Small Craft Advisory 455, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
1230 PM AKDT MON JUN 21 2010

.ANALYSIS AND UPPER LEVELS...A DEEP UPPER LEVEL CLOSED LOW ALONG
THE WESTERN ALEUTIANS IS VERY SLOWLY TRACKING TO THE EAST. THE
SURFACE REFLECTION OF THIS IS A 980S LOW WHICH IS PRODUCING
WIDESPREAD WIND AND RAIN ACROSS THE WESTERN AND CENTRAL BERING.
THE OCCLUDED FRONT LIES FROM THE CENTRAL BERING SOUTHEASTWARD
ACROSS THE EASTERN ALEUTIANS (WEST OF DUTCH HARBOR). THE SOUTHERN
PORTION OF THE FRONT IS MAKING VERY SLOW EASTWARD PROGRESS
WHILE THE SECTION OF FRONT OVER THE BERING REMAINS NEARLY
STATIONARY...SO THE FRONT IS BEGINNING TO STRETCH APART AND WEAKEN.
JUST DOWNSTREAM IS A SHORT-WAVE UPPER LEVEL RIDGE WHICH IS
PRODUCING CLEAR SKIES FROM THE EASTERN BERING TO WESTERN
SOUTHCENTRAL. AN UPPER LEVEL LOW OVER THE COPPER RIVER BASIN
IS PRODUCING AREAS OF RAIN FROM AROUND CORDOVA NORTHWARD ACROSS
THE WRANGELL MOUNTAINS AND UP ALONG THE ALASKA RANGE. A SHORT-WAVE
TROUGH EXTENDS FROM THIS LOW ACROSS THE NORTHERN SUSITNA VALLEY
INTO INTERIOR ALASKA AND IS ROTATING SOUTHWARD. SKIES REMAIN
MOSTLY CLEAR OUT AHEAD OF THIS SHORT-WAVE...BUT AS THE SURFACE
HEATS UP THIS AFTERNOON EXPECT TO QUICKLY BREAK THROUGH ANY CAP
IN PLACE AND DEVELOP CONVECTION.

.MODEL DISCUSSION...MODELS REMAIN IN GOOD AGREEMENT WITH SYNOPTIC
FEATURES. THE TRACK OF THE UPPER LOW OVER THE COPPER BASIN HAS
CHANGED SIGNIFICANTLY SINCE YESTERDAY...BUT ALL SOLUTIONS AGREE
ON THIS AND FORECASTS WILL BE UPDATED TO REFLECT THIS NEW TRACK.

.SHORT TERM FORECAST...

SOUTHERN MAINLAND ALASKA...VISIBLE SATELLITE IMAGERY SHOWS CUMULUS
BEGINNING TO POP UP OVER THE MOUNTAINS OF WESTERN SOUTHCENTRAL
ALASKA. AS THE SURFACE CONTINUES TO HEAT THIS AFTERNOON EXPECT
DEVELOPMENT OVER THE SUSITNA VALLEY AS WELL. SPOKES OF UPPER LEVEL
VORTICITY ALONG WITH COLDER AIR ALOFT WILL COMBINE WITH SURFACE
INSTABILITY TO TRIGGER SHOWERS AND A FEW THUNDERSTORMS...ESPECIALLY
IN THE SUSITNA VALLEY. STORM MOTION WILL TAKE THE CONVECTION
SOUTHWARD TOWARD COOK INLET. AS THE THERMAL TROUGH STRENGTHENS OVER
INTERIOR SOUTHCENTRAL THIS AFTERNOON THE UP INLET FLOW WILL INCREASE.
SO THERE COULD BE SOME INTERACTION BETWEEN THE SEA BREEZE FRONT
AND THE CONVECTION. OF COURSE FLOW OFF THE INLET WILL ALSO TRANSPORT
COOLER MORE STABLE AIR INLAND...SO DON'T EXPECT ANY THUNDERSTORMS
THAT FORM TO SURVIVE VERY LONG AS THEY TRACK TOWARD THE INLET.
MOST OF SOUTHWEST ALASKA IS RIGHT UNDER THE UPPER RIDGE AND
CONVECTION WILL REMAIN CAPPED. CONVECTIVE ACTIVITY WILL PEAK
DURING THE EVENING HOURS AND THEN SLOWLY DISSIPATE OVERNIGHT AS
THE UPPER LOW DROPS INTO THE GULF. EVEN WITH THE DEPARTURE OF THE
UPPER LOW EXPECT AN INCREASE IN CONVECTION ACROSS THE SOUTHERN
MAINLAND AS LOW LEVELS CONTINUE TO GET A LITTLE WARMER. THE RIDGE
OVER SOUTHWEST ALASKA WILL BE PUSHED OFF TO THE SOUTH AS IT IS
SQUEEZED FROM BOTH THE EAST AND THE WEST...WHICH WILL ALLOW
CONVECTION TO DEVELOP...ESPECIALLY OVER INLAND LOCATIONS. ON
WEDNESDAY THE FLOW WILL BEGIN TO SHIFT AROUND TO THE SOUTH AHEAD
OF THE ALEUTIAN LOW SO CONVECTION WILL CONFINED MAINLY TO INLAND
AREAS. SKIES MAY TEMPORARILY CLOUD UP DURING ACTIVE CONVECTION...BUT
EXPECT TO SEE SOME SUN EACH DAY AND FOR TEMPERATURES TO REMAIN WARM.

BERING SEA/ALEUTIANS...THE ALEUTIAN SYSTEM WILL BE SLOW TO MAKE
EASTWARD PROGRESS...PASSING JUST EAST OF ATKA WEDNESDAY AFTERNOON.
THE OCCLUDED FRONT WILL COMPLETELY DISSIPATE AS IT REACHES THE
SOUTHWEST COAST TUESDAY...SO THE BULK OF WIND AND RAIN WILL BE
IN THE VICINITY OF THE LOW CENTER.

.LONG TERM FORECAST...THE ALEUTIAN LOW WILL TRACK INTO THE GULF
BY THE END OF THE WEEK WITH A RIDGE BUILDING IN BEHIND IT OVER
THE BERING AND ALEUTIANS. EXPECT AN INCREASE IN CLOUD COVER AND
SHOWERS OVER THE SOUTHERN MAINLAND THURSDAY AND FRIDAY AS THE LOW
PASSES TO THE SOUTH...BUT WITH IMPROVING CONDITIONS AS WE HEAD
INTO NEXT WEEKEND. ONE FEATURE THAT MAY THROW A WRENCH INTO NEXT
WEEKEND IS AN UPPER LOW THAT MEDIUM RANGE MODELS SHOW DROPPING
SOUTHWARD OUT OF THE CANADIAN ARCTIC AND DOWN ACROSS WESTERN ALASKA.
THIS IS THE ONE FEATURE IN THE EXTENDED FOR WHICH THE MODELS SHOW
POOR RUN-TO-RUN CONTINUITY. THEREFORE...FORECAST CONFIDENCE IS
LOW FOR THAT TIME FRAME OVER MAINLAND ALASKA.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING...155 165 170 179.
FIRE WEATHER...NONE.

SEB JUN 10