FXAK68 PAFC 261502
AFDAFC
AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
500 AM AKST SAT DEC 26 2009
.ANALYSIS AND UPPER LEVELS...
CYCLONIC FLOW CONTINUES TO COVER MUCH OF THE GULF...NORTH
PACIFIC...AND BERING SEA UNDER THE INFLUENCE OF AN UPPER LEVEL LOW
CENTERED 340 MILES SOUTHEAST OF CHIGNIK. TO THE EAST...A BLOCKING
RIDGE STRETCHES ALONG THE WESTERN COAST OF CANADA INTO THE EASTERN
BORDER OF ALASKA. A SEPARATE HIGH PRESSURE IS LOCATED TO THE
NORTHWEST OF MAINLAND ALASKA. AT THE SURFACE...A 980 MB LOW PRESSURE
IS STACKED UNDER THE MAIN UPPER LEVEL LOW WITH A WEAKER LOW JUST TO
THE NORTHWEST. SEVERAL DISTURBANCES ARE MOVING ACROSS THE AREA OF
RESPONSIBILITY IN THIS CYCLONIC FLOW ALOFT AND THE TIMING OF THESE
SOMEWHAT SUBTLE FEATURES REPRESENTS THE GREATEST CHALLENGE IN THE
FORECAST IN THE SHORT TERM.
.MODEL DISCUSSION...
MODELS ARE STILL FAIRLY CONSISTENT IN CAPTURING THE CURRENT PATTERN.
IN THE SHORT TERM MODELS ARE CONSISTENT IN THE PLACEMENT AND
INTENSITY OF THE PRIMARY LOW PRESSURE SYSTEMS. HOWEVER...MODELS ARE
STILL HAVING DIFFICULTY CAPTURING THE SUBTLE QUICK-MOVING
DISTURBANCES AND ASSOCIATED PRECIPITATION. IN THE LONG TERM THE
MODELS BEGIN TO DIVERGE SIGNIFICANTLY AFTER 48 HOURS WITH THE GFS AS
THE MOST NOTEWORTHY OUTLIER...AS IT DEVELOPS A CONCENTRIC LOW SOUTH
OF THE ALASKA PENINSULA BY MONDAY...INSTEAD OF THE PREFERRED
DOUBLE-BARRELED LOW THAT IS DEPICTED IN THE UKMET AND EUROPEAN FROM
MONDAY INTO TUESDAY. THE ECMWF IS THE PREFERRED SOLUTION FROM MONDAY
ONWARD OWING TO ITS HIGHER RESOLUTION AND MORE REALISTIC AND
CONSISTENT DEPICTION OF THIS FEATURE...WHILE ONLY MINOR TWEAKS WERE
MADE IN THE SHORT TERM WITH EXISTING GRIDS.
.SHORT TERM FORECAST...
.SOUTHCENTRAL...A SERIES OF FAIRLY SUBTLE DISTURBANCES ARE MOVING
ACROSS THE REGION THROUGH THE NEXT COUPLE OF DAYS. PERSISTENT ONSHORE
FLOW IN ADDITION TO THESE DISTURBANCES WILL RESULT IN PERIODS OF RAIN
ALONG THE SOUTHCENTRAL GULF COAST. MODELS INDICATE THAT THERE WILL BE
ENOUGH MOISTURE FOR LIGHT PRECIPITATION EVEN IN DOWNSLOPE REGIONS AS
UPWARD VERTICAL MOTION ASSOCIATED WITH THESE DISTURBANCES PASSES BY.
