Winter Storm Watch 3, NWS Medford, OR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
820 PM PDT SAT MAR 29 2014

.DISCUSSION...DEEP MOISTURE ALONG A COLD FRONTAL BOUNDARY IS BEGINNING
TO EXIT THE FORECAST AREA TO THE EAST. TODAY'S PRECIPITATION HAS
BEEN OF THE SHOWERY...HIT OR MISS VARIETY WITH DAYTIME
PRECIPITATION TOTALS GENERALLY A TENTH OF AN INCH OR LESS.
COUNTING YESTERDAY'S AND LAST NIGHT'S PRECIPITATION...ONE HALF TO
ONE INCH TOTALS ARE COMMON ACROSS NORTHERN CALIFORNIA AND SOUTHERN
OREGON. SNOW LEVELS ARE ABOUT 5000 FEET...WITH SISKIYOU SUMMIT
ALONG I-5 WET AND HIGHWAY 140 AT LAKE OF THE WOODS BECOMING SNOW-
COVERED THIS EVENING.

A WEAK UPPER AIR IMPULSE MOVING THROUGH OVERNIGHT WILL MAINTAIN SOME
SHOWER ACTIVITY TONIGHT...ESPECIALLY AT THE COAST AND OVER
HIGH-TERRAIN AREAS. MOUNTAINS WILL SEE ANOTHER SEVERAL INCHES OF
SNOW ABOVE 4000 FEET OR SO.  SUNDAY WILL FEATURE CONTINUED ONSHORE
FLOW AND AT LEAST PARTLY CLOUDY SKIES...WITH DECREASING
PRECIPITATION CHANCES DUE TO A MORE STABLE AIR MASS MOVING OVERHEAD
AND LITTLE IN THE WAY OF SHORTWAVE ACTIVITY.

PLEASE SEE THE PREVIOUS DISCUSSION BELOW FOR DETAILS ON THE RATHER
ACTIVE WEATHER PATTERN THAT SHOULD CONTINUE THROUGH THE
WEEK...ALTHOUGH NO WIDESPREAD PRECIPITATION EVENTS LIKE YESTERDAY'S
ARE EXPECTED.

&&

.AVIATION...POST-FRONTAL SHOWERS WILL CONTINUE OVERNIGHT WITH
MOUNTAIN OBSCURATION AND MVFR CEILINGS MOST COMMON IN THE VICINITY
OF SOUTHWEST AND WEST FACING SLOPES. AREAS OF MVFR CIGS WILL
INCREASE TONIGHT...MOSTLY OVER THE WEST SIDE VALLEYS. WEAK HIGH
PRESSURE WILL DECREASE THE COVERAGE OF SHOWERS ON SUNDAY MORNING
WITH MORE BREAKS AND HIGHER BASES IN THE CLOUD COVER BY AFTERNOON.
VFR CEILINGS AND A FEW SHOWERS WILL THEN LIKELY PERSIST THROUGH
THE AFTERNOON INTO SUNDAY EVENING.

&&

.MARINE...A WEAK DISTURBANCE WILL PRODUCE OCCASIONAL SHOWERS TONIGHT
WHILE A HIGH AND STEEP WEST SWELL SLOWLY AND SLIGHTLY DIMINISHES,
AND SOUTHWEST WINDS DIMINISH AND VEER TO WEST. THE 12Z NAM AND GFS
WERE IN VERY GOOD AGREEMENT THAT WINDS WILL BACK TO SOUTH AGAIN ON
SUNDAY MORNING AND THEN INCREASE AHEAD OF THE NEXT COLD FRONT. WINDS
WILL REACH SMALL CRAFT ADVISORY LEVELS IN THE AFTERNOON THEN GALES
WILL LIKELY DEVELOP IN THE OUTER WATERS SUNDAY EVENING AND CONTINUE
INTO MONDAY MORNING. SEAS WILL LIKELY BECOME HAZARDOUS DURING SUNDAY
EVENING WITH THE LARGE WIND WAVES COMBINING WITH A NORTHWEST SWELL.
WINDS OVER THE WATERS ARE FORECAST TO DIMINISH ON MONDAY WITH SEAS
DOMINATED BY A FRESH SHORT PERIOD SOUTHWEST SWELL. LOW PRESSURE WILL
THEN MOVE SOUTH ALONG THE OUTER WATERS MONDAY NIGHT INTO TUESDAY.
HIGH PRESSURE WILL BUILD IN FROM THE WEST LATE TUESDAY INTO
WEDNESDAY EVENING WITH WEAK WINDS AND DIMINISHING SEAS. ADDITIONAL
FRONTS ARE FORECAST TO CROSS THE COASTAL WATERS LATE WEDNESDAY NIGHT
THROUGH THURSDAY AND FRIDAY THROUGH FRIDAY NIGHT.

