Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 270600
AFDMFR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
1100 PM PDT TUE MAY 26 2015

.UPDATE...
AN UPDATE HAS BEEN SENT TO REMOVE THE THREAT OF THUNDERSTORMS FROM
THE FORECAST FOR THE REMAINDER OF THE EVENING. ISOLATED SHOWERS
ARE STILL POSSIBLE IN SOUTHERN LAKE AND MODOC COUNTIES THROUGH 11
PM PDT.

&&

.DISCUSSION...
70 CLOUD TO GROUND LIGHTNING STRIKES WERE RECORDED BY THE
NATIONAL LIGHTNING DETECTION NETWORK ACROSS SOUTHEASTERN SISKIYOU AND
MODOC COUNTIES THIS AFTERNOON THROUGH EARLY THIS EVENING. SOME OF
THESES STORMS ALSO PRODUCED HEAVY DOWNPOURS AND SOME LOCALLY
GUSTY WINDS, AS WAS EVIDENCED BY THE 0.52" AND WIND GUST OF 29MPH
RECORDED AT THE ROUND MOUNTAIN RAWS BETWEEN 415PM PDT AND 515PM
PDT. OTHER LOCATIONS SUCH AS THE DEVILS GARDEN RAWS AND ALTURAS
AND CANBY RECORDED MEASURABLE RAIN.

WEDNESDAY AFTERNOON AND EVENING THE THREAT FOR THUNDERSTORMS WILL
BE MAINLY ACROSS LAKE COUNTY, AND MOISTURE AND INSTABILITY LOOK
MARGINAL FOR STORM DEVELOPMENT. THURSDAY AFTERNOON AND EVENING THIS
THREAT DRIFTS WESTWARD TO THE CASCADES, THOUGH MOISTURE AND
INSTABILITY ARE ONLY A TAD MORE FAVORABLE FOR THUNDERSTORMS THAN
ON WEDNESDAY.

FRIDAY THROUGH MONDAY A NEW UPPER LEVEL LOW WILL BRING WITH IT A
HEALTHY CHANCE OF SHOWERS AND THUNDERSTORMS ACROSS MUCH OF THE
AREA DURING THIS TIME PERIOD AS IT APPROACHES AND MOVES ACROSS
THE AREA. WE EXPECT MORE BENEFICIAL RAINFALL WITH THESE SHOWERS
AND STORMS. BTL

&&

.AVIATION...FOR THE 27/06Z TAF CYCLE...AREAS OF STRATUS WITH IFR TO
MVFR CIGS ARE EXPECTED ALONG THE COAST ACROSS THE WESTERN UMPQUA
BASIN LATE TONIGHT INTO WEDNESDAY MORNING. ELSEWHERE, VFR CONDITIONS
WILL CONTINUE FOR THE NEXT 24 HOURS. THERE WILL BE A SLIGHT CHANCE
OF THUNDERSTORMS WEDNESDAY AFTERNOON AND EVENING, MAINLY ACROSS LAKE
COUNTY. BTL

&&

.MARINE...UPDATED 130 PM PDT TUESDAY 26 MAY 2015...LOW PRESSURE JUST
INLAND THIS EVENING WILL KEEP MODERATE NORTH WINDS AND STEEP, WIND
DRIVEN SEAS GOING OVER THE WATERS SOUTH OF CAPE BLANCO. LOW PRESSURE
WILL THEN MOVE FURTHER INLAND TONIGHT ALLOWING WINDS TO WEAKEN, BUT
SEAS WILL REMAIN STEEP AND CHOPPY THROUGH THURSDAY. SURFACE HIGH
PRESSURE WILL REMAIN ANCHORED BETWEEN 130 WEST AND 140 WEST THROUGH
THE REMAINDER OF THE WEEK. BTL/WRIGHT

