Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 021748
AFDMFR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
1048 AM PDT MON MAY 2 2016

.AVIATION...FOR THE 02/18Z TAF CYCLE...CUMULUS ARE ALREADY BUILDING
OVER THE CASCADES AND THE CHANCE FOR SHOWERS AND THUNDERSTORMS HAVE
INCREASED. THE MOST LIKELY LOCATION TO HAVE A THUNDERSTORMS IS IN
KLMT AND THAT HAS BEEN INCLUDED IN 18Z TAF, BUT STORMS MAY ALSO
DRIFT OFF MOUNTAINS AND AFFECT KMFR AND KRBG AS WELL. TAFS HAVE BEEN
UPDATED ACCORDINGLY.  AT THE COAST, A COASTAL TRAPPED WIND REVERSAL
IS UNDERWAY AS THE THERMAL TROUGH MOVES INLAND. STRATUS WITH MVFR
CIGS IS NOW APPROACHING THE SOUTH COAST AND WILL MOVE UP THE COAST
LINE THROUGH THE AFTERNOON. ITS INLAND PROGRESS WILL BE IMPEDED BY
SURFACE HEATING, BUT IT WILL SURGE INLAND TONIGHT. -WRIGHT

&&

.PREV DISCUSSION... /ISSUED 911 AM PDT MON MAY 2 2016/

DISCUSSION...TODAY WILL FEATURE A CHANCE OF SHOWERS AND SLIGHT
CHANCE OF THUNDERSTORMS FOR MOST OF THE FORECAST AREA. HIGH
RESOLUTION MODELS ARE IN MODERATE TO GOOD AGREEMENT ON THE
CASCADES AND UMQUA DIVIDE AND BASIN FEATURING THE BEST CHANCE FOR
SHOWERS AND THUNDERSTORMS. MOISTURE AND TRIGGER ARE ONLY MODEST
TODAY BUT INSTABILITY HAS TRENDED HIGHER IN THE MODELS...AND WITH
FULL SUNSHINE FOR MUCH OF THE DAY THIS SEEMS REASONABLE.

ANY SHOWERS AND THUNDERSTORMS SHOULD DIMINISH OVERNIGHT WITH
INCREASING STABILITY AND EXITING TRIGGER...BUT TOMORROW WILL
FEATURE MORE MOISTURE AND INSTABILITY...RESULTING IN A GREATER
CHANCE FOR SHOWERS AND THUNDERSTORMS. PLEASE SEE THE PREVIOUS
DISCUSSION BELOW FOR MORE DETAILS ON THE MORE ACTIVE WEATHER
PATTERN THIS WEEK.

MARINE...UPDATED 600 AM PDT MON 2 MAY 2016...WINDS WILL BECOME
SOUTHERLY AND REMAIN RELATIVELY LIGHT TODAY INTO TONIGHT. WESTERLY
SEAS WILL BUILD SLIGHTLY TODAY BUT REMAIN BELOW SMALL CRAFT
ADVISORY LEVELS. A COLD FRONT WILL MOVE THROUGH THE WATERS ON
TUESDAY. BEGINNING WEDNESDAY...THE THERMAL TROUGH RETURNS...AND
WITH HIGH PRESSURE BUILDING OFFSHORE...AN EXTENDED PERIOD OF
MODERATE TO STRONG NORTH WINDS APPEARS LIKELY THROUGH THE REST OF
THE WEEK. THIS MAY RESULT IN PERIODS OF HAZARDOUS SEAS OR GALES,
MAINLY FOR AREAS SOUTH OF CAPE BLANCO BEYOND 10 NM WEDNESDAY
THROUGH FRIDAY. /CC

