Frost and Freeze in Oregon, December 26, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
910 PM PST FRI DEC 25 2015

.DISCUSSION...AN UPPER RIDGE OVER THE AREA WILL BRING DRY WEATHER
THROUGH SATURDAY. THE LINGERING COLD AIR MASS OVER THE AREA ALONG
WITH RADIATIONAL COOLING TONIGHT WILL RESULT IN VERY COLD
TEMPERATURES ACROSS THE AREA. AREAS WITH SNOW ON THE GROUND AND
WITH CLEAR SKIES THIS EVENING WILL BE PARTICULARLY COLD TONIGHT.
EASTERN VALLEYS ARE EXPECTED TO SEE TEMPERATURES IN THE -5 TO 5
DEGREE F RANGE. WESTERN VALLEYS, WILL SEE TEMPERATURES IN THE 20S
IN OREGON AND IN THE LOWER TEENS IN NORTHERN CALIFORNIA. VALLEY
FOG AND LOW CLOUDS ARE EXPECTED FOR MANY WEST SIDE VALLEYS. LOW
CLOUDS HAVE FORMED IN THE UMPQUA VALLEY THIS EVENING AND A FEW
AREAS OF FOG ARE DEVELOPING IN THE VALLEYS IN JOSEPHINE AND
JACKSON COUNTY. EXPECT INCREASED FOG TO DEVELOP IN THESE WESTERN
VALLEYS TONIGHT AND SATURDAY MORNING. MODELS AND GUIDANCE
CONTINUES TO SHOW CLEARING BY SATURDAY AFTERNOON WITH MAINLY ONLY
HIGH CLOUDS OVER THE AREA. ALONG THE COAST, A FREEZE WARNING
REMAINS IN EFFECT FOR TONIGHT. SEE THE NPWMFR PRODUCT FOR DETAILS.
AN OFFSHORE FLOW PATTERN WILL KEEP SOME COASTAL AREAS CLEAR BUT
EXPECT AREAS WITH FOG ESPECIALLY IN THE COQUILLE VALLEY AND NEAR
NORTH BEND.

THE NEXT FRONT IS FORECAST TO APPROACH THE COAST SATURDAY NIGHT
AND MOVE INLAND SUNDAY. PRECIPITATION IS EXPECTED TO REACH THE
COAST BY SUNDAY MORNING AND SPREAD INLAND SUNDAY AFTERNOON INTO
SUNDAY EVENING. SNOW LEVELS WILL BE A CONCERN WITH THIS FRONT.
AS PRECIPITATION BEGINS, SNOW LEVELS OF AROUND 3000 FEET ARE
FORECAST...LOWERING TO NEAR 2000 TO 2500 FEET LATE SUNDAY
AFTERNOON AND SUNDAY EVENING. THE 00Z NAM INDICATES LOCALLY LOWER
SNOW LEVELS DOWN TO 1500 FEET ARE POSSIBLE SUNDAY EVENING AND
NIGHT, ESPECIALLY IN THE ROGUE AND ILLINOIS VALLEYS. SHOWERS ARE
EXPECTED BEHIND THE FRONT SUNDAY NIGHT AND MONDAY AS AN UPPER
TROUGH MOVES INTO THE AREA. THIS IS A FAIRLY QUICK MOVING SYSTEM
WITH MAINLY ONLY LIGHT TO MODERATE PRECIPITATION. SO FORECAST
SNOW AMOUNTS ARE GENERALLY LIGHT. WILL NEED TO MONITOR FOR THE
POTENTIAL OF SNOWFALL IN LOWER ELEVATION AREAS AND PASSES.

