Frost and Freeze in Oregon, February 26, 2013

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MEDFORD OR
824 PM PST MON FEB 25 2013

.DISCUSSION...HIGH PRESSURE IS MOVING IN AND THE AIR MASS IS
STABILIZING. GUSTY WINDS OVER THE EAST SIDE EARLIER THIS EVENING
ARE DYING DOWN AND WITH MOSTLY CLEAR SKIES TONIGHT, GOOD
RADIATIONAL COOLING SHOULD SEND TEMPERATURES DOWN INTO THE TEENS
AND EVEN SOME SINGLE DIGITS THERE BY SUNRISE TUESDAY. A COLD NIGHT
IS ALSO EXPECTED IN MANY AREAS WEST OF THE CASCADES WITH LOWS IN
THE 20S OVER THE WESTERN VALLEYS. ONLY SOME STRATOCUMULUS WILL
PREVENT AN EVEN COLDER NIGHT. SOME SUB-FREEZING TEMPERATURES ARE
ALSO ANTICIPATED ALONG THE COAST, SO A FROST/FREEZE PRODUCT
(NPWMFR) IS OUT FOR THAT. PREVIOUS DISCUSSION BELOW HANDLES THE
UPCOMING FEATURES WELL AND WILL NOT UPDATE THIS EVENING. SPILDE

&&

.AVIATION...
VFR CONDITIONS ARE EXPECTED TO CONTINUE THROUGH THE NIGHT OVER MUCH
OF THE REGION AS WEAK HIGH PRESSURE MOVES IN. STRATOCUMULUS IS
ALREADY CONSOLIDATING IN THE UMPQUA AND WILL PROBABLY LOWER THROUGH
THE NIGHT RESULTING IN MVFR CIGS BY MORNING. THIS WILL LIMIT THE
POSSIBILITY OF FOG IN THE REGION, HOWEVER. PATCHY FOG AND STRATUS IS
POSSIBLE BY MORNING ELSEWHERE BUT IS OF LOW ENOUGH PROBABILITY NOT
TO INCLUDE IN TAFS. VFR CONDITIONS WITH LIGHT AND VARIABLE WINDS ARE
EXPECTED TOMORROW AS WELL, BUT CLOUDS WILL THICKEN AT THE COAST LATE
IN THE DAY AS A WEAK FRONT APPROACHES. -WRIGHT

&&

.PREV DISCUSSION... /ISSUED 255 PM PST MON FEB 25 2013/

SHORT TERM...A VIGOROUS UPPER TROUGH MOVED THROUGH OVERNIGHT AND
IN ITS WAKE THE REGION HAS BEEN UNDER A BRISK AND UNSTABLE
NORTHWEST FLOW ALOFT. AREAS EAST OF THE CASCADES HAVE GUSTS TO
AROUND 35 MPH. KMAX 88D REVEALS CONTINUING VERY LIGHT SHOWERS
MAINLY IN THE CASCADES OF OREGON...AND THERE HAVE BEEN A FEW
REPORTS OF GRAUPEL.

LITTLE IN THE WAY OF HIGH-IMPACT WEATHER IS FORESEEN THIS WEEK.
HIGH PRESSURE BUILDING IN TONIGHT AND COOL AIR IN PLACE WILL
ALLOW TEMPERATURES TO DIP TO READINGS GENERALLY 5-10 BELOW
NORMAL...BUT SOME AREAS EAST OF THE CASCADES WILL DIP TO BETWEEN
10 AND 15 DEGREES BELOW NORMAL INTO THE UPPER SINGLE DIGITS TO
LOWER TEENS. FROST AND FREEZE HEADLINES CONTINUE FOR THE COAST
REGION LATE TONIGHT INTO TUESDAY MORNING.

A WEAK FRONT WILL MOVE IN TUESDAY NIGHT THROUGH EARLY WEDNESDAY
WHICH WILL BRING SLIGHT CHANCE TO CHANCE POPS ACROSS MAINLY THE
COAST AND UMPQUA. THIS SYSTEM IS TRENDING DRIER IN THE 12Z MODEL
RUNS AND MAY NEED TO BE SCALED BACK YET BY THE MID SHIFT IF TRENDS
CONTINUE. WEDS SHOULD RISE UP A NOTCH TO AROUND NORMAL. DESPITE
BUILDING HIGH PRESSURE THEREAFTER...THE REGION IS NOT QUITE SET
TO REMAIN DRY WED NIGHT INTO THU AS A WARM FRONT MOVES IN. RAIN
CHANCES WILL CONTINUE IN THIS PERIOD WITH MAINLY AREAS WEST OF THE
CASCADES UNDER THE GUN WITH HIGHER SNOW LEVELS. STAVISH

LONG TERM...FRIDAY MORNING, MARCH 1ST THROUGH MONDAY NIGHT,
MARCH 4TH AND BEYOND...

