Gale Warning 590, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
538 AM AKDT TUE SEP 29 2015

.ANALYSIS AND UPPER LEVELS...

THE EARLY MORNING WEATHER MAP IS DOMINATED BY A RAPIDLY DEVELOPING
AREA OF LOW PRESSURE SOUTHEAST OF KODIAK ISLAND AS WELL AS A
DEEPENING UPPER LEVEL TROUGH EXTENDING FROM THE ARCTIC TO THE
ALASKA PENINSULA. WATER VAPOR IMAGERY DEPICTS A POTENT SHORTWAVE
ROUNDING THE BASE OF THE TROUGH AND WITH A WEAK JET STREAK
ACCOMPANYING THIS SHORTWAVE. THIS HAS BEGUN TO INTERACT WITH THE
NEWLY DEVELOPING SURFACE LOW AND SHOULD BE A KEY FACTOR IN THE
RAPID DEVELOPMENT OBSERVED DURING THE DAY. A VERY IMPRESSIVE
BAROCLINIC ZONE HAS SET UP ACROSS MUCH OF SOUTHCENTRAL ALASKA
AHEAD OF THE TROUGH AND DEVELOPING SURFACE LOW...WITH RAINFALL
COMPLETELY OVERSPREADING THE AREA. RAINFALL RATES HAVE RANGED
ANYWHERE FROM 0.05 TO 0.15 INCHES PER HOUR ACROSS MUCH OF
SOUTHCENTRAL ALASKA BUT PRECIPITATION RATES HAVE BEEN GRADUALLY
DIMINISHING AS LOW-LEVEL FLOW SHIFTS FROM THE SOUTHERLY MOISTURE
STREAM TO A MORE EASTERLY/NORTHEASTERLY DRIER STREAM.

LOOKING TOWARD SOUTHWEST ALASKA...PRECIPITATION HAS BEEN GRADUALLY
TAPERING OFF AS THE TROUGH AXIS CONTINUES TO SLIDE EASTWARD. SOME
LINGERING RAIN SHOWERS HAVE QUICKLY TRANSITIONED TO SNOW SHOWERS
DUE TO SIGNIFICANT COLD AIR ADVECTION BEHIND THE TROUGH. THE
DEGREE OF THE COLD AIR ADVECTION HAS BEEN CAPTURED VERY WELL BY
THE PABE RAOBS OVER THE PAST 24 HOURS AS A STEADY DROP IN THE
FREEZING LEVEL AND -20 DEGREES CELSIUS LINE OVER THE TIME FRAME.
GUSTY NORTHWESTERLY WINDS HAVE ALSO BEEN OBSERVED ON THE BACKSIDE
OF THE TROUGH AS A STRONG PRESSURE GRADIENT DEVELOPS BETWEEN
HIGHER PRESSURE OVER THE BERING SEA AND THE RAPIDLY DEVELOPING LOW
IN THE GULF OF ALASKA.

&&

.MODEL DISCUSSION...

WHILE GLOBAL AND SYNOPTIC MODELS ARE PAINTING THE SAME LARGE
SCALE PICTURE...A LOT OF DISCREPANCIES ON THE SMALL SCALE LOOK TO
HAVE SOME CHALLENGING AND POTENTIALLY SIGNIFICANT IMPACTS ON THE
FORECAST. WITH THE RAPID DEVELOPMENT OF THE LOW SOUTH AND EAST OF
KODIAK TODAY...THE LOW PLACEMENT HAS VARIED QUITE A BIT OVER THE
PAST 24 HOURS. PREVIOUS RUNS HAD THE LOW MOVING INTO PRINCE
WILLIAM SOUND WHILE LATER MODELS SUGGEST A MORE WESTWARD TRACK.
THIS SEEMS TO FIT THE CONCEPTUAL MODEL AS LOWS TEND TO SLOW DOWN
WHILE UNDERGOING RAPID CYCLOGENESIS. THIS IMPACTS THE TIMING AND
STRENGTH OF THE NORTHWESTERLY WINDS ACROSS MUCH OF ALASKA BUT ALSO
IMPACTS THE RATE THAT COLDER AIR ADVECTS INTO SOUTHCENTRAL ALASKA.
THE NAM CONTINUES TO BE AN OUTLIER AS IT IS THE MOST AGGRESSIVE AND
QUICKEST WITH USHERING IN THE COLDER AIR TO SOUTHCENTRAL ALASKA.
THE OTHER MODELS KEEP THE COLD AIR POOLED UP ALONG THE ALEUTIAN
RANGE FOR A FEW HOURS AS IT SLOWLY FILTERS THROUGH THE MOUNTAINS.
THIS SEEMS TO BE THE MOST LIKELY SCENARIO BASED ON MODEL CONTINUITY.
THIS WOULD CHANGE PRECIPITATION (IF IT IS STILL ONGOING) FROM RAIN
TO SNOW LATE THIS AFTERNOON INTO THIS EVENING.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...

