FXUS66 KEKA 111222
AFDEKA
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE EUREKA CA
422 AM PST FRI DEC 11 2009
.SYNOPSIS...LOW PRESSURE AND AN ASSOCIATED FRONTAL BOUNDARY WILL
BRING RAIN TO MUCH OF THE REGION TODAY. SNOW IS POSSIBLE IN THE
INTERIOR VALLEYS THROUGH EARLY AFTERNOON...AND ALSO OVER THE
HIGHEST ELEVATIONS. WARMER AIR WILL OVERTAKE THE REGION BY EARLY
SATURDAY IN ADVANCE OF THE NEXT LOW PRESSURE SYSTEM TO AFFECT THE
STATE.
&&
.DISCUSSION...EARLY MORNING SURFACE ANALYSIS SHOWS LOW PRESSURE
AND AN ASSOCIATED COLD FRONT WELL OFFSHORE NORTHERN CA WHILE A
WARM FRONT IS APPROACHING THE IMMEDIATE COAST. THIS FEATURE IS
UNDERCUTTING A LARGE GULF OF ALASKA RIDGE. MEANWHILE...COLD
SURFACE HIGH PRESSURE REMAINS IN PLACE OVER THE GREAT BASIN.
THE MAIN FORECAST CHALLENGE TODAY REVOLVE AROUND
TEMPERATURES...PRECIPITATION TYPE AND INTENSITY. RECENT RADAR
TRENDS AND OBSERVATIONS SHOW THAT LIGHT...MAINLY WARM AIR
ADVECTION-DRIVEN...PRECIPITATION HAS EXPANDED OVER MUCH OF THE
AREA WITHIN THE PAST FEW HOURS. WE EXPECT THIS TREND TO CONTINUE
THROUGH DAYBREAK ALTHOUGH IT STILL APPEARS THAT MUCH OF THE
HEAVIEST PRECIPITATION WILL REMAIN OFF TO OUR WEST AND SOUTH NEAR
THE BEST DEEP LAYER MOISTURE...VORT MAXIMA AND LIFT. MORE
CONCERNING ARE TEMPERATURES...WHICH HAVE BEEN SLOW TO WARM IN THE
INTERIOR VALLEYS BUT FORTUNATELY ARE FINALLY BEGINNING TO NUDGE
UPWARDS. AT 4AM...TEMPERATURES REMAINED AT 32F OR BELOW ACROSS
THE VALLEYS OF TRINITY AND MENDOCINO COUNTIES...AND ALSO IN
HIGHER TERRAIN ABOVE ROUGHLY 3000 FEET. THE LATEST RUC AND NAM
GUIDANCE ARE INSISTENT THAT H85-H925 TEMPS WILL REMAIN AT OR
BELOW 0C IN SOME VALLEY LOCATIONS THROUGH AT LEAST
MIDDAY...PERHAPS UNTIL LATE AFTERNOON. THIS SEEMS REASONABLE
GIVEN THE EASTERLY FLOW AT THE SURFACE AND STRENGTH OF THE
INVERSION. WITH PRECIPITATION OVERSPREADING THE AREA...WE EXPECT
SNOW TO BE THE DOMINANT PTYPE IN THESE LOCALES THROUGH THE
MORNING HOURS. THE LOW LEVEL COLD AIR SHOULD BECOME SUFFICIENTLY
SCOURED OUT BY LATE AFTERNOON AND EARLY EVENING...AT WHICH TIME
SNOW WILL BE CONFINED SOLELY TO THE HIGHER ELEVATIONS OF
NORTHERN/EASTERN TRINITY COUNTY. DUE TO THE CONVECTIVE NATURE OF
THIS SYSTEM SOME HEAVIER BURSTS OF SNOW WILL BE POSSIBLE BEFORE
THE CHANGEOVER TO RAIN. WHILE WE GENERALLY EXPECT ANY SNOW
ACCUMULATIONS THROUGH THIS EVENING TO BE 2 INCHES OR LESS...A FEW
LOCAL 3 TO 4 INCH AMOUNTS CAN BE EXPECTED. HPC GUIDANCE AGREES
WITH THIS THINKING AND SUGGESTS THAT 4 INCH AMOUNTS WILL
OCCUR...BUT MAINLY ABOVE 5000-6000 FT IN NE TRINITY COUNTY.
