Heavy Snow in Alaska, February 19, 2009

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
500 AM AKST THU FEB 19 2009

.ANALYSIS AND UPPER LEVELS...
IN THE UPPER LEVELS...A WEAKENING UPPER LOW OVER EASTERN SIBERIA IS
ACTING AS A PIVOT POINT FOR VARIOUS WAVES MOVING ACROSS THE BERING
SEA. A DIGGING TROUGH EXTENDS FROM THIS VORTEX DOWN THROUGH BRISTOL
BAY AND ACROSS THE ALASKA PENINSULA. THIS TROUGH IS THE MAIN FOCUS
ACROSS THE MAINLAND OVER THE NEXT SEVERAL DAYS. ANOTHER WAVE IS
DEVELOPING OVER THE WESTERN BERING...AND WILL SERVE AS THE FOCUS FOR
THE NEXT SYSTEM MOVING INTO THE AREA OVER THE WEEKEND. ELSEWHERE...A
RIDGE OVER THE EASTERN MAINLAND IS MOVING INTO WESTERN CANADA AND A
TRANSITORY RIDGE IS BUILDING OVER THE CENTRAL AND EASTERN ALEUTIANS.

AT THE SURFACE...A 994 MB LOW OVER THE NORTHERN BERING CONTINUES TO
FILL IN WHILE MOVING NORTHEAST. A STRONG WEATHER FRONT EXTENDING FROM
THIS LOW STRETCHES FROM NOME SOUTHWARD THROUGH THE KUSKOKWIM VALLEY
AND ALASKA PENINSULA. A 1033 MB HIGH OVER THE NORTH PACIFIC CONTINUES
TO BUILD NORTHWARD TOWARD THE CENTRAL AND EASTERN ALEUTIANS.

.MODEL DISCUSSION...
MODELS ONCE AGAIN INITIALIZED WELL AND ARE IN GOOD AGREEMENT THROUGH
12Z SATURDAY. MODELS CONTINUE TO SLOW DOWN THE PROGRESSION OF THE
FRONT ACROSS SOUTHCENTRAL TODAY...WITH THE UPPER TROUGH NOW PROGGED
TO SWEEP THROUGH THE AREA FRIDAY MORNING. ALL MODELS ALSO AGREE ON A
WEAK SURFACE LOW DEVELOPING OVER MONTAGUE ISLAND THURSDAY NIGHT
BEFORE MOVING SOUTHEAST TOWARD THE PANHANDLE. THE MORNING FORECAST
PACKAGE IS BASED ON THE GFS/NAM SOLUTIONS... AND PRIMARILY FOCUSED ON
THE NEXT 36 TO 48 HOURS WITH LITTLE CHANGE TO THE EXISTING FORECAST
THEREAFTER.

.SHORT TERM FORECAST...
.SOUTHCENTRAL...
PRECIPITATION ASSOCIATED WITH THE FRONT IS OVERSPREADING SOUTHCENTRAL
THIS MORNING. TEMPERATURES ACROSS THE AREA ARE COOLER THAN ORIGINALLY
EXPECTED...WITH THIS MORNINGS 12Z ANCHORAGE SOUNDING REVEALING A SUB
FREEZING ISOTHERMAL LAYER UP TO 3000 METERS. WITH THE WARMEST AIR YET
TO MOVE INTO THE REGION...STILL THINK RAIN WILL OCCASIONALLY MIX IN
WITH SNOW OVER THE ANCHORAGE BOWL. THE WINTER STORM WARNING FOR SNOW
IN THE SUSITNA VALLEY HAS ALSO BEEN UPDATED TO INCLUDE TALKEETNA AND
POINTS SOUTH...AS THESE AREAS ARE NOW EXPECTED TO REMAIN ALL SNOW. A
RAIN AND SNOW MIX WILL OCCUR OVER AREAS ALONG THE COAST...WITH VALDEZ
EXPECTED TO REMAIN ALL SNOW. HIGH WINDS WILL CONTINUE OVER HIGHER
TERRAIN AND THROUGH CHANNELED AREAS TODAY...BUT WILL BE DECREASING
THIS AFTERNOON AS A WEAK LOW FORMS IN MONTAGUE ISLAND. PRECIPITATION
WILL BEGIN CLEARING OUT OF SOUTHCENTRAL TONIGHT...AS AN UPPER TROUGH
SWINGS THROUGH AND COLD AIR ADVECTION KICKS IN. OFFSHORE WINDS
THROUGH CHANNELED TERRAIN WILL INCREASE AFTER THE TROUGH PASSAGE
THROUGH AREAS LIKE THOMPSON PASS...SEWARD...AND WHITTIER. STRONG
WINDS WILL ALSO BE POSSIBLE ON KODIAK ISLAND...AS COLD AIR ADVECTION
WILL HAVE THE SUPPORT OF A 150 KT JET MOVING OVERHEAD. ASIDE FROM THE
WINDS...TRANQUIL WEATHER WILL SETTLE BACK OVER SOUTHCENTRAL THROUGH
THE FIRST PART OF THE WEEKEND.

