Small Craft Advisory 648, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
520 AM AKDT TUE AUG 18 2015

.ANALYSIS AND UPPER LEVELS...
CURRENTLY...OVER THE ALASKAN AREA...A LONG WAVE TROUGH EXIST IN
THE BERING SEA WITH A RIDGE OF HIGH PRESSURE OVER THE EASTERN
GULF OF ALASKA INTO THE PANHANDLE. A SECONDARY WAVE JUST EAST OF
THE KAMCHATKA PENINSULA STARTING TO SWING INTO THE TROUGH OVER THE
BERING. A CLOSED LOW IN THE NORTHERN BERING SEA NEAR THE NORTON
SOUND WITH A STRONG VORTICITY MAX IN THE BASE OF THE TROUGH NEAR
THE AKPEN IS POISED TO MOVE EAST TODAY AND TONIGHT. THE TROUGH
OVER THE BERING SEA ALSO HAS A STRONG JET...120KT...ORIENTED FROM
THE BASE OF THE TROUGH NEAR 53N 165W TO THE APEX OF THE RIDGE NEAR
66N 140W.

WITH THE INCREASED STRENGTH IN THE UPPER LEVELS...THE SURFACE
REFLECTION FOR WINDS AND PRECIP WERE INCREASED AS WELL. THE
ORIENTATION AND STRENGTH OF THE TROUGH AND RIDGE OVER THE ALASKA
AREA RESULTED IN TAPPING THE MOISTURE FROM THE REMNANTS OF MOLAVE.
NUMEROUS AREAS OF SOUTHCENTRAL ALASKA EXPERIENCING OVER AN INCH
OF PRECIPITATION IN THE PAST 24 HOURS. DUE TO THE SLOW
PROGRESSION...THE PRECIPITATION RATE SHOULD REMAIN THE SAME
THROUGH TOMORROW MORNING. FURTHER WEST...PRECIPITATION HAS BEEN A
BIT LIGHTER BUT WIDESPREAD.

THE SURFACE LOW WITH ASSOCIATED WITH THE JET IS OVER SOUTHWEST
ALASKA/SHELIKOF STRAIT THIS AFTERNOON. OVER THE GULF AND ADJACENT
WATERS THERE ARE AREAS EXPERIENCING GALE FORCE WINDS. HIGH
PRESSURE BEHIND THE LOW IS BUILDING ACROSS THE SOUTHWEST BERING
SEA.

&&

.MODEL DISCUSSION...
MODELS ARE IN GOOD AGREEMENT THROUGH THE FIRST FEW DAYS OF THE
FORECAST.

AT UPPER LEVELS THE CLOSED LOW OVER NORTHERN BERING SEA WITH ITS
ASSOCIATED TROUGH IS ANTICIPATED TO MOVE INLAND TONIGHT AND SWING
THROUGH THE STATE TUESDAY. A MORE ZONAL PATTERN IS EXPECTED FOR
WEDNESDAY OVER THE MAINLAND AS ANOTHER TROUGH BUILDS INTO THE
BERING SEA ON WEDNESDAY AMPLIFYING A RIDGE OVER THE GULF OF
ALASKA. AS THE TROUGH PROGRESSES ACROSS THE BERING WEDNESDAY IS
WHEN THE DIFFERENCES IN NUMERICAL GUIDANCE STARTS TO SHOW. SEE
THE LONG TERM DISCUSSION.

AS THE TROUGH SWINGS EAST TONIGHT THE TROPICAL MOISTURE TAP MOVES
EAST AS WELL. HOWEVER...AS THE JET STREAKS MOVE THROUGH THE AREA
TONIGHT...THERE COULD BE SOME ENHANCED PERIODS OF PRECIPITATION.
IN THE COLDER NORTHWEST FLOW ON THE BACKSIDE OF THE TROUGH...THE
AIR MASS IS UNSTABLE ACROSS SOUTHWEST AND SOUTHCENTRAL ALASKA.
THE QUESTION IS HOW UNSTABLE CAN IT GET WITH CLOUD COVER AND
COOLER AIR. OVER THE BERING AND ALEUTIANS...HIGH PRESSURE BUILD
IN AT THE SURFACE WITH NORTHWEST FLOW IN THE EASTERN BERING. THIS
COULD PROVIDE SOME SHOWERS ON THE BERING SIDE OF THE ALEUTIAN
CHAIN. AN UPPER LEVEL TROUGH MOVES THROUGH THE BERING TUESDAY AND
OVER THE MAINLAND WEDNESDAY. IT WILL PROVIDE SOME ADDITIONAL LIFT
TO KEEP SHOWER ACTIVITY AROUND WEDNESDAY.

