Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

000
FXUS66 KSEW 212152
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT SAT MAR 21 2015

.SYNOPSIS...SHOWERS WILL DECREASE THIS EVENING BEHIND AND UPPER
TROUGH. A COLD FRONT WILL SPREAD RAIN INTO WESTERN WASHINGTON ON
SUNDAY. ANOTHER UPPER LEVEL TROUGH WILL BRING SHOWERS ON MONDAY. A
GENERALLY WET WEATHER PATTERN WILL CONTINUE OVER THE AREA THROUGH
MID NEXT WEEK.

&&

.SHORT TERM...THE AIR MASS IS WEAKLY UNSTABLE THIS AFTERNOON
RESULTING IN SHALLOW TOPPED CU AND SCATTERED SHOWERS. MOST THE
ACTIVITY IS DIURNALLY DRIVEN AS MOST THE DYNAMIC LIFT HAS SHIFTED TO
THE NE WITH THE MAIN UPPER TROUGH. A WEAK CONVERGENCE ZONE OVER WRN
SNOHOMISH COUNTY IS EVIDENT ON RADAR BUT SO FAR HAS NOT DEVELOPED
INTO A SIGNIFICANT PRECIPITATION BAND. THIS REMAINS A POSSIBILITY
THROUGH EARLY THIS EVENING AS MOST MESO MODELS ARE INDICATING A
CONVERGENCE ZONE SIGNATURE...MAINLY OVER WRN SNOHOMISH COUNTY.
OUTSIDE ANY CONVERGENCE BAND...A FEW RESIDUAL SHOWERS WILL LINGER
BUT ALL PRECIPITATION SHOULD COME TO AN END BY LATE THIS EVENING.

A BREAK IN PRECIPITATION IS LIKELY LATER TONIGHT AND EARLY SUNDAY
MORNING. THE NEXT SYSTEM THAT WILL AFFECT THE AREA CAN BE SEEN ON
SATELLITE IMAGERY ALREADY TAKING ON A NEGATIVE TILT. THIS SYSTEM IS
EXPECTED TO SPLIT AS THE SURFACE TROUGH SPREADS RAIN INTO WRN WA ON
SUNDAY. THIS MAY SLOW THE TIMING WITH RAIN REACHING THE COAST IN THE
MORNING BUT PROBABLY NOT SPREADING DEEPER INTO THE INTERIOR UNTIL
AFTERNOON. MODELS TAKE THE BULK OF DEEP MOISTURE AND BEST DYNAMICS
INTO OREGON/NRN CA SO RAIN AMOUNTS ACROSS WRN WA THROUGH THE EVENING
WILL GENERALLY BE UNDER A QUARTER INCH.

MOIST WLY FLOW CONTINUES INTO MONDAY WITH THE HEAVIEST RAINFALL TO
THE S. WRN WA WILL REMAIN ON THE SHOWERY NRN FRINGE OF THE TROUGH.
IT WILL BE COOL WITH SNOW LEVELS HOVERING AROUND 4000 FEET. SOME
ACCUMULATIONS ARE EXPECTED BUT AMOUNTS SHOULD REMAIN UNDER A FEW
INCHES. HIGHS ACROSS THE LOWLANDS WILL BE COOL IN THE LOWER TO MID
50S.

THE FLOW WILL START TO SHIFT NWD ON TUESDAY WITH A WARM FRONT
OFFSHORE. SHOWERS WILL DECREASE BUT STEADIER RAINFALL IS LIKELY TO
PICK UP ACROSS THE AREA AS THE WARM FRONT ARRIVES. MODELS DIFFER ON
TIMING BUT GENERALLY HOLD OFF ON THE STEADIER RAINFALL UNTIL TUESDAY
NIGHT OR WEDNESDAY. GIVEN THE HIGHLY UNCERTAIN TIMING...CHANCE POPS
ARE INDICATED IN THE FORECAST TUESDAY.

