FXUS66 KMFR 010524
AFDMFR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
924 PM PST MON NOV 30 2009

.DISCUSSION...
DENSE FOG AND FREEZING WILL FORM AGAIN TONIGHT IN LOWER
VALLEYS...FOCUSED IN AND AROUND MEDFORD...GRANTS PASS...THE
ILLINOIS VALLEY AND ROSEBURG. A DENSE FOG ADVISORY MAY NEED TO BE
ISSUED FOR THESE AREAS FOR TUESDAY MORNING...GIVEN CURRENT TRENDS.
SINCE IT WILL TAKE A BIT FOR THE FOG TO GET TO VISIBILITIES OF A
QUARTER OF A MILE OR LESS WILL LET THE NIGHT SHIFT HANDLE THIS.

AN AIR STAGNATION ADVISORY IS IN EFFECT FOR
JACKSON...JOSEPHINE...KLAMATH...AND LAKE COUNTIES THROUGH FRIDAY
MORNING DUE TO AIR STAGNATING IN THE LOWER VALLEYS. WHILE THE BASE
OF THE INVERSION AND MOIST FOG LAYER HAS GENERALLY BEEN CONFINED
TO ELEVATIONS BELOW 2000 FEET MSL...THE TOP OF THE INVERSION HAS
BEEN AROUND 6350 FEET MSL...OR ABOUT THE BASE OF MOUNT ASHLAND SKI
AREA...FOR THE LAST 12 HOURS OR MORE. THUS...WHILE SKIES REMAIN
CLEAR ABOVE THE FOG LAYER...THE AIR IS MIXING VERY LITTLE BELOW
ABOUT 6350 FEET ELEVATION. THIS EVENING'S AIR QUALITY VALUES HAVE
FALLEN TO UNHEALTHY LEVELS FOR SENSITIVE GROUPS FOR PM2.5 IN
MEDFORD AND KLAMATH FALLS...AND TO MODERATE FOR LAKEVIEW.

THE HIGH PRESSURE RIDGE IS LIKELY TO REMAIN OVER THE REGION
THROUGH THE END OF THE WEEK. THEREAFTER THE MODELS DIVERGE ON
DETAILS...BUT AGREE THAT A WESTERLY WIND BURST ASSOCIATED WITH A
MADDEN JULIAN OSCILLATION AND EL NINO ARE LIKELY TO BRING ABOUT A
MAJOR PATTERN CHANGE TO THE WEST COAST. ITEMS OF SIGNIFICANCE TO
WATCH IN THE PATTERN SHIFT...AMONG OTHERS DISCUSSED THE PREVIOUS
TWO EVENINGS...WILL BE THE EXTENT OF TROUGHING AND COLD AIR THAT
SLIPS OUT OF WESTERN CANADA AND INTO THE WESTERN UNITED STATES
EARLY NEXT WEEK...AND THEN EXACTLY WHERE THE STORM TRACK SETS UP.
THE ECMWF OPERATIONAL RUN IS THE MOST AGGRESSIVE WITH A RAPID
PATTERN BREAKDOWN...WHEREAS THE GFS40 HAS TENDED TO BE SLOWER WITH
MORE ENERGY FOCUSED FARTHER SOUTH. THE BOTTOM LINE IS THAT AS
EARLY AS THIS WEEKEND...BUT MORE LIKELY EARLY NEXT WEEK...STORMS
ARE LIKELY TO IMPACT THE WEST COAST. THESE WILL PACK SIGNIFICANT
MOISTURE...AND ARE LIKELY TO COME IN RAPID SUCCESSION. THE CLIMATE
PREDICTION CENTERS MADDEN JULIAN OSCILLATION EXPERT UPDATE FROM TODAY
INDICATES ENHANCED CHANCES FOR ABOVE AVERAGE PRECIPITATION FOR
PRIMARILY THE CENTRAL US WEST COAST DURING THE SECOND WEEK OF
DECEMBER. LUTZ

&&

.AVIATION...
HIGH CLOUDS MOVING THROUGH TONIGHT WILL NOT BE THICK OR EXTENSIVE
ENOUGH TO STAVE OF FORMATION OF EXTENSIVE FOG IN THE WESTERN
VALLEYS AGAIN TONIGHT. IN MOST PLACES THIS FOG HAS ALREADY FORMED
AND WILL ONLY THICKEN. THE BASE OF THE INVERSION ON THIS
AFTERNOON'S MEDFORD SOUNDING WAS LESS THAN 100 FEET LOWER THAN WAS
OBSERVED YESTERDAY...THOUGH THE MOIST LAYER IS ABOUT 1000 FEET
LOWER. ULTIMATELY THIS MEANS THAT THE LOWER VALLEYS ARE LIKELY TO
BE IN FOR DENSE FOG MOST OF THE NIGHT...WHILE ELEVATIONS ABOVE 2000
FEET ARE LIKELY TO REMAIN MOSTLY FOG FREE. THEREFORE...EXPECT
MEDFORD...GRANTS PASS...ROSEBURG...AND THE ILLINOIS VALLEY TO BE
SOCKED IN WITH IFR TO LIFR CIG AND VIS THROUGH EARLY AFTERNOON. IT
IS UNLIKELY THAT GRANTS PASS AND ROSEBURG WILL CLEAR OR GO ABOVE
IFR CONDITIONS AT ALL ON TUESDAY...AND QUESTIONABLE FOR MEDFORD.
VALLEY EDGES LIKE ASHLAND AND OTHER AREAS ABOVE 2000 FEET
ELEVATION...HOWEVER...WILL LIKELY REMAIN VFR THROUGHOUT THE
PERIOD.

