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000
FXUS66 KMFR 092235
AFDMFR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
235 PM PST MON DEC 9 2013

.SHORT TERM...A DRY AIRMASS ALOFT REMAINS OVER US THIS AFTERNOON
AND SKIES ARE GENERALLY CLOUD FREE AS RIDGING TO THE WEST
GRADUALLY MOVES OUR WAY. HOWEVER, MOISTURE TRAPPED IN THE
BOUNDARY LAYER UNDER THE BUILDING INVERSION IS KEEPING THE VALLEYS
ON THE HAZY SIDE. SPEAKING OF THE INVERSION: IT WILL CONTINUE TO
STRENGTHEN OVER THE NEXT FEW DAYS AS THE UPPER LEVEL RIDGE MOVES
OVERHEAD AND IT WILL CONTINUE TO TRAP POLLUTION. THUS, THE AIR
STAGNATION ADVISORY WILL CONTINUE THROUGH THURSDAY MORNING.

TEMPERATURES ARE NOT QUITE AS COLD AS YESTERDAY, RUNNING ROUGHLY
5 DEGREES WARMER THAN 24 HOURS AGO ON THE WEST SIDE AND UP TO 15
DEGREES WARMER EAST OF THE CASCADES. THERE IS A WEAK SYSTEM MOVING
INTO WESTERN WA AND NW OREGON THIS AFTERNOON AND IT MAY SPREAD
SOME CLOUD DOWN THE COAST AND INTO THE UMPQUA TONIGHT, BUT IT
ISN`T EXPECTED TO DO ANY MORE THAN THAT. OTHERWISE, SKIES WILL
REMAIN MOSTLY CLEAR TONIGHT. THE EXCEPTION WILL BE IN THE VALLEYS
WHERE THE AFOREMENTIONED MOISTURE WILL ALMOST CERTAINLY TURN INTO
FOG AGAIN TONIGHT. TONIGHT`S LOWS WILL STILL BE QUITE COLD
COMPARED TO NORMAL, BUT THE AIRMASS IS MODERATING, SO THE COLD
WILL GRADUALLY RELEASE ITS GRIP.

ASSUMING THE FOG BURNS OFF AS EXPECTED, TOMORROW SHOULD BE OUR
FIRST RETURN TO ABOVE FREEZING TEMPERATURES SINCE LAST WEEK IN THE
WEST SIDE VALLEYS. THIS WILL ALLOW SNOW TO BEGIN MELTING IN
ERNEST, WHICH AT FIRST GLANCE, SEEMS LIKE GOOD NEWS. HOWEVER, THIS
MELTING AND/OR EVAPORATING SNOW WILL INJECT MORE MOISTURE INTO
THE LOW LEVELS, AND INVERSIONS WILL REMAIN EVERY BIT AS STRONG, SO
THIS MAY INCREASE OUR CHANCES FOR MORE PERSISTENT FOG TOMORROW
NIGHT INTO WEDNESDAY. FOR THIS REASON, WEDNESDAY`S HIGHS AND
CONDITIONS ARE HIGHLY DEPENDENT ON FOG FORMATION AND PERSISTENCE.

THE MODELS ALL SHOW A WEAK SYSTEM BREAKING DOWN THE RIDGE ON
THURSDAY. AS COLD AIR REMAINS TRAPPED IN THE VALLEYS, THERE IS THE
CHANCE FOR SOME FREEZING RAIN BEFORE THE FRONT IS ABLE TO FORCE
OUT THE COLD AIR. THE GOOD NEWS IS THAT PRECIPITATION SHOULD NOT
REACH THE MEDFORD AREA UNTIL AFTERNOON AT WHICH POINT IT WOULD
LIKELY BE JUST RAIN. WEAK SYSTEMS TRYING TO FORCE OUT TRAPPED COLD
AIR ARE NOTORIOUS FOR STARTING AS FREEZING RAIN AND/OR SNOW SINCE
THEY, BY NATURE, LACK THE WIND FIELDS NEEDED TO DISPLACE THE COLD
AIR. IN OTHER WORDS THE WEAKER THE SYSTEM, THE MORE LIKELY
PRECIPITATION STARTS OUT IN SOME FREEZING FORM. THE SECOND CONCERN
IS THAT SINCE IT`S MOVING INTO A RATHER ROBUST RIDGE, IT COULD END
UP BEING LATER THAN EXPECTED, WHICH MAY MEAN A MORE WIDESPREAD
FREEZING RAIN THREAT FOR THURSDAY NIGHT/FRIDAY MORNING. THIS WILL
HAVE TO BE WATCHED CLOSELY. -WRIGHT

.LONG TERM...FRIDAY THROUGH MONDAY NIGHT...

