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000
FXAK68 PAFC 182107
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
105 PM AKDT SAT MAY 18 2013

.ANALYSIS AND UPPER LEVELS...A NEGATIVELY TILTED HIGH AMPLITUDE
UPPER LEVEL TROUGH EXTENDS FROM AN ANCHOR LOW OVER THE CHUKCHI
SEA SOUTHEASTWARD TO SOUTHCENTRAL ALASKA AND THE GULF OF ALASKA.
A STRONG VORT-MAX MOVING INTO THE BOTTOM OF THE TROUGH IS LEADING
TO FORMATION OF AN UPPER LOW CENTER OVER THE NORTHWEST GULF. THIS
IN TURN HAS SLOWED DOWN THE NORTHEASTWARD PROGRESSION OF THE TROUGH.
A WAVE MOVING UP THE FRONT SIDE OF THE TROUGH IS PRODUCING THE BULK
OF THE VERTICAL MOTION AS INDICATED BY THE COLD CLOUD TOPS JUST OUT
AHEAD OF IT. THIS CORRESPONDS WITH THE AREA OF HEAVIEST PRECIPITATION
WHICH IS NOW OVER THE COPPER RIVER BASIN. THERE IS STILL SOME WEAK
LIFT BEHIND THIS WAVE...PRIMARILY LOW LEVEL LIFT INDUCED BY COLD
ADVECTION. THIS IS CAUSING AREAS OF LIGHT SNOW TO LINGER FROM THE
WESTERN KENAI TO ANCHORAGE AND THE MAT-SU. STRONG GUSTY WINDS ARE
BEING OBSERVED BEHIND THE ARCTIC FRONT PRIMARILY ALONG THE ALASKA
PENINSULA AND THE  WESTERN GULF (INCLUDING KODIAK ISLAND).

LOOKING UPSTREAM...A LARGE MATURE LOW OVER THE NORTHWEST PACIFIC
IS TRACKING SLOWLY TOWARD THE ALEUTIANS WITH A RIDGE BEGINNING TO
BUILD OUT AHEAD OF IT.

.MODEL DISCUSSION...MODELS ARE ALL IN GENERAL AGREEMENT WITH
EVOLUTION OF MAJOR FEATURES THROUGH THE NEXT THREE DAYS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE UPPER WAVE MOVING THROUGH THE COPPER RIVER
BASIN WILL QUICKLY EXIT TO THE NORTH LATER THIS AFTERNOON BRINGING
AN END TO THE HEAVIEST PRECIPITATION. THE FORMATION OF AN UPPER LOW
OVER THE NORTHWEST GULF IS CAUSING UPPER LEVEL VERTICAL MOTION TO
BECOME WEAK AND DIFFUSE ACROSS THE REST OF SOUTHCENTRAL. EXPECT THIS
TO CONTINUE AS THE LOW BECOMES MORE DEFINED THIS AFTERNOON AND EXITS
TO THE EAST-NORTHEAST. THE MAIN TROUGH AXIS WHICH IS LAGGING BACK
ALONG THE ALASKA RANGE WILL FINALLY BE ABLE TO CROSS THE REGION
OVERNIGHT THROUGH SUNDAY MORNING...HELPING TO KICK ANY REMAINING
PRECIP EASTWARD AND CLEAR OUT SKIES ACROSS MOST OF THE REGION.

WITH AN ANOMALOUSLY COLD AIR MASS IN PLACE RECORD OR NEAR RECORD
TEMPERATURES ARE POSSIBLE TONIGHT...ESPECIALLY ACROSS WESTERN
PORTIONS OF SOUTHCENTRAL WHERE SKIES WILL LIKELY CLEAR OUT DURING
THE EVENING. TEMPERATURES ON SUNDAY AND SUNDAY NIGHT WILL REMAIN ON
THE COOL SIDE AS THE AIR MASS VERY SLOWLY MODERATES. HIGH PRESSURE
WILL BUILD OVER THE REGION SUNDAY AND THEN HOLD IN PLACE THROUGH
THE EARLY PART OF NEXT WEEK. NOT ONLY WILL THIS PROVIDE A PERIOD
OF DRY WEATHER...BUT TEMPERATURES WILL MODERATE TO NEAR NORMAL.

SOUTHWEST ALASKA...THE KUSKOKWIM DELTA WILL REMAIN MOSTLY CLOUDY
WITH SCATTERED SNOW SHOWERS THROUGH SUNDAY MORNING AS WEAK UPPER
WAVES MOVING OVER TOP OF THE BUILDING BERING SEA RIDGE PASSES
THROUGH. THE REST OF SOUTHWEST ALASKA WILL REMAIN DRY AND COLD
THROUGH THE REMAINDER OF THE WEEKEND. THE UPPER RIDGE WILL BUILD
OVERHEAD SUNDAY AND THEN SLIDE EASTWARD MONDAY. THIS WILL LEAD TO
A SIGNIFICANT PATTERN CHANGE DURING THE EARLY TO MIDDLE OF NEXT
WEEK AS SOUTHERLY FLOW BRINGS MUCH WARMER TEMPERATURES AND RAIN
TO MOST OF THE REGION.

BERING SEA/ALEUTIANS...THE BERING SEA AND ALEUTIANS ARE QUIET UNDER
THE INFLUENCE OF HIGH PRESSURE. THAT WILL CHANGE TONIGHT THROUGH
SUNDAY AS A VERY LARGE LOW OVER THE NORTHWEST PACIFIC HEADS FOR
THE BERING SEA. EXPECT MODERATING TEMPERATURES AND PERIODS OF RAIN
AS THIS SYSTEM MOVES IN.

.LONG TERM FORECAST...WITH A HIGH AMPLITUDE RIDGE SETTING UP OVER
MAINLAND ALASKA EARLY NEXT WEEK THE SYSTEM OVER THE BERING WILL
BE FORCED NORTHWARD...KEEPING RAIN CONFINED TO THE BERING SEA AND
SOUTHWEST ALASKA. SOUTHCENTRAL WILL STEADILY GET WARMER UNDERNEATH
THE RIDGE...WITH TEMPERATURES POSSIBLY MOVING TO ABOVE NORMAL BY
THE END OF THE WEEK. MODELS INDICATE ANOTHER SYSTEM MOVING UP FROM
THE PACIFIC INTO THE BERING BY NEXT WEEKEND...BUT THERE IS A PRETTY
LARGE SPREAD IN THE TRACK OF THIS SYSTEM SO CONFIDENCE IS VERY LOW.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING 130 170 171 172 175 176 179.
FIRE WEATHER...NONE.

SEB MAY 13