Small Craft Advisory 572, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
1245 PM AKDT TUE AUG 16 2011

.ANALYSIS AND UPPER LEVELS...CONSIDERING A TROUGH IS SITTING RIGHT
OVER MAINLAND ALASKA CONDITIONS ARE QUITE NICE AT THE MOMENT. A
CLOSER LOOK SHOWS AN UPPER LOW EXITING INTO THE YUKON WITH VERY
WEAK RIDGING BEHIND OVER WESTERN SOUTHCENTRAL ALASKA WHERE MOSTLY
CLEAR SKIES DOMINATE. THE 12Z ANCHORAGE SOUNDING SHOWS A DEEP LAYER
OF INSTABILITY ABOVE 4000 FEET. THIS IS REPRESENTATIVE OF THE AIR
MASS ACROSS SOUTHCENTRAL...SO AFTERNOON HEATING WILL HELP REALIZE
SOME OF THIS CONVECTIVE POTENTIAL. WATER VAPOR IMAGERY SHOWS A
COUPLE WEAK SHORT-WAVES MOVING DOWN THE BACK SIDE OF THE TROUGH...
ONE OVER INTERIOR SOUTHWEST ALASKA AND ANOTHER OVER THE FAR NORTHERN
BERING SEA. BOTH OF THESE WILL AID IN CONVECTIVE INITIATION THIS
AFTERNOON AND EVENING. THERE ARE ALSO A COUPLE WEAK WAVES ON THE
BACK SIDE OF THE UPPER LOW ROTATING THROUGH THE COPPER RIVER BASIN.
IN FACT CUMULUS CLOUDS ARE ALREADY POPPING UP ACROSS THE MOUNTAINS
SURROUNDING THE COPPER BASIN IN RESPONSE TO ONE OF THESE WAVES.

A RIDGE IS SITTING TO THE WEST OVER THE CENTRAL BERING WITH A
TROUGH SOUTH OF THE CENTRAL AND EASTERN ALEUTIANS. MOST OF THE
MOISTURE WITH THIS TROUGH IS JUST SOUTH OF THE CHAIN. A DEEPENING
LOW EAST OF THE KAMCHATKA PENINSULA IS PUSHING A FRONT TOWARD THE
WESTERN ALEUTIANS AND CAUSING THE DOWNSTREAM RIDGE TO GRADUALLY
WEAKEN.

.MODEL DISCUSSION...MODEL CONSISTENCY REMAINS POOR WITH HANDLING
OF LOW MOVING ACROSS THE BERING SEA WEDNESDAY NIGHT THROUGH FRIDAY.
THE ROOT OF THE PROBLEM WITH TRACK OF THE SURFACE LOW STEMS FROM
THE COMPLEXITY OF THE UPPER LEVEL PATTERN. AS TROUGH MOVES INTO
THE BERING SEA MODELS INDICATE MULTIPLE STRONG WAVES MOVING THROUGH
IT WHICH WILL AFFECT WHERE THE MAIN UPPER CENTER SETS UP AND THUS
WHERE TH SURFACE LOW WILL TRACK TO. LOOKS LIKE IT MAY TAKE ANOTHER
FEW MODEL CYCLES BEFORE THEY CAN RESOLVE THESE FEATURES CORRECTLY.
WILL STICK CLOSE TO THE EXISTING FORECAST WHICH IS CLOSEST TO THE
NAM SOLUTION. ASIDE FROM HANDLING OF THIS SYSTEM...MODELS ARE IN
GOOD AGREEMENT THROUGH THE NEXT FEW DAYS.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...UNSTABLE AIR MASS IN PLACE COMBINED WITH
FORCING FROM UPPER LEVEL WAVES WILL PRODUCE ISOLATED TO SCATTERED
CONVECTION ACROSS SOUTHCENTRAL THIS AFTERNOON. SUBSIDENCE FROM THE
WEAK RIDGE MOVING THROUGH MAY LOCALLY CAP CONVECTION ESPECIALLY
OVER VALLEYS. MODEL STABILITY INDICES FOR THIS AFTERNOON/EVENING
SUPPORT DEVELOPMENT OF ISOLATED THUNDERSTORMS...PRIMARILY THE
TYPICALLY FAVORED MORE UNSTABLE AREAS INLAND AND ALONG THE MOUNTAINS.
STORM MOTION WILL ALSO BE KEY TO WHERE CONVECTION IS OBSERVED.
THE MEAN FLOW IS OUT OF THE NORTH THIS MORNING BUT WILL BECOME
MORE WESTERLY THIS AFTERNOON AS SHORT-WAVE MOVES IN FROM SOUTHWEST
ALASKA. THIS MEANS CONVECTION THAT FORMS ALONG THE CHUGACH WILL MOVE
AWAY FROM ANCHORAGE AND MORE STABLE AIR WILL LIKELY MOVE IN OFF THE
INLET. BASED ON THIS REASONING...ACTIVITY FROM THE KENAI PENINSULA
TO ANCHORAGE WILL LIKELY BE CONFINED TO RIGHT ALONG AND OVER THE
MOUNTAINS.

