Area Forecast Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
000
FXAK67 PAJK 042338
AFDAJK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
338 PM AKDT TUE AUG 4 2015

.SHORT TERM...AFTERNOON IR SATELLITE IMAGE REVEALS A WEAKENING
INVERTED TROUGH OF LOW PRESSURE OVER THE NORTHERN PANHANDLE AND A
LOW PRESSURE SYSTEM SOUTHWEST OF KODIAK ISLAND. AN OCCLUDED FRONT
STRETCHES NORTHEAST TO EAST FROM THE LOW CENTER ACROSS THE CENTRAL
GULF. MARINE STRATUS THIN OUT AND SHRINK OVER THE OUTER WATERS.
WATER VAPOR SATELLITE IMAGE SHOWS AN UPPER LEVEL LOW OVER BRITISH
COLUMBIA AND ANOTHER UPPER LOW OVER THE WESTERN GULF WITH AN UPPER
RIDGE IN BETWEEN THE TWO UPPER LOWS. AROUND THE UPPER LOW OVER
BRITISH COLUMBIA...MID TO HIGH LEVEL CLOUDS WRAP AROUND THE LOW AND
OUTSKIRT ACROSS THE PANHANDLE. CLOUDS WILL CONTINUE TO INCREASE
OVER THE NORTHERN PANHANDLE TONIGHT...LEADING TO SCATTERED SHOWERS
FOR WEDNESDAY. OVER THE SOUTHERN AREAS...SCATTERED SHOWERS WILL
DIMINISH TO ISOLATED BY WEDNESDAY. SMALL CRAFT LEVEL
NORTHWESTERLY WINDS PREVAIL OVER THE OUTSIDE WATERS FROM PORT
ALEXANDER SOUTHEAST TO DIXON ENTRANCE. ELSEWHERE...NO SIGNIFICANT
WINDS ARE OBSERVED. A PLENTY OF SUNSHINE SOAR UP THE AFTERNOON
TEMPERATURES TO THE MID TO UPPER 70S OVER THE NORTHERN AND NORTH
CENTRAL PANHANDLE...WHILE CONTINUED GLOOMY SKIES KEEP DAYTIME
HIGHS IN THE UPPER 50S TO THE LOWER 60S OVER THE SOUTHERN
PANHANDLE.

MODELS REMAIN IN GENERAL AGREEMENT FOR THE WEAKENING INVERTED
TROUGH OF LOW PRESSURE SLOWLY DISSIPATE AS A RIDGE OF HIGH
PRESSURE ACROSS THE CENTRAL GULF GRADUALLY SHIFTS EAST OVER THE
PANHANDLE BY LATE WEDNESDAY. THE LOW SOUTHWEST OF KODIAK WILL MOVE
EAST THROUGH LATE WEDNESDAY AND A SECONDARY LOW DEVELOP OVER THE
SOUTHEASTERN GULF. REFRESHED THE INHERITED FORECAST WITH THE ECMWF
AND NAM12 FOR THEIR GOOD CONSENSUS AND CONSISTENCY. THE
NORTHWESTERLY LOW LEVEL FLOW OVER THE OUTER WATERS WILL BECOME
LIGHT AND VARIABLE TONIGHT IN TRANSITION THEN SOUTHEASTERLY WINDS
WILL GRADUALLY INCREASE LATE WEDNESDAY MORNING INTO THE AFTERNOON
HOURS IN RESPONSE TO THE INCOMING FRONTAL PASSAGE. AS THE RIDGE OF
HIGH PRESSURE MOVES OVER THE PANHANDLE...SOUTHERLY PRESSURE
GRADIENT WILL BECOME TIGHT AGAIN OVER THE NORTHERN PANHANDLE AND
PRODUCE SMALL CRAFT LEVEL WINDS FOR NORTHERN LYNN CANAL BY LATE
AFTERNOON. OVERNIGHT TEMPERATURES WILL BE MOSTLY IN THE LOWER TO
MID 50S...EXCEPT YAKUTAT WHERE LESS CLOUD COVER MAKES A BIT
COOLER LOWS IN THE UPPER 40S. WITH INCREASING CLOUDS AND SCATTERED
SHOWERS RETURN TO THE AREAS...WEDNESDAY HIGH TEMPERATURES WILL BE
A LITTLE COOLER TO THE MID TO UPPER 60S.