SOME RESIDUAL PRECIPITATION WILL LIKELY REMAIN IN THE AREA FRIDAY
BEHIND THE DEPARTING FRONTAL SYSTEM. THERE IS A POSSIBILITY THAT
COLD AIR REMAINING TRAPPED IN SOME VALLEYS COULD RESULT IN SOME
PRECIPITATION FREEZING ON CONTACT...BUT MUCH WILL DEPEND ON THE
STRENGTH OF THE FRONT AND THE TIMING OF SHOWERS. HAVE ADDED SLIGHT
CHANCE OF FREEZING RAIN TO THE FORECAST FOR PORTIONS OF THE WEST
SIDE INLAND VALLEYS IN OREGON AND THE VALLEYS OF WESTERN SISKIYOU
COUNTY IN CALIFORNIA...BUT CONFIDENCE IS LOW IN SUCH OF AN EVENT AT
THIS TIME. ANY RESIDUAL PRECIPITATION SHOULD COME TO AN END FRIDAY
AFTERNOON OR EVENING.

THE FATE OF THE EXTENDED TERM PAST FRIDAY DEPENDS ON THE STRENGTH OF
A RIDGE BUILDING INTO THE REGION FROM THE EASTERN PACIFIC. THE
ECMWF AND GFS SOLUTIONS FORM A CLOSED LOW TO THE SOUTH WITH RIDGING
TO THE NORTH...CREATING A BLOCKING PATTERN JUST OFF THE WEST COAST.
THE DIFFERENCE IN THE TWO IS THE LOCATION OF THE LOW...WITH THE EC
PLACING IT SLIGHTLY FARTHER WEST. THE END RESULT IS MUCH THE
SAME...HOWEVER...BUILDING THE RIDGE IN OVER THE FORECAST AREA
SATURDAY...THEN BROADENING AND KEEPING IT IN PLACE FOR THE REMAINDER
OF THE FORECAST PERIOD. THIS RESULTS IN WESTERLY TO NORTHWESTERLY
FLOW ALOFT. THIS TYPE OF FLOW COULD PUSH A FEW SHORTWAVES INTO THE
REGION...BUT THERE IS LITTLE CONFIDENCE IN SUCH FEATURES AT THIS
TIME RANGE. MODELS DO RESOLVE A WEAK TROUGH WITHIN THE FLOW PUSHING
ONSHORE TO OUR NORTH SUNDAY...BUT THE ASSOCIATED FRONT WASHES OUT AS
IT MOVES ONSHORE...RESULTING IN MUCH LESS CHANCE FOR PRECIPITATION.
THIS KEEPS WITH A RUN TO RUN MODEL TREND OF A STRONGER RIDGE AND
WEAKER SYSTEMS IN THE LONG TERM...SO HAVE ADJUSTED POPS TO FIT MORE
IN LINE WITH THIS IDEA. HOWEVER...HAVE LEFT SLIGHT CHANCE POPS IN
PLACE ALONG AND WEST OF THE CASCADES TO ACCOUNT FOR THE UNCERTAINTY.

AS ALLUDED TO EARLIER...INVERSIONS WITHIN THE VALLEYS COULD KEEP
COLD AIR TRAPPED THERE FOR QUITE A WHILE...ESPECIALLY IF FIRMLY
LOCKED UNDER HIGH PRESSURE. WEST SIDE VALLEY FOG AND AIR STAGNATION
COULD BE A CONCERN...AND TEMPERATURES WITHIN THE VALLEYS COULD STAY
WARMER AT NIGHT AND COOLER DURING THE DAY DUE TO FOG AND LOW CLOUD
FORMATION. THAT BEING SAID...TEMPERATURES DURING THE EXTENDED PERIOD
WILL REMAIN STEADY AT OR SLIGHTLY ABOVE SEASONAL NORMALS. BPN

&&

.AVIATION...BASED ON THE 09/18Z TAF CYCLE.

MOST AREAS OF WEST SIDE MVFR/IFR IN LOW CLOUDS AND/OR FREEZING FOG
HAVE CLEARED...BUT SOME SPOTTY AREAS REMAIN. MOST IF NOT ALL OF THE
FORECAST AREA SHOULD REMAIN IN VFR FOR THE REMAINDER OF THE DAY AND
INTO THIS EVENING. TONIGHT AND TOMORROW MORNING...MANY WEST SIDE
VALLEYS WILL LIKELY DROP INTO MVFR/IFR IN FREEZING FOG AND LOW
STRATUS...AND COASTAL AREAS COULD SEE SOME AREAS OF LOW CLOUDS.
AGAIN...MOST AREAS SHOULD CLEAR BY LATE MORNING. ELSEWHERE...VFR
WILL PREVAIL THROUGH THE REMAINDER OF THE TAF PERIOD.


&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...AIR STAGNATION ADVISORY UNTIL 4 AM PST THURSDAY FOR
     ORZ023>026-028>031.
     FREEZE WARNING FROM 10 PM THIS EVENING TO 8 AM PST TUESDAY FOR
     ORZ021-022.

CA...NONE.

PACIFIC COASTAL WATERS...SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL
     10 PM PST TUESDAY FOR PZZ350-356-370-376.
     SMALL CRAFT ADVISORY FOR WINDS UNTIL 10 PM PST TUESDAY FOR
     PZZ376.

$$