FXUS66 KPQR 120454
AFDPQR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
850 PM PST FRI DEC 11 2009
.SYNOPSIS...A COLD AIR MASS WILL BE RELUCTANT TO MOVE OUT OF THE AREA
THROUGH SATURDAY. IN THE MEANTIME A SYSTEM MOVING INTO CALIFORNIA
WILL ROTATE SOME PRECIPITATION NORTHWARD OVER THE COLD AIR MASS. THE
PRECIP THAT DOES FALL WILL BE FROZEN...WITH MAINLY FREEZING RAIN
SOUTH OF PORTLAND AND A SNOW AND FREEZING RAIN MIX FROM PORTLAND
NORTH. DIFFICULT TO SAY IF ALL OF THE COLD AIR WILL SCOUR BY SUNDAY
AS ANOTHER WEAK SYSTEM SWINGS DOWN FROM THE NORTH...THOUGH WITH
ONSHORE FLOW BELIEVE ANY FROZEN PRECIP WITH THIS SYSTEM WILL BE
SHORT LIVED BEFORE CHANGING OVER TO RAIN. NEXT WEEK LOOKS VERY WET
WITH A LARGE TROPICAL SYSTEM MOVING INTO THE REGION WHICH WILL
LIKELY BRING QUITE A BIT OF RAIN.
&&
THE FORECAST CONTINUES TO BE A CHALLENGING ONE THIS EVENING. AS FAR
AS PRECIPITATION TYPE...THE SOUTH VALLEY AND SOUTH COAST RANGE WILL
LIKELY CONTINUE TO SEE FREEZING RAIN. NO BIG WARM UP IS EXPECTED
THERE AT THE SURFACE AND NO COOLING IS ANTICIPATED ALOFT. THE CENTRAL
VALLEY IS PROBLEMATIC. I THINK WHATEVER FALLS WILL BE MAINLY FREEZING
RAIN AND SLEET. HOWEVER...HEAVIER PRECIP...SEE BELOW...COULD ALLOW
SNOW TO MIX IN AT TIMES THROUGH SATURDAY MORNING. FROM PORTLAND NORTH
AND EAST INTO THE GORGE THE MAIN PRECIP TYPE WILL BE SNOW.
NOW FOR PRECIP AMOUNTS...THE 00Z MODEL RUNS HAVE FOR THE MOST PART
TRENDED DRIER DURING THE OVERNIGHT HOURS AND FURTHER SOUTH WITH THE
BULK OF THE MOISTURE. THAT BEING SAID...THE MM5 NAM...CANADIAN...AND
THE GFS STILL INDICATE DECENT AMOUNTS. IN FACT...THE CENTRAL VALLEY
AND CASCADE FOOTHILLS MAY END UP UNDER THE MAIN DEFORMATION ZONE. I
PLAN ON MAINTAINING THE CURRENT ADVISORIES AND WARNINGS BUT WILL
MASSAGE AMOUNTS TO BETTER REFLECT CURRENT MODEL THINKING.
REMAINDER OF PREVIOUS FORECAST DISCUSSION UNCHANGED...
.SHORT TERM...NOT A LOT OF CHANGE FROM WHAT I SPOKE ABOUT THIS
MORNING SO WILL ELABORATE ON SOME EXTRA FUN DETAILS. THE UPPER HOOD
RIVER VALLEY IS IN A FAVORABLE POSITION FOR DECENT UPSLOPE COMBINED
WITH THE EASTERN EDGE OF DEFORMATION WHICH WILL ENHANCE SNOWFALL IN
THAT AREA. BECOMING CLEAR THAT DEFORMATION BAND WILL BE OVER CENTRAL
OR NORTHERN WILLAMETTE VALLEY THUS UPGRADED ADVISORIES TO WARNINGS
FOR CENTRAL AND NORTH WILLAMETTE VALLEY. ALSO GORGE WILL SEE SOME OF
THE DEFORMATION BANDS INFLUENCE. ALREADY SEEING SOME SNOWFLAKES FROM
OBSERVERS NEAR EUGENE. STILL BELIEVE 4 TO 8 INCHES AND POSSIBLY MORE
IS NOT OUT OF THE QUESTION FOR THE AREA UNDER THE DEFORMATION
BAND...STILL MOST LIKELY JUST SE OF THE PORTLAND METRO AREA THOUGH
STILL NOT EXTREMELY CLEAR. 2 TO 4 INCHES STILL SEEMS REASONABLE FOR
PORTLAND METRO AREA EVEN IF THE BAND DOES NOT PASS DIRECTLY OVER THE
AREA. SALEM WILL BE A MESS WITH WHEN TO TRANSITION OUT OF
FZRA...LIKELY AROUND 6 PM SATURDAY. NORTH COAST SHOULD NOT SEE MUCH
PRECIPITATION THOUGH IT IS COLD ENOUGH THAT ANY THAT DOES FALL WILL
BE FROZEN ESPECIALLY IF WE MAINTAIN OFFSHORE FLOW AS MODELS SUGGEST.
