Frost and Freeze in Oregon, February 5, 2014

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
800 PM PST TUE FEB 4 2014

.DISCUSSION...05/00Z NAM IN. 05/00Z GFS IN THROUGH 72HR.

PARTLY CLOUDY SKIES PREVAIL OVER THE MEDFORD CWA THIS EVENING
UNDER BRISK NORTHWEST FLOW ALOFT. THIS TYPE OF PATTERN IS USUALLY
COOL...AND THE LACK OF SHORT WAVES EMBEDDED IN THE FLOW WILL KEEP
CONDITIONS MOSTLY DRY. A FREEZE WARNING IS IN EFFECT FOR THE COAST
TONIGHT...AND A WATCH IS IN EFFECT FOR WEDNESDAY NIGHT.

THE NORTHWEST FLOW WILL GRADUALLY BECOME ZONAL THROUGH THURSDAY AS
THE RIDGE WEAKENS AND A LONG WAVE TROUGH DIGS IN PLACE ALONG THE
WEST COAST. AS THE FRONT DIGS A COLD FRONT WILL DEVELOP AND MOVE
INTO THE AREA WEDNESDAY NIGHT. AT THE SAME TIME ANOTHER LOW WILL
MOVE INTO CENTRAL CALIFORNIA AND SOME OF THE WRAPAROUND
PRECIPITATION WILL MOVE INTO NORTHERN CALIFORNIA. THE FRONTAL
ZONE WILL REMAIN OVER THE AREA THROUGH THURSDAY NIGHT AND MOVE OUT
FRIDAY...SO THERE WILL BE SOME PRECIPITATION AROUND THROUGH THAT
PERIOD...ESPECIALLY OVER NORTHERN CALIFORNIA. SNOW LEVELS IN THE
WETTEST AREAS WILL BE AROUND 2000 FEET...RISING TO 4000 FEET OVER
NORTHERN CALIFORNIA THURSDAY EVENING BEFORE PLUNGING AGAIN
THURSDAY NIGHT. THE MODELS HAVE BEEN SHOWING SIGNIFICANT RUN TO
RUN VARIABILITY ON JUST HOW COLD THE INCOMING AIR MASS WILL
BE...SO THERE IS SOME UNCERTAINTY ON WHAT THE SNOW LEVELS WILL
BE...THE CURRENT FORECAST TENDS TO THE COLD SIDE OF THE ENVELOPE.

THE UPPER TROUGH WILL MOVE EAST FRIDAY AND A LONG WAVE RIDGE WILL
BUILD IN FROM THE WEST...BUT NOT FOR LONG AS THIS PATTERN WILL BE
HIGHLY PROGRESSIVE. THE 04/18Z GFS HAS A STRONG SHORT WAVE ALOFT
DRIVING A WET AND WINDY FRONT ONSHORE FRIDAY NIGHT...A BIT EARLIER
THAN PREVIOUS MODELS HAVE STATED. THE 04/12Z EC IS ABOUT SIX HOURS
SLOWER THAN THIS FEATURE.

EXTENDED TERM DISCUSSION FROM THE TUESDAY AFTERNOON AFD...SATURDAY
THROUGH TUESDAY NIGHT.  MODELS ARE IN PARTICULARLY GOOD AGREEMENT
DURING THE LONG TERM...LENDING HIGHER THAN AVERAGE CONFIDENCE TO
THE FORECAST OF A WETTER AND MORE ACTIVE PATTERN. THIS IS
ESPECIALLY TELLING GIVEN THE ABRUPT PATTERN CHANGE...AND AS A
RESULT...THERE IS VERY LITTLE REASON TO CHANGE THE HIGH END POPS
THAT WERE INHERITED FROM PREVIOUS SHIFTS. HOWEVER...SOME
ADJUSTMENTS WERE NEEDED TO BRING TEMPERATURES AND SNOW LEVELS INTO
LINE WITH THE EXPECTED PATTERN.

THE FIRST OF SEVERAL RELATIVELY SIGNIFICANT WAVES WILL ENTER THE
AREA SATURDAY EVENING. IVT AND OTHER MOISTURE PROFILES INDICATE A
CONNECTION BETWEEN THIS SYSTEM AND TROPICAL MOISTURE SOURCES. WHILE
THIS "ATMOSPHERIC RIVER" FEATURE  DOES NOT APPEAR TO BE A STRONG
ONE...IT WILL BE SIGNIFICANT IN THAT THERE HAVE NOT BEEN MANY SUCH
EVENTS THIS SEASON. WITH SUCH A FEATURE...TEMPERATURES AND SNOW
LEVELS WILL JUMP UPWARDS AS THE MOISTURE MOVES IN...SO HAVE UPPED
THOSE PARAMETERS FROM THE COOLER PREVIOUS FORECAST. SNOW LEVELS
SHOULD RISE TO ABOVE 5000 FEET...WHILE TEMPERATURES APPROACH
SEASONAL NORMALS. ONE CAVEAT TO THIS EVENT IS THAT MODEL SOLUTIONS
HAVE BEEN QUICK TO PUSH THE BULK OF THE RAINFALL THROUGH THE AREA
OVER A PERIOD OF 6 TO 12 HOURS. THIS DOES NOT LEND ITSELF TO HEAVY
RAINFALL AMOUNTS...BUT THERE IS STILL GOOD REASON TO EXPECT AN INCH
OR MORE OF LIQUID PRECIPITATION ALONG THE COAST AND OVER THE HIGHER
TERRAIN ALONG AND WEST OF THE CASCADES. AS THE PREDOMINANT FLOW WILL
BE WEST TO EAST...NORTH/SOUTH TRENDING RIDGELINES CAN EXPECT HEAVIER
PRECIPITATION.

