Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 250415
AFDMFR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
915 PM PDT SUN MAY 24 2015

.SHORT TERM...THE FORECAST IS ON TRACK FOR THE TONIGHT INTO
MONDAY. SHOWERS HAVE DIMINISHED OVER THE AREA AND EXPECT LITTLE
IF ANY ACTIVITY OVERNIGHT. ON MONDAY...AN UPPER LOW OVER BRITISH
COLUMBIA WILL SHIFT SLIGHTLY SOUTH WITH A SHORTWAVE MOVING INTO
THE AREA. MODELS CONTINUE TO SHOW SOME AFTERNOON SHOWERS AND
ISOLATED THUNDERSTORMS, MAINLY EAST OF THE CASCADES AND OVER
NORTHERN CALIFORNIA. EXPECT BEST CHANCE FOR SHOWERS AND
THUNDERSTORMS ON MONDAY WILL BE OVER SOUTHEAST KLAMATH, SOUTHERN
LAKE, SOUTHEAST SISKIYOU AND MODOC COUNTIES. THIS IS WHERE MODELS
SHOW INSTABILITY AND MOISTURE CONVERGENCE WITH THE SOUTHWEST FLOW
IN THE SOUTHERN PORTION OF THE AREA MEETING THE NORTHWEST FLOW.

&&

.AVIATION...FOR THE 25/00Z TAF CYCLE...ALONG THE COAST AND IN THE
UMPQUA BASIN...VFR CONDITIONS WILL PREVAIL INITIALLY...BUT AREAS
OF MVFR CIGS WITH LOCAL IFR CIGS WILL DEVELOP THIS EVENING. THOSE
AREAS WILL CLEAR TO VFR BY MONDAY AFTERNOON. OVER THE REMAINDER OF
THE AREA...VFR CONDITIONS WILL PREVAIL THROUGH MONDAY...BUT
SCATTERED AFTERNOON AND EVENING SHOWERS MAY OBSCURE SOME HIGHER
TERRAIN. ISOLATED THUNDERSTORMS WILL DEVELOP MONDAY AFTERNOON OVER
SISKIYOU AND MODOC COUNTIES IN CALIFORNIA AND SOUTHERN KLAMATH AND
LAKE COUNTIES IN OREGON.

&&

.MARINE...UPDATED 130 PM PDT SUNDAY 24 MAY 2015...HIGH PRESSURE
OFFSHORE AND A THERMAL TROUGH NEAR THE COAST WILL STRENGTHEN INTO
SUNDAY AFTERNOON...THEN GRADUALLY WEAKEN MONDAY NIGHT THROUGH
THURSDAY. THIS WILL BRING MODERATE TO STRONG NORTH WINDS TO THE
WATERS AND STEEP TO VERY STEEP WIND DRIVEN SEAS THROUGH WEDNESDAY
EVENING. THE STRONGEST WINDS AND HEAVIEST SEAS WILL OCCUR IN THE
AFTERNOONS AND EVENINGS SOUTH OF GOLD BEACH AND BETWEEN 10 NM AND 40
NM OFFSHORE.

&&


.PREV DISCUSSION... /ISSUED 413 PM PDT SUN MAY 24 2015/

UPDATE...RADAR IS SHOWING ISOLATED SHOWERS OVER SOUTHERN AND
EASTERN SISKIYOU COUNTY AND MODOC COUNTY AS WELL AS NORTH INTO
KLAMATH AND LAKE COUNTIES. HAVE INCREASED THE COVERAGE OF
ISOLATED SHOWERS BASED ON THE RADAR AND LATEST RAP MODEL OUTPUT.
SHOWERS ARE LOW TOPPED AND EXPECT ONLY SOME LIGHT RAIN WITH THESE
SHOWERS.