&&

.PREV DISCUSSION... /ISSUED 239 PM PDT SAT MAR 29 2014/

DISCUSSION...A DRY LAYER ABOVE 700 MB IS DRYING OUT THE TOP OF
CONVECTION THIS AFTERNOON IN SPITE OF BEING 5 DEGREES ABOVE THE
CONVECTIVE TEMP. THE RESULT IS FORMING CUMULUS DON-KINGUS...WITH
GOOD LOWER BUILD-UPS...BUT FUZZING OUT AT THE TOPS. ADDITIONAL
MOISTURE FEED INTO THIS LAYER WITH THE CONVECTIVE ACTIVITY MAY BE
ENOUGH TO PUSH PAST THIS LAYER AND HAVE KEPT A SLIGHT CHANCE OF
THUNDERSTORMS IN FOR LATE AFTERNOON. THE COLD AIR MASS WITH 850
TEMPS NEARING ZERO IN PLACES KEEP SHOWERS OVERNIGHT...BUT
GRADUALLY DECLINES AS INLAND AREAS COOL. THE 700 FLOW BECOMES MORE
SOUTHERLY TOMORROW AHEAD OF THE NEXT FRONT. THIS SYSTEM MOVES IN
ON A CLOSE UPPER LEVEL LOW THAT MOVES DOWN THE COAST OFFSHORE AND
MODELS HAVE TRENDED THE ONSET OF PRECIP AT THE COAST BACK ABOUT
6-10 HOURS MONDAY MORNING. THE GFS HAS SOUTHERLY BOUNDARY LAYER
WINDS OF 50 KT AT SPOTS IN THE COASTAL WATERS AHEAD OF THE
FRONT...BUT THE PRESSURE GRADIENT IS STRAIGHT OFFSHORE RESULTING
IN STRETCHING INSTEAD OF FUNNELING A COASTAL JET. AS THE CLOSED
UPPER LOW SLOWLY MEANDERS OFF THE CENTRAL CALIFORNIA COAST BY
WEDNESDAY MORNING IT SHOULD SPIN VORT LOBE AFTER VORT LOBE AROUND
IT...BRINGING BAND AFTER BAND OF SHOWERS THOUGH THE REGION THROUGH
TUESDAY AFTERNOON. SVEN

EXTENDED DISCUSSION...WEDNESDAY THROUGH SATURDAY.

THE FIRST PART OF THE EXTENDED PERIOD WILL BE A CONTINUATION OF THE
LOW AMPLITUDE PROGRESSIVE PATTERN NOW IN PLACE.  THE OVERALL FLOW
PATTERN ALOFT WILL BE ZONAL...BUT WILL VARY A BIT AS EMBEDDED SHORT
WAVES MOVE THROUGH...ONE WEDNESDAY NIGHT INTO THURSDAY AND THE OTHER
THURSDAY NIGHT INTO FRIDAY.

THE EC AND GFS DIVERGE AFTER THAT.  THE 29/12Z GFS SOLUTION SHOWS AN
OFFSHORE RIDGE BUILDING IN FRIDAY INTO SATURDAY. HOWEVER...IT KEEPS
THE AREA WET WITH AN ACTIVE WARM FRONT MOVING THROUGH SATURDAY INTO
SATURDAY NIGHT.  MEANWHILE THE 29/00Z EC SHOWS ANOTHER SHORT WAVE
MOVING THROUGH FRIDAY NIGHT INTO SATURDAY...FOLLOWED BY A BUILDING
RIDGE. OVERALL IT IS THE DRIER SOLUTION.  GIVEN THE UNCERTAINTY IN
THE TIMING OF THE SHORT WAVES...THE CURRENT EXTENDED FORECAST
REPRESENTS A REASONABLY GOOD ATTEMPT AT SHOWING THE VARIATIONS...SO
WILL TWEAK THE POP GRIDS FOR THE MOST PART. JRS

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...NONE.

CA...NONE.

PACIFIC COASTAL WATERS...SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 5 PM PDT SUNDAY
     FOR PZZ350-356-370-376.
     GALE WATCH FROM SUNDAY AFTERNOON THROUGH MONDAY MORNING FOR
     PZZ370-376.
     SMALL CRAFT ADVISORY FOR WINDS FROM 2 PM TO 5 PM PDT SUNDAY FOR
     PZZ370-376.
     HAZARDOUS SEAS WATCH FROM SUNDAY AFTERNOON THROUGH MONDAY
     AFTERNOON FOR PZZ370-376.

NSK/JRS/DW