Small Craft Advisory 191, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
111 PM AKST THU FEB 24 2011

.SHORT TERM...EARLY AFTERNOON SATELLITE LOOP REVEALS A LARGE
HIGH PRESSURE ACROSS THE GULF WITH A WEAK INVERTED TROUGH OF LOW
PRESSURE OVER OUTSIDE AND OFFSHORE COASTAL WATERS. AN UPPER LEVEL
RIDGE TILTING POSITIVELY BRINGS A HIGH CLOUD SHIELD ACROSS
SOUTHEAST ALASKA FROM THE YUKON. HIGH PRESSURE OVER THE
NORTHWESTERN CANADA SLOWLY MOVES SOUTHEAST ALONG COAST MOUNTAINS
WITH PRESSURE GRADIENT RELAXING ACROSS THE FORECAST DOMAIN. SMALL
CRAFT WINDS ARE STILL AFFECTING THE OUTSIDE WATERS BUT EXPECT THE
WINDS WILL FURTHER DECREASE THIS AFTERNOON WITH SUBSIDING SEAS AS
WELL. COLD AIR MASS CONTINUES AFFECTING THE PANHANDLE WITH
MERCURY READINGS RANGING IN THE TEENS ACROSS THE NORTHERN AND
CENTRAL PANHANDLE INTO THE 20S ACROSS THE SOUTHERN AREAS.

 MODELS ARE IN FAIR AGREEMENT WITH THE WEAK INVERTED TROUGH
ACROSS THE OFFSHORE AND OUTSIDE COASTAL WATERS DRIFTING SLIGHTLY
EAST OVER THE PANHANDLE BEFORE DISSIPATING BY LATE TONIGHT. A
DEVELOPING WEAK LOW IN THE FAR NORTHERN GULF LATE TONIGHT WILL
PUSH A WARM FRONT AHEAD OF ITS TRACK TO THE EAST ON FRIDAY. THE
NAM HANDLED THE TRANSITION OF ONSHORE FLOW THE BEST AND WAS USED
TO REFRESH THE INHERITED GRIDS FOR TONIGHT AND FRIDAY. THE
FRONTAL BOUNDARY APPEARS PRETTY WEAK AND THE ACCOMPANIED WARM AIR
MASS DOES NOT SEEM TO MAKE A SIGNIFICANT WARMING ACROSS THE
FORECAST DOMAIN. NONETHELESS...THE FRONT WILL ARRIVE YAKUTAT AREA
FRIDAY MORNING WITH UP TO AN INCH OF SNOW THROUGH THE DAY...AND
THE PANHANDLE COAST IN THE EARLY AFTERNOON. NO CONSIDERABLE SNOW
ACCUMULATION IS EXPECTED.

 THE RELAXED PRESSURE GRADIENT WILL REMAIN ACROSS THE AREAS
THROUGH TONIGHT. AHEAD OF THE INCOMING FRONT...WESTERLY ONSHORE
FLOW WILL GRADUALLY INCREASE LATE TONIGHT INTO EARLY FRIDAY
MORNING ACROSS THE OUTSIDE AND OFFSHORE WATERS...GENERATING SMALL
CRAFT WINDS/SEAS BY LATE FRIDAY MORNING INTO THE EARLY AFTERNOON
HOURS. SOUTHERLY PRESSURE GRADIENT WILL ALSO INCREASE ACROSS THE
NORTHERN INNER CHANNELS WITH SMALL CRAFT WINDS ACROSS NORTHERN
LYNN CANAL ON FRIDAY AFTERNOON. A BIT MORE ZONAL MIDDLE TO UPPER
LEVEL FLOW COMBINED WITH INCREASING ONSHORE LOW LEVEL FLOW WILL
MAINTAIN OVERNIGHT TEMPERATURES MOSTLY IN THE LOWER TO MIDDLE
TEENS INLAND AND LOWER TO MIDDLE 20S ALONG THE COAST...WITH
SUBSTANTIAL WARMING THROUGH FRIDAY AFTERNOON TO THE UPPER 20S TO
THE LOWER 30S...EXCEPT AROUND 40 NEAR THE IMMEDIATE PANHANDLE
COAST.

