Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 271844
AFDMFR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
1045 AM PST SAT DEC 27 2014

.DISCUSSION...NO UPDATES PLANNED THIS MORNING. LOW CLOUD AND
PATCHY FOG IS WIDESPREAD IN THE ROGUE AND ILLINOIS VALLEYS THIS
MORNING. MODELS ARE INDICATING THAT IT MAY BE A BIT MORE STUBBORN
TODAY, BUT SHOULD BREAK OUT THIS AFTERNOON. ELSEWHERE, SKIES ARE
PARTLY TO MOSTLY SUNNY. A COLD FRONT WILL BRING LIGHT RAIN TO THE
NORTH COAST AND UMPQUA LATE THIS AFTERNOON AND EVENING WITH LIGHT
SNOW BREAKING OUT IN THE CASCADES NORTH OF CRATER LAKE AROUND THE
SAME TIME. ALL OF THIS WILL SPREAD SOUTH THROUGH THE NIGHT INTO
SUNDAY MORNING. PRECIPITATION AMOUNTS WILL BE LIGHT, BUT
OROGRAPHIC ENHANCEMENT SHOULD PROVIDE ENOUGH EXTRA LIFT TO PRODUCE
MODERATE ACCUMULATIONS IN THE CASCADES TONIGHT, PRIMARILY NORTH OF
HIGHWAY 140. ALL OF THIS IS COVERED IN THE GOING FORECAST. -WRIGHT

&&

.AVIATION...FOR THE 27/18Z TAF CYCLE... IFR TO MVFR CEILING
CONDITIONS WITH LIFR FOG ALONG THE HILLSIDES WILL CONTINUE LATE THIS
MORNING, IMPROVING TO MOSTLY VFR BY EARLY AFTERNOON FOR THE
ILLINOIS, ROGUE, AND LOWER KLAMATH RIVER VALLEYS. OTHERWISE VFR WILL
PREVAIL UNTIL A FRONTAL SYSTEM ARRIVING FROM THE NORTHWEST ARRIVES
WITH MVFR TO IFR CONDITIONS AND TERRAIN OBSCURATIONS. THE FRONT WILL
BEGIN TO AFFECT THE NORTHWEST COAST NEAR NORTH BEND WITH MVFR
BETWEEN 19Z AND 21Z AND ROSEBURG BETWEEN 22Z AND 00Z, MEDFORD AROUND
03Z, AND KLMT AROUND 05Z. SNOW LEVELS WILL BE NEAR 4000 FEET. BTL &&

&&

.MARINE...UPDATED 835 AM PST DECEMBER 27, 2014... LIGHT WINDS TODAY
WILL TRANSITION TO NORTHWEST THIS AFTERNOON THROUGH THIS EVENING AS
A COLD FRONT MOVES THROUGH. NORTHWEST SEAS WILL BUILD ON SUNDAY INTO
THE 8 TO 10 FOOT RANGE WITH A DOMINANT PERIOD AROUND 13 SECONDS. A
COLDER FRONT IS EXPECTED TO MOVE IN FROM THE NORTH ON MONDAY
BRINGING SHOWERS AND GUSTY NORTHWE4ST THROUGH NORTHEAST WINDS.
MODERATE NORTH THROUGH NORTHEAST WINDS AND STEEPER SEAS ARE EXPECTED
MONDAY NIGHT THROUGH WEDNESDAY. BTL

&&

.PREV DISCUSSION... /ISSUED 630 AM PST SAT DEC 27 2014/

DISCUSSION...THERE WILL BE TWO PRIMARY CONCERNS DURING THE
COMING FORECAST WEEK, THE FIRST BEING SNOWFALL OVER THE CASCADES
DURING THE TRAVEL WEEKEND, AND THE SECOND BEING VERY COLD
TEMPERATURES MIDWEEK. WHILE MODELS ARE IN GOOD AGREEMENT WITH THE
OVERALL PATTERN AND TRENDS, THERE ARE SOME DIFFERENCES, MAINLY IN
THE PRECIPITATION FORECASTS, BETWEEN THE LARGER AND SMALLER SCALE
SOLUTIONS. BUT WE WILL GET TO THAT IN A MOMENT.

