National Weather Service Weather Forecast Office

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

000
FXAK67 PAJK 182339
AFDAJK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
339 PM AKDT TUE JUN 18 2013

.SHORT TERM...AFTERNOON SATELLITE IMAGE SHOWS A WEAKENING TROUGH
OF LOW PRESSURE ACROSS THE CENTRAL AND EASTERN GULF...SLOWLY
DRIFTING WESTWARD. NUMEROUS CONVECTIVE CELLS DEVELOPING OVER
BRITISH COLUMBIA SLOWLY PROPAGATE WESTWARD CROSS THE BORDER ALONG
COAST MOUNTAIN RANGE...INCREASING THUNDERSTORM THREATS OVER THE
SOUTHERN PANHANDLE AREAS. A CUT-OFF UPPER LEVEL LOW RESPONSIBLE
FOR LAST NIGHT THUNDERSTORMS HAS MOVED OVER THE NORTHERN GULF AND
WEAKENED. DAYTIME HIGHS SORE TO THE LOWER 70S AT SKAGWAY AND
ANGOON. SEA BREEZES DEVELOPING MANY LOCATIONS.

 MODELS REMAIN FAIRLY CONSISTENT FOR THE WEAKENING TROUGH OF LOW
PRESSURE ACROSS THE GULF OF ALASKA CONTINUES TO MOVE WESTWARD
THROUGH WEDNESDAY MORNING BEFORE DISSIPATING BY WEDNESDAY
AFTERNOON. MODELS ALSO SHOW A GOOD AGREEMENT FOR A RIDGE OF HIGH
PRESSURE BUILDING OVER THE SOUTHERN GULF TONIGHT. THE RIDGE WILL
CONTINUE TO EXPAND NORTHWARD ACROSS THE CENTRAL AND EASTERN GULF
THROUGH WEDNESDAY. USED THE NAM TO UPDATE FORECAST PACKAGE FOR
TONIGHT AND WEDNESDAY.

 PRESSURE GRADIENT WILL REMAIN RELATIVELY WEAK AND THUS EXPECTING
NO SIGNIFICANT WINDS. AS THE RIDGE OF HIGH PRESSURE STARTS
BUILDING ACROSS THE GULF...A WEAK INVERTED TROUGH OF LOW PRESSURE
WILL REMAIN ALONG COAST MOUNTAIN RANGE. NO SIGNIFICANT SURFACE
FEATURES BUT UPPER LEVEL LOW OVER BRITISH COLUMBIA COAST WILL
CONTINUE TO PLAY A MAJOR ROLE BRINGING UPPER LEVEL VORTICITY
FIELDS WITH INCREASING INSTABILITIES FOR POTENTIAL THUNDERSTORMS
INTO THE SOUTHERN PANHANDLE THEN EXPAND NORTHWARD UP TO
PETERSBURG THROUGH THE AFTERNOON.

.LONG TERM...AS ADVERTISED, A PATTERN CHANGE WILL EMERGE BEGINNING
LATE IN THE WORK WEEK, JUST IN TIME FOR THE WEEKEND AS THE
EASTERLY FLOW THAT HAS WROUGHT THE WARM TEMPERATURES AND THE
CONVECTIVE WEATHER SOUTHEAST ALASKA HAS BEEN EXPERIENCING OVER THE
LAST FEW DAYS WILL YIELD TO MORE SOUTHERLY FLOW...ESPECIALLY BY
FRIDAY. AN UPPER LOW WILL SCOOT EASTWARD INTO THE WESTERN GULF
WHILE THE LOW TO THE SOUTHEAST OF THE PANHANDLE WILL MOVE EAST AS
WELL. CONVECTIVE PRECIPITATION WILL BECOME MORE STRATIFORM AND
TEMPERATURES WILL COOL JUST IN TIME FOR THE WEEKEND. AT THE
SURFACE, A RIDGE WILL DEVELOP BEYOND THE AFOREMENTIONED WEAK
TROUGH MENTIONED IN THE SHORT-TERM WEDNESDAY NIGHT INTO THURSDAY.
IN ADDITION, SLIGHT WARM AIR ADVECTION FROM BC OVER SE AK THURSDAY
WILL ONCE AGAIN ESTABLISH A THERMAL TROUGH OVER SE. WHILE THINK
AREAS OF FOG AND LOW CLOUDS WILL BEGIN APPEARING OVER THE OFFSHORE
AND NORTHERN OUTSIDE WATERS, THE THERMAL TROUGH SHOULD SHUNT ANY
ENCROACHMENT ONSHORE OFF TO THE SOUTHEAST...AT LEAST THIS IS THE
THINKING FOR NOW. BOTH NAM AND GFS INDICATED CAPE VALUES ARE STILL
HIGH ENOUGH FOR AN ISOLATED RISK OF THUNDERSTORMS FOR THE SOUTHERN
THIRD OF THE PANHANDLE, ZONE 26, 28, AND 29 FOR WEDNESDAY EVENING.
AS PREVIOUS SHIFT HAS MENTIONED, THIS LIKELY IS ASSOCIATED WITH A
LOBE OF POSITIVE VORTICITY MIGRATING WESTWARD ACROSS THE AREA.
WHILE THE LOW MIGRATES INTO THE MIDDLE NORTHERN HALF OF THE
PANHANDLE EARLY THURSDAY MORNING, NOT CONFIDENT ENOUGH TO HOLD
TOGETHER CONVECTION THAT LATE IN THE NIGHT, FOR SUCH A POORLY
DEFINED FEATURE. FOR THURSDAY, TEMPERATURES MAY BE SLIGHTLY
WARMER, BUT CAPE VALUES NOT AS IMPRESSIVE AND THUS HAVE DECIDED
NOT TO CONTINUE THUNDERSTORMS FOR NOW.

WHILE MINOR SHORTWAVES WILL ROTATE AROUND A MAIN UPPER LEVEL LOW
FEATURE FROM FRIDAY THROUGHOUT THE WEEKEND GIVING SOUTHEAST ALASKA
CHANCES OF RAIN, EVEN MAKING RAIN LIKELY FOR FRIDAY INTO FRIDAY
NIGHT, MODELS ARE INDICATING THE MAIN LOW WILL MIGRATE EASTWARD
EARLY NEXT WEEK WITH MORE CHANCES OF RAIN. WHILE TEMPERATURES
INDICATED NOW ARE MORE FITTING FOR SOUTHEAST CONVECTIVE FLOW,
LATER SHIFTS WILL LIKELY NEED TO COOL TEMPERATURES FOR THE WEEKEND
EVEN FURTHER AND ESPECIALLY FOR NEXT WEEK AS WELL, AS TREND LOOKS
MORE LIKELY TO BE A MORE MARINE INFLUENCED AIR MASS.

USED A BLEND OF NAM, ECMWF, AND GFS TO HANDLE THE BUILDING THERMAL
TROUGH WEDNESDAY NIGHT AND THURSDAY. FOR PRECIPITATION, DECIDED
EXISTING FORECAST LOOKED GOOD FOR WEDNESDAY NIGHT AND THURSDAY,
BUT INSTEAD EXPANDED LIKELY POPS FOR FRIDAY AND FRIDAY NIGHT WITH
SHORTWAVE MOVING NORTH BECOMING MORE PROBABLE. MADE FEW CHANGES TO
LONG TERM, DID EMPLOY SOME WPC TO SMOOTH PATTERN FOR MONDAY AND
TUESDAY. CONFIDENCE IN FORECAST JUST AVERAGE SINCE PATTERN CHANGES
SOMETIMES ARE DIFFICULT TO NAIL.

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

&&

$$

AHN/JWA