018
FXUS66 KMFR 180312
AFDMFR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
812 PM PDT FRI MAY 17 2013
DISCUSSION
THE AXIS OF AN UPPER LEVEL TROUGH WILL CONTINUE TO
SHIFT EAST OF THE AREA TONIGHT. EVIDENCE OF THIS IS IN THE LAST
FEW IMAGES OF THE VISIBLE SATELLITE PRIOR TO SUNSET...WHERE STRONG
SUBSIDENCE TO THE LEE OF THE CASCADES LED TO RAPID CLEARING.
THERE STILL MAY BE AN ISOLATED SHOWER FROM NEAR MOUNT SHASTA
EASTWARD TO ALTURAS, THEN UP TO LAKEVIEW THROUGH ABOUT 11PM, BUT
THAT'S ABOUT IT. MOISTURE AROUND 5KFT WILL KEEP SOME CLOUDS ALONG
AND WEST OF THE CASCADES THROUGH THIS EVENING AND OVERNIGHT. THE
NEXT FRONT WILL APPROACH THE COAST TONIGHT, WITH MID LEVEL
CLOUDS RAPIDLY INCREASING WEST OF THE CASCADES BY MORNING. LIGHT
PRECIPITATION WILL LIKELY REACH THE COAST SATURDAY MORNING, THEN
INLAND TO THE CASCADES BY NOON. EAST SIDE AREAS COULD SEE SOME
SHOWERS DEVELOP SATURDAY AFTERNOON. SLIGHT THUNDERSTORM CHANCES
APPEAR TO BE ACROSS THE FAR EAST SIDE...MAINLY FROM THE WARNERS
EASTWARD. CURRENT FORECAST HANDLES THIS WELL SO HAVE MADE NO
CHANGES HERE. SPILDE
AVIATION
BASED ON THE 18/00Z TAF CYCLE.
VFR CONDITIONS WILL PREVAIL THIS EVENING. AREA OF MVFR CIGS WILL
DEVELOP AT THE COAST LATER TONIGHT AND SPREAD INLAND INTO THE
UMPQUA BASIN LATE TONIGHT AND EARLY SATURDAY. A COLD FRONT WILL
MOVE ONSHORE LATE SATURDAY MORNING...BRINGING ADDITIONAL MVFR CIGS
TO THE COAST AND UMPQUA BASIN. THESE AREAS WILL CLEAR TO VFR
SATURDAY AFTERNOON...BUT MVFR CIGS WILL RETURN TO THE SAME AREAS
SATURDAY NIGHT. STOCKTON
MARINE
HAVE GONE AHEAD AND ISSUED A GALE WATCH FOR THE SOUTHERN
OUTER MARINE WATERS WITH THE EXPECTED INCREASE IN NORTH WINDS
SUNDAY. STRONG GRADIENT BETWEEN HIGH PRESSURE OFFSHORE AND A
THERMAL TROUGH ALONG THE NORTHERN CALIFORNIA COAST WILL LEAD TO THE
PERIOD OF STRONG NORTH WINDS. IT GOES WITHOUT SAYING THAT VERY STEEP
SHORT PERIOD SEAS WILL ALSO ACCOMPANY THE RAMP UP IN WINDS. WINDS
AND SEAS SHOULD EASE A BIT MONDAY EVENING AS THE HIGH WEAKENS AND
ANOTHER FRONT APPROACHES FROM THE NORTH AND WEST. SPILDE
PREV DISCUSSION
/ISSUED 311 PM PDT FRI MAY 17 2013/
SHORT TERM...TONIGHT THROUGH MONDAY NIGHT...
AN UPPER TROUGH IS EXITING INTO EASTERN OREGON THIS AFTERNOON
WHILE A WEAK SHORTWAVE TROUGH IN A WESTERLY FLOW ALOFT HAS MOVED
INTO WESTERN OREGON. THE RESULT HAS BEEN A DECREASE OF CLOUDS OVER
THE EAST SIDE AND IN NORTHERN CALIFORNIA WHILE THIN BUT WIDESPREAD
MID-LEVEL CLOUDS HAVE PERSISTED IN SOUTHWEST OREGON. THERE WILL BE
A BRIEF PERIOD OF SHORTWAVE RIDGING THIS EVENING THEN LATE IN THE
EVENING THROUGH LATE TONIGHT ANOTHER SHORTWAVE TROUGH WILL
INCREASE CLOUD COVER AND BRING A SLIGHT CHANCE OF SHOWERS TO COOS
AND DOUGLAS COUNTIES.
