Flooding and Heavy Rain in Washington, December 1 to 4, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
318 AM PST SAT DEC 01 2007

.SYNOPSIS...DECEMBER TO START...WILD. COLD AIR MASS OVER THE REGION
TODAY WILL BE REPLACED WITH A MILD WET PACIFIC AIR MASS BY SUNDAY.
HOWEVER...TRANSITIONS ARE ALWAYS MESSY AS THIS WILL BE...WITH SNOW
POTENTIAL SAT AND SAT NIGHT ACROSS THE INTERIOR. BY SUN...STRONG
WARM WINDS WILL BUFFET THE REGION...WITH A LOT OF RAIN ARRIVING AS
WELL.
&&

.SHORT TERM...A LOT TO CONSIDER...BUT RUNNING LOW ON TIME TO
DISCUSS...SO WILL TRY TO JUST HIT ON SOME OF THE HIGHLIGHTS. SOME
LIGHT SHOWERS OVER THE AREA EARLY THIS MORNING ASSOCIATED WITH THE
UPPER LEVEL TROUGH...NOT AMOUNTING TO MUCH...BUT WITH THE CURRENT
TEMPERATURES...IT DOES INDICATE SOME LOW ELEVATION SNOW POSSIBLE IN
THE EARLY GOING AS WE SAW A FEW SNOWFLAKES EARLIER IN THE NIGHT HERE
AT THE OFFICE WITH A TEMP AROUND 35. MODELS STILL CONSISTENTLY
DEVELOPING A LOW OFF THE N WA COAST...WITH LIGHT PRECIPITATION
DEVELOPING TODAY. INITIALLY AIR MASS IS COLD ENOUGH TO SUPPORT SNOW
AT LOW ELEVATIONS...THEN AS SOUTHERLY FLOW KICKS IN LATER TODAY AIR
MASS BEGINS TO MODIFY...ALTHOUGH SNOW LEVELS BY THE END OF THE DAY
ONLY LOOK TO GET UP TO AROUND 1000 FEET IN THE N WILLAMETTE VALLEY
AND LOWER COLUMBIA VALLEY. EXPECT MAIN ACCUMULATING SNOW TODAY TO BE
GENERALLY ABOVE 500 FEET. SNOW LEVELS TO GRADUALLY RISE
OVERNIGHT...WITH THE BIGGER JUMP EXPECTED SUN WITH THE ARRIVAL OF THE
FIRST WARM FRONT IN THE DEVELOPING FLOW SW FLOW FROM ACROSS THE
PACIFIC.

ELSEWHERE...EXPECT TO LEAVE REMAINING HEADLINES ALONE FOR NOW. HIGH
WINDS LIKELY ALONG THE COAST AND COAST RANGE SUN INTO MON...WITH PEAK
WINDS LIKELY OCCURRING WITH THE FRONTS SUN MORNING AND LATE SUN NIGHT
INTO MON MORNING. MODELS HAVE NOT BACKED OFF ON THE STRENGTH OF THE
FLOW...SO SOME GUSTS TO 100 MPH NOT OUT OF THE QUESTION ALONG THE
COAST WITH THE SUN NIGHT/MON FRONT...AND NOT MUCH LESS INTO THE COAST
RANGE. SOME GUSTY WINDS INTO THE INLAND VALLEYS ARE POSSIBLE
TOO...ESPECIALLY SUN WITH THE EXPECTED TRACK OF THE LOW COMING INTO
WESTERN WA AND TRACKING ACROSS PUGET SOUND. MODELS HAVE TRENDED
TOWARDS A LITTLE MORE FAVORABLE N TO S SURFACE PRESSURE GRADIENT WITH
THE SUN LOW...WHICH AT THIS POINT LOOKS LIKE WINDS WOULD ULTIMATELY
FALL INTO THE WIND ADVISORY RANGE FOR THE VALLEY.

WINTER STORM WARNINGS ALREADY OUT FOR THE CASCADES AND HOOD RIVER
VALLEY...WITH THE S WA CASCADES LIKELY TO SEE THE GREATEST AMOUNTS
DUE TO OROGRAPHICS ENHANCING THE SW FLOW. IN THE GORGE FOR LATER
TODAY THROUGH SUN MORNING PREFER TO LEAVE IT AS A WATCH FOR ONE MORE
FORECAST CYCLE AS TEMPERATURES ARE CRITICAL TO SNOWFALL POTENTIAL.
EARLY THIS MORNING TEMPERATURES IN THE LOWER TO MID 30S WOULD
PROBABLY SUPPORT SNOW FOR THE GORGE...BUT IT IS NOT CLEAR HOW LONG
THE COLD AIR WOULD LAST. MOSTLY LIKELY CASE SCENARIO WOULD BE FOR AIR
TO WARM WITH THE WARM FRONT LIFTING THROUGH SUN MORNING...BUT NOT
ENOUGH CONFIDENCE TO GO WITH WARNING QUITE YET.

SUN WARM FRONT SEES THE ARRIVAL OF THE FIRST TROPICALLY INFLUENCED
AIR MASS. STRONG SW FLOW SUN THROUGH MON SUGGEST POTENTIAL FOR HEAVY
RAINFALL...ESPECIALLY ALONG THE COAST AND INTO THE COAST RANGE...
WHERE FLOODING POTENTIAL WILL NEED TO BE WATCHED.

