Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 011049
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
348 AM PDT SUN MAY  1 2016

.SYNOPSIS...THE MONTH OF MAY IS STARTING OFF WITH YET ANOTHER ROUND
OF UNSEASONABLY WARM WEATHER FOR SOUTHWEST WASHINGTON AND NORTHWEST
OREGON. A REX BLOCK PATTERN IS SETTING UP OVER WESTERN NORTH
AMERICA...WITH STRONG UPPER RIDGING OVER WESTERN CANADA AND A BROAD
UPPER LOW OVER THE SOUTHWESTERN UNITED STATES. THIS WILL USHER IN
WARM OFFSHORE FLOW TODAY...WITH TEMPERATURES WELL ABOVE NORMAL ALL
THE WAY TO THE COAST. THIS PATTERN BEGINS TO BREAK DOWN A BIT MONDAY
AS A PACIFIC COLD FRONT SLOWLY APPROACHES THE COAST. THIS SHOULD COOL
DOWN THE COAST MONDAY...WITH COOLER TEMPERATURES SPREADING INLAND
TUESDAY AND WEDNESDAY. SHOWER CHANCES WILL INCREASE AS THE FRONT
APPROACHES...WITH ISOLATED THUNDERSTORMS POSSIBLE FOR MUCH OF THE
FORECAST AREA LATE MONDAY INTO TUESDAY. SHOWERS CONTINUE INTO
MIDWEEK...THEN MODELS HINT AT ANOTHER WARMING AND DRYING TREND
DEVELOPING BY NEXT WEEKEND.

&&

.SHORT TERM...AFTER THE WARMEST APRIL ON RECORD SINCE THE 1920S OR
1930S...MAY BEGINS WITH ANOTHER ROUND OF UNSEASONABLY WARM WEATHER
FOR SOUTHWEST WASHINGTON AND NORTHWEST OREGON. A REX BLOCK PATTERN IS
DEVELOPING AS UNSEASONABLY STRONG UPPER RIDGING TAKES RESIDENCE OVER
WESTERN CANADA AND A BROAD UPPER LOW SITS OVER THE SOUTHWESTERN
UNITED STATES. OFFSHORE PRESSURE GRADIENTS APPEAR TO BE HOLDING OFF
UNTIL A WEAK AND DRY UPPER DISTURBANCE RETROGRADES EAST TO WEST
ACROSS OREGON. THE VORTMAX ASSOCIATED WITH THIS DISTURBANCE IS TOUGH
TO PICK OUT ON WATER VAPOR IMAGERY DUE TO HOW DRY THE AIR MASS IS...
BUT IT PRESENTLY APPEARS TO BE OVER BAKER AND MALHEUR COUNTIES IN
SOUTHEAST OREGON. EXPECT AN UPTICK IN EAST WINDS TODAY ONCE THIS
VORTMAX MOVES THROUGH LATER THIS MORNING. THE AFTERNOON SHOULD HAVE A
BIT OF A WARM BREEZE FOR MOST OF THE CWA...THE 06Z NAM REMAINS
CONSISTENT WITH PREVIOUS RUNS IN BRINGING 25-35 KT OF EASTERLY FLOW
AT 850 MB ALONG WITH GOOD MIXING. EXPOSED AREAS MAY SEE SOME 30-40
MPH GUSTS...BUT MOST OF THE DISTRICT SHOULD SEE GUSTS IN THE 20-30
MPH RANGE THIS AFTERNOON. THE EAST WINDS WILL BRING A QUICK RISE IN
TEMPS ONCE THEY BEGIN. WITH 850 MB TEMPS RISING TO +11 DEG C...HIGHS
TODAY SHOULD BE IN THE MID 70S TO LOWER 80S TODAY ALL THE WAY TO THE
COAST.

