Frost and Freeze in Oregon, August 27, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
100 AM PDT MON AUG 27 2012

.DISCUSSION...27/00Z NAM/GFS IN.

LOTS OF LOW CLOUDS OVER THE COAST AND COAST RANGE THIS MORNING DUE
TO A WEAK FRONT THAT MOVED ONSHORE LAST EVENING AND STALLED.
MEASURABLE PRECIPITATION HAS BEEN RECORDED AT NUMEROUS STATIONS IN
THIS AREA. AMOUNTS ARE LIGHT...GENERALLY 0.10 OR LESS...EXCEPT IN
THE BROOKINGS AREA. THEY ARE GETTING QUITE A BIT OF PRECIPITATION...
GENERALLY AROUND 0.50 INCHES IN THE PAST 6-12 HOURS. RED MOUND HAS
MORE THAN AN INCH SO FAR. CLOUDS ASSOCIATED WITH THE FRONTAL BAND
ARE PUSHING INTO THE UMPQUA BASIN AND ILLINOIS VALLEY...BUT NO
PRECIPITATION HAS BEEN RECORDED IN THOSE AREAS.

INLAND...COLD LOW TEMPERATURES WERE EXPECTED EAST OF THE
CASCADES...BUT TEMPERATURES ARE DROPPING MORE SLOWLY THAN
EXPECTED. MAY NEED TO CANCEL THE FREEZE WARNING/FROST ADVISORY
THERE IF THEY DON'T START DROPPING FAST.

THE MODELS ARE IN GENERAL AGREEMENT THROUGH WEDNESDAY...SO
CONFIDENCE IN THE FORECAST IS GOOD IN THE NEAR TERM.

A CLOSED UPPER LEVEL LOW IS NOW SPINNING NEAR 48N 130W...OR ABOUT
150NM WEST OF THE WASHINGTON COAST. SOME ENERGY DIVING DOWN THE
BACK SIDE OF THE TROUGH WILL CAUSE A FUJIWARI EFFECT...AND THE
WAVE AND THE LOW WILL ROTATE AROUND EACH OTHER THROUGH MONDAY.
THIS WILL SLOW THE EASTWARD PROGRESS OF THE TROUGH. MOST OF THE
ENERGY ASSOCIATED WITH THIS TROUGH WILL REMAIN OFFSHORE...BUT
SOUTHWEST FLOW ALOFT WILL INCREASE IN FITS AND STARTS DURING THIS
INTERVAL.

THE TROUGH WILL BECOME PROGRESSIVE AGAIN TUESDAY AND IT WILL MOVE
ONSHORE TUESDAY EVENING AND TO POINTS EAST WEDNESDAY. SOUTHWEST
FLOW ALOFT LOOKED LIKE IT WOULD GET STRONGER AT THIS POINT ON THE
MODEL RUNS LAST NIGHT...BUT THE NEWER RUNS HAVE BACKED OFF ON
THIS. TUESDAY WILL STILL BE A WINDY DAY OVER THE RIDGES AND EAST
OF THE CASCADES...JUST NOT AS WINDY AS PREVIOUSLY FORECAST.

A SHORT WAVE DIVING DOWN THE BACKSIDE OF THE TROUGH WILL MOVE
ONSHORE WEDNESDAY AND DEPART WEDNESDAY NIGHT.

AS FAR AS WEATHER ON THE SURFACE GOES...THE WEAK FRONT OVER THE
COAST RANGE DISSIPATE IN PLACE LATER TODAY. THE ASSOCIATED
PRECIPITATION WILL BE LIGHT AND LIMITED TO THE COASTAL AREAS AS
THERE IS WEAK UPPER LEVEL SUPPORT FOR THIS FRONT SO FAR FROM THE
PARENT LOW.

THE MAIN IMPACT OF THE INCOMING TROUGH WILL BE IN THE FIRE
WEATHER DEPARTMENT. THE INCREASING WINDS ALOFT WILL MAKE FOR GUSTY
WINDS OVER THE RIDGES AND THESE WINDS WILL SURFACE IN SOME OF THE
EAST SIDE VALLEYS...WHERE RELATIVE HUMIDITIES WILL BE QUITE LOW.
THE WINDS ON MONDAY WILL BE LIGHTER THAN SUNDAY...BUT THE WINDS ON
TUESDAY WILL BE ABOUT THE SAME AS SUNDAY WAS. NO PRECIPITATION IS
EXPECTED FROM MONDAY AFTERNOON ON.

