Frost and Freeze in Oregon, December 4 to 5, 2013

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
936 PM PST WED DEC 4 2013

.DISCUSSION...BASED ON CURRENT TEMPERATURES, DEW POINTS AND THE 24
HOUR TREND, AN UPDATE HAS BEEN ISSUED TO ADJUST THURSDAY MORNING
LOW TEMPERATURES LOWER. THE EXTENT WILL BE GENERALLY BY 3 TO 7
DEGREES ON THE EAST SIDE AND 1 TO 2 DEGREES ON THE WEST SIDE.

THE FOCUS OF THE FORECAST IS ON TWO ISSUES...TEMPERATURES THROUGH THE
NEXT WEEK AND THE TIMING/STRENGTH OF AN OCCLUDED FRONT FRIDAY INTO
FRIDAY NIGHT.

THIS EXTREME COLD SNAP WITH NEAR RECORD TO RECORD LOWS WILL
CONTINUE AT LEAST THROUGH TUESDAY NIGHT. THE EXTENT OF THE COLD
AND THE DURATION ARE BOTH NOTABLE. WITH THE CLOUD COVER AND
PRECIPITATION FORECAST FOR FRIDAY INTO FRIDAY NIGHT, THE 00Z GFS
HAS TRENDED SLIGHTLY HIGHER (1 TO 3 DEGREES) WITH THOSE
TEMPERATURES BUT SLIGHTLY LOWER OTHERWISE THROUGH TUESDAY. WHILE
LOW TEMPERATURES HAVE BEEN FREQUENTLY COLDER TO MUCH COLDER THAN NORMAL
SINCE NOVEMBER 21ST...THE ADDITION OF HIGH TEMPERATURES REMAINING
BELOW FREEZING ON MOST OF THE WEST SIDE SATURDAY, SUNDAY AND
MONDAY COULD CAUSE INSUFFICIENTLY PROTECTED PIPES TO FREEZE AND BURST.

BASED UPON THE 00Z GFS AND 00Z NAM, THE NIGHT SHIFT WILL LIKELY
SLOW DOWN THE BEGINNING OF THE SNOWFALL EVENT BY 3 TO 6 HOURS...TILL
AFTER MIDNIGHT THURSDAY NIGHT WITH ONLY A FEW FLURRIES BEFORE 4 AM.
THE GFS AND NAM ARE EACH STRONGER/WETTER THAN THEIR 18Z RUNS, BUT
THIS SIMPLY TOOK THEM BACK TOWARD THEIR 12Z SOLUTIONS AND CLOSER TO
THE CURRENT FORECAST. ONE NOTABLE DIFFERENCE IS THAT THE NAM
SPREADS SNOW ACROSS ALMOST ALL OF THE WEST SIDE BY 10AM WHILE THE
GFS IS SLOWER WITH SNOWFALL MAINLY WEST AND NORTH OF JACKSON
COUNTY UNTIL THEN. THE MODEL DIFFERENCES ARE SMALLER FOR FRIDAY
AFTERNOON AND EVENING. TOTAL SNOWFALL FRIDAY MORNING THROUGH
FRIDAY EVENING IS FORECAST TO BE GENERALLY 2 TO 4 INCHES IN
VALLEYS AND 4 TO 8 INCHES IN THE MOUNTAINS. ANOTHER IMPORTANT FACT
IS THAT THE GFS HAS TRENDED TOWARD THE NAM IN CONTINUING/LINGERING
LIGHT SNOW SHOWERS THROUGH THE REMAINDER OF FRIDAY NIGHT WITH A
FOCUS ON NORTH AND NORTHEAST FACING SLOPES.

&&

.AVIATION...BASED ON THE 05/00Z TAF CYCLE.

LOCAL LATE NIGHT THROUGH MORNING IFR CIGS/VSBYS WILL DEVELOP IN LOW
CLOUDS AND FOG IN THE UMPQUA VALLEY...AND TO A LESSER EXTENT IN THE
ILLINOIS AND ROGUE VALLEYS. ELSEWHERE...VFR CONDITIONS WILL PREVAIL
THROUGH THURSDAY EVENING.  /FB

&&

.PREV DISCUSSION... /ISSUED 241 PM PST WED DEC 4 2013/

DISCUSSION...CURRENT VISIBLE IMAGE SHOWS CLEAR SKIES OVER MOST OF
THE CWA. THE EXCEPTION IS IN EASTERN LAKE COUNTY. EVEN THEN THE
CLOUDS ARE DECREASING THERE. THE COMBINATION OF CLEAR SKIES AND A
COLD AIR MASS IN PLACE TONIGHT WILL ALLOW TEMPERATURES TO PLUMMET
FAIRLY QUICKLY AFTER DARK. THE FORECAST FOR MEDFORD PROPER IS 16
DEGREES TONIGHT. IF THIS VERIFIES WILL BREAK THE OLD RECORD OF 18
DEGREES SET BACK IN 1972

