Flood in Alaska, September 27 to 30, 2015

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
458 PM AKDT SUN SEP 27 2015

.ANALYSIS AND UPPER LEVELS...
THE MAIN DRIVER OF THE UPPER LEVELS IS AN UPPER LOW CENTERED WAY
UP IN THE ARCTIC NORTH OF SIBERIA AND ANOTHER SHORTWAVE ROUNDING
THE BASE OF THE TROUGH IN THE NORTHERN BERING. THIS SETUP PUTS
MOST OF THE SOUTHERN MAINLAND IN A BROAD...MOIST...SOUTHWESTERLY
FLOW THROUGHOUT MOST OF THE COLUMN. A WARM FRONT IS CURRENTLY
DRAPED OVER THE SOUTHERN MAINLAND BRINGING STEADY RAIN TO MUCH OF
THE AREA. A DISTURBANCE EMBEDDED IN THE FLOW ALOFT IS APPROACHING
THE COOK INLET REGION FROM THE ALASKA PENINSULA...BRINGING ANOTHER
AREA OF RAIN OVER SOUTHCENTRAL ALASKA. THE BIGGER STORY WITH THE
SYNOPTIC PATTERN IS THE DIVISION OF WARM AIR OVER THE NORTHERN
PACIFIC AND SIBERIAN AIR ON THE OTHER SIDE OF THE COLD FRONT. THE
INTERACTION OF THESE TWO AIR MASSES WILL SHAPE THE FORECAST IN
COMING DAYS.

&&

.MODEL DISCUSSION...
GUIDANCE IS IN GOOD GENERAL AGREEMENT WITH THE SYNOPTIC
PICTURE...HOWEVER THE DEVIL IS IN THE DETAILS. THE BIGGEST
DIFFERENCE IN MODELS COME WITH THE INTERACTION OF THE MOIST
TROPICAL AIRMASS IN PLACE OUT AHEAD OF THE APPROACHING COLD FRONT
AND TROUGH. THE LATEST TREND IN GUIDANCE...ESPECIALLY THE GLOBAL
MODELS IS TO CLOSE OFF THE UPPER TROUGH AS IT MOVES INTO THE GULF.
WHETHER THE TROUGH STAYS OPEN OR CLOSES OFF WILL HAVE MAJOR
IMPLICATIONS ON OVERALL SPEED OF THE SYSTEM AND PRECIPITATION
TIMING. THE ONE THING GUIDANCE AGREES ON ACROSS THE BOARD IS THE
PRESENCE AND MOVEMENT OF ARCTIC AIR THROUGH THE ALASKA RANGE AND
INTO SOUTHCENTRAL. THE TIMING OF THE COLD AIR AND AMOUNT OF LIFT
STILL AROUND WILL DETERMINE SNOWFALL CHANCES AROUND THE COOK INLET
REGION.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THIS AFTERNOON INTO THE EVENING HOURS A WARM FRONT ALREADY BRINGING
RAIN TO MUCH OF THE AREA WILL BE REINFORCED BY A DISTURBANCE
ALOFT. ONCE THE INITIAL WARM FRONT MOVES ASHORE AND TO THE EAST
TONIGHT A SHOWERY REGIME WILL STAY IN PLACE THROUGH MONDAY
AFTERNOON. PLENTIFUL MOISTURE WILL HANG AROUND MAKING FOR LOW
CLOUDS AND GENERALLY LIGHT WIND EXCEPT FOR ALONG TURNAGAIN ARM AND
HIGHER ELEVATIONS WHERE GUSTY SOUTHEASTERLIES REMAIN IN THE
FORECAST. MONDAY AFTERNOON THE AREA COMES UNDER THE INFLUENCE OF
AN APPROACHING DISTURBANCE/JET STREAK OUT AHEAD OF THE MAIN UPPER
TROUGH...MEANING MORE STEADY RAINFALL FOR MOST OF THE AREA.