AT THE SURFACE...A LOW NEAR SHELIKOF STRAIT TO SWING THROUGH THE
PRINCE WILLIAM SOUND AREA TONIGHT AND INTO THE YUKON TUESDAY
AFTERNOON. AS THE LOW PASSES AND WITH NORTHWEST FLOW...SOME STRONG
GAP WINDS WILL DEVELOP ON THE WEST SIDE OF THE GULF OF ALASKA AND
THEN INTO PRINCE WILLIAM SOUND WEDNESDAY AS THE SURFACE LOW GETS
ENHANCED BY THE SHORT WAVE TROUGH PASSING THROUGH THE AREA. BY
THURSDAY...A LOW DEVELOPING ALONG THE JET FROM 50N 180 TO THE
RIDGE APEX NEAR 61N 160W WILL BRING RAIN ACTIVITY TO THE CENTRAL
AND WESTERN ALEUTIANS. THE UPPER TROUGH OVER THE WESTERN BERING AT
THIS TIME WILL ALSO BRING A LOW SKIRTING THROUGH THE NORTHERN
BERING SEA.

&&

.FIRE WEATHER...
WETTING RAINS WILL CONTINUE OVER SOUTHCENTRAL THROUGH TONIGHT INTO
TOMORROW MORNING. RAIN ACTIVITY WILL DECREASE FROM WEST TO EAST
ACROSS SOUTHWEST AND SOUTHCENTRAL ALASKA TOMORROW.
HOWEVER...HUMIDITY LEVELS WILL BE REMAIN RATHER HIGH THROUGH
THURSDAY. SOME GAP WINDS ALONG THE COAST OF THE GULF OF ALASKA AND
THE AKPEN COULD POSE THREATS TO ANY FIRE ACTIVITY CURRENT OR
PLANNED.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (THROUGH WED NIGHT)...
RAIN MOVES EAST ACROSS SOUTHCENTRAL THIS MORNING. SUBSIDENCE WITH
WEAK RIDGING ON THE BACKSIDE OF THE EASTWARD MOVING WAVE WILL
CONTINUE/GIVE RISE TO STRATUS AND PATCHY FOG ACROSS THE COOK INLET
REGION THIS MORNING. A TRAILING UPPER TROUGH MOVING IN FROM THE
WEST THIS AFTERNOON WILL BRING MORE SHOWERY CONDITIONS FOR AREAS
WEST OF THE COPPER RIVER BASIN. IN FACT STRONG UPPER
DYNAMICS(STRONG VORTICITY MAX/COOLING ALOFT) IN COMBINATION WITH
FAVORABLE STABILITY INDICES WILL INCREASE CHANCES OF THUNDERSTORMS
ACROSS THE SUSITNA VALLEY...MATANUSKA VALLEY...ANCHORAGE
BOWL...AND INTERIOR KENAI PEN THIS AFTERNOON. THUNDERSTORM
POTENTIAL IS ALSO POSSIBLE ALONG THE NORTH GULF COAST AND
TALKEETNA MOUNTAINS OF THE COPPER RIVER BASIN THIS EVENING. MOST
SHOWER ACTIVITY SHOULD DIMINISH OVERNIGHT WITH WEAK UPPER
RIDGING...CHANCES OF SHOWERS INCREASE SOME OVER INLAND AREAS WED
AS A WEAK UPPER TROUGH MOVES IN FROM THE WEST. WEST TO NORTH
OUTFLOW WINDS ARE EXPECTED TO DEVELOP OVER THE EASTERN KENAI PEN
TODAY AS PRESSURE GRADIENTS INCREASE WITH A SURFACE LOW MOVING
INTO THE NORTHERN GULF. OUTFLOW WINDS WILL BECOME MORE WIDESPREAD
ACROSS THE NORTH GULF COAST WED NIGHT.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
RAINFALL WILL GRADUALLY TAPER OFF THROUGH WEDNESDAY AS THE AXIS OF
THE UPPER TROUGH HAS MOVED EAST INTO SOUTHCENTRAL ALASKA AND
MULTIPLE WEAK SHORTWAVES WILL MOVE THROUGH THE AREA IN THE
WESTERLY FLOW. MOST OF THE PRECIPITATION WILL BE IN THE FORM OF