OTHER INTERESTING MODEL PARAMETERS...A LOT OF VERTICAL VELOCITY AT
700 MB SETS UP RIGHT OVER PORTLAND INDICATING LIFT FROM DEFORMATION
BAND. FURTHER DECENT LIFT IN THE DENDRITIC GROWTH ZONE WITH 6
MICROBAR PER SECOND LIFT IN SATURATED AIR BETWEEN -10 AND -15 C OVER
MUCH OF THE VALLEY TONIGHT. WRFGFS BRINGS LITTLE SNOW TO VALLEY FLOOR
BUT BELIEVE IT IS WAY TO FAST IN SCOURING COLD LAYER. AND IF COLD AIR
STAYS AND PRECIP IS CORRECT...WRFGFS WILL GIVE PORTLAND A GOOD DOSE
OF SNOW BRINGING DEFORMATION BAND RIGHT OVER THE METRO AREA. WOLFE
THATS ALL FOR THE UPDATE.
PREVIOUS DISCUSSION FOLLOWS...
ALL FOCUS TURNS TO THE NEXT 36 TO 48 HOURS. IT APPEAR MODELS ARE
BEGINNING TO COME INTO AGREEMENT WITH THE TIMING OF THE ONSET OF
HEAVIER PRECIP BEING SOMETIME SATURDAY...HOWEVER...THERE WILL BE A
WEAKER BAND THAT MOVES THROUGH TONIGHT THAT WILL LIKELY BE IN THE
FORM OF SNOW THOUGH ACCUMULATIONS ARE NOT EXPECTED TO BE MUCH WITH
THIS WEAKER BAND. THOUGH TOMORROW WILL BE A DIFFERENT STORY.
ON SATURDAY MODELS HAVE REALIZED...AT LEAST A LITTLE...THE RELUCTANCE
OF THE COLD AIR TO LEAVE...AND THEREFORE THE PRECIP WILL LIKELY FALL
AS SNOW FROM SALEM NORTH. THAT BEING SAID THERE WILL BE A LOT OF
ACCUMULATION SOMEWHERE THOUGH GIVEN THE DYNAMIC FORCING MECHANISM...A
DEFORMATION BAND...WHICH ACTS TO INCREASE INSTABILITY OVER A NARROW
BAND...IT IS DIFFICULT TO NAIL DOWN WHERE THE HEAVIEST SNOW WILL
FALL. THE ORIENTATION OF THIS BAND WILL BE FROM THE SOUTHWEST TO
NORTHEAST AND AS OF NOW LOOKS LIKE IT WILL BE SOMEWHERE TO THE
SOUTHEAST OF THE PORTLAND METRO AREA. AS DETAILS UNFOLD...WILL TRY TO
ASSESS A MORE ACCURATE PLACEMENT THOUGH. THIS IS NOT TO SAY THAT
PORTLAND WILL NOT GET ANY ACCUMULATION AND ON THE OTHER HAND THE BAND
MAY VERY WELL SET UP OVER PORTLAND. SO BELIEVE A WARNING IS NECESSARY
FOR A LOCATION IN THE NORTH VALLEY OR CENTRAL VALLEY...AND WILL
REANALYZE THIS THIS MORNING BEFORE ISSUING.
