Small Craft Advisory 88, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
204 PM AKST MON JAN 30 2012

.SHORT TERM...OCCLUDED FRONT OVER THE ERN GULF WILL CONTINUE NNE
THRU THE AREA TONIGHT AND TUE MORNING. LOW PRESSURE WILL MOVE NE
TO THE N-CENTRAL GULF BY TUE MORNING. A SMALL LOW PRESSURE AREA
WILL MOVE NE TO THE SRN PANHANDLE BY LATE TUE AFTERNOON. USED THE
12Z ECMWF DUE TO ITS SUPERIOR INITIALIZATION ON THE FRONT AND
LOWS.

 MAIN FORECAST ISSUE WILL BE TIMING THE PTYPE TRANSITION. AIRMASS
BEHIND THE FRONT IS RATHER MILD SO AS FRONT MOVES THRU EXPECT
PRECIP TO GO OVER TO RAIN. HOWEVER...THE FRONT MAY GO ALOFT AS IT
MOVES OVER THE FAR NRN INNER CHANNEL AREA TONIGHT...SLOWING DOWN
THE NWD TIMING OF PRECIP CHANGEOVER SOME THERE. THE PAYA AREA IS
EVEN TRICKIER AS THEIR MIX/CHANGEOVER WILL BE PEGGED TO WHEN LOW
LEVEL WINDS SHIFT FROM ENE TO ESE OR SE. MODELS DIFFER ON TIMING
THIS...AND BASED ON THE 12Z ECMWF...WINDS WOULD SHIFT TO SE BY
12Z TONIGHT. THINK THIS WILL KEEP THEM FROM GETTING TO 6+ INCHES
OF SNOWFALL TONIGHT SO WILL BE DROPPING THE WS WATCH FOR THEM. DO
NOT EVEN THINK THEY WILL GET TO ADVY LEVELS AS HEAVIEST PRECIP
WILL ARRIVE DURING TIME OF THE WINDS TURNING SELY. THE PAHN AREA
IS A DIFFERENT STORY...AND WILL BE ISSUING A WW ADVY FOR THEM
TONIGHT INTO EARLY TUE MORNING. FURTHER S ALONG THE PAJN-PAEL
CORRIDOR...SNOW MAY ACCUMULATE UP TO 3 INCHES BEFORE IT MIXES
WITH OR CHANGES TO RAIN LATE TONIGHT. S OF THAT AREA SHOULD BE
ALL RAIN BY TIME FORECAST GOES OUT LATE THIS AFTERNOON.

 OTHERWISE...WILL HAVE TEMPS SLOWLY RISING OVER THE NRN HALF OF
THE AREA TONIGHT...WITH LITTLE CHANGE OVER THE S. WINDS SHOULD
PEAK AT SCA LEVELS AHEAD OF THE FRONT AND ON E SIDE OF LOW MOVING
INTO THE N-CENTRAL GULF. ONLY GOING SCA WINDS FOR THAT SMALL LOW
MOVING NE INTO THE SRN AREA TUE...BUT IT BEING A RATHER SMALL LOW
THERE COULD BE A LOCALLY STRONGER BAND OF WINDS ON ITS SE
SIDE...SO LATER SHIFTS WILL HAVE TO WATCH THIS CLOSELY.

&&

.LONG TERM...MAIN FOCUS OF THE EXTENDED PERIOD IS THE STRONG
SYSTEM APPROACHING THE PANHANDLE ON WED. CURRENTLY A WEAK
DISTURBANCE SW OF THE ALEUTIANS THIS FEATURE WILL START INTERACTING
WITH THE LEFT EXIT REGION OF A 100 KT 500 MB JET STREAK EARLY
TUESDAY. THE CLOSE ASSOCIATION OF THE MID LEVEL FEATURE WILL STICK
WITH THE SYSTEM UNTIL WED WHEN THE SURFACE LOW SEPARATES FROM THE
EXIT REGION AND GETS LEFT BEHIND IN THE WESTERN GULF. BY THIS TIME
THE LOW WILL HAVE STRENGTHENED TO AROUND 944 MB. THE LOW WILL
DRIFT NORTH AND FILL IN THROUGH FRI.