THE BIG CONCERN AHEAD IS THE POTENTIAL FOR LOW ELEVATION SNOW FOR
MOST OF THE WESTSIDE EARLY FRIDAY MORNING. THE MODELS REMAIN IN
FAIRLY GOOD AGREEMENT WITH THE TRACK OF THE STORM. IF ANYTHING THEY
HAVE TRENDED A BIT SLOWER WITH THE ARRIVAL TIME COMPARED TO THE
PREVIOUS RUNS AND TRACKING SLIGHTLY FURTHER SOUTHEAST. THIS COULD
PREVENT ANY WARMING, ALBEIT BRIEF FROM MOVING INTO THE REGION. MODEL
SOUNDING AND 1000-850 MB THICKNESS AND 850 MB TEMPERATURES ON THE
MODELS INDICATE PRECIPITATION WILL BE ALL SNOW. OF NOTE THE NAM DOES
INDICATE THE POTENTIAL FOR DOWNSLOPING WHICH ALSO SHOWS UP ON THE
CROSS SECTION, BUT THE OMEGAS DO INCREASE DURING THE MORNING, SO
ITS POSSIBLE PRECIPITATION COULD BE DELAYED UNTIL AFTER DAYBREAK.
HOWEVER WITH THESE TYPES OF SYSTEMS COMING IN FROM THE NORTHWEST,
OFTENTIMES THEY COME IN FASTER THEN WHAT THE MODELS SUGGEST, SO FELT
IT WAS BEST TO STAY THE COURSE AND NOT MAKE ANY SIGNIFICANT CHANGES.
THE NIGHT SHIFT WILL NEED TO TAKE ANOTHER LOOK AT THIS.

HAVE MAINTAINED THE WINTER STORM WATCH WITH SOME MINOR ADJUSTMENTS
FOR THE REASONS NOTED ABOVE. FIRST EXPANDING THE WATCH TO COVER ALL
OF ZONES 23, 80, 81 AND 82 AND ABOVE 500 FEET FOR THE COASTAL ZONES.
DETAILS ON THIS CAN BE FOUND AT WSWMFR. MOST LIKELY THESE WILL BE
UPGRADED TO A COMBINATION OF ADVISORIES AND OR WARNINGS EITHER
TONIGHT OR TOMORROW MORNING.

KEEP IN MIND THAT FURTHER FINE TUNING AND ADJUSTMENTS WILL BE MADE
TO THE FORECAST OVER TIME. PARTICULARLY WITH SNOW AMOUNTS AND SNOW
LEVELS, ESPECIALLY ALONG THE COAST WHICH WILL GREATLY DEPEND ON
TRACK OF THE LOW.

THE LOW WILL CONTINUE TO TRACK INTO NORTHERN CAL FRIDAY EVENING WITH
A RE-ENFORCING SHOT OF COLD AIR BEHIND IT. PRECIPITATION IS EXPECTED
TO DECREASE FROM NORTH TO SOUTH EARLY IN THE EVENING. HOWEVER THE
PATTERN STILL LOOKS FAVORABLE FOR UPSLOPE PRECIPITATION ALONG THE
EAST FACING SLOPES OF THE CASCADES. THIS ALONG WITH A COLDER AIR MASS
MOVING FROM THE NORTH COULD RESULT IN A PROLONGED PERIOD OF SNOW AND
HIGHER SNOWFALL ACCUMULATIONS DUE TO HIGHER SNOW RATIOS NEAR KLAMATH
FALLS AND CHEMULT. -PETRUCELLI

EXTENDED DISCUSSION...SATURDAY THROUGH WEDNESDAY.

AN UPPER TROUGH WILL DEPART THE AREA SATURDAY...LEAVING BEHIND A
LAYER OF SNOW OVER MOST OF THE AREA.  SKIES WILL CLEAR AS DRY
NORTHERLY FLOW ALOFT DEVELOPS OVER THE AREA.  THE CLEARING SKIES
COMBINED WITH THE FRESH SNOW WILL MAKE SATURDAY NIGHT A VERY COLD
ONE OVER THE AREA.  HAVE LOWERED THE MINIMUM TEMPERATURES QUITE A
BIT FOR THAT NIGHT.

SATURDAY THROUGH MONDAY WILL BE DRY AND COLD WITH A WEAK RIDGE OVER
THE AREA.  GOING WITH A WARMING TREND DURING THAT INTERVAL...BUT IT
WILL REMAIN QUITE COLD...BELOW FREEZING OVER MOST OF THE MEDFORD
CWA.  A SHORT WAVE MOVING OVER THE RIDGE WILL MOVE ONSHORE
TUESDAY...AND THIS WILL BRING THE NEXT CHANCE OF PRECIPITATION TO
THE AREA. BY THAT TIME SNOW LEVELS WILL BE BACK UP AROUND 3000
FEET.  ADDITIONAL SHORT WAVES WILL FOLLOW...AND THIS WILL BREAK DOWN
THE RIDGE BY THE MIDDLE OF NEXT WEEK. THE CHANCE OF PRECIPITATION
WILL GRADUALLY INCREASE AFTER TUESDAY...BECOMING LIKELY ALONG THE
COAST AND IN THE UMPQUA BASIN BY THURSDAY.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...FREEZE WATCH FROM THURSDAY EVENING THROUGH FRIDAY MORNING FOR
     ORZ021-022.
     FREEZE WARNING UNTIL 9 AM PST THURSDAY FOR ORZ021-022.
     WINTER STORM WATCH FROM THURSDAY EVENING THROUGH FRIDAY EVENING
     ABOVE 500 FEET FOR FOR ORZ021-022.
     WINTER STORM WATCH FROM THURSDAY EVENING THROUGH FRIDAY EVENING
     FOR ORZ023.
     WINTER STORM WATCH FROM LATE THURSDAY NIGHT THROUGH FRIDAY
     EVENING FOR ORZ025.
     WINTER STORM WATCH FROM LATE THURSDAY NIGHT THROUGH FRIDAY
     EVENING FOR ORZ024-026.

CA...WINTER STORM WATCH FROM LATE THURSDAY NIGHT THROUGH FRIDAY
     EVENING FOR CAZ080-081.

PACIFIC COASTAL WATERS...SMALL CRAFT ADVISORY FOR WINDS UNTIL 10 PM PST
     THIS EVENING FOR PZZ376.
     HAZARDOUS SEAS WARNING UNTIL 10 PM PST THIS EVENING FOR PZZ376.

DW/FB