.LONG TERM...THE MAJORITY OF 12Z MODEL RUNS BRING WARM FRONTAL RAIN
INTO WRN WA TUESDAY NIGHT AND WEDNESDAY. POPS WERE BOOSTED TO THE
LIKELY CATEGORY GIVEN THE GOOD MODEL AGREEMENT. A BUILDING RIDGE OF
HIGH PRESSURE SHOULD EVENTUALLY SHIFT MOST THE PRECIPITATION TO THE
N. MODELS GENERALLY AGREE THAT SOME RAIN MAY LINGER OVER NRN
PORTIONS OF WRN WA INTO THURSDAY. HIGH TEMPERATURES SHOULD WARM INTO
AT LEAST THE LOW 60S...BUT COULD BE WARMER DEPENDING ON THE
AMPLITUDE OF THE RIDGE.

A COLD FRONT OFFSHORE IS EXPECTED TO PUSH INTO WRN WA FRIDAY AND
FRIDAY NIGHT. THIS SYSTEM SHOULD BRING A SHOT OF RAIN TO WRN WA.
TIMING REMAINS THE MAIN UNKNOWN SO POPS OVER THE INTERIOR ARE
LIMITED TO CHANCE CATEGORY. BOTH THE GFS/ECMWF SHOW RIDGING AND
DRIER WEATHER NEXT WEEKEND. HOWEVER...MODELS HAVE NOT PERFORMED WELL
LATELY AND THE STORM TRACK WILL NOT BE THAT FAR TO THE N. IF THE
RIDGE IS SLIGHTLY WEAKER...MOIST FLOW COULD REACH WRN WA WITH SOME
RAIN. WILL KEEP A CHANCE OF RAIN IN THE FORECAST FOR NOW.  MERCER

&&

.HYDROLOGY...FLOODING IS UNLIKELY OVER THE NEXT 7 DAYS. MERCER

&&

.AVIATION...WEAK HIGH PRES WILL BUILD ALOFT TONIGHT...THEN SOUTHERLY
FLOW ALOFT WILL DEVELOP SUNDAY AS A FRONT APPROACHES FROM THE WEST.
AT THE SURFACE MODERATE ONSHORE FLOW WILL EASE THIS EVENING. HIGH
PRES WITH LIGHT WINDS WILL PREVAIL TONIGHT. THE AIR MASS IS MOIST
AND UNSTABLE BUT WILL STABILIZE TONIGHT. A PUGET SOUND CONVERGENCE
ZONE IS ORIENTED SOUTHWEST-NORTHEAST IN SNOHOMISH AND SKAGIT
COUNTIES. OTHERWISE THERE ARE SCATTERED SHOWERS.

CEILINGS ARE VFR WITH OCCASIONAL DIPS TO MVFR IN SHOWERS. THIS
PATTERN WILL LAST INTO THE EVENING THEN SOME CLEARING IS LIKELY
TONIGHT. PATCHY FOG IS POSSIBLE SUNDAY MORNING. ANOTHER FRONT WILL
BRING CLOUDS AND RAIN SUNDAY AFTERNOON.

KSEA...DISCUSSION ABOVE APPLIES. THE PSCZ IS UNLIKELY TO REACH KSEA
BUT SCATTERED SHOWERS ARE POSSIBLE THIS EVENING. SW WIND 10 KT WILL
BECOME SE AND FALL BELOW 5 KT AFTER 06Z. CHB

&&

.MARINE...GUSTY SOUTHWEST WINDS HAVE EASED AND ALL OF THE SMALL
CRAFT ADVISORIES HAVE ENDED. WINDS ARE GENERALLY LESS THAN 20 KT
THOUGH A FEW SITES MIGHT HAVE GUSTS ABOVE 20 KT FOR A FEW HOURS.

HIGH PRES WILL BUILD OVER THE AREA TONIGHT GIVING LIGHT WINDS...THEN
ANOTHER FRONT WILL MOVE INTO THE COASTAL WATERS SUNDAY MORNING AND
INTO WESTERN WASHINGTON SUNDAY AFTERNOON. SOME SMALL CRAFT ADVISORY
STRENGTH WINDS ARE LIKELY WITH THIS FRONT AS WELL.

ANOTHER WEAK FRONT WILL ARRIVE MONDAY. A STRONGER FRONT IS FORECAST
TO AFFECT THE AREA TUESDAY NIGHT. FOR NOW HAVE KEPT WINDS WITH THIS
SYSTEM IN THE 20-30 KT OR LESS RANGE. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

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













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