Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 162348
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PST TUE DEC 16 2014

.SYNOPSIS...A WEAK TROUGH WILL BRING SPOTTY LIGHT RAIN TO WESTERN
WASHINGTON THROUGH WEDNESDAY NIGHT. A STRONGER PACIFIC FRONTAL
SYSTEM IS FORECAST TO ARRIVE LATER THURSDAY INTO FRIDAY WITH MORE
WIDESPREAD RAIN. YET ANOTHER SYSTEM MAY BRING HEAVY RAIN AT TIMES TO
THE MOUNTAINS AND COAST SUNDAY INTO EARLY NEXT WEEK.
&&

.SHORT TERM...RADAR AND SATELLITE IMAGERY INDICATE CLOUDS AND SOME
SPOTTY LIGHT RAIN ACROSS WRN WA THIS AFTERNOON. THE UPPER LEVEL
TROUGH AXIS STILL OFFSHORE IS PROVIDING MOIST SLY FLOW OVER THE
AREA. DYNAMIC LIFT IS WEAK GIVEN THE SPLIT FLOW ALOFT AND WITH THE
SURFACE TROUGH. THEREFORE...MEASURABLE RAIN IS NOT VERY WIDESPREAD
AND IS MAINLY CONFINED TO THE S AND W OF PUGET SOUND. LIGHT
PRECIPITATION SHOULD EASE OFF OR END FOR A TIME THIS EVENING AS THE
BULK OF MOISTURE AND LIFT DIVES SE INTO CA/OREGON.

THE NRN BRANCH UPPER TROUGH AXIS MOVES THROUGH WRN WA LATER TONIGHT
AND WEDNESDAY. MODELS GENERALLY DEPICT WEAK LIFT AND ONLY LIGHT
SPOTTY QPF FIELDS OVER THE HIGHER TERRAIN OF WRN WA. HOWEVER...THE
21Z HRRR SHOWS MOISTURE FROM THE SRN STREAM SYSTEM BECOMING
ENTRAINED NWD AHEAD OF THE NRN BRANCH TROUGH LATE TONIGHT. THIS IS
DIFFERENT FROM THE GFS40 WHICH SHOWS A MORE PRONOUNCED SPLIT AND
MEAGER QPF FIELDS. SINCE MODELS HAVE NOT SETTLED ON THE DEGREE OF
SPLITTING...SOME CHANCE POPS WILL REMAIN IN THE FORECAST FOR WRN WA
THROUGH WEDNESDAY. THE TROUGH AXIS SHIFTS EWD WEDNESDAY NIGHT WITH
SHOWERS ENDING.

A WEAK RIDGE ALOFT IS STILL EXPECTED TO SHIFT ACROSS WA ON THURSDAY.
MODELS CONTINUE TO DIFFER ON THE TIMING OF THE NEXT SOMEWHAT
STRONGER SYSTEM TO ARRIVE. THE 18Z GFS/12Z ECMWF AND NAM12 SHOW RAIN
HOLDING OFF UNTIL LATE THURSDAY AFTERNOON. THIS IS SLOWER THAN SOME
PAST RUNS AND SLOWER THAN THE 12Z CANADIAN. A STRONG 180+ KT JET
ACROSS THE PACIFIC PUTS THE SLOWER TIMING INTO QUESTION AND WILL OPT
TO LEAVE A CHANCE OF RAIN IN THE FORECAST.

THE FRONT PUSHES THROUGH WRN WA THURSDAY NIGHT BUT WILL
SPLIT...THOUGH LATER THAN PREVIOUS SYSTEMS. QPF AMOUNTS SHOULD BE
MORE WIDESPREAD AND COULD APPROACH A HALF INCH IN THE MOUNTAINS.
SNOW LEVELS WILL BE LOWER THAN WITH PAST SYSTEMS SO A FEW INCHES
DOWN TO 4000 FEET IS POSSIBLE. HIGHER ELEVATIONS SUCH AS THE SKI
RESORTS COULD GET 6 INCHES IF THE SYSTEM DOES NOT SPLIT TOO
SEVERELY. LOWLAND RAIN WILL CHANGE OVER TO SHOWERS LATE FRIDAY OR
FRIDAY NIGHT AHEAD OF THE NEXT STORM SYSTEM.