.LONG TERM...INITIAL PERIOD...WED NIGHT...FEATURES SOME MAJOR
MODEL DISAGREEMENT FOR A SHORT TERM FORECAST. DUST SETTLED A BIT
WITH THE NEW EC RUN WEAKENING ITS ENHANCED 00Z MARINE GRADIENTS
AND COMING IN WITH A GENERAL WEAK TO MODERATE MARINE LOW MOVING TO
THE SOUTHEAST TO A POSITION SOUTHWEST OF HAIDA GWAII BY EARLY SAT.
GFS WAS AN OUTLIER DURING THE EARLY PERIOD WITH A MARINE LOW MUCH
FURTHER NORTH...AND THE NAM CONTINUED TO SHOW A DIFFUSE MARINE LOW
WITH VERY LITTLE GRADIENT. CHOSE TO LEAN WITH THE NEW EC SOLUTION
AND THIS WAS ALL THAT TIME ALLOWED...AFTER WEAKENING WINDS ON THE
ANCHORAGE BORDER THE TWEAKS REQUIRED FOR INHERITED WINDS WERE
MINOR. THE MODEL DISAGREEMENT FOR WED PM/THU WAS A MYSTERY AND WAS
NOT APPARENT IN THE MODEL INITIALIZATIONS. STILL HAVE NORTHERN
LYNN IN SCA- LEVEL WINDS FOR WED EVENING BUT IT IS SOMEWHAT
MARGINAL AND BRIEF. 20KT WINDS CONTINUE TO BE THE PROGGED MAXIMUM
THROUGHOUT THE OTHER MARINE ZONES (BOTH OFFSHORE AND INSHORE)
THROUGH THE STORM EPISODE UNTIL THE NEXT EVENT EARLY NEXT WEEK.
FOR THERMODYNAMIC UPDATES DURING DAYS 2-3 UTILIZED A BLEND OF THE
MODELS WITH INHERITED GRIDS. TEMPERATURES WERE MODIFIED USING
PRIMARILY MOS AND THIS RESULTED IN A 1-3 DEGREE WARMUP FOR
THURSDAY AT MOST LOCATIONS.

FOR THE THU PM-FRI PERIOD CONTINUED TO USE 12Z EC AS PRIMARY
GUIDANCE FOR DYNAMIC FIELDS...GFS SURFACE PRESSURE PATTERN WAS A
COMPLETE OUTLIER FOR THIS TIME PERIOD AND THE EC MAINTAINED A
STRONGER MARINE LOW COMPARED TO YESTERDAY`S GUIDANCE; THE NAM
PREDICTION WAS THE MOST PROGRESSIVE OF ALL OF THE MODELS.
CONTINUED USING A BLEND FOR THERMODYNAMIC UPDATES WITH INHERITED.
CONTINUED THIS APPROACH RIGHT INTO BLENDING IN WITH WPC FIELDS BY
DAY 5. THE WPC GUIDANCE UTILIZED A COMBO OF EC DETERMINISTIC (70%)
WITH EC ENSEMBLES (30%). THIS SEEMED REASONABLE BASED ON ANALYSIS
OF RECENT MODEL PERFORMANCE. PRECIPITATION COVERAGE IN THIS EC
SCENARIO SHIFTS SLOWLY SOUTHWARD IN THE PANHANDLE ON FRI/SAT AS
THE UPPER CLOSED LOW CONTINUES TO TAKE A SOUTHEASTERLY TREK TO THE
WEST OF HAIDA GWAII BY SAT. SHORTWAVE RIDGING DOMINATES AFTERWARDS
FOR THE NORTHERN PANHANDLE BY SAT PM INTO THE SOUTHERN PANHANDLE
BY LATE SUNDAY. THIS DOMINATES THROUGH MONDAY ON THE EC. THE
RESULT IS A MOSTLY CLOUDY, PRECIPITATION- FREE OUTLOOK FOR THE SAT
PM THROUGH MONDAY PERIOD IN THE CENTRAL AND NORTHERN PANHANDLE
(SUN/MON FOR MUCH OF THE SOUTHERN PANHANDLE).

MEANWHILE A NEW PROGRESSIVE TROUGH IS CENTERED OVER THE ALEUTIANS
BY TUE, AND THEN THIS TRANSFORMS INTO A NEW NEGATIVELY- TILTED
CLOSED LOW OVER THE NORTHERN GULF BY WED. THE 18Z GFS RUN IS LESS
PROGRESSIVE WITH THIS TROUGH...MAINTAINING MOSTLY PRECIP- FREE
CONDITIONS OVER MUCH OF THE PANHANDLE THROUGH TUE. ASIDE FROM THE
TIMING DIFFERENCE WITH THE GULF TROUGH THE UPPER PATTERN OVER THE
GULF AGREES REASONABLY BETWEEN THE 2 MODELS...HOWEVER OVER THE
BERING THERE ARE MORE MAJOR DIFFERENCES ON DAY 8 (GFS...NEW
DIGGING TROUGH; EC...RIDGING). MODEL CONFIDENCE A BIT ABOVE
AVERAGE OVERALL AFTER THE DAY 2-3 DYNAMIC DIFFERENCES.


&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ012.

&&

$$

AHN/WESLEY

VISIT US AT HTTP://WWW.WEATHER.GOV/JUNEAU


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.