&&

.AVIATION...FOR THE 26/00Z TAF CYCLE...OFFSHORE FLOW WITH DECENT
GROUND MOISTURE WILL BRING VALLEY FOG WEST OF THE CASCADES TONIGHT
AND SATURDAY MORNING...WITH WIDESPREAD IFR AND ISOLATED LIFR
CONDITIONS. EXPECT A MIX OF MFR AND IFR CIGS IN WESTERN VALLEYS
THIS EVENING...BECOMING IFR CIGS AND VISIBILITIES TONIGHT AND
CONTINUING THROUGH SATURDAY MORNING. CONDITIONS ARE EXPECTED TO
GRADUALLY IMPROVE TO VFR IN THE LATE MORNING . /CC

&&

.MARINE...UPDATED 210 PM PST FRIDAY 25 DEC 2015...MODERATE NORTHWEST
SWELL WILL CONTINUE TODAY BEFORE EASING BELOW SMALL CRAFT ADVISORY
LEVELS THIS EVENING. MODELS ARE TRENDING A BIT STRONGER WITH A
FRONTAL SYSTEM THAT WILL PASS THROUGH THE COASTAL WATERS LATE
SATURDAY INTO SUNDAY...AND ARE NOW SUGGESTING GALES AND VERY STEEP
WIND DRIVEN SEAS ARE POSSIBLE AHEAD OF THE FRONT AS IT MAKES ITS WAY
ONSHORE. ANOTHER LONG PERIOD NORTHWEST SWELL AROUND 17 SECONDS WILL
ARRIVE ON MONDAY. -BPN

&&

.PREV DISCUSSION... /ISSUED 241 PM PST FRI DEC 25 2015/

SHORT TERM...THE PRIMARY CHALLENGES ARE COLD TEMPERATURES
SATURDAY MORNING AND THE DEVELOPMENT OF LOW CLOUDS AND FREEZING
FOG IN THE VALLEYS...AND THEN THE INCOMING WEATHER SYSTEM ON
SUNDAY.

A LONG NIGHT...CLEARING SKIES...AND FRESH SNOW IN MANY AREAS WILL
ALLOW FOR IMPRESSIVE RADIATIONAL COOLING TONIGHT. THE EASTSIDE
WILL BE QUITE COLD ESPECIALLY FARTHER EAST IN MODOC AND LAKE
COUNTIES. THE SHASTA AND SCOTT VALLEYS WILL ALSO BE AROUND 10
DEGREES BY SATURDAY MORNING. THE WESTSIDE VALLEYS IN OREGON SHOULD
SEE FREEZING FOG AND LOW CLOUDS DEVELOP WHICH WILL KEEP LOW
TEMPERATURES FROM BOTTOMING OUT BUT FREEZING FOG COULD MAKE ROADS
SLICK. THE FREEZE WARNING FOR THE COAST CONTINUES FOR SATURDAY
MORNING WITH LOWS NEAR FREEZING EVEN IN THE BANANA BELT OF
BROOKINGS.

THE GFS AND NAM ARE IN DECENT AGREEMENT WITH THE INCOMING
COLD FRONT REACHING THE COAST SUNDAY AFTERNOON. THE GFS IS A
LITTLE SLOWER WITH THE TRAILING UPPER TROUGH AND KEEPS DEEPER
MOISTURE FARTHER WEST ON MONDAY IN THE COLDER AIR...BUT HIGH
PRESSURE IS ALREADY BUILDING NEAR THE COAST BY MONDAY AFTERNOON.

THE INCOMING FRONT ON SUNDAY IS WEAK COMPARED WITH THE STORMS WE
HAVE SEEN THE PAST FEW WEEKS AND EVEN CRATER LAKE SHOULD SEE LESS
THAN 5 INCHES OF SNOW SUNDAY INTO MONDAY AS THE PATTERN IS NOT
GOOD FOR UPSLOPE ENHANCEMENT.

