National Weather Service National Headquarters

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

000
FXUS66 KSEW 200948
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
248 AM PDT MON MAY 20 2013

.SYNOPSIS...AN UPPER LEVEL RIDGE WILL MOVE ACROSS THE AREA TODAY
GIVING A DRY AND WARM DAY TO THE INTERIOR. CONDITIONS ON THE COAST
WILL BE COOLER WITH CLOUDS AND ONSHORE FLOW. BIG CHANGES TO A COOL
AND WET PATTERN WILL BEGIN LATE TONIGHT AND PERSIST INTO NEXT
WEEKEND AS AN UPPER LEVEL LOW MOVES SOUTHEAST FROM THE GULF OF
ALASKA THEN SPINS AROUND OVER THE PACIFIC NORTHWEST.

&&

.SHORT TERM...ENJOY TODAY. WHILE THERE WILL BE AN INCREASE IN HIGH
CLOUDS THIS AFTERNOON...A PASSING UPPER LEVEL RIDGE WILL BRING DRY
AND WARM WEATHER TO THE INTERIOR. CONDITIONS WILL BE COOLER AND
CLOUDIER ON THE COAST AS LOW LEVEL ONSHORE FLOW INCREASES WITH PRE
FRONTAL RIDGING AHEAD OF A WEAK LOW NEAR 46N 136W. TEMPERATURES
ALONG THE COAST WILL REMAIN AROUND 60 TODAY WHILE TEMPERATURES IN
THE INTERIOR RISE TO 65 TO 75 DEGREES.

MODELS ALL ADVERTISE A MAJOR PATTERN CHANGE BEGINNING LATE TONIGHT
AS A STRONG UPPER LEVEL RIDGE OFF SOUTHERN CALIFORNIA RETROGRADES
AND AMPLIFIES ALONG 145W-150W. AS THE RIDGE BUILDS OFFSHORE...AN
UPPER LEVEL LOW OVER THE YUKON WILL DROP SOUTHWARD OFF WESTERN
VANCOUVER ISLAND AND TO JUST OFFSHORE. THIS UPPER LOW AND THE COLD
AIR ASSOCIATED WITH IT GAVE A LATE SEASON SNOWFALL TO PARTS OF
CENTRAL AND EASTERN ALASKA OVER THE PAST COUPLE OF DAYS. THE NAM12
IS SLIGHTLY FASTER THAN THE GFS AND ECMWF SOLUTIONS IN BRINGING
RAINFALL AND COOLING INLAND TONIGHT...BUT THE MODELS OVERALL ARE IN
GOOD AGREEMENT.

THE LOW IS FORECAST TO LINGER AROUND THE PACIFIC NORTHWEST THROUGH
THE END OF THE WEEK. 850 MB TEMPERATURES WILL FALL TO BELOW 0C
AROUND TUESDAY MORNING AND STAY -2C TO -4C THROUGH WED...THEN ONLY
SLOWLY MODERATE AFTER THAT. SNOW LEVELS IN THE MOUNTAINS ARE
EXPECTED TO DROP TO AROUND 3500 FEET LATER TUE INTO WED. MOISTURE
COMBINED WITH THE COOL AND UNSTABLE AIR MASS WILL GIVE ABUNDANT
SHOWER ACTIVITY THROUGH MIDWEEK. TEMPERATURES TUE-WED WILL STRUGGLE
TO GET INTO THE MID 50S AND WILL FALL INTO THE 40S AT NIGHT. THIS IS
5-10 DEGREES BELOW NORMAL FOR LATE MAY.

NAM12 BUFR SOUNDINGS DO NOT SHOW THUNDERSTORMS TUE/WED AS CAPE
REMAINS BELOW ABOUT 300 J/KG IN A SKINNY CAPE PROFILE. CONVECTIVE
TEMPERATURES WILL BE LOW HOWEVER...IN THE UPPER 40S TO AROUND
50...SO GRAUPEL IS LIKELY AROUND HEAVIER SHOWERS STARTING LATER
TUESDAY.  ALBRECHT

.LONG TERM...TEMPERATURES ARE LIKELY TO SLOWLY MODERATE LATE IN
THE WEEK THROUGH NEXT WEEKEND AS THE UPPER LOW SPINS AROUND THE
AREA. SHOWERS MAY DECREASE SOMEWHAT TOWARD THE END OF THE WEEK AS
WELL. FORECASTING THE DETAILS WILL BE QUITE DIFFICULT AS SMALL
CHANGES IN THE POSITION AND STRENGTH OF THE UPPER LOW CAN RESULT IN
BIG SENSIBLE WEATHER CHANGES. NONETHELESS IT APPEARS THAT
TEMPERATURES WILL REMAIN AT LEAST 5 DEG BELOW NORMAL THROUGH NEXT
WEEKEND AND THERE WILL BE QUITE A FEW SHOWERS AROUND THE
AREA...ESPECIALLY OVER THE MOUNTAINS. ALBRECHT

&&

.AVIATION...THE RIDGE IS FLATTENING ACROSS THE PAC NW THIS MORNING
AS A DEEP UPPER LEVEL LOW SPINS INTO NORTHERN B.C. THE FLOW ALOFT IS
WESTERLY. EXPECT SCT-BKN CIRRUS OVER WESTERN WA TODAY WITH FEW-SCT
CUMULUS FORMING THIS AFTERNOON. VFR CONDITIONS WILL PREVAIL. CLOUDS
WILL INCREASE THIS EVENING AS THE UPPER LOW SHIFTS FARTHER SOUTH.
MODELS SHOW DIFFLUENCE OVER THE REGION WITH RAIN DEVELOPING ALONG
THE COAST BY MIDNIGHT. THE INTERIOR WILL SEE PRECIP EARLY TUE
MORNING AS THE UPPER LOW SPINS OFFSHORE AND THE SURFACE LOW MOVES
INLAND. 33

KSEA...VFR CONDITIONS WILL PREVAIL TODAY WITH LIGHT NORTHERLY
SURFACE WINDS. CLOUDS WILL INCREASE TONIGHT WITH RAIN IN THE VC
EARLY TUE MORNING. 33

&&

.MARINE...WEST TO NORTHWEST FLOW WILL PREVAIL ACROSS WESTERN WA
TODAY. PRES GRADIENTS WILL INCREASE TONIGHT WITH SMALL CRAFT
ADVISORY WINDS LIKELY THROUGH THE STRAIT OF JUAN DE FUCA.

A 1010-1016MB LOW WILL TRACK OVER THE OFFSHORE WATERS TONIGHT...THEN
SLIDE INLAND EARLY TUE MORNING OVER SOUTHERN OREGON. ONSHORE FLOW
WILL INCREASE ACROSS WESTERN WA TUE NIGHT AS THIS SYSTEM MOVES
FARTHER INLAND. MODELS ARE SHOWING ANOTHER WEAK SURFACE LOW SPINNING
OVER THE COASTAL WATERS TOWARD THE END OF THE WEEK. HOWEVER EXACT
TIMING AND TRACKING OF THIS FEATURE IS STILL UNCLEAR. MODEL OUTPUT
IS RATHER SWIRLY WITH THE WINDS. FOR NOW WILL STICK WITH THE
PREDOMINANT WEST TO NORTHWEST FLOW ACROSS THE REGION. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

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