Winter Storm in Alaska, November 20 to 22, 2013

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
623 AM AKST WED NOV 20 2013

.SHORT TERM...SATELLITE IMAGERY THIS MORNING SHOWING A HIGH OVER
THE YUKON AND A WEAKENING LOW IN THE NORTHEASTERN GULF THAT MOVED
SNOW SHOWERS OVER THE PANHANDLE. SNOW AMOUNTS FOR THE SOUTHERN
PANHANDLE WERE IN THE 1 TO 3 INCH RANGE WITH 1 TO 2 INCHES FOR
THE CENTRAL PANHANDLE. THESE SHOWER BANDS WILL TRACK EASTWARD
THROUGH THE DAY AND DIMINISH WITH ADDITIONAL 1 TO 2 INCHES OF
SNOW ACCUMULATION POSSIBLE. COLD ARCTIC AIR IN PLACE WILL STILL
PRODUCE FINE LIGHT SNOW WHICH KEEPS TOTAL ACCUMULATION A BIT
LOWER.

 TO THE WEST A STRONG LOW DEVELOPING SOUTH OF KODIAK IS PUSHING A
WARM FRONTAL BOUNDARY ACROSS THE EASTERN GULF. THIS FRONT WILL
REACH THE PANHANDLE TONIGHT BRINGING IN CATEGORICAL PRECIP FOR
THE SOUTHERN TO CENTRAL PANHANDLE. FOR NOW PRECIP BOUNDARY ALONG
THE ZONAL FLOW LOOKS TO JUST REACH ICY STRAIT. THE FRONT IS
BRINGING IN LONG FETCH OF TROPICAL MOISTURE WHICH WILL PRODUCE
HIGH QPF VALUES STARTING TONIGHT. THE RESULTING OVER RUNNING SNOW
EVENT WITH THE COLD ARCTIC AIR IN PLACE AND WARM MOIST AIR MOVING
OVER WILL PRODUCE HEAVY SNOW FALL STARTING LATE WEDNESDAY NIGHT.
12 HOUR SNOW FALL AMOUNTS TONIGHT IN THE 3 TO 6 INCH RANGE WITH
HEAVIEST SNOW FALL IN THE SOUTH CENTRAL PANHANDLE. SNOW WILL
CONTINUE THURSDAY INTO FRIDAY MORNING. WINTER STORM WARNING HAVE
BEEN ISSUED FOR A MAJORITY OF THE CENTRAL AND SOUTHERN PANHANDLE.

 WARM AIR WILL PUSH IN FROM THE SOUTH AS WINDS SHIFT TO THE SOUTH
WITH OVERNIGHT TEMPS REMAINING STEADY OR INCREASING FOR THE
SOUTHERN AND COASTAL AREAS. SNOW WILL TRANSITION TO RAIN FOR
THESE AREAS WITH THE RAIN BECOMING HEAVY DUE TO THE INFLUX OF
TROPICAL MOISTURE. ARCTIC AIR WILL KEEP TEMPS FOR THE CENTRAL AND
NORTHERN PANHANDLE IN THE TEENS TO LOW 20S.

 WINDS OVER THE AK GULF WIND INCREASE TO GALE FORCE WITH
ADVANCING FRONT AND A BARRIER JET DEVELOPING ALONG THE NORTHERN
GULF. TIGHTENING PRESSURE GRADIENT BETWEEN THE YUKON HIGH AND THE
ADVANCING FRONT WILL INCREASE WINDS OVER THE NORTHERN INNER
CHANNELS TO GALE FORCE WITH FREEZING SPRAY DEVELOPING DUE TO COLD
AIR IN PLACE.

 00Z MODELS WERE IN FAIR AGREEMENT WITH NEW RUNS SHOWING BIT
BETTER AGREEMENT. ECMWF WAS PUSHING PRECIP BIT FURTHER NORTH THAN
THE NAM/GFS. MOS GUIDANCE WAS SHOWING SOME LARGE SPREAD FOR
TEMPERATURES ADDING UNCERTAINTY FOR PRECIP TYPE AND EVENTUAL SNOW
TOTALS. USED A BLEND OF GFS/NAM. FORECAST CONFIDENCE IS AVERAGE
WITH MAIN ISSUES BEING SNOW TOTALS FOR TONIGHT, TIMING OF SHOWERS
ENDING THIS MORNING AND RATE OF TEMPERATURES INCREASES TONIGHT.

