Small Craft Advisory 578, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
130 PM AKDT WED AUG 17 2011

.ANALYSIS AND UPPER LEVELS...WATER VAPOR IMAGERY SHOWS THE PRESENCE
OF SOME VERY DYNAMIC FEATURES ACROSS THE ALASKA REGION. A SHORT-WAVE
TROUGH IS DIGGING INTO SOUTHCENTRAL ALASKA WHERE A VERY UNSTABLE
AIRMASS IS STILL IN PLACE. STABILITY INDICES ON THE 12Z ANCHORAGE
SOUNDING WERE QUITE IMPRESSIVE WITH TOTAL TOTALS OF 56...LIFTED
INDEX OF -4.9...AND CAPE OF 743 J/KG. THE RESULT OF THE SYNOPTIC
FORCING ON THIS UNSTABLE AIRMASS IS HEAVY SHOWERS WITH A FEW EMBEDDED
THUNDERSTORMS...PRIMARILY FROM ANCHORAGE NORTHWARD. THE SHORT-WAVE
IS BEGINNING TO SLOWLY SWING NORTHEASTWARD SO EXPECT SOUTHERN END
OF SHOWERS TO FOLLOW SUIT. THERE WILL STILL BE SOME SYNOPTIC FORCING
AS ADDITIONAL VORT-MAXES MOVE IN FROM THE WEST...BUT TEMPERATURES
ALOFT WILL BEGIN TO WARM SO ANY SHOWERS THAT DO DEVELOP WILL BE
LIGHTER THAN WHAT IS BEING OBSERVED THIS MORNING.

THERE ARE ALSO A FEW SHOWERS OVER THE SOUTHERN KUSKOKWIM VALLEY
ALONG THE UPPER TROUGH AXIS. DRIER CONDITIONS PREVAIL JUST UPSTREAM

WITH A SHORT-WAVE RIDGE FROM THE CENTRAL BERING TO SOUTHWEST ALASKA
COAST. THIS RIDGE IS WEAKENING AS A TROUGH DIGS OUT OF ASIA INTO
THE WESTERN BERING SUPPORTED BY A 100 TO 130 KT SUBTROPICAL JET.
A VERTICALLY STACKED LOW IS NEARLY STATIONARY EAST OF THE KAMCHATKA
PENINSULA WITH A STRONG SHORT-WAVE SOUTH OF IT HEADED FOR THE BERING
SEA. EXPECT DEVELOPMENT OF A SURFACE LOW JUST AHEAD OF THIS
SHORT-WAVE TODAY. THIS WILL THEN DEVELOP INTO A MAJOR SYSTEM OVER
THE BERING SEA WHICH WILL BE THE PRIMARY WEATHER MAKER OVER THE
NEXT SEVERAL DAYS.

.MODEL DISCUSSION...THERE IS FAIRLY HIGH CONFIDENCE IN THE GENERAL
EVOLUTION OF THE PATTERN THE NEXT FEW DAYS. HOWEVER...THERE IS STILL
QUITE A BIT OF UNCERTAINTY IN TIMING AND PLACEMENT OF FEATURES
ASSOCIATED WITH DEVELOPING BERING SEA SYSTEM. EACH MODEL SHOWS SOME
CONSISTENCY WITH ITSELF...BUT THERE IS STILL A SPREAD AMONG ALL THE
SOLUTIONS. WILL CONTINUE TO STICK CLOSE TO THE INHERITED FORECAST
WHICH IS CLOSEST TO THE NAM SOLUTION. THIS PROVIDES A NICE MIDDLE
GROUND BETWEEN THE GFS AND ECMWF. DUE TO UNCERTAINTY IN A LOT OF
DETAILS WILL TEND TO TAKE MORE OF A BROAD-BRUSH APPROACH...ESPECIALLY
FOR PRECIP CHANCES AND TIMING.

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...SHORT-WAVE TROUGH WILL BE SLOW TO LIFT
NORTHEASTWARD ACROSS SOUTHCENTRAL...NOT COMPLETELY EXITING THE
AREA UNTIL THURSDAY AFTERNOON. VORT-MAXES SLIDING ALONG THE AXIS
WILL CONTINUE TO PROVIDE LIFT FOR SHOWERS WHICH WILL INCREASINGLY
BE FOUND FARTHER NORTH OVER INTERIOR SECTIONS OF SOUTHCENTRAL.
MUCH WARMER AIR ALOFT WILL MOVE IN BEHIND THE TROUGH AND STABILIZE
THE ATMOSPHERE. A WARM FRONT WILL PROGRESS NORTHWARD ACROSS THE GULF
WEDNESDAY WAY OUT AHEAD OF THE DEVELOPING BERING SEA SYSTEM WHICH
WILL SPREAD LIGHT RAIN OVER MOST OF THE REGION BY THURSDAY AFTERNOON.