ANOTHER IMPACT WILL BE CLOUD COVERAGE FROM THE CASCADES WEST. ALL
THIS TROUGH ACTIVITY ALOFT WILL CRANK UP THE MARINE LAYER DEPTH.
WOULD NOT BE SURPRISED TO SEE MARINE STRATUS ALL THE WAY TO THE
CASCADES AND SISKIYOUS WEDNESDAY MORNING. THIS WILL HELP MAKE
WEDNESDAY THE COOLEST DAY OF THE COMING WEEK OVER THE INLAND
AREAS.

LONG TERM DISCUSSION FROM THE SUNDAY AFTERNOON AFD...THURSDAY
THROUGH SUNDAY. THE TREND IN THE SECOND HALF OF NEXT WEEK IS FOR
H5 HEIGHTS TO START RISING AS THE TROUGH LIFTS OUT OF THE AREA.
HIGH TEMPERATURES WILL REBOUND TO SEASONAL NORMS, WHICH FOR
MEDFORD IS IN THE MID TO UPPER 80S. BOTH THE GFS AND THE EC BRING
ANOTHER CLOSED LOW TOWARD THE BAY AREA WITH STRONG SOUTHERLY FLOW
INTO NORTHERN CALIFORNIA AND SOUTHERN OREGON. TIMINGS ARE OFF BY
ABOUT A DAY FROM ONE ANOTHER BUT THE SYNOPTIC PATTERNS ARE
SIMILAR. WE WILL HAVE TO KEEP A CLOSE EYE ON THIS FEATURE AS HIGHS
COULD RISE INTO THE MID 90S AGAIN NEXT WEEKEND WITH POTENTIAL FOR
THUNDERSTORMS. /FB

&&

.AVIATION...THE COLD FRONT IS DRAPED OVER THE COAST/COASTAL
MOUNTAINS AND PRODUCING BOUTS OF LIGHT RAIN, REDUCED VISIBILITIES,
AND IFR CEILINGS. GUSTY SOUTH TO SOUTHWEST WINDS ARE ALSO
ASSOCIATED WITH THE FRONT...BUT THEY ARE WEAKENING AT THIS TIME.
AS WINDS SUBSIDE ALONG THE COAST...CEILINGS WILL BECOME IFR UNTIL
LATE MONDAY MORNING. THE FRONT WILL PROVIDE ENOUGH INSTABILITY TO
KEEP CEILINGS IN THE UMPQUA BASIN PREDOMINANTLY JUST ABOVE MVFR.
FURTHER INLAND...KMFR WILL REMAIN VFR WHILE KLMT WILL CONTINUE TO
SEE MVFR CEILINGS AND VISIBILITY DUE TO HEAVY SMOKE FROM WILDFIRES.

&&

.FIRE WEATHER...THE LONG-DURATION WIND EVENT FOR EAST OF THE
CASCADES CONTINUES AND A FIRE WEATHER WATCH IS OUT AT RFWMFR. THE
TROUGH AND ASSOCIATED FRONT WILL WEAKEN MONDAY...AND WINDS WILL
TREND LOWER DURING THE AFTERNOON. HOWEVER...HUMIDITIES WILL DROP
BY 5 PERCENT OR SO. AT THIS POINT...CONDITIONS WILL LIKELY NOT
REACH RED FLAG CRITERIA MONDAY AFTERNOON...BUT WILL HEADLINE IN
OUR FIRE WEATHER FORECAST PRODUCT TO KEEP ATTENTION ON AREAS EAST
OF THE CASCADES. ON TUESDAY...SOME GUIDANCE STILL SUGGESTS
INCREASING WINDS...AND THEREFORE A FIRE WEATHER WATCH CONTINUES
FOR FOR FIRE WEATHER ZONES 624...625...285...AND 284.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...FROST ADVISORY FROM 3 AM TO 8 AM PDT MONDAY FOR ORZ029>031.
     FREEZE WARNING FROM 3 AM TO 8 AM PDT MONDAY FOR ORZ029>031.
     FIRE WEATHER WATCH FROM TUESDAY AFTERNOON THROUGH TUESDAY
        EVENING FOR ORZ029>031.

CA...FIRE WEATHER WATCH FROM TUESDAY AFTERNOON THROUGH TUESDAY
        EVENING FOR CAZ083>085.
     FROST ADVISORY FROM 3 AM TO 8 AM PDT MONDAY FOR CAZ082.

PZ...NONE.

15/15/16