 .LONG TERM...THURSDAY RIDGE AMPLIFYING OVER THE PANHANDLE WITH
TROUGH SOUTH OF THE ALASKA PENINSULA WELL OUT INTO THE NORTH
PACIFIC. THIS SYSTEM WILL PUMP THE SUBTROPICAL MOISTURE BEGINNING
LATE WEDNESDAY NIGHT IN THE SHORT-TERM. HOWEVER, A CONVERGENCE
ZONE INDICATED IN THE 500 MB VORTICITY PATTERN IN THE YUKON WILL
HELP ANCHOR THE COLD AIR FROM THE SOUTH CENTRAL PANHANDLE
NORTHWARD. THE AMPLIFYING RIDGE WILL DIRECT MOISTURE STRAIT INTO
THE COAST RANGE AND FOLKS PARTICULARLY IN THE INTERIOR SOUTH
CENTRAL PANHANDLE WILL SEE A POSSIBLE 2 FEET OF SNOW BY THURSDAY
NIGHT, PARTICULARLY IN WIND-SHELTERED LOCATIONS SUCH AS
PETERSBURG WHERE THE COLD AIR WILL BE DIFFICULT TO SCOUR OUT.
COASTAL LOCATIONS SUCH AS SITKA AND HYDABURG WILL HAVE TURNED TO
RAIN ALREADY, WITH SOME BANDS HEAVY AT TIMES. AS THE RIDGE
AMPLIFIES THE PRECIPITATION BANDS AND THE HIGHER QPF WILL SHIFT
NORTHWEST OVER THE NORTHERN PANHANDLE. THIS IS WHERE JUNEAU,
GUSTAVUS, HOONAH, AND PELICAN WILL EXPERIENCE THE GREATEST
SNOWFALL TOTALS WITH PELICAN RECEIVING UP TO 8 INCHES THURSDAY
NIGHT AND JUNEAU UP TO 7 THURSDAY NIGHT. SNOW TOTALS WERE BASED
MAINLY ON QPF VALUES DERIVED FROM A GRAND COALITION OF MODELS, AS
THERE WERE SUBTLE DIFFERENCES AMONG THEM. HOWEVER, OVERALL, QPFS
WERE RAISED A LITTLE FROM THE LAST PACKAGE. TO OBTAIN SNOW
TOTALS, DID TONE DOWN PURE 10:1 RATIOS AS IN THE LATER TIME
FRAMES, THE SNOW WILL BE A WETTER, AND COMPACT MORE. IT SHOULD BE
STRESSED THAT EVEN IF THE SNOW TOTALS ARE LESS, THE RATES AND
VISIBILITY DEGRADATION WILL BE THERE, AND IT WILL BE A HAZARDOUS
MESS ALONG THE STREETS, THROUGH THE CHANNELS AND ALONG THE
SIDEWALKS. THE ENDING OF THIS EVENT DOES NOT INSPIRE CONFIDENCE.
THINK IN SPOTS, THE RAIN WILL DEVELOP SOONER, POSSIBLY LATE
THURSDAY NIGHT, WHILE IN SHELTERED AREAS LIKE PETERSBURG THERE IS
A CHANCE THAT THE SNOW COULD LAST WELL INTO FRIDAY. IN ADDITION
SOME COMPACTION MAY KEEP THE SNOW TOTALS DOWN, BUT A GOOD
POTENTIAL IS OUT THERE...ENOUGH NOT TO IGNORE IT. IN GENERAL,
WENT WITH NUMBER LESS THAN THE GFS BY 1 TO 3 INCHES IN SEVERAL
KEY SPOTS FOR THURSDAY NIGHT.

 WINDS WILL ALSO BE A BIG CONCERN. STORM FORCE WINDS REMAIN IN
THE FORECAST FOR FRIDAY AS A STRONG COLD FRONT APPROACHES THE
PANHANDLE. WINDS ALONG LYNN CANAL AND STEPHENS PASSAGE AS WELL AS
CHATHAM STRAIT WERE RAISED TO 30 KT BUT DO NOT BE SURPRISED TO
SEE SOME GALES BE EVENTUALLY POSTED AS FRONTS TEND TO LEND AN
EXTRA UMPH TO THE SURGE OF SOUTHERLIES WITH THE WARM FRONT. THINK
MANY OF THE INTERIOR LOCATIONS ON LAND WILL SEE GOOD WINDS ON
FRIDAY AS WELL. EXPECT JUNEAU'S WINDS TO BE 15 TO 25 MPH WITH
HIGHER GUSTS ON FRIDAY, BUT IN REALITY, I WANTED TO START A
TREND. THE POTENTIAL IS THERE TO BE STRONGER.

 FOR THE WEEKEND...PERIOD IS ACTIVE BUT LESS SEVERE WITH WARMER
CONDITIONS AND RAIN FOR MOST AREAS OF THE PANHANDLE WITH THE
EXCEPTION OF THE FAR NORTH. TEMPERATURES WILL BE BACK TO NORMAL
LEVELS. TOOK A GRAND BLEND OF MODELS...USED NAM, GFS, AND ECMWF
AT VARIOUS TIMES. WAS CONSERVATIVE IN USHERING THE WARM AIR
NORTH, HELD TEMPERATURES BACK AS PREVIOUS SHIFTS HAVE DONE.
CONFIDENCE AT BEST AVERAGE, BUT GOOD WITH OVERRUNNING
MOISTURE...TEMPERATURES LESS CONFIDENT ABOUT WHICH UNFORTUNATELY
IS THE KEY TO THE FORECAST.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER STORM WARNING FROM MIDNIGHT TONIGHT TO 5 AM AKST FRIDAY
     FOR AKZ020>022-024>026-029.
         STRONG WIND LATE TONIGHT FOR AKZ018.
MARINE...HEAVY FREEZING SPRAY WARNING FOR PKZ012.
         GALE WARNING FOR PKZ011>013-021-022-042-043-051-052.
         SMALL CRAFT ADVISORY FOR PKZ031>036-041-053.

&&

PRB/JWA