Freeze Warning 2, NWS Medford, OR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
845 AM PST WED JAN 9 2013

.DISCUSSION...A POTENT COLD FRONT IS ALREADY MOVING ONSHORE AND
WILL MOVE THROUGH THE FORECAST AREA TODAY. TEMPERATURES AND SNOW
LEVELS START OUT AT THEIR HIGHEST THIS MORNING...THEN WILL COME
DOWN DURING THE DAY. THE QUESTION WILL BE HOW LOW THEY GET BY THIS
EVENING. CURRENT FORECAST HAS SNOW LEVELS LOWERING BETWEEN 1000
AND 1500 FEET THIS EVENING....BUT THIS COULD VARY DEPENDING ON THE
INTENSITY OF THE PRECIPITATION AND HOW QUICKLY THE COLD AIR MASS
MOVES IN. A PRELIMINARY LOOK AT THE GFS AND NAM SUGGEST THE
COLDEST AIR WILL MOVE OVERHEAD TONIGHT. SO SUSPECT ANY
ACCUMULATION IN THE VALLEYS WILL PROBABLY NOT OCCUR UNTIL AFTER
DARK. WE'LL LOOK AT THIS IN MORE DETAIL THIS MORNING. WINDS ALONG
THE COAST...SHASTA VALLEY AND EAST OF THE CASCADES ARE NOT AS
STRONG AS ORIGINALLY THOUGHT....BUT STILL THEM TO INCREASE SOME AS
THE FRONT PUSHES THROUGH.

THE GFS AND NAM SHOW THE TROF AXIS SHIFTING EAST TONIGHT WHICH
SHOULD RESULT IN DECREASING SHOWERS. HOWEVER SEVERAL SMALL
DISTURBANCES THAT THE MODELS ARE NOT PICKING UP ON VERY WELL COULD MOVE
THROUGH THE AREA WHICH COULD ENHANCE SHOWER ACTIVITY. THE GFS
SUGGEST SHOWER ACTIVITY WILL BE AT A MINIMUM TONIGHT INTO THURSDAY
MORNING THEN INCREASING IN THE AFTERNOON.

PREVIOUS FORECAST FOR TODAY IS ON TRACK AND NO CHANGES ARE NEEDED.
-PETRUCELLI

&&

.AVIATION...CONDITIONS ARE MOSTLY  A MIX OF MVFR AND IFR WEST OF
THE CASCADES WITH GENERALLY VFR CONDITIONS EAST OF THE CASCADES. A
SHARP COLD FRONT IS APPROACHING WITH BREEZY SOUTHERLY SURFACE
WINDS AT 15 TO 25 KT AND STRONGER SOUTH TO SOUTHWEST WINDS
ALOFT...INCLUDING SPEEDS OF 25 TO 45 KT BETWEEN 2000 AND 8000 FT
AGL. THIS MAY CREATE A PERIOD OF LOW LEVEL SPEED SHEAR JUST AHEAD
OF THE FRONT WEST OF THE CASCADES. THE FRONT WILL REACH THE COAST
AROUND 12Z WEDNESDAY THEN TRACK ACROSS THE AREA THROUGH WEDNESDAY
AFTERNOON. A PERIOD OF LIGHT TO MODERATE PRECIPITATION WILL
ACCOMPANY THE FRONTAL PASSAGE WEDNESDAY ALONG WITH RAPIDLY
PLUNGING FREEZING LEVELS. STRATUS WILL REMAIN LIKELY LOCKED IN THE
VALLEYS UNTIL IT IS MIXED OUT BY THE FRONT WEDNESDAY MORNING.
WIDESPREAD MOUNTAIN OBSCURATION ALONG AND WEST OF THE CASCADES IS
EXPECTED TOMORROW MORNING AND BY WEDNESDAY EVENING OR WEDNESDAY
NIGHT...VISIBILITIES WILL DROP ACROSS MUCH OF THE AREA AS RAIN
CHANGES TO SNOW SHOWERS.