SNOW LEVELS ARE TRICKY AHEAD OF THE FRONT. HIGH CLOUDS SHOULD
MOVE IN SATURDAY NIGHT KEEPING SUNDAY MORNING LOWS HIGHER THAN
SATURDAY MORNING. THE AIRMASS IS DRY AHEAD OF THE FRONT AND THERE
WILL BE A LOT OF WET-BULB COOLING (EVAPORATION) ON SUNDAY WHICH
WILL SIGNIFICANTLY LOWER SNOW LEVELS. BY THE TIME LIGHT
PRECIPITATION BEGINS BY MID MORNING TO MIDDAY SUNDAY IN THE
WESTSIDE VALLEYS THE SNOW LEVEL COULD FALL TO NEAR 2000 FEET AND
PROBABLY ALL THE WAY TO 1500 FEET ALONG HIGHWAY 96 AROUND HAPPY
CAMP.

HAVE CONSIDERED THE POSSIBILITY OF BRIEF FREEZING RAIN IN
THE WESTSIDE VALLEYS SUNDAY MORNING BUT CONFIDENCE IS TOO LOW TO
MENTION IT SINCE THE FRONT IS MOVING ALONG AT A GOOD PACE AND ANY
FREEZING RAIN SHOULD BE QUITE BRIEF AT BEST. BY SUNDAY AFTERNOON
THE SNOW LEVELS SHOULD BE CLOSER TO 3000 FEET. LITTLE SNOW
ACCUMULATION SHOULD OCCUR AROUND SEXTON AND CANYON PASS ON I-5
LATE SUNDAY MORNING BUT IT COULD BE ENOUGH TO MAKE IT SLICK.

BEHIND THE FRONT SUNDAY NIGHT INTO MONDAY THE SNOW LEVELS WILL BE
RELATIVELY LOW AROUND 2500 FEET BUT STILL WELL ABOVE THE WESTSIDE
VALLEY FLOORS. OVERALL...PRECIPITATION TOTALS WILL BE LIGHT WITH
MOST AREAS FROM THE CASCADES WEST RECEIVING BETWEEN ONE-QUARTER
AND ONE-HALF INCH...AND ONLY AROUND ONE-TENTH OF AN INCH EAST OF
THE CASCADES.
SANDLER

LONG TERM...THE LONG-TERM FORECAST (TUESDAY TO FRIDAY)...BOTH
THE GFS AND THE ECMWF SHOW A DISTURBANCE MOVING SOUTH ALONG THE
EAST SIDE OF THE RIDGE ON TUESDAY...BRINGING LIGHT RAIN TO NW
OREGON DOWN TO THE UMPQUA BASIN. THE ECMWF IS ABOUT 12 HRS LATER
BUT BOTH MODELS EVENTUALLY SHOW PRECIPITATION ACROSS SOUTHERN
OREGON BY TUESDAY NIGHT...THEN INTO NORTHERN CALIFORNIA BY
WEDNESDAY.

BY THURSDAY...UPPER RIDGING BUILDS INTO THE EASTERN PACIFIC AND THE
WEST COAST WITH THE ATMOSPHERE BECOMING DRIER AND MORE STABLE. THIS
IS SOMETHING WE HAVE NOT HAD TO CONTEND WITH THIS FALL AND WINTER,
UNLIKE IN THE PREVIOUS TWO WINTERS. EVENTUALLY WE'LL HAVE TO START
THINKING ABOUT AN AIR STAGNATION ADVISORY FOR THE SECOND HALF OF
NEXT WEEK.
FB

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...FREEZE WARNING FROM 3 AM TO 9 AM PST SATURDAY FOR ORZ021-022.

CA...NONE.

PACIFIC COASTAL WATERS...GALE WATCH FROM SATURDAY EVENING THROUGH SUNDAY AFTERNOON FOR
     PZZ350-356-370-376.
     HAZARDOUS SEAS WATCH FROM SATURDAY EVENING THROUGH SUNDAY
     AFTERNOON FOR PZZ350-356-370-376.
     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 7 AM PST SATURDAY
     FOR PZZ356-376.
     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 1 AM PST SATURDAY
     FOR PZZ350-370.

CC