THIS SHORTWAVE WILL BE IN ADVANCE OF A COLD FRONT. THE COLD FRONT
WILL REACH THE COAST TOWARD NOON ON SATURDAY THEN MOVE EAST
ACROSS OUR AREA INTO EARLY EVENING. THE 12Z GFS QPF FIELD FOR
SATURDAY MORNING LOOKS TOO WET GIVEN THE RELATIVELY SHALLOW HEIGHT
AT WHICH THE AIR MASS WILL BE SATURATED (AROUND 750 MB), WEAK LIFT
WITH OMEGA VALUES OF 1 TO 2 MICROBARS PER SECOND, AND SOUTHERLY
WINDS OVER THE COASTAL WATERS PEAKING AT ONLY ABOUT 18 KT. THAT
SAID, THERE IS A HIGH PROBABILITY OF LIGHT RAIN DURING THE
AFTERNOON AND EVENING WEST AND NORTH OF GRANTS PASS WITH A SLIGHT
CHANCE TO CHANCE ACROSS THE REST OF OUR AREA. THE FRONT WILL ALSO
CREATE BREEZY AFTERNOON AND EVENING NORTHWEST WINDS. WINDS WILL BE
STRONGEST EAST OF THE CASCADES WITH GUSTS UP TO 30 MPH. TOWARD
PEAK HEATING IN THE LATE AFTERNOON, THE FRONT IS EXPECTED TO BE
EAST OF THE CASCADES WITH A SLIGHT CHANCE OF THUNDERSTORMS OVER
THE HIGHER TERRAIN OF EASTERN LAKE AND EASTERN MODOC COUNTIES. THE
COLD FRONT WILL BE THE MAIN FEATURE, BUT A COUPLE OF SHORTWAVES
WILL FOLLOW IN A NORTHWEST FLOW BEHIND THE FRONT SATURDAY NIGHT
AND CONTINUE A SLIGHT CHANCE TO CHANCE OF LIGHT SHOWERS.
LOW LEVEL CLOUDS WILL LINGER AT THE COAST AND IN THE UMPQUA BASIN
SUNDAY MORNING BUT CONDITIONS WILL BE IMPROVING WITH A HIGH
PRESSURE RIDGE CENTERED OFFSHORE FROM CALIFORNIA BUILDING TOWARD
OUR AREA. WINDS WILL TURN TO NORTH TO NORTHEAST AND THE AIR MASS
WILL BE DRYING SUNDAY INTO SUNDAY NIGHT. THIS WILL LEAD TO
CLEARING SKIES...INCLUDING AT THE COAST WITH HIGHS RETURNING TO
NEAR NORMAL EXCEPT ABOUT 10 DEGREES ABOVE NORMAL NEAR BROOKINGS.
THE RIDGE WILL CONTINUE TO INFLUENCE OUR WEATHER ON MONDAY WITH
THE 2-DAY WARMING TREND RESULTING IN HIGH TEMPERATURES A FEW TO
SEVERAL DEGREES ABOVE NORMAL. THE GFS INDICATES HIGH CLOUDS
INCREASING MONDAY AFTERNOON AS THE FIRST SIGN OF A SIGNIFICANT
CHANGE IN THE WEATHER PATTERN. A COLD UPPER LOW WILL BE MOVING
SOUTH TO SOUTHEAST ALONG THE WEST COAST OF CANADA WITH THE FIRST
IN A SERIES OF ASSOCIATED DISTURBANCES APPROACHING THE OREGON
COAST EARLY TUESDAY MORNING.
LONG TERM...TUESDAY, MAY 21ST THROUGH SATURDAY MORNING, MAY 25TH...