EAST WINDS APPEAR TO BECOME A BIT MORE CONSTRICTED THROUGH THE GORGE
TONIGHT AND EARLY MONDAY AS THE SYNOPTICALLY DRIVEN EASTERLY FLOW
ALOFT BREAKS DOWN AND VEERS MORE TO THE SOUTH...LEAVING SURFACE
PRESSURE GRADIENTS AS THE ONLY DRIVER FOR THE EAST WINDS. THIS SHOULD
ALLOW THERMAL LOW PRESSURE...INITIALLY PUSHED OFFSHORE BY TODAYS
OFFSHORE FLOW...TO LIFT NORTHWARD OFF THE COAST AS A CLOSED THERMAL
LOW MONDAY. IN TURN...MODELS SHOW A SIGNIFICANT SOUTHERLY WIND
REVERSAL SPREADING UP THE COAST BEHIND THE THERMAL LOW AS MONDAY
PROGRESSES. SINCE THERE IS PRESENTLY VERY LITTLE STRATUS ALONG THE
CALIFORNIA COAST...IT IS TOUGH TO TELL JUST HOW MUCH IN THE WAY OF
FOG AND LOW CLOUDS WILL COME WITH THE WIND REVERSAL. CLOUD COVER OR
NOT...THE SWITCH BACK TO S-SW FLOW WILL PROBABLY COOL THE COAST
CONSIDERABLY ON MONDAY. 500 MB HEIGHTS WILL NOT BE ALL THAT HIGH
ALONG THE COAST MONDAY...AT LEAST NOT WHEN COMPARED TO THE PAST
COUPLE WARM SPELLS. MODEL AVERAGE SEEMS TO BE SOMEWHERE NEAR 570 DAM.
THE LOWER HEIGHTS MAY ALLOW THE MARINE LAYER TO DEEPEN ENOUGH TO COOL
DOWN EUGENE ON MONDAY AS LOW-LEVEL FLOW TURNS SOUTHWESTERLY...OR
MAYBE NOT. MOS GUIDANCE SEEMS FAIRLY INSISTENT ON KEEPING EUGENE
UNSEASONABLY WARM...SO WE LEFT TEMPS NEAR 80 DEGREES AT EUGENE
MONDAY. CONFIDENCE IS HIGHER FOR AREAS SALEM NORTHWARD THAT MONDAY
WILL STAY IN THE 80S FOR THE INLAND VALLEYS...AS IT APPEARS DOUBTFUL
THE MARINE LAYER WILL HAVE A SUBSTANTIAL IMPACT THAT FAR NORTH BY MON
AFTERNOON.

BY MONDAY AFTERNOON...MODELS SHOW ENOUGH INSTABILITY AND MOISTURE
ARRIVING TO CONSIDER CONVECTION. AS USUAL...THE BEST CHANCE FOR
THUNDER WILL BE ALONG THE CASCADES...WHERE ODDS ARE BEST THAT
SURFACE-BASED CONVECTION CAN BREAK A WEAK CAP DUE TO THE ELEVATED
HEAT SOURCE. MODEL MIXED-LAYER CAPE IS NOT QUITE AS IMPRESSIVE ON THE
NAM AS THE MODEL WAS SHOWING LAST NIGHT...BUT 500-1000 J/KG IS STILL
IMPRESSIVE FOR OUR AREA. NOCTURNAL INVERSIONS WILL PROBABLY RESULT IN
A BIT OF A CAP FOR THE WILLAMETTE VALLEY...WITH 100-200 J/KG OF CIN
PER THE 06Z NAM. THIS MAY BE DIFFICULT TO OVERCOME...SO LOWLANDS MAY
HAVE TO RELY ON CONVECTION STARTING IN CASCADES OR COAST RANGE THEN
DRIFTING OVER THE COAST OR INLAND VALLEYS VIA S-SE FLOW ALOFT. ALSO
NOTABLE IN MODEL SOUNDINGS IS HOW DRY THE AIR MASS REMAINS MON
AFTERNOON BELOW 700 MB...00Z GFS SHOWS PORTLAND NEARLY MIXING DOWN
DRY ADIABATICALLY ALL THE WAY FROM 700 MB. SHOULD THERE BE ANY
ORGANIZATION TO CONVECTION MONDAY...THIS COULD ASSIST IN PRODUCING
SOME STRONG DOWNDRAFTS AND OUTFLOW. THE POTENTIAL FOR ELEVATED
CONVECTION IS NOT ZERO MONDAY NIGHT/TUE MORNING...BUT ALSO DOES NOT
LOOK QUITE AS GOOD AS IT DID 24 HOURS AGO WITH LATEST RUNS ONLY
SHOWING WEAK...THIN CAPE ALOFT OR A NEUTRALLY STABLE AIR MASS. LATEST
RUNS ARE SLOWING THE APPROACH OF THE PACIFIC COLD FRONT
SLIGHTLY...WHICH SEEMS REASONABLE GIVEN THE BLOCKING PATTERN. THIS
WOULD LIKELY KEEP WARMER/DRIER AIR ALOFT A LITTLE LONGER AND MAKE IT
MORE DIFFICULT FOR ELEVATED CONVECTION TO DEVELOP. REGARDLESS...
WHETHER IT BE FROM REMNANTS OF SFC-BASED CONVECTION IN THE EVENING OR
ELEVATED CONVECTION OVERNIGHT...FELT IT WOULD BE PRUDENT TO CARRY A
SLIGHT CHANCE OF THUNDER OVERNIGHT MON NIGHT THROUGHOUT THE CWA.