THE ASSOCIATED FRONT WILL BE THE MAIN WORRY FOR US AS IT PLOWS
INTO THE PANHANDLE WED THROUGH WED NIGHT. PRECIP WILL BE ONE HALF OF
THAT PROBLEM AS FOR MOST AREAS OF THE PANHANDLE TEMPERATURES WILL
BE NEAR FREEZING WITH 850 MB TEMPS WARMING UP ABOVE -6 C AS EARLY
AS WED MIDDAY FOR MOST OF THE PANHANDLE. THIS IS WARM ENOUGH THAT
MOST AREAS SHORT OF THE EXTREME NORTH MAY START OUT AS SNOW BUT
QUICKLY CHANGE OVER TO RAIN. AND WITH FORECAST QPF VALUES RANGING
FROM 3 TENTHS TO UP TO .75 INCHES, AREAS THAT STILL HAVE A
SIGNIFICANT SNOW PACK MAY BE FACING HEAVY SNOW LOADS ON ROOFS AND
PONDING OF WATER WHERE ICE AND SNOW BLOCK DRAINAGE. NORTH OF HAINES
AND SKAGWAY, THE PROBLEM SHIFTS FROM RAIN ON SNOW PACK TO PUTTING
MORE SNOW ON TOP OF WHAT IS ALREADY THERE AS AREAS UP THE HAINES
AND KLONDIKE HIGHWAYS MAY SEE SOME SIGNIFICANT SNOW OUT OF THIS
SYSTEM. PRECIP LIGHTENS UP AND CHANGES TO SHOWERS BY THURS.

THE SECOND HALF OF PROBLEM OF THE FRONT WILL BE THE WIND THAT IT
BRINGS WITH IT. AREAS OUT IN THE GULF WILL BE EXPERIENCING STORM
FORCE WINDS WED PARTICULARLY IN THE NORTHERN GULF WHERE THE USUAL
BARRIER JET WILL BE SETTING UP AS THE FRONT COMES NORTHWARD.
FARTHER SOUTH THE INTERACTION BETWEEN THE ENCROACHING FRONT AND A
BUILDING RIDGE OVER BRITISH COLUMBIA WILL BUILD UP THE PRESSURE
GRADIENT OVER THE SOUTHERN PANHANDLE WED EVENING. 850 MB WINDS ARE
FORECAST TO EXCEED 70 KT WHICH COULD MEAN GUST TO 60 MPH AT THE
SURFACE WED EVENING. OTHER INNER CHANNEL AREAS MAY SEE HIGH ENOUGH
GUSTS FOR STRONG WIND HEADLINES TO BE NEEDED WED NIGHT.

MODEL AGREEMENT IS FAIRLY GOOD FOR THIS SYSTEM WITH THE NAM BEING
A BIT OF AN OUTLIER WITH FASTER FRONTAL SPEED. DECIDED TO APPLY
A BLEND WITH MORE WEIGHT TO THE 12Z EC WITH SOME LESSER AMOUNTS OF
THE GFS THROWN IN TO GET A DECENT AVERAGE FROM THE SMALL
DIFFERENCES STILL PRESENT BETWEEN THE MODELS. PAST FRI MODEL
AGREEMENT IS STILL IN LIMBO WITH THE MODEL STILL NOT AGREEING ON
THE NUMBER AND TRACK OF VARIOUS WEAK SMALL SYSTEMS. THE ONLY PLACE
THEY SOMEWHAT AGREE IS ON THE OVERALL MID LEVEL PATTERN OF A
RIDGE BUILDING OVER THE PANHANDLE AND INTERIOR OVER THE WEEKEND
WITH A TROUGH DIGGING INTO THE NORTH CENTRAL PACIFIC. WENT WITH
HPC DATA FOR THE EXTENDED RANGE FOR ITS USE OF THE MORE CONSISTENT
ENSEMBLE MEANS. FORECAST CONFIDENCE IS ABOUT AVERAGE OUT TO FRI
THEN DROPS TO POOR AFTER THAT.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER WEATHER ADVISORY UNTIL 9 AM AKST TUESDAY FOR AKZ019.
MARINE...SMALL CRAFT ADVISORY FOR PKZ021-022-033>036-041>043-051-052.

&&

RWT/EAL

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