MODEL GUIDANCE AGREES WELL ON A DEEP LOW PRESSURE SYSTEM OF 540
DECAMETERS AT THE 500 MILLIBAR LEVEL BEING LOCATED OFF OF THE
COAST OF BRITISH COLUMBIA AT THE BEGINNING OF THIS PERIOD. THIS
PARENT LOW WILL DRIVE A COLD FRONT THROUGH THE FORECAST AREA ON
TUESDAY YIELDING BREEZY CONDITIONS AND BRINGING A SHARP HIGH
TEMPERATURE DROP OF 10 TO 20 DEGREES FOR HIGHS TUESDAY AFTERNOON
AS COMPARED TO MONDAY AFTERNOON. MOST OF THE PRECIPITATION WILL BE
LOCATED OVER NORTHWESTERN PORTIONS OF THE FORECAST AREA ON
TUESDAY.
TUESDAY NIGHT THROUGH SATURDAY THE PARENT LOW AT 500MB WILL MOVE
INTO THE PACIFIC NORTHWEST, BUT HOW FAR SOUTH THE LOW MAKES IT,
HOW STRONG IT IS WHEN IT ARRIVES, AND HOW LONG IT LINGERS ARE
ALL STILL LOWER CONFIDENCE ITEMS IN THE FORECAST RIGHT NOW. THE
GENERAL CONSENSUS IS THAT THE INFLUENCE OF THE LOW WILL LEAD TO
TEMPERATURES ABOUT 10 DEGREES BELOW NORMAL FOR HIGHS THROUGH THE
PERIOD WITH A CHANCE FOR SHOWERS ACROSS THE AREA, MOST LIKELY ON
WEDNESDAY THROUGH FRIDAY, AND MOST LIKELY OVER THE MOUNTAINS, THE
COAST, AND UMPQUA BASIN. ANY PERIODS OF CLEARING AT NIGHT COULD
CERTAINLY YIELD FROST OR FREEZE CONCERNS NEAR AND EAST OF THE
CASCADES TO INCLUDE THE SHASTA VALLEY.
MODEL UNCERTAINTY TOOLS AND RAW MODEL FIELDS DO INDEED INDICATE
ANOMALOUSLY COLD CONDITIONS FOR THE MIDDLE TO THE END OF THE WEEK.
ON THURSDAY THE GFS MODEL LIFTS THE PARENT LOW BACK TO THE
NORTH, INTO INTERIOR BRITISH COLUMBIA THROUGH THE EARLY PART OF
THE WEEKEND. THE ECMWF AND THE NAEFS ENSEMBLE INSTEAD TRACK THE
LOW SOUTHWARD ALONG THE COAST, AND THEN INTO NORTHERN CALIFORNIA
FROM THURSDAY INTO FRIDAY.THE GFS DOES HAVE A WEAKER RENDITION OF
SECONDARY LOW PRESSURE OVER OR NEAR THE SAME AREA AS THE ECMWF
PARENT LOW IN THE THURSDAY THROUGH FRIDAY TIME FRAME, BUT NOT
NEARLY AS STRONG AS THE ECMWF. SINCE THERE APPEARS TO BE A FAIR
AMOUNT OF SPREAD ON THE DETAILS, BUT MORE DATA SUGGESTING THE
ECMWF IS LIKELY TO BE CLOSER TO WHAT VERIFIES, I'VE KEPT ENHANCED
PROBABILITIES OF PRECIPITATION AND COOL TEMPERATURES IN CENTERED
ON THURSDAY. THUS, LEANED ON THE ECMWF SIDE OF CONSENSUS. THIS
MEANS THAT IT WILL BE UNSEASONABLY COOL LATE NEXT WEEK AND THERE
WILL HIGHER MOUNTAIN SNOWFALL AND VALLEY RAINS. EXPECT DETAILS TO
BECOME CLEARER AS EARLY AS TOMORROW. BTL
MFR WATCHES/WARNINGS/ADVISORIES
OR...NONE.
CA...NONE.
PACIFIC COASTAL WATERS...GALE WATCH FROM SUNDAY AFTERNOON THROUGH
MONDAY EVENING FOR PZZ376.
HAZARDOUS SEAS WATCH FROM SUNDAY AFTERNOON THROUGH MONDAY
EVENING FOR PZZ376.
MAS/MAS/JRS
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