&&

.PREV DISCUSSION... /ISSUED 254 PM PST MON NOV 30 2009/ 

SYNOPSIS...AN UPPER LEVEL RIDGE EXTENDS FROM SOUTHERN OREGON TO
OFF THE COAST OF CALIFORNIA. A WEAK UPPER TROUGH WILL MOVE BY TO
THE NORTH AND EAST OF THE CWA TOMORROW, BUT WILL HAVE LITTLE OR NO
NOTICEABLE IMPACT ON THE OVERALL WEATHER PATTERN. FOR THE MOST
PART...THE RIDGE WILL REMAIN IN CONTROL FOR THE REST OF THE WORK
WEEK. THE NEXT STORM SYSTEM WILL MOVE INTO THE AREA FRIDAY NIGHT
INTO SATURDAY.

SHORT TERM...THE NEXT FEW DAYS WILL BE DOMINATED BY AN UPPER
LEVEL RIDGE. THIS WILL RESULT IN A PERSISTENT STABLE WEATHER PATTERN 
OVER SOUTHERN OREGON AND NORTHERN CALIFORNIA. CALM/LIGHT 
WINDS WILL LEAD TO OVERNIGHT TEMPERATURE INVERSIONS TONIGHT THROUGH 
FRIDAY MORNING. THIS WILL TRAP MOISTURE NEAR THE GROUND. AS A 
RESULT, THE LOW CLOUDS AND FOG THAT HAVE BEEN AROUND OVER THE WEST 
SIDE VALLEYS AND IN THE UMPQUA BASIN THE LAST COUPLE OF NIGHTS WILL 
REMAIN/REFORM IN THOSE AREAS AGAIN. ALSO...AN AIR STAGNATION 
ADVISORY HAS BEEN ISSUED TO HIGHLIGHT THE POTENTIAL FOR POOR AIR 
QUALITY DUE THE LIMITED MOVEMENT OF THE AIR MASS. VALLEY CLOUDS AND 
FOG WILL GENERALLY BREAK FOR SOME SUNSHINE DURING THE LATE-MORNINGS 
OR EARLY AFTERNOONS...WITH SUNNY CONDITIONS ELSEWHERE. 

LONG TERM...BY THE END OF THE WEEK, THE MEDIUM RANGE GUIDANCE 
CONTINUES TO INDICATE CHANGES WILL BE UNDERWAY AND THE PERSISTENT 
PATTERN ALONG THE WEST COAST WILL TRANSITION TO ONE THAT IS MORE 
ACTIVE. BUT, JUST HOW ACTIVE IS STILL IN QUESTION. IT WILL BEGIN 
WITH A DISTURBANCE THAT DIGS SOUTHEASTWARD FROM THE NORTH PACIFIC 
FRIDAY NIGHT. THIS SYSTEM IS FORECAST TO MOVE INTO THE PACNW 
SATURDAY. MODEL SOLUTIONS IN REGARDS TO WHAT KIND OF WEATHER 
THIS LOW BRINGS TO SOUTHERN OREGON AND NORTHERN CALIFORNIA THIS 
WEEKEND AND EARLY NEXT WEEK STILL DIFFER SIGNIFICANTLY. THE 
CANADIAN/GFS CONTINUE TO SUGGEST BELOW AVERAGE TEMPERATURES AND 
LIGHT PRECIPITATION WHILE THE ECMWF MERGES THIS LOW WITH ANOTHER LOW 
UNDERCUTTING A HIGH AMPLITUDE RIDGE OVER THE NORTHEAST PACIFIC. THIS 
WOULD BRING A PERIOD OF SIGNIFICANT PRECIPITATION TO THE AREA WITH 
TEMPERATURES NEAR NORMAL FOR THIS TIME OF YEAR. DUE TO THE 
UNCERTAINTY WILL NOT MAKE A LOT OF CHANGES AT THIS TIME.

AVIATION...LOW CLOUDS AND FOG HANGING TOUGH IN A FEW OF THE LOWER
WEST SIDE VALLEYS AND THE UMPQUA BASIN THIS AFTERNOON. EXPECT
THESE MVFR/IFR CIGS TO PERSIST THROUGH TONIGHT, WHILE THE VALLEYS
THAT HAVE CLEARED THIS AFTERNOON WILL SEE THE FOG AND CLOUDS MOVE
BACK IN AROUND 06Z. LIFR CONDITIONS WILL LIKELY DEVELOP AGAIN AT
MFR AND LAST UNTIL LATE TOMORROW MORNING/EARLY TOMORROW AFTERNOON.
MOSTLY VFR ELSEWHERE THROUGH TOMORROW.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...AIR STAGNATION ADVISORY UNTIL NOON PST FRIDAY FOR ORZ024-ORZ026-
     ORZ029-ORZ030-ORZ031.

CA...NONE.

PACIFIC COASTAL WATERS...SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS 
     UNTIL 4 AM PST TUESDAY FOR PZZ350-PZZ356.
     SMALL CRAFT ADVISORY FOR WINDS UNTIL 4 AM PST WEDNESDAY FOR 
     PZZ376.
     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 10 PM PST TUESDAY 
     FOR PZZ370-PZZ376.

$$

BTL/SPILDE