&&

.PREV DISCUSSION... /ISSUED 406 AM PST WED JAN 9 2013/

SHORT TERM...A COLD FRONT IS LOCATED JUST OFFSHORE AND WILL MOVE
INLAND THIS MORNING INTO THIS AFTERNOON. THIS WILL BRING A SHARP
TRANSITION TO COLDER TEMPERATURES TODAY WITH LOWERING SNOW LEVELS. THEN
TAN UPPER LEVEL TROUGH WILL MOVE INTO THE AREA LATE THIS AFTERNOON
INTO THIS EVENING. THIS TROUGH WILL REMAIN OVER THE REGION THROUGH
THURSDAY AS IT DIGS SOUTHWARD. THEN THE UPPER TROUGH WILL SLOWLY
SHIFT EAST THURSDAY NIGHT AND FRIDAY.

OBSERVATIONS ALONG THE COAST ARE BEGINNING TO REGISTER LIGHT
PRECIPITATION AHEAD OF THE FRONT. ALSO WINDS HAVE BEGUN TO PICK UP
OVER THE MOUNTAINS AND INTO THE SUMMER LAKE AREA. SNOW LEVELS ARE
CURRENTLY HIGH BUT WILL QUICKLY LOWER AS THE FRONT PUSHES INLAND.
SNOW LEVELS ARE EXPECTED TO LOWER TO 4000 TO 4500 FEET THIS
MORNING THEN DOWN TO 2000 TO 3000 FEET THIS AFTERNOON FOR AREAS
WEST OF THE CASCADES. BY EVENING EXPECT MOST AREAS TO SEE SNOW
SHOWERS AS SNOW LEVELS LOWER FURTHER TO 1000 TO 1500 FEET. SNOW
SHOWERS BEHIND THE FRONT DURING THE EVENING AND TONIGHT ARE
EXPECTED TO BE LIGHT. HOWEVER HEAVIER SNOW SHOWERS ARE FAVORED
OVER THE MOUNTAINS AND ALONG THE WESTERN TO NORTHWESTERN
SLOPES...ESPECIALLY ALONG THE CASCADES AND SISKIYOUS. GUSTY SOUTH
TO SOUTHWEST WINDS OVER THE MOUNTAINS AND INTO THE SUMMER LAKE
AREA WILL CONTINUE AHEAD OF THE FRONT...THEN EXPECT WINDS TO
DECREASE AND BECOME WEST TO NORTHWEST LATE THIS AFTERNOON INTO
THIS EVENING.

MODELS ARE IN GOOD AGREEMENT WITH THE TIMING AND STRENGTH OF THE
FRONT. THE GFS CONTINUES TO SHOW SNOW LEVELS LOWERING SLIGHTLY SLOWER
THAN THE NAM AND SREF DURING THE AFTERNOON THOUGH. ALSO THE GFS
SHOWS LIGHTER PRECIPITATION THIS EVENING AND TONIGHT BEHIND THE
FRONT. HOWEVER BASED ON THE BROAD AREA CONVECTIVE ACTIVITY
ASSOCIATED WITH THE UPPER TROUGH SHOWN ON SATELLITE...HAVE LEANED
TOWARDS THE WETTER NAM AND SREF. THERE ARE DIFFERENCES IN THE
MODEL FORECASTS TONIGHT INTO THURSDAY. THE NAM AND SREF KEEP
WIDESPREAD SHOWER ACTIVITY INTO THURSDAY MORNING ESPECIALLY FOR
AREAS FROM THE CASCADES WEST WHILE THE GFS DECREASES THE SHOWER
ACTIVITY IN THE OVERNIGHT AND EARLY MORNING HOURS. HAVE KEPT THE
WINTER WEATHER ADVISORIES GOING THROUGH 4 AM PST THURSDAY MORNING
FOR NOW. HOWEVER...IF THE NAM AND SREF ARE CORRECT...MAY NEED TO
EXTEND THESE ADVISORIES FOR THE WEST SIDE INTO THURSDAY.

