Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 300324
AFDMFR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
824 PM PDT FRI MAY 29 2015

.UPDATE...ASIDE FROM A SMALL SHORT-LIVED SHOWER DIRECTLY OVER THE
MT ASHLAND RADAR SITE, CONVECTION HAS CONFINED ITSELF TO THE
CASCADES AND EAST SIDE. WHILE MOST OF THE THUNDERSTORMS HAVE
DISSIPATED OR WEAKENED CONSIDERABLY AS OF THIS WRITING, A
STRAY SHOWER OR CLAP OF THUNDER IS STILL POSSIBLE OVER PORTIONS
OF THE EAST SIDE. UPDATES TONIGHT HAVE CONCENTRATED MOSTLY ON
TRIMMING DOWN PRECIPITATION CHANCES TO THOSE AREAS. OTHERWISE, NO
CHANGES ARE NEEDED. SEE THE PREVIOUS DISCUSSION BELOW FOR MORE
INFORMATION. -BPN &&

.PREV DISCUSSION... /ISSUED 618 PM PDT FRI MAY 29 2015/

SHORT TERM...29/12Z NAM/GFS IN.

LOTS OF STRATUS ALONG THE COAST WITH ISOLATED SHOWERS AND
THUNDERSTORMS OVER PORTIONS OF KLAMATH AND NORTHERN LAKE COUNTIES
THIS EVENING. THESE SHOULD WANE OVER THE NEXT FEW HOURS AS WE LOSE
DAYTIME INSTABILITY.

A REX BLOCK REMAINS IN PLACE OFFSHORE WITH THE RIDGE ALONG THE
ALASKA/CANADA BOUNDARY AND THE LOW NEAR 40N 150W. THERE IS ANOTHER
WEAKER LOW OVER WEST CENTRAL BRITISH COLUMBIA. HOWEVER...THE END
IS IN SIGHT. LOWS WILL BEGIN UNDERCUTTING THE RIDGE SATURDAY.

UNTIL THEN...A WEAKER UPPER LEVEL RIDGE WILL REMAIN IN PLACE ALONG
THE WEST COAST WITH NEARLY ZONAL FLOW ALOFT. THERE WILL BE
ISOLATED SURFACE HEATING DRIVEN THUNDERSTORM ACTIVITY IN THE
AFTERNOONS AND EVENINGS...MOSTLY OVER HIGHER TERRAIN. INLAND HIGH
TEMPERATURES WILL BE 10 TO 15 DEGREES ABOVE NORMAL OVER THE WEST
SIDE AND 5 TO 10 DEGREES ABOVE NORMAL EAST OF THE CASCADES
THROUGH SUNDAY.

THE PATTERN WILL BEGIN TO CHANGE SUNDAY AS THE FIRST AND
STRONGEST OF SEVERAL TROUGHS UNDERCUTTING THE RIDGE APPROACHES THE
COAST. THIS WILL BRING SOUTH TO SOUTHWEST FLOW ALOFT TO THE AREA.
A STRONG SHORT WAVE ROUNDING THE BOTTOM OF THE TROUGH WILL MOVE
ONSHORE SUNDAY EVENING...WITH THE BEST DYNAMICS NEAR THE COAST.
THE COOLING ALOFT COMBINED WITH CONTINUED STRONG SURFACE HEATING
WILL ENHANCE THE POTENTIAL FOR CONVECTION. AT THIS TIME MODEL
LIFTED INDICES AND SOUNDINGS INDICATE THE BEST CHANCE OF
THUNDERSTORMS SUNDAY AFTERNOON WILL BE OVER THE SISKIYOUS...THE
WESTERN VALLEYS...AND THE CASCADES...ESPECIALLY NORTH OF THE
UMPQUA DIVIDE. THE BEST AREA WILL MIGRATE A BIT TO THE NORTH AND
EAST SUNDAY EVENING.

THE OTHER CONCERN WITH THE SUNDAY SYSTEM IS WIND. IT WILL GET
QUITE BREEZY OVER THE RIDGES AND EAST OF THE CASCADES...BUT NOT AS
MUCH IN THE SHASTA VALLEY...AS THE GRADIENTS WILL BE PRETTY WEAK.

THE UPPER TROUGH WILL REMAIN OVER THE PACIFIC NORTHWEST MONDAY.
THUNDERSTORM ACTIVITY WILL MOVE OFF TO THE EAST AS COOLING AT ALL
LEVELS WILL STABILIZE THE AIR MASS...BUT THERE WILL STILL BE A
SLIGHT CHANCE OVER LAKE AND MODOC COUNTIES. MOST OF THE AREA WILL
JUST SEE SHOWERS. INLAND HIGH TEMPERATURES WILL COOL
DRASTICALLY...10 TO 20 DEGREES...DOWN TO NEAR OR A BIT BELOW
SEASONAL NORMALS.