TUESDAY WILL BE A TRANSITION DAY BACK TO MORE SEASONAL EARLY MAY
WEATHER AS ONSHORE FLOW RETURNS. THIS SHOULD RESULT IN A 10-15 DEGREE
COOLDOWN INLAND TUESDAY. INCREASING LOW-LEVEL MOISTURE AND THE
APPROACH OF THE FRONT WILL LIKELY KICK OFF MORE SHOWER ACTIVITY
TUESDAY...THOUGH MODEL SOUNDINGS CONTINUE TO LIMIT THE POTENTIAL FOR
DEEP CONVECTION WEST OF THE CASCADES. WILL HOLD ON TO THE SLIGHT
CHANCE THUNDER TUESDAY COAST RANGE EASTWARD...BUT THE BEST CHANCE
APPEARS TO BE ALONG THE CASCADES AS LOW LEVELS BECOME INCREASINGLY
STABILIZED BY THE MARINE LAYER.

ANOTHER CLIMATE NOTE THIS MORNING...SINCE PORTLAND AIRPORT RECORDS
ONLY GO BACK TO 1940...PDX SHATTERED THEIR RECORD FOR THE ALL-TIME
WARMEST APRIL ON RECORD WITH AN AVERAGE TEMPERATURE OF 57.8 DEGREES
FOR THE MONTH. THIS IS 5.7 DEGREES ABOVE NORMAL AND 1.5 DEGREES
WARMER THAN THE PREVIOUS RECORD OF 56.3 DEGREES SET IN 2004. SITES
WITH LONGER PERIODS OF RECORD DID NOT BREAK THE ALL-TIME APRIL
RECORD...IT APPEARS 1926 AND 1934 ALSO HAD EXTREMELY WARM APRIL
TEMPERATURES. WILL TRY TO GET A PUBLIC INFORMATION STATEMENT OR
RECORD EVENT REPORT OUT WITH DETAILS LATER THIS MORNING.  WEAGLE

.LONG TERM...NO CHANGES. PREVIOUS DISCUSSION FOLLOWS...
TUESDAY NIGHT THROUGH SATURDAY. BROAD UPPER TROUGH REMAINS OVER THE
EASTERN PACIFIC JUST OFF THE CALIFORNIA COAST...PROVIDING FOR SOUTH
TO SOUTHWEST FLOW ALOFT OVER THE REGION. THIS WILL KEEP SHOWER
CHANCES IN PLACE THROUGH THURSDAY. THUNDER CHANCES TUESDAY NIGHT AND
WEDNESDAY AFTERNOON/EVENING WILL BE GREATEST IN THE CASCADES AND
FOOTHILLS...BUT LATEST MODELS SUGGEST ENOUGH INSTABILITY TO KEEP A
SLIGHT CHANCE MENTION FOR THE WILLAMETTE VALLEY. MAY NOT BE ENOUGH OF
A TRIGGER TO TAP INTO THIS...BUT GIVEN THE POTENTIAL IN THE MODEL
SOUNDINGS FEEL A SLIGHT CHANCE REMAINS WARRANTED. EXPECT SOME WARMING
ON FRIDAY AND SATURDAY...WITH PERHAPS SOME DRY CONDITIONS... AS THE
UPPER LOW MIGRATES EASTWARD INTO THE GREAT BASIN AND HIGH PRESSURE
BUILDING INTO THE GULF OF ALASKA AS REX BLOCK DEVELOPS OVER THE
REGION. HOWEVER...MAINTAINED THE FORECAST ALONG THE LINES SUGGESTED
BY A BLEND OF MODELS AS THE ECMWF CONTINUES TO REMAIN WITH A MORE WET
SOLUTION AND SOME SPREAD IN THE GFS ENSEMBLE MEMBERS.

