIES AT THIS POINT IN
TIME. HOWEVER...HAVE ISSUES A SPECIAL WEATHER STATEMENT TO HIGHLIGHT
THE RETURN OF WARMER TEMPERATURES AND WINTRY PRECIPITATION.

CONFIDENCE IS FAIRLY HIGH THAT COLDER AIR WILL RAPIDLY MOVE BACK
IN BEHIND THE SURFACE LOW AND CHANGE ANY REMAINING PRECIPITATION
TO SNOW FRIDAY AFTERNOON THROUGH FRIDAY NIGHT. IN FACT MODELS
INDICATE A FAIRLY POTENT SHORT-WAVE ROUNDING THE UPPER LOW AND
CROSSING SOUTHCENTRAL FRIDAY NIGHT WHICH COULD BRING A QUICK SHOT
OF MODERATE TO HEAVY SNOW. COLD ADVECTION ON THE BACK SIDE WILL
ALSO LEAD TO STRONG WINDS FROM THE EASTERN BERING ACROSS THE ALASKA
PENINSULA AND INTO THE GULF. WITH RELATIVELY WARM SEA SURFACE
TEMPERATURES OUT THERE RIGHT NOW EXPECT GENERATION OF STEADY SNOW
SHOWERS AS THE MUCH COLDER AIR MOVES OVER TOP OF THE OPEN WATER AND
POTENTIAL FOR BLOWING SNOW/BLIZZARD ISSUES ALONG THE SOUTHERN ALASKA
PENINSULA.

THE PATTERN BEYOND THIS POINT LOOKS LIKE IT WILL REMAIN ACTIVE...BUT
THERE IS QUITE A SPREAD IN SOLUTIONS WITH HANDLING OF NEXT COUPLE
SYSTEMS DEVELOPING OVER THE NORTH PACIFIC AND FORECAST CONFIDENCE
IS LOW.

.LONG TERM FORECAST...MEDIUM RANGE MODELS REFLECT  PROGRESSIVE AND
DYNAMIC FLOW THROUGH THE DAY 4 THROUGH 7 TIME PERIOD. THIS MAKES IT
VERY DIFFICULT TO HAVE CONFIDENCE IN TRACK OF ANY PARTICULAR SOLUTION
OR STORM SYSTEM. WITH THIS TYPE OF PATTERN EXPECT CONTINUES ACTIVE
WEATHER ACROSS MUCH OF THE FORECAST AREA.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...120 131 132 138 150 155 160 165 170 172 173
                        174 175 176 177.
         HEAVY FREEZING SPRAY WARNING...121 126 127 139 140 180 181.
FIRE WEATHER...NONE.

SEB NOV 13