HEAVY RAINFALL IN THE COOK INLET REGION NORTHWARD THROUGH THE
SUSITNA VALLEY HAS CAUSED STREAMS AND RIVERS IN THE AREA TO RISE
CONSIDERABLY. THE SUSITNA VALLEY HAS A FLOOD ADVISORY IN EFFECT
THROUGH TUESDAY NIGHT DUE TO SOME MINOR FLOODING IN THE
AREA...ESPECIALLY AROUND MONTANA CREEK. THE ANCHORAGE AREA HAS
ALSO SEEN THE AREA STREAMS COME UP TO ADVISORY LEVELS IN ADDITION
TO LARGE AREAS OF PONDING ON THE ROADWAYS DUE TO THE HEAVY
OVERNIGHT RAINFALL. THE RAINFALL ACROSS THE REGION WILL BE
HEAVIEST DURING THE EARLY MORNING HOURS.

RAIN IS STILL EXPECTED TO MIX WITH SNOW THIS AFTERNOON AROUND THE
COOK INLET THROUGH THE SUSITNA VALLEY REGION AND THEN TURN TO ALL
SNOW LATE THIS AFTERNOON OR EARLY EVENING. THE INFLUX OF WARM AIR
INTO SOUTHCENTRAL OVERNIGHT WILL HELP DELAY THE ONSET OF SNOWFALL.
BY THE TIME IT DOES GET COLD ENOUGH TO SNOW THE HEAVIEST
PRECIPITATION WILL BE NORTHEAST OF THE AREA. THEREFORE SEA LEVEL
AREAS AROUND SOUTHCENTRAL ARE EXPECTED TO ONLY SEE AROUND AN INCH
OR SO OF SNOW WITH HIGHER ACCUMULATIONS AT HIGHER ELEVATIONS.
THERE IS ALWAYS THE POSSIBILITY IN A SYSTEM SUCH AS THIS ONE THAT
THE COLD AIR DOES NOT MOVE IN UNTIL ALMOST ALL OF THE
PRECIPITATION IS OUT OF THE AREA LEAVING ONLY A DUSTING OF
SNOWFALL. FORECASTS HAVE BEEN LEANING HEAVILY TOWARD LIMITED
PRECIPITATION AFTER THE COLD AIR ARRIVES. THE BIGGEST THREAT OF A
SERIOUS FORECAST ERROR IS IF THE COLD AIR ARRIVES SOONER AND IS
STRONGER THAT WHAT HAS BEEN INDICATED BY THE MODELS. COMPARING THE
12Z 850 MB TEMPERATURES FROM THE UPPER AIR SOUNDINGS TO THE MODELS
IT SHOWS THE 06Z NAM AS HAVING A VERY GOOD HANDLE ON THE LOCATION
AND DEPTH OF THE COLD AIR. THIS DOES ADD CONFIDENCE TO THE TIMING
FOR THE CHANGE IN PRECIPITATION FROM RAIN TO SNOW.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...

GUSTY NORTHWESTERLY WINDS WILL DEVELOP THIS MORNING THROUGHOUT THE
SOUTHWEST MAINLAND...WITH THE STRONGEST WINDS EXPECTED ALONG THE
BRISTOL BAY COAST. THIS IS DUE TO THE STRONG PRESSURE GRADIENT
AND COLD AIR ADVECTION BEHIND THE DEEP LOW MOVING INTO THE
GULF...AS WELL AS AN UPPER LEVEL TROUGH MOVING BEHIND THE LOW.
THIS UPPER TROUGH WILL CAUSE WINDS TO KICK UP BY MID MORNING ALONG
THE BRISTOL BAY COAST...WHILE STRONG COLD AIR ADVECTION BEHIND THE
UPPER LEVEL TROUGH WILL ALLOW IT TO CONTINUE THROUGH TONIGHT.
UNDER THIS UPPER LEVEL TROUGH...RAIN AND SNOW WILL CONTINUE TO
FALL FROM NORTHEASTERN BRISTOL BAY INTO THE ALASKA
RANGE...ALLOWING FOR A FEW INCHES OF ADDITIONAL SNOW ACCUMULATION
(MAINLY ALONG THE ALASKA RANGE). RAIN AND SNOW IN THE AREA WILL
BEGIN TO DIMINISH BY THIS EVENING AS THE UPPER LEVEL TROUGH PASSES
INTO THE GULF...THOUGH ISOLATED TO SCATTERED RAIN AND SNOW
SHOWERS AND GUSTY NORTHWESTERLY WINDS WILL PERSIST INTO TONIGHT
WITH CONTINUED COLD AIR ADVECTION.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...