.SOUTHWEST...
THE FRONT THAT BROUGHT ALL THE ACTIVE WEATHER TO SOUTHWEST LAST NIGHT
AND YESTERDAY CONTINUES TO MOVE OUT OF THE AREA. COLD AIR ON THE BACK
SIDE OF THE FRONT WILL CHANGE LINGERING PRECIPITATION BACK TO ALL
SNOW THIS MORNING. HEAVIER PRECIP WILL BE SLOWER TO CLEAR THE
KUSKOKWIM VALLEY...WITH THE WINTER WEATHER ADVISORY FOR SNOW AND
FREEZING RAIN STILL IN EFFECT UNTIL 6 AM THIS MORNING. THE
AFOREMENTIONED TROUGH MOVING IN FROM THE BERING WILL SWING THROUGH
SOUTHWEST LATE TONIGHT...HELPING TO CLEAR OUT ANY REMNANT SNOW
SHOWERS. HIGH PRESSURE WILL BRIEFLY BUILD OVER THE AREA FROM THE
SOUTH ON FRIDAY...BEFORE ANOTHER FRONT APPROACHES FROM THE WEST ON
SATURDAY.

.BERING SEA AND ALEUTIANS...
SCATTERED RAIN AND SHOW SHOWERS WILL PERSIST ACROSS THE ISLANDS
TODAY. SOUTHEASTERLY WINDS TO STORM FORCE WILL DEVELOP FRIDAY AS HIGH
PRESSURE BUILDS OVER THE ALASKA PANHANDLE AND A NORTH PACIFIC LOW
APPROACHES THE WESTERN ALEUTIANS. RAIN AND SNOW WILL INCREASE IN
COVERAGE AND INTENSITY ACROSS THE CENTRAL AND WESTERN ALEUTIANS AS
THE LOW CROSSES INTO THE BERING SEA FRIDAY NIGHT.

.LONG TERM FORECAST...
RIDGING IS EXPECTED TO BUILD OVER SOUTHWESTERN AND SOUTHCENTRAL
ALASKA THIS WEEKEND AND INTO THE FIRST PART OF NEXT WEEK. AT THE SAME
TIME...A BROAD TROUGH WILL ENCOMPASS THE ALEUTIANS. THE RESULT WILL
BE FAIRLY BENIGN CONDITIONS ACROSS THE MAINLAND...WITH A SERIES OF
NORTH PACIFIC LOWS AND UNSETTLED WEATHER AFFECTING THE BERING SEA AND
ALEUTIANS THROUGH THE BEGINNING OF NEXT WEEK.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...WINTER STORM WARNING...145
         WINTER WEATHER ADVISORY...151
MARINE...GALE WARNING...120 121 129 132 140 141 170 171 176 179 185
                        351 352 414
         STORM WARNING...130 172 175 411 412 413
FIRE WEATHER...NONE

NRH FEB 09