THIS IS FOR CERTAIN...ALL MODELS HAVE INCREASED THE LONGEVITY OF THE
COLD AIR. THEY ALL HAVE INCREASED QPF AMOUNTS TO NEAR 0.40 INCHES TO
0.8 INCHES BY SUN MORNING WHICH TRANSLATES TO 4 TO 8 INCHES OF SNOW
IF IT ALL FALLS AS SNOW...EVEN IF IT FALLS AS ICE THIS WILL BE A
MESS. THE GOOD NEWS IS THE HEAVIEST AMOUNTS WILL BE SOMEWHAT LOCAL IN
NATURE. BECOMING FAIRLY CONFIDENT IN PRECIP TYPE. TIMING APPEARS TO
BE LIGHT PRECIP MOST OF THE DAY SATURDAY WITH A HEAVIER BAND SAT
NIGHT. LOW TO MODERATE CONFIDENCE IN WHERE THE HEAVIEST PRECIPITATION
WILL BE. LOW TO MODERATE CONFIDENCE IN WHEN PRECIP WILL CHANGE TO
FZRA OR RAIN IN THE NORTH IF EVER. MODERATE CONFIDENCE IN THE SOUTH
VALLEY.
QUICK PARAGRAPH ABOUT THE COAST...IF FLOW REMAINS OFFSHORE THERE WILL
LIKELY BE SOME FREEZING PRECIP THERE AS WELL THOUGH ACCUMULATIONS
SHOULD NOT BE TO THE MAGNITUDE OF THE VALLEY...MORE LIKE AN EIGHTH OF
AN INCH OF ICE.
PLEASE STAY TUNED AS THIS COMPLEX SCENARIO UNFOLDS IT HAS A LOT OF
POTENTIAL TO CREATE HAVOC IN SOME AREAS AND WILL LIKELY AFFECT MOST
OF THE FORECAST AREA IN ONE WAY OR ANOTHER. WOLFE
.LONG TERM...NO CHANGES. PREVIOUS DISCUSSION FOLLOWS...EXTENDED
MODELS REASONABLY CONSISTENT THIS TIME AROUND.
UPPER FLOW TURNS TO THE W OR SW FOR MUCH OF NEXT WEEK...WITH THE
INITIAL PACIFIC FRONT PUSHING IN BY MON NIGHT. CHANCES FOR RAIN RUN
HIGHER THAN CLIMATOLOGY...WITH A RETURN TO MORE SEASONAL TEMPS.
OFFSHORE FLOW THROUGH THE GORGE AHEAD OF THE FIRST COLD FRONT
SUGGESTS STILL SOME POTENTIAL FOR FREEZING RAIN MON NIGHT AND
POSSIBLY INTO EARLY TUE. OVERALL LITTLE CHANGE PLANNED TO PREVIOUS
FORECAST.
&&
.AVIATION...A BAND OF WINTRY MIX PRECIPITATION CONTINUES TO MOVE
SLOWLY N AND WAS E-W ACROSS THE CENTRAL WILLAMETTE VALLEY FRI
EVENING. FREEZING RAIN AND SOME SNOW ALONG WITH PERIODS OF MVFR CIGS
WERE REPORTED WITH THIS BAND OF PRECIPITATION AS IT SPREAD N FROM
THE SOUTH SOUTH WILLAMETTE VALLEY FRI AFTERNOON INTO THE CENTRAL
VALLEY BY FRI EVENING. FREEZING RAIN IS EXPECTED TO MODERATE TO JUST
RAIN IN THE S WILLAMETTE VALLEY SAT MORNING AND WILL LIKELY BE
ACCOMPANIED BY WIDESPREAD MVFR CIGS IN THE S VALLEY THROUGH MUCH OF
SAT MORNING BEFORE IMPROVING TO VFR CIGS WITH RAIN BY SAT AFTERNOON.
IN THE CENTRAL VALLEY PERIODS OF MVFR CIGS AND FREEZING RAIN OR SNOW
SHOULD PERSIST THROUGH THE NIGHT INTO SAT MORNING. IT IS POSSIBLE
THAT THE PRECIPITATION WILL CHANGE TO RAIN BY SAT AFTERNOON IN THE
CENTRAL VALLEY WITH SPOTTY MVFR CIGS MIXED WITH THE VFR CONDITIONS.
IN THE N WILLAMETTE VALLEY THE PRECIPITATION SHOULD BE
SNOW...POSSIBLY MIXED WITH SOME FREEZING RAIN...BEGINNING AROUND OR
JUST AFTER MIDNIGHT. CIGS SHOULD DROP TO MVFR LEVELS WITH THE SNOW
AND PERSIST THROUGH MUCH OF THE DAY SAT--ALTHOUGH THERE WILL BE
PERIODS OF VFR CIGS IN THE N VALLEY LATER SAT AFTERNOON WITH THE
MIXED SNOW AND SPOTTY FREEZING RAIN CONTINUING.