.LONG TERM...GLOBAL MODELS REMAIN IN GOOD AGREEMENT THAT A MORE
CONSOLIDATED WLY FLOW PATTERN DEVELOPS ACROSS THE PACIFIC OVER THE
WEEKEND. THE STRONG WLY JET WILL DIRECT AN ATMOSPHERIC RIVER OF
MOISTURE INTO WRN WA LATER SATURDAY THROUGH  EARLY NEXT WEEK. THE
INITIAL WARM FRONTAL WAVE WILL BRING STRONG SW FLOW ALOFT AND HEAVY
RAIN TO THE MOUNTAINS SATURDAY AFTERNOON AND NIGHT. MODELS DIFFER ON
HOW LONG THE TRAILING MOISTURE PLUM REMAINS DIRECTED INTO WRN WA.
HOWEVER...MOST MODELS SHOW AT LEAST 24-36 HOURS OF HEAVY MOUNTAIN
RAINFALL BEFORE TAPERING OFF WHICH COULD CAUSE SOME CONCERNS FOR
FLOODING ON PRONE RIVERS. SEE THE HYDRO DISCUSSION FOR FURTHER
DETAILS. RAIN AMOUNTS WILL NOT BE NEARLY HAS HEAVY IN THE LOWLANDS
WHICH SHOULD SHADOW EFFICIENTLY FROM SEATTLE/TACOMA NORTH TO EVERETT
GIVEN THE STRONG W-WSW FLOW. HIGH POPS ARE STILL WARRANTED EVEN WITH
LESS QPF AND IT WILL ALSO BE BREEZY AT TIMES...ESPECIALLY ACROSS THE
N INTERIOR/COAST WHICH IS MORE SUSCEPTIBLE TO STRONG SE SURFACE
GRADIENTS.

THE GFS/ECMWF BOTH SHOW A LULL OR BREAK IN PRECIPITATION AROUND
MONDAY OR MONDAY NIGHT. ANOTHER SURFACE LOW FEATURE TRACKS INTO WRN
WA TUESDAY WITH MORE RAIN. IF THERE IS A BREAK AS SHOWN BY CURRENT
MODELS...THEN ADDITIONAL RAINFALL WILL PROBABLY NOT BE
HYDROLOGICALLY SIGNIFICANT. IF THE LOW IS FASTER WITH LITTLE OR NO
BREAK...THEN IT WILL ADD TO FLOODING CONCERNS.

RAIN TRANSITIONS TO SHOWERS SOMETIME MID WEEK. THE 00Z RUNS OF THE
GFS/ECMWF SHOWED A COLDER NLY FLOW PATTERN DEVELOPING. 12/18Z RUNS
ARE COOL AND DRY BY WEDNESDAY BUT NOT NEARLY AS COLD. IT WILL ALL
DEPEND ON THE DEVELOPMENT OF A HIGH OVER KONA LOW. RIDGING OUTSIDE
OF 140 W WOULD DRIVE COLDER NLY FLOW OVER THE PAC NW. INSIDE 140 W
WOULD STILL LIKELY BE DRY BUT NOT AS COLD. TEMPERATURES ARE TRENDED
DOWNWARD SLIGHTLY WITH HIGHS IN THE LOWER 40S BY WEDNESDAY AND LOWS
IN THE 30S. MERCER

&&

.HYDROLOGY...RIVER FLOODING IS NOT EXPECTED THROUGH SATURDAY. A
MOIST WESTERLY FLOW PATTERN WILL DEVELOP SATURDAY NIGHT THROUGH
EARLY NEXT WEEK. THE COMBINATION OF STRONG FLOW AND A LONG MOISTURE
FETCH MEANS HEAVY MOUNTAIN RAIN IS POSSIBLE IF THE FRONTAL SYSTEM IS
DIRECTED INTO WRN WA. MOST MODEL RUNS SHOW A POTENTIAL FOR 3-6+
INCHES OF RAIN IN 36 HOURS FROM SATURDAY NIGHT THROUGH EARLY MONDAY.
THIS IS POSSIBLY ENOUGH TO CAUSE FLOODING ON SOME OF THE MORE FLOOD
PRONE RIVERS. THE EXTENDED FORECAST SHOULD BE MONITORED CLOSELY. AN
EXTENDED FLOOD OUTLOOK WILL BE ISSUED IF FLOODING BECOMES MORE
CERTAIN.  MERCER

&&

.AVIATION...AN UPPER LEVEL TROUGH OVER THE COASTAL WATERS COMBINED
WITH HIGH PRESSURE WELL EAST OF THE AREA IS GIVING LIGHT SOUTHERLY
FLOW ALOFT. AT THE SURFACE...HIGH PRESSURE EAST OF THE CASCADES
COMBINED WITH A TROUGH OF LOW PRESSURE OFFSHORE IS GIVING LOW LEVEL
OFFSHORE FLOW. THE TROUGH WILL MOVE INLAND ON WEDNESDAY. THE AIR
MASS IS MOIST AND STABLE...EXCEPT IS DRY AT LOW LEVELS OVER THE
INTERIOR VALLEYS. THE AIR MASS WILL BECOME DRIER TONIGHT THROUGH
WEDNESDAY.