&&

PREV DISCUSSION... /ISSUED 232 PM PDT TUE MAY 26 2015/

SHORT TERM...AN UPPER LOW IS CENTERED OVER WESTERN WASHINGTON
AND EXTREME NORTHEAST OREGON. THIS LOW WILL BRING WEAK SHORTWAVES
AND WESTERLY MOISTURE INTO SOUTHEAST PORTIONS OF THE AREA THIS
AFTERNOON AND EVENING AND MAY ALLOW SOME SHOWERS AND
THUNDERSTORMS TO DEVELOP, MAINLY IN SOUTHEAST SISKIYOU AND MODOC
COUNTIES. MODELS CONTINUE TO INDICATE THAT THE THERMODYNAMICS,
ALTHOUGH SLIGHTLY WEAKER THAN YESTERDAY, WOULD SUPPORT SOME
SHOWERS AND THUNDERSTORMS OVER THESE AREAS IN NORTHERN CALIFORNIA
THIS AFTERNOON AND EVENING. HOWEVER, LIGHTNING GUIDANCE AND HIGH
RESOLUTION HRRR MODEL OUTPUT INDICATES THAT ACTIVITY MAY REMAIN
ISOLATED OVER THE CWA. SOME ISOLATED STORMS ARE ALSO POSSIBLE IN
THE SOUTHEAST LAKE COUNTY BUT ELSEWHERE IN SOUTHERN OREGON EXPECT
DRY WEATHER THIS EVENING. SHOWERS AND THUNDERSTORMS WILL DIMINISH
BY LATE EVENING.

THEN AS THE UPPER LOW GRADUALLY SHIFTS EASTWARD AND BECOMES
CENTERED OVER IDAHO ON WEDNESDAY, EXPECT MAINLY DRY CONDITIONS
WITH ONLY A SLIGHT CHANCE FOR SOME ISOLATED SHOWER OR
THUNDERSTORM ACTIVITY IN THE AREA ON WEDNESDAY. THE POTENTIAL FOR
ACTIVITY IS MAINLY LIMITED TO NORTHERN CALIFORNIA AND INTO THE
CASCADES AND NORTHERN KLAMATH COUNTY ON WEDNESDAY AFTERNOON AND
EVENING. LATE WEDNESDAY INTO THURSDAY AN UPPER RIDGE WILL MOVE
INTO THE AREA. THIS WILL BRING WARMING TEMPERATURES ACROSS INLAND
AREAS. A SLIGHT CHANCE FOR SHOWER AND THUNDERSTORM ACTIVITY
CONTINUES THOUGH ON THURSDAY AS A SHORTWAVE MOVES INTO THE AREA
AND WEAK DAYTIME INSTABILITY IS FORECAST OVER THE AREA. THIS
SLIGHT CHANCE FOR THUNDERSTORMS IS MAINLY OVER THE CASCADES AND
OVER SOME OF THE HIGHER TERRAIN EAST OF THE CASCADES.

THEN FRIDAY THROUGH SATURDAY...A PACIFIC LOW WILL APPROACH FROM
THE WEST...REACHING THE COAST ON SUNDAY. AHEAD OF THIS LOW EXPECT
A SLIGHT CHANCE FOR SHOWERS AND THUNDERSTORMS FRIDAY AND SATURDAY
ACROSS MUCH OF THE AREA AS SHORTWAVES MOVE INTO THE AREA WITH
MOIST SOUTHERLY FLOW. CURRENT MODEL RUNS INDICATE THE AREA FOR
POTENTIAL SHOWERS AND THUNDERSTORMS ON FRIDAY AND SATURDAY WILL
BE MAINLY OVER WESTERN AND CENTRAL SISKIYOU COUNTY...NORTHWARD
INTO JACKSON COUNTY...AND INTO THE SOUTHERN OREGON CASCADES AND
EASTWARD INTO NORTHERN KLAMATH COUNTY. HOWEVER CONFIDENCE IN THE
EXACT AREA FOR SHOWER AND THUNDERSTORM DEVELOPMENT IS RELATIVELY
LOW SINCE THE TIMING AND TRACK OF ANY SHORTWAVES MAY ALTER.
CONFIDENCE IS SOMEWHAT HIGHER FOR SHOWERS AND THUNDERSTORMS TO
DEVELOP ON SATURDAY, DUE TO THE LOW MOVING CLOSER TO THE COAST AND
INCREASING MOISTURE AND SHORTWAVES EXPECTED AT THIS TIME.