THE FORECAST FOR THIS PERIOD IS BASED ON A COMBINATION OF
UNCERTAINTY TOOLS...TO INCLUDE EUROPEAN MODEL ENSEMBLE PLOTS,
REFORECAST ANALOGS, AND RELATIVE MEASURE OF PREDICTABILITY PLOTS.
ADDITIONALLY, THE GFS40 AND ECMWF 12Z OPERATIONAL RUNS PLAYED A
LARGE ROLE IN THE FORECAST. THE PRIMARY FORECAST CHALLENGE FOR THE
PERIOD REVOLVES AROUND A RATHER COMPLEX LOW PRESSURE SYSTEM EXPECTED
TO ARRIVE NEXT WEEKEND. A STRONG JET STREAM IN THE WEST-CENTRAL
PACIFIC ON THE FRONT SIDE OF A TROUGH COMING OFF OF THE EAST ASIAN
COAST WILL MIGRATE ACROSS THE PACIFIC THIS WEEK. IT WILL THEN
AMPLIFY IN THE EASTERN PACIFIC, AND THE POLAR JET STREAM ASSOCIATED
WITH IT IS LIKELY TO COMBINE WITH THE SUBTROPICAL JET STREAM BEFORE
THE ASSOCIATED FRONTAL SYSTEM ARRIVES AT OUR COAST AND THEN MOPES
THROUGH THE AREA. WHILE OPERATIONAL MODEL GUIDANCE HAS TRENDED
GENERALLY WETTER FOR OUR AREA OVER THE LAST 48 HOURS, THERE IS A
HIGH AMOUNT OF SPREAD IN THE ENSEMBLES, AND THE ANALOGS POINT AWAY
FROM A SIGNIFICANT EVENT. THUS, THE SUMMARY IS THAT THIS STORM
SYSTEM BEARS SOME WATCHING, AS CONFIDENCE IN THE DETAILS IS
LOW. SAID ANOTHER WAY, THERE CONTINUES TO BE MODERATE TO HIGH
CONFIDENCE THAT THIS SYSTEM WILL ARRIVE BUT LOW CONFIDENCE IN THE
COVERAGE AND PRECIPITATION AMOUNTS ASSOCIATED WITH IT.

FOR FRIDAY INTO SATURDAY WE ARE STILL VERY MUCH ON TRACK FOR ABOVE
NORMAL SPRING-LIKE HIGH TEMPERATURES ACROSS THE FORECAST AREA. THE
AMOUNT OF HIGH CLOUD AND THE TIMING THEREOF WILL INFLUENCE HOW WARM
WE GET, BUT IT APPEARS FRIDAY IS LIKELY TO BE THE SUNNIEST AND
WARMEST DAY OF THE WEEK. SATURDAY SHOULD BE QUITE MILD AS WELL,
ESPECIALLY IF THE INCOMING FRONTAL SYSTEM SLOWS.

BEYOND ABOUT A WEEK OUT THERE ARE STILL MANY COMPETING FORCES IN THE
FORECAST. THE GFS HAS BEEN CONSISTENTLY LEANING ON WEEK TWO BEING
WETTER THAN WEEK ONE FOR THE WEST SIDE. THE ECMWF SHOWS A CLOSED LOW
MEANDERING OUR WAY EARLY TO MI NEXT WEEK, AS WELL. EVENS, AT THIS
POINT IT APPEARS THAT PRECIPITATION IS STILL LIKELY TO FALL IN AT
NEAR TO BELOW NORMAL FOR THE PERIOD ACROSS THE FORECAST AREA. BTL

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...FROST ADVISORY FROM 1 AM TO 8 AM PST TUESDAY FOR ORZ021-022.
     FREEZE WARNING FROM 1 AM TO 8 AM PST TUESDAY FOR ORZ021-022.

CA...NONE.

PACIFIC COASTAL WATERS...SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS
     UNTIL 10 PM PST WEDNESDAY FOR PZZ350-356-370-376.

MAS/TRW