THE NEXT SHORT-WAVE CURRENTLY OVER THE NORTHERN BERING WILL DIG
SOUTHWARD AS IT REACHES SOUTHCENTRAL EARLY WEDNESDAY MORNING. THIS
WILL RESULT IN MORE CLOUDS AND STABILIZATION OF THE AIR MASS. THERE
WILL STILL BE ENOUGH MID TO UPPER LEVEL INSTABILITY FOR PRECIPITATION
TO TAKE THE FORM OF SHOWERS. AN ISOLATED THUNDERSTORM OR TWO IS STILL
POSSIBLE INLAND MAINLY OVER THE COPPER RIVER BASIN WHICH WILL SEE
SOME SUNSHINE BEFORE CLOUDS INCREASE. BASED ON THE SHARPNESS OF THIS
SHORT-WAVE PLAN TO INCREASE POPS...ESPECIALLY FROM THE WESTERN KENAI
PENINSULA TO THE SUSITNA VALLEY. THE SHORT-WAVE TROUGH WILL STALL
OVER SOUTHCENTRAL WEDNESDAY NIGHT AND THURSDAY...GRADUALLY ELONGATING
AND WEAKENING. THUS SHOWERS WILL DIMINISH WITH TIME. ON THURSDAY A
FRONTAL SYSTEM EXTENDING FROM A BERING SEA LOW WILL MAKE ITS WAY
TOWARD SOUTHCENTRAL. THERE IS UNCERTAINTY IN THE EXACT TIMING AND
STRENGTH OF THIS FRONT...BUT EXPECT RAIN TO AFFECT THE REGION LATE
THURSDAY THROUGH FRIDAY TIME FRAME.

SOUTHWEST ALASKA...THE SHORT-WAVE OVER THE NORTHERN BERING WILL HELP
TRIGGER SCATTERED SHOWERS AND POSSIBLY A FEW THUNDERSTORMS THIS
AFTERNOON AND EVENING OVER INTERIOR SOUTHWEST ALASKA. IT WILL THEN
STALL OVERHEAD OF THE KUSKOKWIM VALLEY AND PROVIDE A CHANCE OF
SHOWERS THROUGH WEDNESDAY. MEANWHILE...THE BERING RIDGE WILL SHIFT
EASTWARD TOWARD SOUTHWEST ALASKA AND PROVIDE DRY WEATHER UNTIL THE
INTENSIFYING STORM SYSTEM REACHES THE COAST SOME TIME ON THURSDAY.
ALTHOUGH THERE IS SOME UNCERTAINTY WITH THIS STORM IT APPEARS LIKELY
THAT ALL OF SOUTHWEST ALASKA WILL SEE RAIN AND WIND AT THE END OF
THE WEEK.

BERING SEA/ALEUTIANS...THE FRONT ENTERING THE WESTERN ALEUTIANS
WILL RAPIDLY WEAKEN AS IT PROGRESSES EASTWARD. HOWEVER...EXPECT
RAPID DEVELOPMENT OF A LOW TRACKING ACROSS THE BERING SEA WEDNESDAY
NIGHT THROUGH THURSDAY NIGHT. THERE IS QUITE A BIT OF UNCERTAINTY
IN THE EXACT TRACK OF THE LOW...BUT EXPECT IMPACTS TO BE WIDESPREAD
THROUGH THE END OF THE WEEK.

.LONG TERM FORECAST...GFS/ECWMF SHOW EXCELLENT AGREEMENT WITH BASIC
EVOLUTION OF LONG-WAVE PATTERN THROUGH MOST OF THE DAY 4 THROUGH 7
PERIOD. THE LARGE LOW OVER THE EASTERN BERING WILL MOVE INTO THE
GULF OVER THE WEEKEND LIKELY PROVIDING A FAIRLY WET WEEKEND ACROSS
THE SOUTHERN MAINLAND. A RIDGE WILL BUILD IN BEHIND IT ACROSS THE
BERING SEA AND ALEUTIANS BRINGING A RETURN TO BENIGN CONDITIONS.
THE OTHER FEATURE WHICH MAY AFFECT THE SOUTHERN MAINLAND BY EARLY
NEXT WEEK IS A LARGE LOW OVER THE ARCTIC...ALTHOUGH MODEL SPREAD
WITH SHORT-WAVES ROTATING AROUND THIS IS FAIRLY LARGE.

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

SEB AUG 11