FXUS66 KMFR 280524
AFDMFR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
924 PM PST FRI NOV 27 2009
.DISCUSSION...
THE EXITING UPPER TROUGH IS LIKELY THE LAST REASONABLE
PRECIPITATION PRODUCER ACROSS THE REGION FOR AT LEAST THE NEXT 5
TO 7 DAYS...THOUGH A WEAK AND RATHER COOL SYSTEM COULD BRUSH BY
NORTHWEST SECTIONS AT THE END OF THIS NEXT WEEK AS HIGH PRESSURE
AMPLIFIES INTO THE GULF OF ALASKA. THIS...OF COURSE...DOES NOT
BODE WELL FOR THOSE LOOKING FOR STORM ACTIVITY AND MOUNTAIN
SNOWFALL. ADDITIONALLY...AIR STAGNATION IS LIKELY TO BECOME
SOMETHING OF AN ISSUE THIS NEXT WEEK DUE TO PERSISTENT VALLEY
INVERSIONS RESULTING IN ACCUMULATING PARTICULATES.
HIGH PRESSURE WILL CONTINUE TO BUILD INTO THE AREA FROM THE
EASTERN PACIFIC...SETTING UP OVER THE NORTHERN ROCKIES EARLY NEXT
WEEK. THEREAFTER...THIS HIGH IS MOST LIKELY TO MEANDER BETWEEN THE
CANADIAN ROCKIES AND THE GULF OF ALASKA. BY THE END OF NEXT WEEK
MODEL SOLUTIONS INDICATE THE HIGH PRESSURE RIDGING TO MOST LIKELY
END UP OVER THE GULF OF ALASKA AND ALASKA...BECOMING UNDERCUT BY
INCREASING SOUTHERN STREAM ENERGY.
ULTIMATELY...IT IS UNLIKELY THAT MODEL DETAILS...AND POSSIBLY EVEN
THEIR GENERAL INDICATIONS CAN BE TRUSTED BEYOND DAYS 5 TO 7. A
COHERENT MADDEN JULIAN OSCILLATION THAT HAS RECENTLY MOVED INTO
THE WESTERN PACIFIC IS LIKELY TO HAVE INCREASING EFFECTS ON THE
STORM TRACK OVER THE PACIFIC OVER THE NEXT WEEK OR TWO AS IT MOST
LIKELY PROGRESSES EAST OF THE INTERNATIONAL DATELINE. THERE ARE
STILL QUESTIONS AS TO WHETHER OR NOT THIS WILL OCCUR DURING THIS
WEEK OR NEXT WEEK...BUT...APPROXIMATLEY WHEN IT DOES...THE
CENTRAL WEST COAST IS LIKELY TO BE AFFECTED BY SUBSTANTIAL STORM
ACTIVITY.
FOR THOSE INTERESTED IN THIS EVOLUTION PLEASE REFER TO THE CLIMATE
PREDICTION CENTERS MADDEN JULIAN OSCILLATION WEEKLY UPDATE AND
ASSOCIATED GRAPHICS...LIKELY TO BE UPDATED AGAIN ON MONDAY. LUTZ
&&
.AVIATION...
VFR CONDITIONS CURRENTLY EXIST AT VALLEY AIRFIELDS THROUGHOUT THE
AREA...EXCEPT IN LAKEVIEW WHERE MVFR CONDITIONS EXIST DUE TO
LINGERING CLOUD COVER FROM THE EXITING TROUGH OF LOW PRESSURE AND
A DEVELOPING SUBSIDENCE INVERSION UNDER NORTH TO NORTHEAST FLOW.
EXPECT THAT MARGINAL INSTABILITY PERSISTING THIS EVENING ACROSS
THE AREA TO BECOME INCREASINGLY MORE STABLE AS THE SUBSIDENCE
INVERSION LOWERS AND THE SURFACE COOLS. THUS...MVFR TO IFR CONDITIONS
ARE EXPECTED TO DEVELOP FOR MOST VALLEY LOCATIONS TONIGHT THROUGH
SATURDAY MORNING...PARTICULARLY ON THE WEST SIDE. SOME PATCHY FOG
IS POSSIBLE...THOUGH LIMITING CEILINGS ARE LIKELY TO BE MORE
COMMON DUE TO SOME LINGERING INSTABILITY UNDER THE BUILDING
INVERSION. EXPECT VFR AT ALL LOCATIONS EXCEPT POSSIBLE GRANTS PASS
BY LATE AFTERNOON.
