FXUS66 KMFR 260437
AFDMFR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
837 PM PST FRI DEC 25 2009

.DISCUSSION...
CURRENT FORECAST SEEMS ON TRACK FOR SYSTEM AFFECTING THE CWA 
SATURDAY NIGHT INTO SUNDAY. A SPLIT FLOW PATTERN IS RESULTING IN A 
WEAKENING FRONTAL SYSTEM APPROACHING SOUTHERN OREGON.  BLENDED TPW 
IMAGERY INDICATES THE SYSTEM IS LOSING SOME DEEP MOISTURE AS IT RUNS 
INTO A FAIRLY POTENT UPPER RIDGE THAT HAS BEEN OCCUPYING THE PAC NW 
RECENTLY. ADDITIONALLY...ANOMALOUSLY DEEP LOW OVER THE MIDWEST IS
HELPING TO PREVENT ANY INITIATION OF A PROGRESSIVE WEATHER
PATTERN.  NAM AND FORECAST REFLECT A WEAKENING SYSTEM AND LOW QPF
AS LIGHT PRECIPITATION AFFECTS SOUTHERN ORE AND EXTREME NORTHERN
CALIFORNIA SATURDAY NIGHT THROUGH SUNDAY. CONFIDENCE IS FAIRLY
HIGH TEMPS ALOFT WILL COOL SOME TOMORROW...ALLOWING FOR A
WEAKENING INVERSION AND THEREFORE EVENTUAL WARMING AT THE SURFACE
IN SHELTERED VALLEYS. THIS SHOULD PREVENT ANY FROZEN PRECIPITATION
FROM OCCURRING IN THE ROGUE VALLEY THIS WEEKEND. EAST SIDE AREAS
WILL REMAIN MOSTLY DRY THROUGH THE WEEKEND WITH TEMPERATURES AT
OR BELOW NORMAL.

EXPANDED DENSE FOG ADVISORY TO INCLUDE UMPQUA BASIN AS RBG VISIBILITY 
LOWERED QUICKLY THIS EVENING. WHILE CURRENT VISIBILITY AT RBG HAS 
IMPROVED SOME...BELIEVE THAT RBG WILL LOWER AGAIN AND LOCALIZED 
AREAS OF ONE QUARTER MILE VISIBILITY WILL OCCUR IN THE UMPQUA BASIN 
GIVEN STRONGER SUBSIDENCE TONIGHT PER MFR SOUNDING. FOG SHOULD LIFT 
TO ABOVE ONE QUARTER MILE IN MOST LOCATIONS LATE TOMORROW MORNING AS 
AFOREMENTIONED INCREASING INSTABILITY AIDS IN SOME LOW LEVEL MIXING. 
NK

&&

.AVIATION...DENSE FOG AND VLIFR CEILINGS AND VISIBILITY WILL IMPACT 
RBG AND MFR TONIGHT THROUGH SAT MORNING WITH SOME IMPROVING 
CONDITIONS BY THE AFTERNOON.  EAST SIDE AREAS SHOULD REMAIN MOSTLY 
VFR THROUGH SATURDAY.  CLOUDS WILL GRADUALLY LOWER SATURDAY ON THE 
WEST SIDE...AND EXPECT MOUNTAIN OBSCURATION TO BE PREVALENT BY THAT 
TIME.
NK

&&

.PREV DISCUSSION... /ISSUED 244 PM PST FRI DEC 25 2009/ 

SHORT TERM...A STRONG UPPER RIDGE REMAINS OVER THE REGION
HOWEVER AN UPPER TROUGH IS NEARING FROM THE EASTERN PACIFIC. AS
THE UPPER RIDGE WEAKENS AND SHIFTS EAST...THE TROUGH IS EXPECTED
TO SPREAD CLOUDS AND SOME LIGHT RAIN/SNOW ACROSS THE REGION
SATURDAY NIGHT THROUGH SUNDAY. MEANWHILE...CONDITIONS WILL REMAIN
STATUS-QUO. THUS...WE'VE HOISTED ANOTHER DENSE FOG ADVISORY FOR
THE ROGUE BASIN TONIGHT...BUT HAVE NOT INCLUDED THE UMPQUA.
VISIBILITY REMAINED GOOD AT ROSEBURG LAST NIGHT...AND WE DO NOT
EXPECT A SIGNIFICANT IMPACT THERE AGAIN TONIGHT. THE EVENING
SHIFT WILL HAVE TO EVALUATE TRENDS AND DETERMINE IF THE ADVISORY
SHOULD BE EXPANDED. THE FREEZING FOG IN THE ROGUE BASIN WILL AGAIN
RESULT IN ICY SURFACES AND MAYBE SOME EARLY-MORNING SNOW AT THE
MEDFORD AIRPORT PENDING MORE GROUND-BASED FOG SEEDING. THE AIR
STAGNATION ADVISORY CONTINUES UNCHANGED.