SHOWERS WITH THE WESTERN SLOPES OF THE ALASKA/ALEUTIAN RANGES AND
THE KILBUCK MOUNTAINS BEING THE FAVORED AREAS FOR PRECIPITATION.
ONSHORE FLOW IS EXPECTED TO CONTINUE THROUGH THE PERIOD...KEEPING
MARINE STATUS AND FOG IN THE FORECAST FOR COASTAL AREAS.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
CONDITIONS WILL BE QUIETING DOWN OVER THE REGION TODAY AS HIGH
PRESSURE BUILDS INTO THE SOUTHERN BERING SEA. PRECIPITATION IS
EXPECTED MAINLY FROM DUTCH HARBOR EASTWARD TODAY AS THE NORTHWEST
FLOW ALONG THE HIGHER TERRAIN WILL PRODUCE ISOLATED SHOWERS.
WINDS AND SEAS WILL INCREASE OVER THE WESTERN BERING SEA ON
WEDNESDAY AS A LOW MOVING OUT OF THE KAMCHATKA PENINSULA WILL
DEEPEN AND MOVE INTO THE NORTH CENTRAL BERING SEA BY TOMORROW
NIGHT. WINDS OVER THE BERING SEA ARE EXPECTED TO REMAIN BELOW GALE
FORCE WITH THIS SYSTEM. A SECOND LOW WILL APPROACH THE EASTERN
ALEUTIANS AND THE ALASKA PENINSULA LATE THURSDAY...BRINGING A
CHANCE OF RAIN TO THE AREA.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THE PATTERN ALOFT STARTS OFF PROGRESSIVE WITH A SHORTWAVE TROUGH
EXITING THE MAINLAND INTO THE GULF ON WEDNESDAY AFTERNOON. THIS
WILL STRENGTHEN AN OUTFLOW WIND PATTERN ACROSS THE NORTH GULF
COAST WITH DECREASING CLOUDS AND PRECIPITATION. THE WINDS WILL BE
SHORT LIVED AS A TROUGH FROM THE BERING SEA WILL DROP SOUTH AND
TAP SOME BAROCLINIC ENERGY OVER THE NORTHERN PACIFIC RESULTING IN
A STRONGER DEVELOPING SURFACE LOW PRESSURE SYSTEM MOVING INTO THE
GULF OF ALASKA. IT IS STILL A LITTLE SOON FOR SPECIFICS BUT GIVEN
THE ANTECEDENT CONDITIONS THE SCENARIO SEEMS MORE THAN PLAUSIBLE
WITH THE BAROCLINIC ZONE ALREADY IN PLACE FROM THE NORTH PACIFIC
TO THE GULF OF ALASKA. THE PATTERN THEN TRENDS TO A HIGHER
AMPLITUDE AS SEVERAL UPPER LEVEL TROUGHS SHAPE THE PATTERN...ONE
NEAR KAMCHATKA WHICH WILL PUMP UP AN UPPER LEVEL RIDGE OVER THE
BERING SEA AND ANOTHER DROPPING IN FROM THE CHUKCHI...EVENTUALLY
SETTING UP SHOP IN THE GULF.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES 120 130 131 351 352.
FIRE WEATHER...NONE.

&&

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

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE ANCHORAGE AK
304 PM AKDT TUE AUG 18 2015

AKZ111-190039-
MATANUSKA VALLEY-
INCLUDING THE CITIES OF...PALMER...WASILLA...SUTTON...CHICKALOON
304 PM AKDT TUE AUG 18 2015

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
337 PM AKDT TUE AUG 18 2015

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

0330 PM     HAIL             HOUSTON                 61.62N 149.78W
08/18/2015  E0.25 INCH                          AK   PUBLIC

            PEA SIZE HAIL REPORTED

&&

RDUELL