STRONG NORTHWESTERLY WINDS WILL DEVELOP ALONG THE AKPEN THIS
MORNING AS THE UPPER LEVEL TROUGH PASSES AND STRONG COLD AIR
ADVECTION DEVELOPS UNDER STRONG UPPER LEVEL FORCING. THE STRONGEST
WINDS ARE EXPECTED TO THE EAST OF COLD BAY WHERE THE STRONGEST
WINDS ALOFT ARE EXPECTED TO MIX DOWN THIS MORNING THROUGH LATE
TONIGHT. OVER THE NORTHERN BERING A WAVE WILL BRING RAIN AND GALE
FORCE WINDS TO THE PRIBILOF ISLANDS THIS EVENING. THIS WAVE WILL
THEN TRACK TOWARDS THE AKPEN ON MONDAY MORNING...REINVIGORATING
RAIN AND GUSTY WINDS OVER THE AREA INTO THE AFTERNOON. RIDGING
OVER THE WESTERN AND CENTRAL ALEUTIANS WILL KEEP WINDS MUCH
LIGHTER TODAY...HOWEVER WINDS WILL THEN INCREASE LATE TONIGHT OVER
THE WESTERN ALEUTIANS AS THE NEXT FRONTAL SYSTEM APPROACHES. THIS
FRONT WILL BRING RAIN AND STRONG WINDS TO THE WESTERN ALEUTIANS ON
WEDNESDAY...AND THEN SPREAD INTO THE CENTRAL ALEUTIANS ON
WEDNESDAY NIGHT.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...

BY THURSDAY MORNING THE COLD UPPER LEVEL TROUGH WILL BE CENTERED
OVER EASTERN INTERIOR ALASKA AND THE COPPER RIVER BASIN ON ITS WAY
TO SHIFTING EAST INTO THE YUKON TERRITORY. WITH THE ASSOCIATED
LOW LEVEL POOL OF COLD AIR AND A SURFACE RIDGE OVER THE BASIN
EXPECTING TO SEE SOME NORTHERLY GAP WINDS OUT OVER OF THE COPPER
RIVER BASIN AND THROUGH THOMPSON PASS AS WELL AS DIMINISHING
WESTERLIES IN THE WHITTIER AREA. A FAST MOVING BUT HIGH AMPLITUDE
UPPER LEVEL RIDGE WILL PUSH OVER SOUTHWEST ALASKA THURSDAY AND
SOUTHCENTRAL ALASKA THURSDAY NIGHT BRINGING A SHORT DRIER BREAK
WHILE ALSO QUICKLY PUSHING THE UPPER LEVEL TROUGH AND SURFACE
RIDGE OFF TO THE EAST AND OUT OF THE AREA.

A STRONG FRONT BRINGING STORM FORCE SOUTHERLIES WILL CROSS THE
EASTERN BERING AND BRISTOL BAY THURSDAY AND THEN PUSH ONSHORE AND
INTO SOUTHWEST ALASKA THURSDAY EVENING. WHILE THE FRONTAL SYSTEM
WILL WEAKEN SOMEWHAT AS IT MAKES LANDFALL IT WILL STILL BRING
PLENTY OF WIDESPREAD PRECIPITATION TO SOUTHWEST ALASKA THURSDAY
NIGHT AND FRIDAY AND TO SOUTH CENTRAL ALASKA FRIDAY AND SATURDAY.
ADDITIONALLY THIS SYSTEM WILL TIGHTEN THE SOUTHEASTERLY PRESSURE
GRADIENT ACROSS SOUTH CENTRAL ALASKA WITH GAP WINDS BUILDING
THROUGH TURNAGAIN ARM...THE KNIK RIVER VALLEY AND ALONG THE COPPER
RIVER BEGINNING FRIDAY MORNING AND CONTINUING THROUGH SATURDAY.

ON SUNDAY A FLAT TRANSIENT RIDGE WILL CROSS MAINLAND ALASKA BUT
QUICKLY BE FOLLOWED BY THE NEXT SYSTEM SPINNING OFF THE BERING.
PERSISTENT UPPER LEVEL TROUGHING OVER THE BERING WILL PUSH INTO
THE WESTERN GULF EARLY NEXT WEEK WITH A STRONG JET STREAM BETWEEN
45 AND 50N CONTINUING TO BRING ADDITIONAL FRONTAL SYSTEMS IN FROM
THE WEST ACROSS THE BERING AND INTO SOUTHWEST ALASKA AND GULF OF
OF ALASKA. BROAD RIDGING OVER THE WESTERN US AND CANADA WILL TURN
THE SYSTEMS THAT MAKE IT INTO GULF TO THE NORTH AND TEND TO BRING
THEM INLAND ALONG THE GULF COAST.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORM WARNING 130 131 132 137 138 150.
         GALE WARNING 119 121 136 155 160 165 170 172
                      175 176 177 178 180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MC
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
LONG TERM...JER

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1252 PM AKDT TUE SEP 29 2015

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1244 PM     FLOOD            1 SW TRAPPER CREEK      62.23N 150.44W
09/29/2015                                      AK   PUBLIC

            SOCIAL MEDIA REPORT OF WATER HALFWAY UP TIRES ON OIL WELL
            ROAD AT MOOSE CREEK. REPORTED THAT ROAD STARTING TO WASH
            OUT NEAR THE BRIDGE.

&&

RDUELL

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
112 PM AKDT TUE SEP 29 2015

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0111 PM     FLOOD            WILLOW                  61.74N 150.06W
09/29/2015                                      AK   BROADCAST MEDIA

            MEDIA REPORTS HOMES AND ROADS FLOODED IN THE WILLOW AREA.

&&

RDUELL