&&

.AVIATION...INCREASING OFFSHORE FLOW WILL BRING VFR CONDITIONS TO
ALL TAF SITES THROUGH 12Z MONDAY. HOWEVER THERE IS INCREASING
CHANCES FOR IFR STRATUS MAY REACH KONP CLOSE TO 12Z MON BUT WILL
NOT INCLUDE IN FORECAST AT THIS TIME.  5SM VISIBILITY AT KAST
SEEMS SUSPICIOUS BASED ON WEB CAMERAS SO KEPT THEM VFR EVEN THOUGH
THE OBSERVATIONS SUGGEST OTHERWISE.

KPDX AND APPROACHES...INCREASING OFFSHORE FLOW WILL BRING VFR
CONDITIONS THROUGH 12Z MONDAY. EASTERLY WINDS WILL INCREASE MIDDAY
WITH GUSTS TO 20 KT OVER EASTERN APPROACHES THROUGH EARLY
EVENING. /MH

&&

.MARINE...A THERMAL TROUGH OVER THE SOUTH OREGON COAST WILL WEAKEN
THIS MORNING EASING WINDS OVER THE WATERS. CURRENTLY NORTH WINDS
ARE GUSTING 25 TO 30 KT SOUTH OF CASCADE HEAD AND WEST OF 10 NM.
06Z ASCAT PASS SHOWED THIS WELL. THE ASCAT DATA SO SHOWED WEAKER
WINDS NORTH OF CASCADE HEAD SO HAVE DROPPED THE SMALL CRAFT
ADVISORY IN THAT AREA. DOMINANT PERIODS CONTINUE AROUND 7 SECONDS
AT BUOY 46050 OFF OF NEWPORT SO WILL CONTINUE THE ADVISORY FOR
STEEP SEAS THROUGH 9 AM FOR THE WATERS SOUTH OF CASCADE HEAD.

THE THERMAL TROUGH SHIFT OFF THE COASTLINE AND OVER THE WATERS
TODAY SETTING UP FOR A SOUTHERLY WIND REVERSAL TO PUSH UP THE
COAST TONIGHT INTO MONDAY AND COULD BRIEFLY BRING A PERIOD OF 20
TO 25 KT WIND GUSTS NOT A CLEAR SIGNAL FOR ADVISORY AT THIS TIME
BUT WILL HAVE TO MONITOR. OTHERWISE...AN UPPER LEVEL TROUGH
SPLITTING AND HEADING SOUTH OF THE REGION SHOULD BRING A RETURN TO
A NORTHERLY WIND PATTERN TOWARDS THE MIDDLE OF NEXT WEEK WITH
SMALL CRAFT ADVISORY LEVEL WIND GUSTS OF 25 TO 30 KT POSSIBLE.
HIGH PRESSURE THEN MAY SHIFT MORE SQUARELY OVER THE NORTHEAST
PACIFIC LATE IN THE WEEK AND FOCUS NORTHERLY WINDS ACROSS THE
CENTRAL OREGON WATERS. EVEN THOUGH SEAS LOOK TO REMAIN UNDER 10 FT
INDEFINITELY...SEAS WILL LIKELY BECOME FRESH SWELL/WIND DOMINATED
DURING THE MIDDLE TO LATTER HALF OF THE WORK WEEK SO IT STILL
LOOKS A BIT CHOPPY AND ROUGH FOR SMALLER BOATS. /MH /NEUMAN

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...NONE.
WA...NONE.
PZ...SMALL CRAFT ADVISORY FOR WINDS UNTIL 11 AM PDT THIS MORNING FOR
     WATERS FROM CASCADE HEAD TO FLORENCE OR FROM 10 TO 60 NM.

     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 9 AM PDT THIS
     MORNING FOR COASTAL WATERS FROM CASCADE HEAD OR TO
     FLORENCE OR OUT 60 NM.

&&

$$

INTERACT WITH US VIA SOCIAL MEDIA
WWW.FACEBOOK.COM/NWSPORTLAND
WWW.TWITTER.COM/NWSPORTLAND

THIS DISCUSSION IS FOR NORTHWEST OREGON AND SOUTHWEST WASHINGTON
FROM THE CASCADE CREST TO 60 NAUTICAL MILES OFFSHORE. THE AREA IS
COMMONLY REFERRED TO AS THE FORECAST AREA.


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