Frost and Freeze in Oregon, January 2, 2016

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
909 PM PST FRI JAN 1 2016

.DISCUSSION...A HIGH PRESSURE RIDGE REMAINS LOCATED NORTH OF THE
AREA WITH A LOW PRESSURE TROUGH TO THE SOUTH. A VERY COLD AIR MASS
REMAINS IN PLACE, ESPECIALLY FOR LOCATIONS FROM THE CASCADES EAST.
A STRONG SURFACE PRESSURE GRADIENT IS FORECAST TO REMAIN IN PLACE
AND DECREASE SOME ON SATURDAY.

THIS PATTERN WILL BRING CONTINUED OFFSHORE FLOW AND GUSTY EAST
WINDS OVER THE MOUNTAINS, ESPECIALLY IN THE CASCADES AND SISKIYOUS
THIS EVENING INTO TONIGHT. SOME BREEZY AND GUSTY WINDS ARE ALSO
LIKELY IN THE ASHLAND AREA AS WELL. WINDS HAVE LOWERED IN OTHER
PARTS OF THE ROGUE VALLEY AND ARE EXPECTED TO CONTINUE TO
GRADUALLY LOWER TONIGHT. OTHER VALLEY LOCATIONS, WILL SEE MAINLY
LIGHT AND VARIABLE WINDS. IN THE UMPQUA BASIN, EXCEPT CALM WINDS
AND PERSISTENT LOW LEVEL MOISTURE. THE MAIN CONCERN IS THAT
FREEZING FOG WILL FORM ONCE AGAIN TONIGHT AND CONTINUE SATURDAY
MORNING. THIS MAY BRING VERY LOW VISIBILITIES. MODEL GUIDANCE
CURRENTLY SUPPORTS ONE HALF TO ONE QUARTER MILE VISIBILITIES IN
FOG IN THE ROSEBURG AREA, IN FOG DEVELOPING LATE TONIGHT AND EARLY
SATURDAY MORNING. ALSO LIGHT ICE MAY FORM ON SURFACES AND BRING
SLIPPERY CONDITIONS. WILL NEED TO MONITOR THE UMPQUA VALLEY FOR
DENSE FREEZING FOG TONIGHT.

WITH THE COLD AIR MASS IN PLACE, ALONG THE COAST, EXPECT  AREAS
OF FREEZING TEMPERATURES TONIGHT ALONG THE CURRY COAST WITH
WIDESPREAD FREEZING TEMPERATURES ALONG THE COOS AND DOUGLAS
COASTS. A FREEZE WARNING IS IN EFFECT FOR THESE AREAS.

&&

.AVIATION...FOR THE 02/00Z TAF CYCLE...
SOUTHEAST WINDS ALOFT PENETRATED INTO MOST OF THE WEST SIDE VALLEYS
THIS AFTERNOON. THESE WINDS HAVE MIXED OUT THE AIR IN MOST OF THE
VALLEYS, REPLACING THEM WITH MUCH DRIER AIR. THUS, FOG AND LOW
CLOUDS WILL BE LESSER TONIGHT THAN WAS OBSERVED LAST NIGHT, EXCEPT
IMMEDIATELY ALONG AND NEAR THE EDGES OF WARMER WATERWAYS. THERE IS
NOW HIGH CONFIDENCE THAT MEDFORD WILL NOT GET FOG TONIGHT INTO
SATURDAY MORNING. ENOUGH MOISTURE STILL EXISTS FOR FOG IN THE
ROSEBURG AREA, HOWEVER, SO EXPECT FREEZING FOG THERE FROM AS EARLY
AS 06Z THROUGH ABOUT 19Z. OTHERWISE, VFR IS EXPECTED. BTL