THE COLD TROUGH LINGERS ON THURSDAY AND MODELS SHOW ANOTHER
DISTURBANCE MOVING INTO THE AREA THURSDAY AFTERNOON AND EVENING
BRINGING ANOTHER ROUND OF WIDESPREAD TO SCATTERED LIGHT SNOW
SHOWERS TO THE AREA. PREVIOUS RUNS HAVE SHOWN VARIABILITY DURING
THIS TIME PERIOD THOUGH. OF THE CURRENT RUNS...THE NAM REMAINS
THE WETTER SOLUTION. HOWEVER...ALL THE MODELS SHOW LIGHT
PRECIPITATION WITH GENERALLY LESS THAN A TENTH OF AN INCH OF
LIQUID EQUIVALENT PRECIPITATION FORECAST EXCEPT FOR LOCATIONS
ALONG THE COASTAL MOUNTAINS...CASCADES AND SISKIYOUS WHERE UP TO
TWO TENTHS OF AN INCH ARE SHOWN IN THE NAM. SO...ANOTHER 1 TO 3
INCHES OF SNOW MAY BE POSSIBLE IN SHOWERS ON THURSDAY WITH THIS
DISTURBANCE.

THEN LIGHT SHOWER ACTIVITY MAY CONTINUE INTO FRIDAY...ESPECIALLY
FOR AREAS FROM THE CASCADES WEST AS MODELS SHOW AN ELONGATED
SHORTWAVE MOVING INTO THE AREA ON THE BACK SIDE OF THE UPPER
TROUGH.

LOW SNOW LEVELS AND COLD TEMPERATURES ARE EXPECTED TO CONTINUE
THROUGH FRIDAY.

PREV EXTENDED DISCUSSION... /ISSUED 324 PM PST TUE JAN 8 2013/

EXTENDED TERM...SATURDAY MORNING THROUGH TUESDAY NIGHT...

BY SATURDAY MORNING...THE LARGE SCALE TROUGH SHOULD BEGIN ITS
SLOW PROGRESSION EAST...KEEPING THE CWA UNDER NORTHERLY FLOW AND
UNDER THE GUN OF ANY FAST MOVING SHORTWAVES TRAVERSING THE MAIN BELT
OF WESTERLIES. THE TROUGH SHOULD SHIFT FAR ENOUGH EAST DURING THE
TERM TO PLACE SOUTHERN OREGON AND NORTHERN CALIFORNIA UNDER THE
PERIPHERY OF A LARGE AMPLITUDE RIDGE THAT CURRENTLY RESIDES OVER THE
THE EASTERN PACIFIC. MAIN CONCERN DURING THE EXTENDED PERIOD WILL BE
THE VIGOROUS SHORTWAVE EXPECTED TO DROP SOUTH INTO THE REGION
SATURDAY NIGHT INTO SUNDAY.

MODELS SOLUTIONS HAVE BEEN RATHER SKETCHY ON THE DETAILS CONCERNING
THE SATURDAY NIGHT WAVE. YESTERDAY...THE GFS SOLUTION PRODUCED A
MORE VIGOROUS WAVE...WHILE THE ECMWF PROVIDED A WEAKER SOLUTION.
TODAY...THE MODELS HAVE SWITCHED...WITH THE GFS MUCH WEAKER THAN THE
ECMWF. OTHER MODELS...SUCH AS THE GEM AND DGEX HAVE BEEN EQUALLY AS
UNCERTAIN. HOWEVER...ENSEMBLE RUNS AND REFORECAST ANALOGS SUGGEST
THAT THERE IS ENOUGH CONFIDENCE OF PRECIPITATION TO WARRANT AN
INCREASE IN POPS AND QPF FOR SATURDAY NIGHT INTO SUNDAY. AMOUNTS
SHOULD BE RELATIVELY LIGHT...WITH SNOW LEVELS OF JUST BELOW 2000
FEET ON THE WEST SIDE RISING SLOWLY AND STEADILY AS THE EVENT
UNFOLDS. EAST SIDE SNOW LEVELS WILL BE LOWER...PRODUCING ALL
SNOW...BUT DUE TO THE NATURE OF THE SYSTEM...OVERALL SNOW SHOULD
ONLY AMOUNT TO ABOUT AND INCH IN THE LOWER ELEVATIONS.