MONDAY EVENING IS WHEN THE FORECAST BECOMES REALLY CHALLENGING.
THE UPPER LEVEL DYNAMICS CROSSING THE ALASKA RANGE WILL BEGIN A
STEADY CYCLOGENSIS OVER THE REGION BEFORE THE SURFACE LOW
PRESSURE REALLY DEEPENS IN PRINCE WILLIAM SOUND. TUESDAY MORNING
IS WHEN THE ARCTIC AIR ARRIVES THROUGH THE ALASKA RANGE AND LOOKS
TO TURN RAIN INTO SNOW. MULTIPLE FACETS MAKE THIS A CHALLENGING
FORECAST INCLUDING UPPER LOW TRACK...TIMING OF COLD AIR/WARM AIR
WRAPPING INTO THE SYSTEM...ARRIVAL OF DRY AIR. WHILE SEEING SOME
SNOW MAKE IT DOWN TO SEA LEVEL IN MOST INLAND AREAS SEEMS MORE AND
MORE LIKELY...AMOUNT OF ANY ACCUMULATION IS STILL DEPENDENT ON ALL
THESE VARIABLES. STAY TUNED TO THE FORECAST.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
STEADY RAIN WILL CONTINUE TONIGHT OVER MOST OF SOUTHWEST ALASKA
AS THE FRONT CONTINUES TO PUSH TROUGH THE REGION. PRECIPITATION
WILL TURN FROM A STEADY RAIN INTO MIXED RAIN AND SNOW SHOWERS AS
THE FRONT MOVES THROUGH OVERNIGHT AS THERE WILL BE AMPLE COLD AIR
ADVECTING IN UNDER NORTHWEST FLOW TO BRING A WINTERY MIX TO THE
AREA. TEMPERATURES ARE WARM ENOUGH ON THE SURFACE THAT LITTLE TO
NO SNOW ACCUMULATION IS EXPECTED EXCEPT IN HIGHER TERRAIN. THE
REGION WILL STAY IN A SHOWERY PATTERN THROUGH TUESDAY AS A SERIES
OF SHORTWAVES EMBEDDED IN THE NORTHWEST FLOW WILL HELP PROVIDE
ENOUGH FORCING FOR PRECIPITATION.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE ACTIVE WEATHER PATTERN WILL CONTINUE OVER THE BERING SEA AND
ALEUTIANS THROUGH TUESDAY NIGHT AS THE FRONT PUSHES THROUGH THE
EASTERN BERING OVERNIGHT AND A RIDGE BUILDS OVER THE WESTERN
BERING SEA. AS THE FLOW BEHIND THE FRONT BECOMES NORTHWESTERLY...
SEASONALLY STRONG COLD AIR ADVECTION WILL SETUP OVER THE EASTERN
HALF OF THE FORECAST AREA. THIS...COUPLED WITH THE LOW DEVELOPING
IN THE GULF OF ALASKA AND THE RIDGE TO THE WEST...WILL ALLOW FOR A
SUSTAINED PERIOD OF LOWER TO MID GALE FORCE WINDS ALONG THE
BERING AND PACIFIC SIDES OF THE ALASKA PENINSULA BEGINNING MONDAY
AFTERNOON AND CONTINUING THROUGH THE PERIOD. ELSEWHERE...WINDS
WILL REMAIN AT OR NEAR SMALL CRAFT ADVISORY LEVELS FOR MOST OF THE
BERING SEA WITH LIGHTER WINDS OVER THE CENTRAL ALEUTIANS UNDER THE
CENTER OF THE RIDGE. WINDS WILL BEGIN TO INCREASE AGAIN OVER THE
WESTERN BERING AGAIN TUESDAY NIGHT AS A STRONG LOW APPROACHES THE
REGION.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
AN ARCTIC TROUGH OVER THE SOUTHWEST MAINLAND MONDAY NIGHT WILL
BEGIN TO SHIFT EASTWARD OVER SOUTHCENTRAL ALASKA BY TUESDAY...WHERE
A COLD UPPER LOW WILL TAKE SHAPE OVER THE NORTHERN GULF. THIS
DYNAMIC LOW WILL BE CONTINUING TO SUPPORT AREAS OF SNOW AND SNOW
SHOWERS INLAND...ESPECIALLY ALONG THE CHUGACH MOUNTAINS INTO THE
COPPER RIVER BASIN. THIS SYSTEM WILL RAPIDLY DEPART
WEDNESDAY...LEAVING BEHIND COLD AIR AND MOUNTAIN SNOW
SHOWERS...ALTHOUGH LOWER ELEVATION SNOW SHOWERS WILL REMAIN ON
WEDNESDAY ACROSS THE COPPER BASIN. A BRIEF REPRIEVE IN WEATHER
WILL LAST ACROSS SOUTHERN ALASKA FROM WEDNESDAY NIGHT THROUGH
THURSDAY AFTERNOON...AS RIDGING OVER THE BERING SEA AND ALEUTIAN
CHAIN SHIFTS EASTWARD OVER THE SOUTHERN MAINLAND BY THURSDAY
MORNING. ANOTHER TROUGH OVER THE FAR WESTERN BERING WILL BEGIN
MOVING A STRONG FRONT ACROSS THE BERING AND INTO SOUTHERN ALASKA
THURSDAY EVENING THROUGH FRIDAY...BRINGING ANOTHER ROUND OF RAIN
AND CLOUDS TO SOUTHERN ALASKA.

MODELS BEGIN TO STRUGGLE MUCH MORE WITH THE OVERALL SHORTWAVE
PATTERN BY THE TRANSITION INTO NEXT WEEKEND. THEY ARE HAVING A
HARD TIME WITH THE TIMING AND STRENGTH OF ANOTHER SYSTEM NEAR THE
SOUTHERN KAMCHATKA PENINSULA...AND ITS ASSOCIATED FRONT TRACKING
INTO THE WESTERN ALEUTIANS/BERING BY THE END OF THE WEEK/EARLY
WEEKEND. HOWEVER...THE FLOW LOOKS TO REMAIN PROGRESSIVE WITH
CHANCES FOR PRECIPITATION LASTING THROUGH THE WEEKEND....AND
TEMPERATURES MODERATING BACK TO NEAR NORMAL BY THE END OF THE
WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORM 130 131.
         GALE 119 132 150 155 160 165 172.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ML
SOUTHCENTRAL ALASKA...ML
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RF
LONG TERM...JA