&&

.MARINE...UPDATED 300 PM PST FRIDAY, 01 JAN 2016... STRONG HIGH
PRESSURE INLAND WITH LOWER PRESSURE OFFSHORE WILL CONTINUE TO BRING
SOUTHEAST WINDS TONIGHT THROUGH TOMORROW. A WARM FRONT WILL LIFT
NORTHWARD INTO THE AREA LATE SATURDAY INTO SUNDAY BRINGING RAIN AND
STEEP BUILDING SEAS. BOUTS OF GUSTY SOUTHEAST TO SOUTH WINDS WITH
STEEP SEAS WILL CONTINUE EARLY NEXT WEEK WITH PERIODS OF RAIN.
TUESDAY THROUGH THURSDAY 14 TO 19 FOOT SEAS ARE POSSIBLE AS LONG
PERIOD WEST SWELL ARRIVES. BTL

&&

.PREV DISCUSSION... /ISSUED 555 PM PST FRI JAN 1 2016/

DISCUSSION...

SHORT TERM...TONIGHT THROUGH MONDAY NIGHT... THE REX BLOCK IN
PLACE OVER THE PACIFIC NORTHWEST IS BEGINNING TO BEND UNDER
PRESSURE FROM THE LARGE TROUGH DIGGING INTO THE EASTERN PACIFIC.
THE LARGE TROUGH IS CAUSING THE BLOCK'S ANCHORING UPPER LEVEL
LOW, CURRENTLY RESIDING OVER LAKE TAHOE, TO PUSH NORTH INTO OUR
FORECAST AREA. BY EARLY SATURDAY MORNING, THE CENTER OF THE LOW
SHOULD RESIDE OVER THE EAST SIDE SOMEWHERE BETWEEN BEND AND
KLAMATH FALLS. UNTIL THEN, THE WINDS AROUND THE LOW HAVE ALIGNED
THEMSELVES PERFECTLY TO FLOW DOWN SOME OF THE VALLEYS OF THE WEST
SIDE, INCLUDING THE ROGUE VALLEY FROM ASHLAND THROUGH
JACKSONVILLE AND MEDFORD. THIS IS RESULTING IN THE GUSTY WINDS
THERE THIS AFTERNOON, AND IS THANKFULLY RESPONSIBLE FOR CLEARING
THE VALLEY AIR AND PREVENTING AIR STAGNATION. THE SAME WINDS MAY
WELL PREVENT FOG FORMATION OVERNIGHT, AS DOWNSLOPING WINDS HAVE
PUSHED DEWPOINTS DOWN TO NEAR SINGLE DIGITS. OTHER VALLEYS IN THE
AREA, STILL UNDER INVERSION AND WITHOUT SCOURING WINDS, WILL
LIKELY GO BACK TO FOG TONIGHT, AND WILL REMAIN IN THE AIR
STAGNATION ADVISORY UNTIL THE NEXT FRONTAL SYSTEM ARRIVES SATURDAY
AFTERNOON AND EVENING.

THIS WILL BE A PROBLEMATIC FRONT SATURDAY NIGHT INTO SUNDAY
MORNING, AND NOT JUST IN TERMS OF THE FORECAST SHOULD IT VERIFY.
PRECIPITATION WILL BE LIGHT AND WILL ITSELF RESULT FROM
OVERRUNNING, BUT HOW IT REACHES THE GROUND REMAINS IN QUESTION.
WITH SUCH COLD AIR TRAPPED IN THE VALLEYS, AND WARM AIR MOVING IN
ALOFT, IT IS VERY POSSIBLE THAT FREEZING RAIN COULD OCCUR IN MANY
OF THE WEST SIDE VALLEYS. IF THIS OCCURS AS IT SEEMS IT MAY, THIS
WOULD PRIMARILY BE A TRAVEL CONCERN WITH BLACK ICE FORMING ON AREA
ROADS, INCLUDING INTERSTATE 5. SOMEONE TRAVELING ALONG THE
INTERSTATE BETWEEN MEDFORD AND ROSEBURG COULD EASILY ENCOUNTER
RAIN, SNOW, AND FREEZING RAIN ALONG THE WAY, SWITCHING BACK AND
FORTH DEPENDING ON THE ELEVATION AND OUTSIDE TEMPERATURE.