 &&

.LONG TERM... A VERY BRIEF PRECIPITATION EVENT MOVES SOUTH THROUGH
THE PANHANDLE FRIDAY NIGHT THROUGH SAT NIGHT. THIS FAST MOVING
SHORT WAVE PULLS WARMER AIR INTO COASTAL SECTIONS FRI AFTERNOON
AND EVENING THAT WILL MAKE A TRICKY FORECAST PROBLEM FOR
DETERMINING PRECIP TYPE... THAT IS... RAIN OR SNOW OR POSSIBLY
LOCALIZED FREEZING RAIN. BUT THE MAIN MESSAGE FOR THE EXTENDED
FORECAST PERIOD IS THIS PRECIP EVENT WILL BE SHORT LIVED AND ANOTHER
PROLONGED BLAST OF COLD DRY AIR AND LOCALLY VERY STRONG WIND
STARTING SAT IN THE NORTH AND SUN IN THE SOUTHERN PANHANDLE.

ALL GLOBAL MODELS SHOW A VERY CONSISTENT HIGH AMPLITUDE 4-WAVE
PATTERN AROUND NORTHERN HEMISPHERE THROUGH DAY 10. A STRONG
BLOCKING UPPER LEVEL RIDGE AXIS IS FORECAST TO REMAIN SITUATED
NEAR WESTERN AK AND BERING SEA THROUGH THE EXTENDED PERIOD /NEXT
WED AND BEYOND/. THIS FORCES THE JET STREAM OVER THE PACIFIC TO
MOVE N INTO THE ARCTIC OCEAN WEST OF THE DATELINE AND THEN
SOUTHWARD ALONG THE AK-CANADA BORDER AND OVER THE EASTERN GULF OF
AK BEFORE TURNING EAST INTO WA/OR.

A WEAK WEATHER SYSTEM WILL BE STEERED SOUTHWARD ACROSS THE
PANHANDLE FRI NIGHT THROUGH SAT. RAPID SURFACE PRESSURE FALLS JUST
AHEAD OF THIS SYSTEM WILL CREAT A SOUTHERLY PRESSURE GRADIENT AND
WEAK ONSHORE FLOW FRIDAY NIGHT JUST AS THE PRECIP ARRIVES. OUR
SUB FREEZING AIR TEMPS WILL BE TEMPORARILY SCRUBBED AWAY ALONG
THE COAST BUT MUCH LESS SO FARTHER INLAND. AS THE PRECIP ARRIVES
FRI EVENING IT IS EXPECTED TO FALL IN THE FORM OF RAIN FROM
YAKUTAT S ALG THE COAST BUT REMAIN SNOW IN THE INNER CHANNELS
NORTH OF ABOUT PETERSBURG. THERE MAY BE A NARROW BAND OF RAIN/SNOW
MIXED OR FREEZING RAIN FRI EVENING OVER CHATHAM STRAIT/GLACIER
BAY AND THIS BEARS CLOSER WATCH ON FRI. PRECIP SHOULD LINGER A BIT
LONGER OVER SOTHERN PANHANDLE. USED NAM MODEL TO HELP WITH
PLACEMENT OF SURFACE TROUGH THAT HANGS ALONG SOUTHERN COAST INTO
SUN PARALLEL TO LOW LEVEL NW WINDS. PRECIP EXPECTED TO BE RAIN
SOUTH OF SUMNER STRAIT FRI NIGHT AND SAT THEN MIX WITH SNOW SAT
NIGHT BEFORE ENDING SUN.

THE CONSTATNT REINFORCEMENT OF COLD AIR FROM THE TOP OF THE GLOBE
INTO THE AREA OF MAXIMUM SUBSIDENCE JUST EAST OF THE UPPER RIDGE
AXIS WILL MAINTAIN VERY HIGH SURFACE PRESSURE INLAND FROM THE
COASTAL RANGE OVER THE YUKON AND NORTHERN BC. LOCAL WIND FORECAST
TOOLS HINT AT POSSIBILITY OF A TAKU MOUNTAIN WAVE EVENT SUN-MON.
OTHERWISE NAM/GFS/HPC FORECAST SURFACE PRESSURE GRADIENTS THAT
DEFINTELY SUPPORT VERY STRONG N-NE GAP FLOW AND FREEZING SPRAY.
THE COLD AIR IS EVEN MUCH BELOW NORMAL OVER THE GULF AND WOULD NOT
BE SURPRISED IS FREEZING SPRAY DEVELOPS SUN-TUE OFFSHORE OUTSIDE
OF CAPE SPENCER AND OTHER LOCAL ACCELERATION ZONES.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ012-041>043-051-052.

&&

AHN/TA