AFTER THIS SYSTEM PASSES...THE OVERALL PATTERN SHIFTS TO A VERY
BROAD...LOW AMPLITUDE RIDGE...THROUGH WHICH MANY FAST MOVING WAVES
ARE LIKELY TO TRAVERSE. WHILE CONFIDENCE IS HIGH CONCERNING THE
ACTIVE PATTERN...THERE IS MUCH LOWER CONFIDENCE IN THE TIMING OF
ANY INDIVIDUAL FEATURE. HOWEVER...WITH STRONG ONSHORE FLOW AND
NUMEROUS WAVES EXPECTED...FOUND NO REASON TO DECREASE POPS...AS
SHOWER ACTIVITY WILL MOST LIKELY BE ONGOING BETWEEN EACH SYSTEM.
HAVE THEREFORE KEPT AT LEAST CHANCE POPS IN THE FORECAST THROUGH
THE REMAINDER OF THE TERM. SNOW LEVELS ARE EXPECTED TO HOVER
AROUND 4000 FEET AND TEMPERATURES SHOULD REMAIN AT OR NEAR
SEASONAL NORMALS THROUGHOUT THIS TIME.

&&

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

CONDITIONS WILL REMAIN UNSETTLED ENOUGH TONIGHT THROUGH THURSDAY TO
PREVENT VISIBILITY AND CEILING RESTRICTIONS AT MOST LOW ELEVATION
LOCATIONS. PLENTY OF HIGHER LOW LEVEL TO MID LEVEL CLOUDINESS WILL
RESULT IN PARTIAL TERRAIN OBSCURATIONS OVER THE CASCADES AND PEAKS
FOP THE SISKIYOUS. FOR LOWER ELEVATIONS THE ONE AREA WHERE THERE IS
LIKELY TO BE SOME RESTRICTIONS BY MORNING IS IN THE ROSEBURG AREA,
WHERE MVFR TO IFR STRATUS AND POSSIBLY SOME LIGHT FOG ARE EXPECTED
TO DEVELOP IN THE 12Z TO 17Z TIME FRAME. DUE TO A FAIRLY SOLID VFR
CLOUD DECK ABOVE THE LOW LEVELS, HOWEVER, SUSPECT IT IS UNLIKELY TO
GET LOWER. BTL

&&

.MARINE...
LIGHT NORTH WINDS AND SMALL SEAS WILL PERSIST THROUGH WEDNESDAY.
WEDNESDAY NIGHT THROUGH FRIDAY A WEAK FRONTAL SYSTEM AND RELATED
WAVE OF LOW PRESSURE IS LIKELY TO PROGRESS THROUGH THE AREA, BUT
EXACT TIMING OF THIS FRONTAL SYSTEM IS UNCERTAIN. IT APPEARS MOST
LIKELY THAT THE LOW AND ASSOCIATED TRIPLE POINT WILL RIDE THROUGH
THURSDAY NIGHT WITH A PERIOD OF RAIN AND SHIFTING WINDS. FRIDAY NIGHT
THROUGH SATURDAY A STRONG WARM FRONT IS LIKELY TO BRING GALE FORCE
SOUTH WINDS AND MODERATE TO HEAVY RAINFALL. MODEL GUIDANCE HAS
VARIED QUITE SIGNIFICANTLY WITH THE EXPECTED ARRIVAL TIME OF THIS
WEATHER SYSTEM. RECENT TRENDS INDICATE THAT THE PERIOD GALES AND
MODERATE TO HEAVY RAIN IS LIKELY TO ARRIVE SOONER RATHER THAN LATER-
FRIDAY AFTERNOON AND NIGHT RATHER THAN SATURDAY MORNING. BY SUNDAY
CONDITIONS ARE LIKELY TO BE LESS ACTIVE, WITH SHOWERS IN A GENERAL
SOUTHWEST FLOW. BTL

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...FREEZE WATCH FROM LATE WEDNESDAY NIGHT THROUGH THURSDAY MORNING
        FOR ORZ021-022.
     FREEZE WARNING FROM 1 AM TO 8 AM PST WEDNESDAY FOR ORZ021-022.

CA...NONE.

PACIFIC COASTAL WATERS...NONE.

15/15/15