THE GENERAL PATTERN BEGINS WITH MID LEVEL RIDGING OFFSHORE OF THE
WEST COAST. THE RIDGE WILL AMPLIFY THROUGH THE WEEK, AND IN
RESPONSE, A TROUGH WITHIN THE FLOW WILL PUSH SOUTH TODAY ALONG THE
COAST AND INTO THE PACIFIC NORTHWEST. THIS SYSTEM HAS A MARITIME
ORIGIN, AND WILL THEREFORE HAVE ENOUGH MOISTURE CONTENT TO PRODUCE
LIGHT TO MODERATE PRECIPITATION OVER MUCH OF THE AREA. MOST OF THE
PRECIPITATION WILL BE DRIVEN BY THE TERRAIN, SO HAVE TAKEN A BLEND
OF THE USUAL NAM, GFS, AND ECMWF, THEN ADJUSTED TOWARDS THE HIGHER
RESOLUTION MODELS. SNOWFALL WILL BEGIN ALONG AND NORTH OF CRATER
LAKE THIS AFTERNOON, THEN INTENSIFY AND SPREAD SOUTH THIS EVENING.
SEVERAL INCHES ARE POSSIBLE OVER THE CASCADE PASSES, PARTICULARLY
NEAR CRATER LAKE AND DIAMOND LAKE, WHERE UP TO 10 INCHES OF SNOW
ARE POSSIBLE THROUGH SUNDAY. SISKIYOU SUMMIT COULD SEE AN INCH OR
TWO OF NEW SNOW, WITH MOST OTHER AREAS RECEIVING ONLY LIGHT
AMOUNTS. CONSIDERING THE HIGH TRAVEL WEEKEND, HAVE OPTED TO ISSUE
A WINTER WEATHER ADVISORY FOR THE CASCADE PASSES, AS SOME ISSUES
COULD ARISE DUE TO SNOWY AND ICY ROADS AND INCREASED TRAFFIC.

AFTER A BRIEF, BUT COLDER, BREAK ON SUNDAY, A SECOND TROUGH ON
MONDAY THAT WILL BRING THE TRULY FRIGID AIR TO OUR AREA. AN ARCTIC
CONTINENTAL AIR MASS WILL PUSH SOUTH WITH THIS TROUGH, DROPPING
SNOW LEVELS TO AROUND 1000 FEET BY MONDAY AFTERNOON. WHILE THIS
SECOND SYSTEM WILL NOT HAVE AS MUCH MOISTURE ASSOCIATED WITH IT AS
THE FIRST, IT WILL HAVE ENOUGH DYNAMIC OOMPH TO PRODUCE MORE
PRECIPITATION OVER THE AREA. THE SAME AREAS IN THE CASCADES
IMPACTED BY THE FIRST SYSTEM SHOULD SEE A FEW MORE INCHES WITH
THIS ONE AS WELL, BUT WITH SNOW LEVELS DROPPING CONSIDERABLY, SOME
WINTRY PRECIPITATION MAY MAKE IT TO THE WEST SIDE VALLEY FLOORS.
ACCUMULATIONS THERE, IF ANY, SHOULD BE LIGHT, BUT THE FORECAST
DOES BEAR WATCHING FOR POTENTIAL TRAVEL IMPACTS.

BEHIND THIS TROUGH, TEMPERATURES WILL BE VERY COLD, WITH LOWS ON
THE EAST SIDE DROPPING INTO THE SINGLE DIGITS, AND LOWS IN THE
WEST SIDE VALLEYS DROPPING INTO THE TEENS AND LOWER 20S. EVEN THE
COAST SHOULD SEE TEMPERATURES DROPPING TO BELOW FREEZING BOTH
TUESDAY AND WEDNESDAY MORNINGS. GENERALLY SPEAKING, TEMPERATURES
WILL BEGIN TO INCREASE BEGINNING WEDNESDAY, BUT WITH HIGH PRESSURE
OVERHEAD, VALLEY INVERSIONS ARE LIKELY TO RESULT IN FOG AND COLD
DAYTIME TEMPERATURES IN MANY AREA VALLEYS, A TYPICAL FREEZING FOG
PATTERN FOR THIS TIME OF YEAR. BY THE END OF THE WEEK, MODEL
SOLUTIONS ARE DEPICTING ANOTHER SYSTEM ENTERING THE AREA,
RESULTING IN A CHANCE OF PRECIPITATION THROUGH THE WEEKEND.
HOWEVER, WITH WIDE MODEL SPREADS, AND POSSIBLE BLOCKING UPSTREAM,
CONFIDENCE IS LOW IN THE STRENGTH AND TIMING OF ANY SUCH SYSTEM.
HAVE KEPT SLIGHT TO CHANCE POPS IN THE FORECAST FOR NOW, AS
INHERITED FROM THE PREVIOUS SHIFT. -BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...WINTER WEATHER ADVISORY UNTIL 10 AM PST SUNDAY FOR
     ORZ027-028-617-623.

CA...NONE.

PACIFIC COASTAL WATERS...NONE.

$$


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