FEEL THE NAM12 MIGHT BE THE BETTER MODEL TO FOLLOW WITH THIS
PRECIP EVENT COMING IN LATE SATURDAY THROUGH SUNDAY. IT IS THE
DRIER SOLUTION AND SEEMS MORE REALISTIC GIVEN THE DRY ANTECEDENT
CONDITIONS AND SOUTH TO SOUTHEAST FLOW. THE BEST RAIN/SNOW CHANCES
WILL BE ALONG THE COAST...COAST RANGE...AND SISKIYOU COUNTY. SOME
PRECIP CHANCES WILL CONTINUE MAINLY WEST OF THE CASCADES INTO
MONDAY AS THE UPPER LOW CENTER WRAPS NORTHWARD OFF THE COAST. THE
MODELS ARE STRUGGLING FOR CONSISTENCY IN HOW FAR INLAND THIS LOW
MAKES IT...SO THERE'S NOT A LOT OF CONFIDENCE IN THE FINER DETAILS.
EITHER WAY...THE WHOLE EVENT SHOULD BE LOW QPF AND OF MINOR IMPACT
WITH SNOW LEVELS NEAR THE PASSES. INCOMING CLOUDS AND A LITTLE
WIND SHOULD PREVENT THE WESTERN VALLEY TEMPERATURES FROM FALLING
BELOW FREEZING SATURDAY NIGHT...BUT IT MIGHT BE CLOSE AND THERE IS
A TOUCH OF CONCERN FOR POTENTIAL FREEZING RAIN WHICH BEARS WATCHING.
STAVISH

LONG TERM...ECMWF AND GFS ARE BOTH IN GOOD AGREEMENT WITH THE
SYSTEM THAT MOVES THROUGH THE REGION TUESDAY NIGHT...WITH ALMOST
IDENTICAL JET PROGNOSIS. BOTH REMAIN IN GOOD AGREEMENT WITH WEAK
RIDGE BUILDING BEHIND THIS SYSTEM ON THURSDAY. THE JET SOLUTION
BEGINS TO VARY A BIT WITH THE EC DIGGING THE JET DOWN A BIT
FARTHER SOUTH...DEEPENING THE UPPER TROUGH MORE THAN THE NEAR
ZONAL GFS SOLUTION. HOWEVER...THE TIMING OF FRIDAYS SYSTEM IS
STILL VERY CLOSE WITH BOTH MODELS AND THIS HAS GIVEN ME GREATER
CONFIDENCE TO WALK UP THE POPS SUBSTANTIALLY ON ALL THE SYSTEMS
THAT ARE MOVING THROUGH THE EXTENDED PERIODS. SVEN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...AIR STAGNATION ADVISORY UNTIL 4 PM PST SATURDAY FOR ORZ023-
     ORZ024-ORZ025-ORZ026-ORZ029-ORZ030-ORZ031.
     HIGH SURF ADVISORY FROM 9 AM SATURDAY TO 4 PM PST SUNDAY FOR 
     ORZ021-ORZ022.
     DENSE FOG ADVISORY UNTIL 10 AM PST SATURDAY FOR ORZ023-ORZ024-
     ORZ026.

CA...NONE.

PACIFIC COASTAL WATERS...SMALL CRAFT ADVISORY FOR WINDS FROM 4 PM 
     SATURDAY TO 4 PM PST SUNDAY FOR PZZ370-PZZ376.
     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 4 AM PST TUESDAY 
     FOR PZZ350-PZZ356-PZZ370-PZZ376.

$$