PREV DISCUSSION... /ISSUED 317 PM PDT SUN MAY 24 2015/

SHORT TERM...TONIGHT THROUGH WEDNESDAY NIGHT.
WITH THE LINGERING WEST SIDE CLOUD COVER AND THE AFTERNOON
ONSHORE FLOW ALREADY SPILLING OVER THE CASCADES THE MODELS
INDICATION OF INSTABILITY OVER THE CASCADES LOOKS VERY UNLIKELY.
WITH THAT HAVE JUST KEPT TO SLIGHT CHANCE SHOWERS FOR THE REST OF
THE DAY WITH VERY LIGHT PRECIPITATION EXPECTED FROM ANY THAT DO
DEVELOP. SOUTHERLY FLOW BEGINS TO FEED WARM MOIST AIR UP THE
SACRAMENTO VALLEY THIS EVENING. THE SOUTHERLY FLOW UP THE
SACRAMENTO VALLEY CONTINUES THROUGH THE DAY MONDAY...WITH THE
FLOW MEETING NORTHWESTERLY FLOW OVER THE MOUNT SHASTA REGION AND
EASTWARD. THE STRONG THETA-E CONVERGENCE WILL COMBINE WITH
INSTABILITY OVER THAT AREA WITH WESTERLY STEERING WINDS SHOWING
MARGINAL STRENGTH TO PUSH ANY STORMS THAT DO DEVELOP EAST OFF THE
TERRAIN. THE FLOW PATTERN CONTINUES INTO TUESDAY AND WEDNESDAY
WITH A SIMILAR PATTERN OF INSTABILITY AND CONVERGENCE IN THE SAME
LOCATIONS IN NORTHERN CALIFORNIA. WITH THE THERMAL TROUGH PATTERN
AND OFFSHORE FLOW ON THE EXTREME SOUTHERN COAST, HAVE BUMPED UP
BOTH HIGHS AND LOWS ABOUT HALFWAY FROM GUIDANCE TO FULL ADIABATIC
HEATING IN DOWNSLOPE FLOW. /SVEN

LONG TERM...THURSDAY THROUGH SUNDAY NIGHT.
TEMPERATURES WERE RAISED AND THE PROBABILITY OF PRECIPITATION
DECREASED FOR THURSDAY AS THERE IS EXCELLENT MODEL AGREEMENT ON A
HIGH PRESSURE RIDGE BUILDING INTO THE WEST COAST. HIGH
TEMPERATURES WILL GENERALLY BE 10 TO 15 DEGREES ABOVE NORMAL ON
THURSDAY THEN REACH A PEAK WITH A FEW ADDITIONAL DEGREES OF
WARMING FOR FRIDAY. ON FRIDAY, A LIGHT LOW AND MID LEVEL SOUTHERLY
FLOW IS EXPECTED TO DEVELOP AS THE RIDGE AND SURFACE THERMAL
TROUGH PUSH FARTHER INLAND. BESIDES AIDING THE WARMING, THIS WOULD
BRING A MODEST INCREASE OF 700-500 MB MOISTURE. THE COMBINATION OF
THOSE TWO ELEMENTS WARRANTS THE MENTION OF A SLIGHT CHANCE OF
AFTERNOON AND EVENING SHOWERS AND THUNDERSTORMS AWAY FROM THE
COAST.

FOR NEXT WEEKEND, IT IS UNCERTAIN TO WHAT EXTENT OUR WEATHER WILL BE
INFLUENCED BY BOTH AN UPPER LEVEL TROUGH MOVING INTO BRITISH
COLUMBIA AND ANOTHER TROUGH OFFSHORE FROM OREGON. AS IS ITS TYPICAL
BIAS, THE 12Z GFS SOLUTION IS MORE PROGRESSIVE THAN THE ECMWF. THE
GFS SOLUTION WOULD RESULT IN A FEW TO SEVERAL MORE DEGREES OF
COOLING BUT ALSO AN INCREASED PROBABILITY OF MAINLY AFTERNOON AND
EVENING SHOWERS AND THUNDERSTORMS. THUS, WITH THE MODEL DIFFERENCES,
THE PROBABILITY OF THUNDERSTORMS IS SLIGHTLY HIGHER ON SUNDAY THAN
ON SATURDAY. /DW


&&

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

CA...NONE.

PACIFIC COASTAL WATERS...SMALL CRAFT ADVISORY FOR WINDS UNTIL 5 AM PDT THURSDAY FOR
     PZZ356-376.
     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 5 AM PDT THURSDAY
     FOR PZZ356-376.
     SMALL CRAFT ADVISORY FOR WINDS UNTIL 11 PM PDT MONDAY FOR
     PZZ350-370.
     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 11 PM PDT MONDAY
     FOR PZZ350-370.
     GALE WARNING UNTIL 11 PM PDT MONDAY FOR PZZ376.
     HAZARDOUS SEAS WARNING UNTIL 11 PM PDT MONDAY FOR PZZ376.

$$

CC



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