LIKE YESTERDAY...THE 12Z SOUNDING IN ANCHORAGE INDICATES A DEEP LAYER
OF ABOVE FREEZING TEMPERATURES JUST ABOVE THE SURFACE WHILE MOISTURE
HAS INCREASED. AS A RESULT...ANY PRECIPITATION THAT FALLS IN THE
ANCHORAGE BOWL WILL LIKELY FALL AS LIQUID OR A MIXED BAG OF
PRECIPITATION. THIS WARMER AIR JUST ABOVE THE SURFACE WILL ALSO
RESULT IN MIXED PRECIPITATION IN THE SUSITNA VALLEY AND WESTERN
PORTIONS OF THE KENAI PENINSULA. MEANWHILE SNOW SHOWERS WILL FALL
ACROSS THE COPPER RIVER BASIN AS TEMPERATURES REMAIN BELOW FREEZING
THROUGH THE LOWER ATMOSPHERE. OVER THE NEXT FEW DAYS THE UPPER LEVEL
BLOCKING RIDGE WILL PROGRESS FARTHER EASTWARD ALLOWING A SURFACE HIGH
PRESSURE TO BUILD IN FROM THE NORTH. AS A RESULT THE SOUTHCENTRAL
REGION WILL BRIEFLY DRY OUT NEXT WEEK.
.SOUTHWEST...A SERIES OF WEAK SHORTWAVES WILL MOVE ACROSS SOUTHWEST
ALASKA OVER THE NEXT COUPLE OF DAYS. THESE FEATURES WILL ENHANCE
PRECIPITATION ALONG THE UPSLOPE AREAS AND BRING A CHANCE OF
PRECIPITATION TO OTHER AREAS AS THEY PASS THROUGH. ANY PRECIPITATION
THAT IS ABLE TO FALL OVER SHELTERED VALLEYS WILL HAVE THE BEST CHANCE
OF SEEING POCKETS OF FREEZING RAIN. IN ADDITION...WITH STRONG
PERSISTENT MOIST ADVECTION MOS GUIDANCE VALUES CONTINUE TO
UNDERESTIMATE THE LEVEL OF WARM AIR ADVECTION FROM THE SOUTH...SO
GENERALLY BUMPED TEMPERATURES UP A FEW DEGREES REGIONWIDE OVER MOS
GUIDANCE AND LEANED MORE TOWARDS RAW MODEL TEMPERATURES IN THE GRIDS.
HIGH PRESSURE WILL SLOWLY BUILD OVER THE MAINLAND OVER THE NEXT FEW
DAYS WHICH WILL LIMIT PRECIPITATION. THE CHANCE FOR PRECIPITATION AND
STRONGER WINDS WILL RETURN TO THE ALASKA PENINSULA SUNDAY NIGHT AS A
MORE FORMIDABLE LOW APPROACHES FROM THE NORTH PACIFIC.
.ALEUTIANS/BERING SEA...A DISTURBANCE APPROACHING THE PRIBILOFS WILL
BRING LIGHT SNOW THIS MORNING...WITH PERHAPS A MIX OR CHANGE TO RAIN
IN THE AFTERNOON. OTHERWISE...THE WEATHER PATTERN REMAINS MUCH
QUIETER THAN IN THE PAST FEW WEEKS AS BROAD CYCLONIC FLOW ACROSS THE
REGION PERSISTS. LIGHT PRECIPITATION WILL OCCUR ALONG THE CENTRAL AND
EASTERN ALEUTIANS AS A WEAK LOW RETROGRADES WEST. A MORE POTENT LOW
BRINGS THE CHANCE FOR STRONGER WINDS AND STEADIER PRECIPITATION TO
THE EASTERN ALEUTIANS BY SUNDAY NIGHT.
.LONG TERM FORECAST...MODELS ARE TRENDING TOWARDS A BLOCKING RIDGE TO
PERSIST THROUGH LATE NEXT WEEK. AS A RESULT...THE LONG WAVE TROUGH
PATTERN WILL CONTINUE TO COVER MUCH OF THE NORTH PACIFIC AND BERING
WHICH WILL RESULT IN THE POSSIBILITY OF PRECIPITATION ACROSS MUCH OF
THE COASTAL REGION...WITH DRIER WEATHER IN THE INTERIOR...ESPECIALLY
TOWARDS COPPER RIVER BASIN. SOUTHERLY FLOW SHOULD KEEP TEMPERATURES
NEAR NORMAL TO ABOVE NORMAL THROUGH THE PERIOD.
.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...GALE...130 132 136 137 155
HEAVY FREEZING SPRAY WARNING...185
ELN/CC DEC 09