PREV DISCUSSION... /ISSUED 419 AM PDT MON MAY 2 2016/

DISCUSSION...THE GFS AND ECMWF MODELS REMAIN IN GOOD AGREEMENT
THROUGH FRIDAY. TODAY IS LIKELY TO BE THE WARMEST DAY OF THE WEEK
INLAND...JUST A TAD WARMER THAN YESTERDAY. MEANWHILE A RETURN OF
ONSHORE FLOW WILL BRING COOLING TO THE COAST. LOW PRESSURE OVER
THE EASTERN PACIFIC WILL REACH CLOSE ENOUGH PROXIMITY TO BRING
INCREASING MOISTURE IN A SOUTHERLY FLOW ALOFT. THERE IS ALSO
INCREASING INSTABILITY AND A VERY WEAK SHORTWAVE THAT IS EXPECTED
TO BE IN THE VICINITY OF THE SISKIYOU AND CASCADE MOUNTAINS AROUND
PEAK HEATING LATE THIS AFTERNOON. SHOWERS TODAY ARE NOT EXPECTED
TO BE EXTENSIVE BUT ALL THE ABOVE WARRANTS CONTINUING THE MENTION
OF A SLIGHT CHANCE OF AFTERNOON AND EVENING SHOWERS AND
THUNDERSTORMS WITH THE HIGHEST PROBABILITY OVER THE MOUNTAINS.

THE FORECAST IS ALSO CONSISTENT WITH INCREASED MOISTURE AND
INSTABILITY ON TUESDAY AS BOTH BRANCHES IN A SPLITTING UPPER
LEVEL TROUGH MOVE NEARER THE COAST. THE PROBABILITY OF SHOWERS AND
THUNDERSTORMS IS A NOTCH OR TWO HIGHER THAN TODAY...MAINLY IN THE
CHANCE TO LIKELY CATEGORY WITH A SOUTH STEERING FLOW OF ABOUT 20
MPH.

THE PROBABILITY OF SHOWERS WILL PEAK ON WEDNESDAY AS THE SOUTHERN
BRANCH OF THE TROUGH FORMS A CLOSED LOW OFF THE NORTHERN
CALIFORNIA COAST. THE INCREASED CLOUD COVER AND NUMEROUS SHOWERS
WILL LIMIT HEATING AND THUS LIMIT THE NUMBER OF THUNDERSTORMS.
BUT...EMBEDDED SHORTWAVES ARE LIKELY TO RESULT IN ISOLATED
THUNDERSTORMS.

THE LOW WILL TRACK SOUTH-SOUTHEAST TO THE SOUTHERN CALIFORNIA
COAST ON THURSDAY THIS WILL SHIFT THE FLOW ALOFT TO SOUTHEASTERLY
AND CONTINUE YET ANOTHER DAY OF SHOWERS WITH A SLIGHT CHANCE OF
AFTERNOON AND EVENING THUNDERSTORMS. BY FRIDAY...THE FLOW WILL
CONTINUE TO BACK AS THE LOW MOVES INLAND WITH SHOWERS AND A SLIGHT
CHANCE OF THUNDERSTORMS IN THE EAST TO NORTHEAST FLOW ALOFT MAINLY
EAST OF THE CASCADES.

THE WEEKEND FORECAST HAS BEEN IN FLUX WITH MODEL DIFFERENCES ON
THE EXTENT TO WHICH THE TROUGH OVER THE DESERT SOUTHWEST WILL
CONTINUE TO INFLUENCE OUR WEATHER. THE 00Z GFS HAD A VERY CURIOUS
SOLUTION WITH A STRONG SHORTWAVE IN THE EASTERLY FLOW FORMING A
CLOSED LOW AS IT REACHED THE COAST. A THEME OF SOME EARLIER
SOLUTIONS WAS TO HAVE SATURDAY`S WEATHER RESEMBLE FRIDAY.
BUT...THE 06Z GFS HAS ARRIVED AND HAS TRENDED TOWARD THE 00Z ECMWF
WHICH WOULD BRING WARMING AND DRYING FROM A PACIFIC RIDGE WHILE A
FEW WRAP AROUND SHOWERS FROM THE DESERT SOUTHWEST TROUGH MAY
EXTEND INTO THE FAR EAST SIDE. A MENTION OF A SLIGHT CHANCE OF
SHOWERS AS FAR WEST AS WESTERN SISKIYOU COUNTY WAS MAINTAINED AND
WILL WAIT FOR CONFIRMATION OF ANY DECREASED PRECIPITATION RISK.

EARLY NEXT WEEK...THERE IS UNCERTAINTY IN THE EXTENT TO WHICH
RIDGING WILL SET UP OVER THE WEST COAST WITH THE POSSIBILITY OF A
TROUGH DROPPING SOUTHEAST ACROSS THE PACIFIC NORTHWEST WITH
COOLING...SOME INCREASED CLOUDS AND PERHAPS A FEW SHOWERS OVER THE
EAST SIDE.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...NONE.

CA...NONE.

PACIFIC COASTAL WATERS...NONE.

$$



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