MORE SIGNIFICANT RAIN AND WINDS WILL ARRIVE WITH THE TRIPLE POINT
AND ASSOCIATED FRONTAL SYSTEM ON FRIDAY. SOUTHERLY FLOW ALOFT WILL
SHIFT TO SOUTHEAST AND THERE WILL BE A STRONG EASTERLY LOW LEVEL
WINDS AS SURFACE RIDGE INTENSIFIES ALONG NORTH GULF COASTAL ZONE
AHEAD OF FRONT. THIS OFTEN PRODUCES DOWNSLOPE WINDS. IN THIS
SITUATION WITH LOTS OF MOISTURE AND STRONG DYNAMICS ALOFT EXPECT
EVERYWHERE TO SEE SOME RAIN. ALSO...EXPECT DEVELOPMENT OF TRIPLE
POINT TO STALL THE FRONT JUST OFFSHORE AND PRESSURE GRADIENT TO
SHIFT TO A MORE N-S ORIENTATION WHICH WILL WEAKEN THE E-W GAP WINDS
SUCH AS TURNAGAIN ARM. THE ENTIRE LOW COMPLEX WILL MOVE INTO THE
GULF BY SATURDAY SO COOL AND SHOWERY WEATHER WILL PERSIST.

SOUTHWEST ALASKA...AS UPPER TROUGH LIFTS NORTHWARD FROM THE
KUSKOKWIM VALLEY TONIGHT SHOWERS WILL COME TO AN END. MEANWHILE
A WARM FRONT WILL MAKE QUICK PROGRESS INTO BRISTOL BAY BY EARLY
THURSDAY MORNING AND CONTINUE NORTHWARD DURING THE DAY BRINGING
A RETURN TO RAIN AND MORE WIDESPREAD FOG ALONG THE COAST. HEAVIER
RAIN AND A LITTLE WIND WILL ARRIVE WITH THE OCCLUDED FRONT ON
FRIDAY. THE LOW WILL MOVE INTO THE GULF OVER THE WEEKEND BUT
SOUTHWEST ALASKA WILL REMAIN UNDER UPPER LEVEL CYCLONIC FLOW
AND MID TO UPPER LEVEL INSTABILITY WHICH WILL SUPPORT SHOWERS.

BERING SEA/ALEUTIANS...THE DEVELOPING SURFACE LOW ALONG THE WESTERN
ALEUTIANS WILL INITIALLY MOVE RAPIDLY EASTWARD THANKS TO UPPER
JET OVERHEAD. HOWEVER...IT WILL COME TO A SCREECHING HALT NEAR
THE PRIBILOF ISLANDS THURSDAY AFTERNOON AS UPPER LOW FORMS AND
PULLS IT NORTHWARD WHILE JET CONTINUES OFF TO THE EAST. FURTHER
DEVELOPMENT WILL OCCUR DOWNSTREAM THURSDAY NIGHT AND FRIDAY WHICH
WILL THEN HELP PULL THIS LOW EASTWARD TOWARD THE GULF THURSDAY
NIGHT AND FRIDAY. THIS LOW WILL BRING WITH IT SOME COLDER AIR
ALOFT...SO FOG CONDITIONS WILL FINALLY IMPROVE ACROSS THE REGION.

.LONG TERM FORECAST...AS UPPER LOW WEAKENS OVER THE GULF AND
SOUTHCENTRAL LATE IN THE WEEKEND INTO EARLY NEXT WEEK MODELS
INDICATE A SHARP TROUGH DIGGING SOUTHWARD OUT OF THE ARCTIC AND
TOWARD WESTERN ALASKA. CONSISTENCY IN THE LOCATION AND STRENGTH
OF THIS SHORT-WAVE ARE NOT GREAT AND FORECAST CONFIDENCE IS LOW.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...NONE.
MARINE...GALE WARNING 120 130 170 172.
FIRE WEATHER...NONE.

SEB AUG 11