Small Craft Advisory 515, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
1100 AM AKDT SAT JUN 27 2009

.ANALYSIS AND UPPER LEVELS...
FORECAST ATTENTION IS ON THE POSSIBILITY OF STRONGER CONVECTION IN
THE ANCHORAGE AND MAT-SU VALLEY. 12Z ANCHORAGE SOUNDING SHOWS SURFACE
BASED INVERSION WITH DECENT AMOUNT OF INSTABILITY ABOVE IT. SHOULD
HAVE NO PROBLEM WARMING TO ELIMINATE THIS INVERSION... ALLOWING THIS
INSTABILITY TO BE REALIZED. MODELS INDICATING UP TO 1000 J/KG OF CAPE
AROUND SOUTH CENTRAL ALASKA...INCLUDING ANCHORAGE...AND HAVE REASON
TO BELIEVE THAT THIS WILL BE REALIZED. 12Z ANCHORAGE SOUNDING ALSO
INDICATES LOW FREEZING LEVELS...AROUND 5000 FEET... ALONG WITH A
PREDICTED CLOUD HEIGHT OF 25,000 TO 30,000 FEET SO HAIL POTENTIALLY
LARGER THAN PEA SIZE WILL BE POSSIBLE. THERE IS ALSO A THREAT OF
STRONGER WINDS FROM ALOFT MIXING DOWN TO SURFACE IN ANY THUNDERSTORM.
WE ARE UPDATING THE ZFP NOW TO MENTION THIS THREAT FOR HAIL AND
STRONGER THUNDERSTORMS. IT APPEARS THAT THE ANCHORAGE BOWL ITSELF HAS
A DECENT CHANCE AT THUNDER TODAY...GIVEN THE MODELS FORECASTING
OFFSHORE FLOW AND A STORM MOTION FROM NORTH TO SOUTH. THIS MEANS ANY
TERRAIN INDUCED STORMS SHOULD BE ABLE TO MOVE OVER THE BOWL. THE
OFFSHORE FLOW WILL ALSO INHIBIT ANY SEABREEZE FROM FORMING...WHICH
WILL ALLOW STORMS TO SURVIVE AS THEY MOVE OVER THE BOWL. CONVECTIVE
FORCING WILL FURTHER BE HELPED WITH VORTICITY MAXIMA THAT ARE
SPIRALING ON THE BACK SIDE OF A LOW IN THE GULF OF ALASKA.

VERTICALLY STACKED LOW IS LOCATED IN EASTERN GULF OF ALASKA NEAR THE
PANHANDLE. THE FORCING WITH THIS LOW IS FAR ENOUGH TO THE EAST TO NOT
DIRECTLY IMPACT SOUTH CENTRAL ALASKA...BUT THE ASSOCIATED VORTICITY
MAXIMA SPINNING ON THE BACK SIDE OF THIS FEATURE WILL AID IN THE
ABOVE STATED CONVECTION. BEYOND THIS FEATURE LITTLE IS OCCURRING WITH
THE ALEUTIAN AND WESTERN ALASKA UNDER THE INFLUENCE OF A WEAK UPPER
LEVEL RIDGE THAT SHOULD INHIBIT ANY WIDESPREAD PRECIPITATION THROUGH
THE WEEKEND.

.MODEL DISCUSSION...
MODELS ARE IN GOOD AGREEMENT THROUGH THE SHORT TERM. SMALL
DIFFERENCES EXIST IN THE PLACEMENT AND STRENGTH OF DEVELOPING LOW IN
THE BERING...BUT THESE DIFFERENCES ARE NOT ENOUGH TO SIGNIFICANTLY
CHANGE THE FORECAST.

.SHORT TERM FORECAST...
FORECAST ATTENTION HAS BEEN HEAVILY PLACED ON THE CONVECTIVE CHANCES
AND IT APPEARS THAT THUNDERSTORM CHANCES FOR SOUTH CENTRAL ALASKA
WILL CONTINUE THROUGH THE WEEKEND. THE THUNDERSTORMS WILL NOT BE AS
WIDESPREAD SUNDAY OR MONDAY AS THEY ARE EXPECTED TO BE TODAY...BUT
THEY SHOULD STILL OCCUR AS OVERALL PATTERN REMAINS CONDUCIVE TO
THUNDERSTORMS.

FURTHER WEST...WESTERN ALASKA AND THE EASTERN BERING WILL REMAIN
UNDER HIGH PRESSURE WHICH SHOULD PROVIDE SOME QUIET WEATHER THROUGH
MONDAY. BY MONDAY IT APPEARS THAT THIS HIGH PRESSURE WILL MOVE EAST
THROUGH MAINLAND ALASKA REDUCING THUNDERSTORM AND PRECIP CHANCES.

.LONG TERM FORECAST...
STARTING 00Z TUESDAY IT APPEARS AN UPPER LEVEL LOW WILL DEVELOP AND
STRENGTHEN IN THE BERING. THE MODELS ARE STALLING THIS LOW IN THE
BERING THROUGH THE FORSEEABLE FUTURE WITH LITTLE MOVEMENT TO THE
EAST. THIS LOW WILL CAUSE A MORE AMPLIFIED PATTERN TO DEVELOP OVER
MAINLAND ALASKA... WITH AN AMPLIFIED RIDGE TAKING CONTROL ACROSS ALL
OF SOUTHERN ALASKA BRINGING WITH IT SOME QUIET AND POTENTIALLY WARM
WEATHER.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE
MARINE...NONE
FIRE WEATHER...NONE

TARALDSEN JUN 09