&&
.PREV DISCUSSION... /ISSUED 100 PM PST FRI NOV 27 2009/
DISCUSSION...27/12Z NAM/GFS IN.
THE BACK EDGE OF A COLD FRONT IS NOW OVER FAR EASTERN LAKE COUNTY
AND WILL MOVE OUT OF THE MEDFORD CWA IN THE NEXT HOUR. LIGHT POST-
FRONTAL SHOWERS ARE SCATTERED THROUGHOUT THE CWA. THOSE SHOWERS
ARE BEING FUELED BY COLD AIR ALOFT ASSOCIATED WITH AN UPPER TROUGH
THAT IS NOW IN THE PROCESS OF SHEARING. THE NORTHERN PART WILL
MOVE TO THE EAST OF THE CWA THIS EVENING WHILE THE SHEARED CUTOFF
LOW WILL SET UP OVER CENTRAL CALIFORNIA. EXPECT THE SHOWERS TO END
THIS EVENING.
AN UPPER RIDGE WILL BUILD OVER THE AREA IN THE WAKE OF THE
TROUGH...AND THIS WILL BRING DRY WEATHER TO THE CWA. AS THE RIDGE
MOVES IN...A WARM FRONT WILL MOVE THROUGH THE AREA. AT THIS TIME
IT APPEARS IT WILL BE A DRY ONE. WEAK OFFSHORE FLOW WILL DEVELOP
TONIGHT AND PEAK SATURDAY NIGHT. WHILE THIS WILL BRING MOSTLY
CLEAR SKIES AND WARMER TEMPERATURES TO RIDGES AND PARTS OF THE
COAST...IT WILL ALSO ALLOW STRONG INVERSIONS TO DEVELOP OVER THE
WESTERN VALLEYS...TRAPPING COLD AIR AND CLOUDS IN THOSE LOCATIONS.
THE OFFSHORE FLOW REGIME WILL END MONDAY BUT THE DRY WEATHER WILL
CONTINUE. STRONG OFFSHORE RIDGE PATTERNS SUCH AS THIS USUALLY COME
TO AN END WHEN A LOW UNDERCUTS THE RIDGE...AND THE MODELS INDICATE
THAT THIS WILL BE THE CASE THIS TIME AS WELL. HOWEVER...THERE IS A
LOT OF MODEL-TO-MODEL VARIATION WITH THE LOCATION OF THE LOW AS WELL
AS VARIATIONS FROM RUN-TO-RUN WITHIN THE SAME MODEL. FOR EXAMPLE...
THE 27/12Z MODEL RUNS PLACE THE LOW AT 02/00Z ALONG 130W ANYWHERE
FROM 32N TO 48N...WEST OF LOS ANGELES OR WEST OF VANCOUVER BC.
SO...CONFIDENCE IS LOW IN THE EXTENDED FORECAST...SO WILL CONTINUE
THE CURRENT MOSTLY DRY FORECAST FOR NOW.
FROM THURSDAY ON THE SOLUTION GETS MURKIER WITH THE GFS AND ECMWF
180 DEGREES OUT OF PHASE.
AVIATION...AREAS OF MVFR CIGS IN SHOWERS WITH OBSCURATION OF
HIGHER TERRAIN OVER LAKE AND MODOC COUNTIES WILL CLEAR TO VFR WITH
ISOLATED MVFR CIGS LATER THIS AFTERNOON THIS AFTERNOON. VFR WITH
ISOLATED MVFR CIGS OVER THE REMAINDER OF THE CWA THROUGH THIS
EVENING. AREAS OF IFR CIGS/VSBYS WILL DEVELOP IN THE WESTERN
VALLEY TONIGHT THROUGH SATURDAY MORNING.
&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR...NONE.
CA...NONE.
PACIFIC COASTAL WATERS...SMALL CRAFT ADVISORY FOR WINDS UNTIL 1 AM
PST SATURDAY FOR PZZ356.
SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 4 PM PST TUESDAY
FOR PZZ350-PZZ356-PZZ370-PZZ376.
SMALL CRAFT ADVISORY FOR WINDS UNTIL 10 PM PST SUNDAY FOR PZZ376.
$$
BTL/JRS