ONCE THE WAVE EXITS SUNDAY...THE EASTERN PACIFIC RIDGE WILL BEGIN TO
BUILD TOWARDS THE AREA. ALTHOUGH THE CWA LOOKS TO RESIDE UNDER
NORTHERLY FLOW THROUGHOUT THE REMAINDER OF  THE TERM...THE RIDGE
SHOULD MAINTAIN ENOUGH INFLUENCE TO KEEP ANY PRECIPITATION PRODUCERS
WELL TO THE EAST...AND ALLOW FOR AMPLE INSOLATION. HAVE DRIED OUT
THE FORECAST AND INCREASED TEMPERATURES FOR THE REMAINDER OF THE
TERM TO ACCOUNT FOR THIS. -BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...WINTER WEATHER ADVISORY FROM 1 PM THIS AFTERNOON TO 4 AM PST
     THURSDAY FOR ORZ030-031.
     WIND ADVISORY UNTIL 1 PM PST THIS AFTERNOON FOR ORZ030-031.
     WINTER WEATHER ADVISORY FROM 4 PM THIS AFTERNOON TO 4 AM PST
     THURSDAY ABOVE 1000 FEET FOR FOR ORZ021-022.
     WINTER WEATHER ADVISORY FROM 1 PM THIS AFTERNOON TO 4 AM PST
     THURSDAY ABOVE 1000 FEET FOR FOR ORZ023.
     WINTER WEATHER ADVISORY UNTIL 4 AM PST THURSDAY FOR ORZ025.
     WINTER WEATHER ADVISORY FROM 1 PM THIS AFTERNOON TO 4 AM PST
     THURSDAY FOR ORZ024-026.
     WINTER WEATHER ADVISORY UNTIL 4 AM PST THURSDAY FOR ORZ027-028.

CA...WIND ADVISORY UNTIL 1 PM PST THIS AFTERNOON FOR CAZ085.
     WINTER WEATHER ADVISORY FROM 1 PM THIS AFTERNOON TO 4 AM PST
     THURSDAY FOR CAZ081>083.
     WINTER WEATHER ADVISORY FROM 1 PM THIS AFTERNOON TO 4 AM PST
     THURSDAY ABOVE 1500 FEET FOR FOR CAZ080.
     WIND ADVISORY UNTIL 1 PM PST THIS AFTERNOON FOR CAZ081.
     WINTER WEATHER ADVISORY FROM 1 PM THIS AFTERNOON TO 4 AM PST
     THURSDAY FOR CAZ085.

PACIFIC COASTAL WATERS...SMALL CRAFT ADVISORY FOR WINDS UNTIL 1 PM PST THIS AFTERNOON FOR
     PZZ350-356-370-376.
     HAZARDOUS SEAS WARNING FROM 10 AM THURSDAY TO 4 AM PST FRIDAY
     FOR PZZ350-356-370-376.
     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 10 AM PST THURSDAY
     FOR PZZ350-356-370-376.

MAP/MAP/MAP

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MEDFORD OR
228 PM PST WED JAN 09 2013

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0227 PM     SNOW             N TENNANT               41.59N 121.91W
01/09/2013  M1.0 INCH        SISKIYOU           CA   TRAINED SPOTTER

            SNOW BEGAN 1 HOUR AGO, AND IS ONGOING.

&&

BPN