OVERNIGHT SATURDAY, MODELS SHOW SOME WEAK INSTABILITY...EMBEDDED
SHORTWAVES...AND INCREASED SOUTHERLY MOISTURE OVER THE AREA WITH A
POTENTIAL SHOWERS OR THUNDERSTORMS IN CENTRAL PORTIONS OF THE
AREA, MAINLY NEAR THE SOUTHERN OREGON CASCADES AND THE SISKIYOUS.
SO HAVE KEPT A SLIGHT CHANCE OF SHOWER AND THUNDERSTORMS IN PLACE
SATURDAY NIGHT.

LONG TERM...THE UPPER LOW WILL APPROACH THE COAST OF SOUTHERN
OREGON AND NORTHERN CALIFORNIA SUNDAY AFTERNOON AND MOISTURE AND
DYNAMIC FORCING WILL INCREASE OVER THE AREA. THE PROBABILITY OF
THUNDERSTORMS IS EXPECTED TO REACH A PEAK SUNDAY AFTERNOON AND
EVENING AND HAS BEEN INCREASED OVER THE PREVIOUS FORECAST. A
RELATIVE BROAD BRUSH OF A SLIGHT CHANCE TO A CHANCE OF
THUNDERSTORMS WAS UTILIZED AS MODEL DIFFERENCES INCREASE REGARDING
THE SPEED/POSITIONING OF THE UPPER TROUGH AS IT MOVES INLAND
SUNDAY EVENING. AS ITS TYPICAL BIAS, THE GFS IS MORE PROGRESSIVE
AND WOULD FOCUS ACTIVITY OVER CENTRAL PORTIONS OF THE AREA,
INITIALLY WEST OF THE CASCADES THEN INTO EASTERN AREA BY SUNDAY
EVENING. MEANWHILE, THE SLOWER ECMWF HAS A FOCUS ON THE WEST
SIDE THROUGH SUNDAY EVENING. GIVEN THE ANTECEDENT SPELL OF HOT
TEMPERATURES, THIS IS A LARGE ENOUGH TROUGH WITH A LIKELY
SUBSTANTIAL ENOUGH INCREASE OF MOISTURE THAT THUNDERSTORM ACTIVITY
MAY END UP AS SCATTERED ACROSS ALL OF OUR AREA INLAND FROM THE
COOLER/LESS UNSTABLE COAST. OVERNIGHT SUNDAY...HAVE KEPT SHOWERS
AND THUNDERSTORMS IN THE FORECAST AS THE LOW IS EXPECTED TO BE
CENTERED NEAR OR OVER THE AREA.

WITH BOTH THE GFS AND ECMWF SHOWING EASTWARD PROGRESSION OF THE
TROUGH SUNDAY NIGHT INTO MONDAY, THERE IS AN INCREASED
PROBABILITY THAT THE FOCUS OF THUNDERSTORMS WILL SHIFT TO THE
CASCADES AND EAST SIDE. WITH AT LEAST A PORTION OF THE LOW LIKELY
TO BE OVER THE AREA, THIS WILL BRING COOLING, INCREASED CLOUD
COVER, AND A MORE FREQUENT OCCURRENCE OF SHOWERS.

BY DAY 7, TUESDAY, UNCERTAINTY IS HIGH AS THE GFS SOLUTION SENDS THE
UPPER LOW EAST OF OUR REGION WHILE THE ECMWF SUGGESTS IT WILL LINGER
OVER THE REGION. THUS, A SLIGHT CHANCE OF AFTERNOON AND EVENING
SHOWERS AND THUNDERSTORMS WAS MAINTAINED WITH A FOCUS ON THE EAST
SIDE. -DW/CC

&&

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

CA...NONE.

PACIFIC COASTAL WATERS...
- SMALL CRAFT ADVISORY FOR WINDS UNTIL 5 AM PDT WEDNESDAY FOR
  PZZ356-376.
- SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 11 PM PDT
  THURSDAY FOR PZZ356-376.

$$

BTL/CC/TRW



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