KPDX AND APPROACHES...VFR THROUGH MIDNIGHT...THEN MOSTLY MVFR WITH
SNOW FROM EARLY SAT MORNING THROUGH SAT AFTERNOON. SOME FREEZING
RAIN MAY BE MIXED IN WITH THE SNOW EARLY SAT MORNING AND AGAIN LATER
SAT AFTERNOON. THE MVFR CIGS MAY IMPROVE AT TIMES LATER SATURDAY
AFTERNOON TO VFR LEVELS.
&&
.MARINE...LIGHT TO MODERATE OFFSHORE FLOW WILL CONTINUE THROUGH
TODAY. A LOW PRESSURE SYSTEM OFF CALIFORNIA COAST WILL
DRIFT NE TOWARD CAPE BLANCO TONIGHT. MUCH OF THE WIND AND SWELL
ENERGY ASSOCIATED WITH THIS SYSTEM WILL STAY SOUTH. STRONGER LOW
PRESSURE WILL MOVE INTO NORTHERN CALIFORNIA SAT/SAT NIGHT...AGAIN
AFFECTING MAINLY CALIFORNIA WITH WIND AND SWELL. SCA WINDS WILL
SPREAD TO SOUTHERN WATERS LATE TONIGHT THROUGH SAT AFTERNOON. STORM
TRACK IS EXPECTED TO LIFT FURTHER NORTH EARLY NEXT WEEK...WITH A
PACIFIC FRONTAL SYSTEM LIKELY TO IMPACT THE PACIFIC NORTHWEST
COASTAL WATERS WITH STRONGER WINDS AND SEAS MONDAY AND TUESDAY.
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
PZ...SMALL CRAFT ADVISORY FOR WINDS LATE TONIGHT THROUGH SAT
AFTERNOON CASCADE HEAD TO FLORENCE
OR...WINTER STORM WARNING FROM 8 PM THIS EVENING TO 6 AM PST SUNDAY
FOR GREATER PORTLAND METRO AREA-UPPER HOOD RIVER VALLEY-
WESTERN COLUMBIA RIVER GORGE.
WINTER WEATHER ADVISORY FROM 5 PM THIS AFTERNOON TO 6 AM PST
SUNDAY FOR NORTHERN OREGON CASCADE FOOTHILLS.
WINTER WEATHER ADVISORY UNTIL 8 AM PST SATURDAY FOR CASCADE
FOOTHILLS IN LANE COUNTY-CENTRAL COAST RANGE OF WESTERN
OREGON-SOUTH WILLAMETTE VALLEY.
WINTER WEATHER ADVISORY FROM 8 PM THIS EVENING TO 6 AM PST
SUNDAY FOR COAST RANGE OF NORTHWEST OREGON-LOWER COLUMBIA.
WINTER WEATHER ADVISORY FROM 8 PM THIS EVENING TO 10 PM PST
SATURDAY FOR NORTH OREGON COAST.
WINTER WEATHER ADVISORY UNTIL MIDNIGHT PST TONIGHT FOR CENTRAL
OREGON COAST.
WINTER STORM WARNING FROM 5 PM THIS AFTERNOON TO 10 PM PST
SATURDAY FOR CENTRAL WILLAMETTE VALLEY.
WA...WINTER STORM WARNING FROM 8 PM THIS EVENING TO 6 AM PST SUNDAY
FOR GREATER VANCOUVER AREA-WESTERN COLUMBIA RIVER GORGE.
WINTER WEATHER ADVISORY FROM 8 PM THIS EVENING TO 6 AM PST
SUNDAY FOR I-5 CORRIDOR IN COWLITZ COUNTY-SOUTH WASHINGTON
CASCADE FOOTHILLS.
WINTER WEATHER ADVISORY FROM 8 PM THIS EVENING TO 10 PM PST
SATURDAY FOR SOUTH WASHINGTON COAST.
&&
$$
MORE WEATHER INFORMATION ONLINE AT...
HTTP://WEATHER.GOV/PORTLAND
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.