LONG TERM...TUESDAY THROUGH FRIDAY...AN UPPER TROUGH
OVER THE AREA TUESDAY WILL BRING COOL TEMPERATURES AS WELL AS
AREAS OF SHOWERS AND A SLIGHT CHANCE OF THUNDERSTORMS EAST OF THE
CASCADES, MAINLY ACROSS LAKE COUNTY. THEN THE TROUGH WILL
GRADUALLY SHIFT EAST TUESDAY NIGHT AND EXPECT DECREASING SHOWER
CHANCES.

ON WEDNESDAY...MODELS CONTINUE TO SHOW DIFFERENCES WITH THE TIMING
AND STRENGTH OF A SHORTWAVE TROUGH MOVING INTO THE AREA. HAVE
KEPT A SLIGHT CHANCE FOR SHOWERS IN PLACE FOR MUCH OF THE AREA
WEDNESDAY WITH A SLIGHT CHANCE THUNDERSTORMS IN LAKE AND MODOC
COUNTIES.

BY THURSDAY...MODELS ARE SHOWING AN UPPER TROUGH DEEPENING OVER
THE REGION ...WITH THE TROUGH DEVELOPING INTO A CLOSED LOW OVER
THE AREA. THIS WILL BRING ADDITIONAL CHANCES FOR SHOWERS AS WELL
AS FOR THUNDERSTORMS EAST OF THE CASCADES. THERE IS LOW CONFIDENCE
ON THE STRENGTH AND TRACK OF THE LOW. CURRENT MODELS INDICATE IT
WILL GRADUALLY PUSHING EAST OR SOUTHEAST AS A RIDGE OFFSHORE
BUILDS AND NUDGES INTO THE PACNW COAST. DEPENDING ON HOW QUICKLY
THIS LOW MOVES OUT OF THE AREA, MAY SEE ADDITIONAL CHANCES FOR
SHOWERS OR THUNDERSTORMS, MAINLY EAST OF THE CASCADES INTO FRIDAY.
SO HAVE KEPT A SLIGHT CHANCE FOR SHOWERS AND THUNDERSTORMS IN
PLACE OVER LAKE AND MODOC COUNTIES ON FRIDAY. /CC

AVIATION...FOR THE 30/00Z TAF CYCLE...ISOLATED SHOWERS AND
THUNDERSTORMS EAST OF THE CASCADES AND MOSTLY IN KLAMATH AND
NORTHERN LAKE COUNTIES EARLY THIS EVENING WILL DISSIPATE IN THE
NEXT FEW HOURS. THESE MAY BRIEFLY LOWER CONDITIONS TO MVFR,
OTHERWISE, VFR IS EXPECTED OVERNIGHT INTO SATURDAY. EXPECT
MVFR/IFR CIGS ALONG THE COAST IN THE MARINE LAYER THE REST OF
TONIGHT THROUGH MUCH OF SATURDAY. EXPECT MVFR CIGS TO DEVELOP IN
THE UMPQUA BASIN AROUND KRBG ABOUT 12Z. THESE SHOULD ERODE BY 17Z
OR 18Z. THE REST OF THE AREA WILL BE VFR TONIGHT THROUGH SATURDAY,
THOUGH ISOLATED THUNDERSTORMS WILL DEVELOP AGAIN FROM THE HIGHER
TERRAIN OF SOUTHEAST JACKSON COUNTY TO KLAMATH AND NORTHERN LAKE
COUNTIES. -SPILDE

MARINE...UPDATED 200 PM PDT, FRIDAY, 29 MAY 2015...SEAS OF 4-5 FEET
WITH GENTLE TO MODERATE NORTHERLY BREEZES WILL PERSIST OVER MOST OF
THE WATERS THROUGH SATURDAY NIGHT. WEAK LOW PRESSURE WILL MOVE INTO
THE WATERS SUNDAY AND MONDAY WITH SOME SHOWERS. WINDS WILL SHIFT
INTO THE SOUTH FOR A WHILE MONDAY, THEN SHIFT TO SOUTHWESTERLY AS
THE LOW MOVES NORTHWARD AND ONSHORE. EVEN SO, THEY SHOULD REMAIN
LIGHT ENOUGH FOR SEAS TO REMAIN LOW. A THERMAL TROUGH IS EXPECTED TO
REDEVELOP AS EARLY AS WEDNESDAY AND IS LIKELY TO BRING MODERATE TO
STRONG NORTH BREEZES AND WIND DRIVEN SEAS BACK TO THE AREA
THURSDAY AND FRIDAY. -SPILDE

&&

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

CA...NONE.

PACIFIC COASTAL WATERS...NONE.

$$

BPN/MAS/JRS/CC



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