.LONG TERM...NO CHANGES. PREVIOUS DISCUSSION FOLLOWS...
MODEL TRENDS BEYOND THIS POINT HAVE NOT BEEN CONSISTENT...WITH MID
RANGE MODELS CARRYING DIFFERING SOLUTIONS THROUGH THE END OF THE
WEEK. MODELS VARY AS THEY DEAL WITH THE MOISTURE REMAINS FROM THE
TRANSFORMED TYPHOONS MITAG AND HAGIBIS WHICH ARE NOW IN THE WESTERLY
FLOW OVER THE WESTERN PAC. BOTTOM LINE... A LOT OF TROPICAL MOISTURE
IS POISED TO ARRIVE IN THE PAC NW NEXT WEEK. HAVE KEPT FORECAST THE
SAME FROM TUESDAY ONWARD FOR CONSISTENCY. BROWN
&&

.AVIATION...FLIGHT CONDITIONS ARE ALL OVER THE PLACE THIS MORNING.
COASTAL SITES WILL REMAIN VFR THROUGH ROUGHLY 14Z-16Z BEFORE
DROPPING DOWN TO MVFR CIGS FOR THE REMAINDER OF THE DAY. WILL CONT
TO SEE MIX OF MVFR AND VFR CIGS...WITH PATCHES OF IFR FOG...THROUGH
14Z. AFTERWARD EXPECT PREDOMINANT CIGS TO BE MVFR. LIGHT PRECIP
SHOULD BEGIN TO FALL ACROSS THE NRN HALF OF THE CWA BY 18Z. THIS
PRECIP WILL LIKELY BEGIN AS A RAIN/SNOW MIX AT THE COAST...ALL SNOW
AT NORTH INTERIOR SITES. SHOULD SEE VSBY FALL TO IFR LEVELS DURING
SNOW SHOWERS. BY 18Z RAIN SHOULD TURN TO ALL SNOW...STARTING AT THE
COAST WITH A GRADUAL CHANGE TO RAIN EXPECTED INLAND BY 00Z.

KPDX AND APPROACHES...MVFR CIGS WILL CONTINUE THROUGH MUCH OF THE
DAY. PRECIP SHOULD START FALLING AT KPDX SHORTLY AFTER
15Z...BEGINNING AS -SN. EXPECT CHANGEOVER TO ALL RAIN BY 00Z.
FREEZING LVLS WILL REMAIN VERY LOW SO A RAIN/SNOW MIX WILL BE
POSSIBLE THROUGH LATE THIS EVENING. 70 PERCENT CONFIDENT IN COMPLETE
CHANGEOVER TO RAIN BY 00Z AT KPDX.  DALTON

&&

.MARINE...BENIGN CONDITIONS WILL GIVE WAY TO SMALL CRAFT WINDS EARLY
THIS AFTERNOON. SW WINDS THEN INCREASE TO GALE FORCE ALL WATERS BY
MIDNIGHT. GALES WILL CONTINUE THROUGH SUNDAY MORNING...EXCEPT FOR
POSSIBLY THE FAR NORTHWEST WATERS. STORM FORCE WINDS WILL ACCOMPANY
A ROUGHLY 987MB LOW AS IT TRACKS FROM ABOUT 200 NM W OF ASTORIA SUN
MORNING...ACROSS THE PUGET SOUND BY SUN AFTERNOON. SHOULD SEE STORM
FORCE WINDS FROM 18Z SUN THROUGH 00Z MON. TIMING BETWEEN MODELS IS
SLIGHTLY DIFFERENT...BUT EXPECT TO SEE HURRICANE FORCE WINDS COVER
THE WATERS LATE SUNDAY NIGHT THROUGH LATE MONDAY MORNING. COMBINED
SEAS WILL CLIMB FROM THE LOW TEENS...TO AROUND 22 FT AFTER MIDNIGHT
TONIGHT. SEAS THEN CREEP ABOVE 30 FT SUNDAY NIGHT AND APPROACH 40 FT
BY MONDAY MORNING.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
WA/OR...SMALL CRAFT ADVISORY FOR WINDS THIS AFTERNOON AND EVENING.
        GALE WARNING THIS EVENING THROUGH LATE LATE TONIGHT.
        STORM WARNING FROM LATE TONIGHT THROUGH SUNDAY AFTERNOON.
        HURRICANE FORCE WINDS EXPECTED SUNDAY NIGHT THROUGH MONDAY
        MORNING.
        SNOW ADVISORY THIS AFTERNOON FOR THE N WILLAMETTE VALLEY...
         CLARK COUNTY...AND I-5 CORRIDOR.
        SNOW ADVISORY THIS AFTERNOON AND EVENING FOR THE N OREGON
         COAST RANGE AND THE WILLAPA HILLS.
        HIGH WIND WATCH FOR SUN THROUGH MON FOR THE N AND CENTRAL
          OREGON COAST...S WA COAST...WILLAPA HILLS AND THE N AND
          CENTRAL OREGON COAST RANGE.
        WINTER STORM WARNING FROM SAT AFTERNOON THROUGH SUN FOR THE
          S WA CASCADES AND FOOTHILLS...AND N OREGON CASCADES.
        WINTER STORM WARNING FOR THE UPPER HOOD RIVER VALLEY FOR
          SAT AFTERNOON INTO SUN MORNING.
        WINTER STORM WATCH FROM SAT AFTERNOON INTO SUN MORNING FOR
          THE W COLUMBIA GORGE.
&&

.PRELIMINARY POINT POPS (PROBABILITY OF MEASURABLE PRECIPITATION)
AST +++++   PDX 8++++   SLE 7++++  EUG 6++9+

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.