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AREA FORECAST DISCUSSION

 
 FXUS66 KEKA 242212
 AFDEKA
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE EUREKA CA
 212 PM PST TUE NOV 24 2009
 
 .SYNOPSIS...DRY WEATHER WILL PERSIST ACROSS THE REGION THROUGH
 WEDNESDAY NIGHT. A FRONT WILL APPROACH THE COAST THANKSGIVING
 DAY...BRINGING A CHANCE OF RAIN TO MUCH OF THE REGION THURSDAY
 NIGHT. THIS FRONT WILL DEPART ON FRIDAY. HIGH PRESSURE WILL BUILD
 OVER THE WEEKEND AND BRING A RETURN OF DRY AND STABLE WEATHER.
 
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 .DISCUSSION...ANOTHER NICE DAY ACROSS THE REGION TODAY UNDER
 LIGHT OFFSHORE FLOW AND A BUILDING RIDGE ALOFT. EXPECT PATCHES
 OF FOG AND LOW CLOUDS TO FORM AGAIN TONIGHT IN THE VALLEYS. THE
 SKY FORECAST FOR THE COAST WILL BE A BIT MORE TRICKY TONIGHT...WITH
 SHALLOW FOG LIKELY DEVELOPING IN THE EVENING. VISIBLE SATELLITE
 IMAGERY DID SHOW AN AREA OF FOG/LOW CLOUD FORMING ABOUT 40-50NM
 FROM PT ST GEORGE...SLOWLY CREEPING TOWARDS THE SHORELINE. THE
 OFFSHORE EASTERLY FLOW SHOULD KEEP THE BULK OF THIS STRATUS AND
 FOG OUT OVER THE WATERS THROUGH WEDNESDAY.
 
 PRIMARY FORECAST PROBLEM IS TIMING OF THE RAIN LATER IN THE WEEK.
 A COLD FRONT CURRENTLY CROSSING 140W LONG ALONG 40N LAT WILL
 APPROACH THE NORTH COAST WED NIGHT. LIGHT RAIN FROM THIS FRONT IS
 EXPECTED AS EARLY AS THURSDAY AFTERNOON/EVENING IN THE WARM AIR
 ADVECTION AND SOUTHERLY FLOW IN ADVANCE OF THE WEAKENING BOUNDARY.
 MULTI-STAGGERED FRONTAL PASSAGE IS EXPECTED THURSDAY NIGHT AS THE
 500MB TROUGH AND COOLING ALOFT CATCHES UP TO AND OVER-RUNS THE
 SURFACE BOUNDARIES. BY 4 AM FRIDAY...UP TO THREE-QUARTERS OF AN
 INCH OF RAIN MAY ACCUMULATE IN DEL NORTE COUNTY...WITH LESS THAN A
 TENTH SOUTH TO UKIAH AND EAST TO WEAVERVILLE. SNOW LEVELS WILL
 REMAIN ABOVE ALL THE MAJOR PASSES DURING THE BULK OF THE
 PRECIP...SO THERE ARE NO WINTER WEATHER TRAVEL CONCERNS AT THIS
 TIME. COLD AIR ALOFT AND SHALLOW INSTABILITY WILL LINGER THROUGH
 THE DAY ON FRIDAY AND WE MAY SEE ISOLATED/SCATTERED SNOW ACCUMULATIONS
 OF 1 INCH OR LESS DOWN TO 2500 FEET OVER THE COASTAL RANGES.
 
 HIGH PRESSURE WILL RE-BUILD OVER THE WEEKEND AND HOLD THROUGH
 EARLY NEXT WEEK FOR GENERALLY DRY AND STABLE WEATHER. GFS AND
 ECMWF HINT AT UNDER-CUTTING FLOW MID NEXT WEEK AND PERHAPS MORE
 RAIN/SNOW BY WED OR THU. DJB
 
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 .AVIATION...WEAK OFFSHORE WINDS WILL DEVELOP TONIGHT AS WEAK HIGH
 PRESSURE WILL PERSIST ACROSS THE REGION. FLIGHT CONDITIONS WILL BE
 MOSTLY VFR WITH THE EXCEPTION OF LOW CEILINGS AND VISIBILITY WITH
 RADIATION FOG WITHIN INTERIOR VALLEYS. THE OFFSHORE WINDS SHOULD
 KEEP ANY STRATUS THAT MAY DEVELOP OVER THE OCEAN OFFSHORE. THINK
 THE WINDS WILL BE WEAKER NEAR KACV OVERNIGHT AND THAT STRATUS MAY
 TEMPORARILY IMPACT THE HUMBOLDT BAY/KACV VICINITY. TH
 
 &&
 
 .MARINE...MODERATE NW SWELL SHOULD REMAIN BELOW ADVISORY CRITERIA
 THROUGH WED NIGHT. WEAK HIGH PRESSURE WILL MAINTAIN MOSTLY LIGHT
 NORTH WINDS THROUGH THU. THE WINDS WILL BE SLIGHTLY STRONGER
 ACROSS THE SOUTHERN WATERS WHERE THE PRESSURE GRADIENT WILL BE A
 TAD TIGHTER.
 
 A LARGER AND LONGER-PERIOD STORM WILL BEGIN TO MOVE THROUGH THE 
 WATERS EARLY THU MORNING. THIS SWELL IS DERIVED FROM A LOW PRESSURE 
 SYSTEM CURRENTLY NEAR 46N 150W. SHIP AND BUOY WIND OBSERVATIONS NEAR 
 THIS LOW VARY LITTLE FROM GFS FORECASTED WINDS...AND THE POSITION OF 
 THE LOW OBSERVED ON SATELLITE IMAGERY ALIGNS WELL WITH GFS MSLP 
 ISOBARS. THEREFORE THERE IS NO REASON TO DEVIATE FROM THE SWELL 
 MODELS FOR THIS EVENT WHICH ARE FORECASTING A W TO NW SWELL PEAKING 
 NEAR 16 FT EARLY THU MORNING...AND SLOWLY SUBSIDING THROUGH FRI.    
 
 THE WIND FORECAST FOR THU NIGHT/FRI WAS EXTRA CHALLENGING 
 AS THE DIFFERENT NUMERIC MODELS VARY GREATLY WITH THE POSITION OF 
 THE EXPECTED LOW. THE NAM IS THE OUTLIER AND HAS A FAR MORE 
 SOUTHERN POSITION OF THE SURFACE LOW...AND MUCH STRONGER WINDS. HAVE 
 BLENDED BETWEEN THE 00Z ECMWF AND 12Z GFS WHICH BOTH FORECAST A 
 NORTHERN POSITION OF THE LOW BUT VARY SLIGHTLY IN TIMING. SOUTHERLY 
 WINDS WILL BE WEAKER WITH THIS SOLUTION THAN THE PREVIOUS 
 FORECAST...AND GALES ARE NO LONGER EXPECTED AHEAD OF THE SYSTEM. 
 NORTH WINDS WILL INTENSIFY FRI AS HIGH PRESSURE BUILDS BEHIND THE 
 TROUGH. TH     
 
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 .EKA WATCHES/WARNINGS/ADVISORIES...NONE.
 
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