DESPITE ABUNDANT CLOUD COVER AND LIGHT PRECIPITATION...LOW LEVEL
OFFSHORE FLOW IS GIVING VFR CONDITIONS TO THE INTERIOR OF WESTERN
WASHINGTON WITH GOOD VISIBILITY AND CEILINGS AROUND FLIGHT LEVEL
035. MARGINAL VFR CEILINGS AND VISIBILITIES ARE OCCURRING ON THE
NORTH COAST AND LOCAL CEILINGS AROUND FLIGHT LEVEL 010-015 ARE BEING
OBSERVED AROUND KOLM. THE LATEST NAM12 AND GFS SOLUTIONS SUGGEST
THAT THE LOWER AND MID LEVELS OF THE AIR MASS WILL DRY TONIGHT AS
THE WEAK TROUGH THAT IS IN THE COASTAL WATERS MOVES ONSHORE. ANY
CLEARING MAY RESULT IN LOCAL IFR CEILINGS AND VISIBILITIES IN
STRATUS AND FOG LATE TONIGHT INTO WED MORNING...ESPECIALLY IN FOG
PRONE VALLEYS SUCH AS AT KOLM. THE LATEST HRRR SHOWS PRECIPITATION
TO THE NE OF A LOW MOVING IN AROUND THE EXTREME SOUTH OREGON COAST
WILL SHOOT NE INTO THE AREA OVERNIGHT. WILL MONITOR SATELLITE AND
RADAR FOR THIS POSSIBILITY...BUT THE INCREASING RAIN SCENARIO
APPEARS UNLIKELY AT THIS TIME GIVEN THE STRENGTH OF THE SPLIT FLOW
ALONG THE WEST COAST. AT THIS TIME THE 00Z TERMINALS WILL FOLLOW THE
NAM12/GFS SCENARIO AND WILL BE SIMILAR TO THE PREVIOUS PACKAGE.
ALBRECHT

KSEA...VFR CONDITIONS WILL CONTINUE WITH BKN-OVC035 OVC060...
PERIODS OF VERY LIGHT RAIN AND VISIBILITIES 6KM OR HIGHER. CEILINGS
ARE EXPECTED TO IMPROVE AND RAIN CHANCES DECREASE ABOUT 04Z THIS
EVENING. EXPECT EAST WINDS 6 TO 9 KT TONIGHT. ALBRECHT

&&

.MARINE...HIGH PRESSURE EAST OF THE CASCADES COMBINED WITH A TROUGH
OF LOW PRESSURE OVER THE COASTAL WATERS IS GIVING LIGHT OFFSHORE
FLOW TO THE AREA. THIS OFFSHORE FLOW WILL CONTINUE TO GRADUALLY EASE
OVERNIGHT AS THE TROUGH SLOWLY MOVES TOWARD THE COASTLINE. A SMALL
CRAFT ADVISORY CONTINUES FOR THE COASTAL ZONES FOR THIS
EVENING...EXCEPT FOR THROUGH LATE TONIGHT OVER THE OUTER COASTAL
WATERS FROM 10 TO 60 NAUTICAL MILES OFFSHORE.

WEAKER PRESSURE GRADIENTS WILL GIVE LIGHTER FLOW ON WEDNESDAY. SMALL
CRAFT ADVISORIES MAY NEED TO BE CONTINUED OVER THE OUTER COASTAL
ZONES WITH SEAS CONTINUING TO BE AROUND 10 FEET. BAR CONDITIONS MAY
BECOME ROUGH AGAIN DURING THE EBB MIDDAY WEDNESDAY.

A COLD FRONT WILL APPROACH THE WATERS LATE THURSDAY AND THURSDAY
NIGHT. GALE FORCE WINDS ARE POSSIBLE WITH THIS FRONT...MAINLY OVER
THE COASTAL WATERS. A GALE WATCH HAS BEEN ISSUED FOR THE COAST 18Z
THU THROUGH 12Z FRI FOR THIS FRONT.

A STRONGER WARM FRONT ON SATURDAY COULD PRODUCES GALES OVER THE
NORTHERN INLAND WATERS...ENTRANCES TO THE STRAIT OF JUAN DE FUCA...
AND COASTAL WATERS.

A LONG FETCH OF STRONG WESTERLY FLOW IS NOW DEVELOPING OVER THE
NORTHWESTERN PACIFIC. WAVES BEING GENERATED IN THIS FETCH AREA WILL
MARCH ACROSS THE PACIFIC THIS WEEK AND WILL LIKELY REACH THE
WASHINGTON COAST AS AN ENERGETIC 20 TO 25 FT SWELL ON FRIDAY. LONG
PERIOD SWELL WITH THIS HEIGHT WOULD PRODUCE BEACH EROSION. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COAST.

&&

$$

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML











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