HAVE SPENT CONSIDERABLE TIME WEIGHING THE OPTIONS, AND
UNFORTUNATELY, CONFIDENCE IN THE FREEZING RAIN SCENARIO REMAINS
LOW, ESPECIALLY GIVEN THE DIFFERENCE ONLY ONE OR TWO DEGREES WOULD
MAKE, OR A VARIATION IN THE TIMING OF ONLY AN HOUR OR TWO. A FEW
THINGS TO CONSIDER: SHOULD CLOUDS MOVE IN EARLIER, TEMPERATURES
MAY NOT GET BELOW FREEZING IN SOME AREAS AND RAIN WOULD RESULT.
CONVERSELY, IF DRY AIR REMAINS IN PLACE, EVAPORATIONAL COOLING
COULD DROP FREEZING LEVELS DOWN TO THE SURFACE, PREVENTING SNOW
FROM MELTING IN THE FIRST PLACE. GIVEN THE LOW CONFIDENCE, HAVE
KEPT A SLIGHT CHANCE OF FREEZING RAIN IN PLACE FOR THE VALLEYS OF
THE WEST SIDE EARLY SUNDAY MORNING. AS TEMPERATURES WILL WARM
THROUGH THE MORNING, ANY FROZEN PRECIPITATION WILL COME TO AN END
BY NOON, AND SNOW LEVELS ARE EXPECTED TO RISE FROM AROUND 1000
FEET SATURDAY NIGHT TO ABOVE 4000 FEET BY EARLY SUNDAY AFTERNOON.

SUNDAY EVENING INTO MONDAY WILL SEE THE PASSAGE OF A WEAK COLD
FRONT, ASSOCIATED WITH ANOTHER CLOSED LOW ORBITING WITHIN THE FLOW
OF THE LARGE PACIFIC TROUGH. THIS WILL LOWER SNOW LEVELS SLIGHTLY
INTO MONDAY, WITH SHOWERY PRECIPITATION CONTINUING INTO MONDAY
NIGHT AS THE AXIS OF THE TROUGH MOVES OVERHEAD. THERE ARE SOME
DIFFERENCES BETWEEN THE MODELS CONCERNING TIMING OF THIS LOW, BUT
THEY ARE SMALL, AND CONFIDENCE IS BETTER THAN AVERAGE IN THE
OVERALL SCENARIO. -BPN

LONG TERM...TUESDAY THROUGH FRIDAY...THE MAIN LOW PRESSURE IN THE
EASTERN PACIFIC WILL CONTINUE TO SEND A SERIES OF FRONTS INLAND
THROUGH THE MIDDLE OF THE WEEK. THE JET STREAM PATTERN DURING THIS
TIME FRAME IS STARTING TO FAVOR THE SOUTHERN TRACK WITH NORTHERN
CALIFORNIA. BOTH THE GFS AND THE ECMWF ARE REASONABLY CONSISTENT
THROUGH WEDNESDAY. THERE WILL BE PERIODS OF SOUTHERLY WIND AHEAD OF
EACH LOW PRESSURE TO PROVIDE FOR UPLIFT INTO THE MOUNT SHASTA AREA.

BEYOND WEDNESDAY BOTH MODELS ARE SUGGESTING A RIDGE MOVING INTO THE
EASTERN PACIFIC WITH MAINLY NORTHERLY FLOW ALONG THE WEST COAST.
THIS WILL LEAD TO A DECREASING CHANCE OF SHOWERS, ESPECIALLY FOR
SOUTHERN OREGON, BUT TEMPERATURES WILL ALSO TURN COLDER. /FB

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...FREEZE WARNING UNTIL 8 AM PST SATURDAY FOR ORZ021-022.
     AIR STAGNATION ADVISORY UNTIL 10 AM PST SATURDAY FOR ORZ023-024.

CA...NONE.

PACIFIC COASTAL WATERS...NONE.

CC