MEANWHILE ALONG THE COAST AND OVER THE OPEN WATERS...AN ISOLATED
THUNDERSTORM MAY OCCUR TODAY MAINLY FROM CAPE MENDOCINO AND
POINTS SOUTH WHERE THE GREATEST INSTABILITY WILL BE PRESENT. A
FEW LIGHTNING STRIKES WERE OBSERVED WELL OFFSHORE YESTERDAY
EVENING BUT NONE HAS SHOWN ANY INDICATIONS OF REACHING THE COAST.
A MORE ROBUST SHORTWAVE LOOKS TO AFFECT THE REGION LATE SATURDAY
INTO SATURDAY NIGHT...ALTHOUGH AGAIN IT LOOKS AS IF THE BEST
DYNAMICS WILL REMAIN TO OUR SOUTH...NEAR THE BAY AREA. UPPER
LEVEL FORCING WILL BE MORE SIGNIFICANT WITH THIS SYSTEM AS LOW
PRESSURE ATTEMPTS TO BECOME VERTICALLY STACKED OFF THE COAST. THE
HIGHEST QPF...AROUND 1.5 INCHES...WILL OCCUR OVER OVER MENDOCINO
COUNTY. THE POTENTIAL ALSO EXISTS FOR A SHORT PERIOD OF RATHER
STRONG WINDS AND THIS WILL NEED TO BE CLOSELY MONITORED
ESPECIALLY OVER THE MENDOCINO COAST AND ADJACENT AREAS INLAND. A
FEW THUNDERSTORMS WILL ALSO BE POSSIBLE WITH FORECAST SOUNDINGS
HINTING AT MARGINAL CAPE JUST ALONG AND BEHIND THE FRONT. SNOW
LEVELS WILL RISE THROUGH THE DAY ALTHOUGH THE HIGHEST
ELEVATIONS...MAINLY IN TRINITY COUNTY...WILL STILL EXPERIENCE A
BIT OF SNOW OR MIXED PRECIPITATION.
AS THE SOUTHERN BRANCH OF THE JET REMAINS ACTIVE...A SERIES OF
WEAK DISTURBANCES WILL BE PUSHED THROUGH THE REGION SUNDAY INTO
TUESDAY. IT IS DIFFICULT TO PINPOINT THE EXACT TIMING OF THESE
INDIVIDUAL SHORTWAVES AND THE LATEST IR SAT IMAGERY SEEMS TO
DEPICT AT LEAST TWO SEPARATE SHORTWAVES THAT WERE NOT INITIALIZED
IN RECENT MODEL RUNS. CONFIDENCE IN ONE SOLUTION IS NOT HIGH AND
DIFFERENCES REMAIN...BUT WE DID TREND SLIGHTLY TOWARD THE NAM AND
ECMWF. DESPITE RATHER WEAK SYNOPTIC FORCING...OCCASIONAL BOUTS OF
SHOWERS ARE EXPECTED TO CONTINUE LATE SATURDAY NIGHT INTO EARLY
SUNDAY...ESPECIALLY WITH REMNANT INSTABILITY ALOFT.
GIVEN MODEL INCONSISTENCIES AND RECENT PERFORMANCE...WE GENERALLY
MADE FEW CHANGES TO THE EXTENDED FORECAST. A PERIOD OF WEAK
RIDGING IS STILL ANTICIPATED LATE SUNDAY INTO MONDAY...BEFORE
POTENTIALLY MORE ACTIVE WEATHER RETURNS FOR MID WEEK. BETTWY
&&
.AVIATION...LOW PRESSURE APPROACHING THE COAST WILL PRODUCE PERIODS
OF LIGHT RAIN AND PERHAPS LIGHT SNOW ABOVE 1000 FEET TODAY.
SOUTHEAST WINDS RAMP UP SOME TODAY...BUT MORE AGGRESSIVELY ON
SATURDAY AS ANOTHER LOW DEVELOPS. DJB
&&
.EKA WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
NORTHWEST CALIFORNIA COASTAL WATERS...
GALE WATCH FROM SATURDAY MORNING THROUGH SATURDAY AFTERNOON FOR
PZZ470.
SMALL CRAFT ADVISORY UNTIL 7 PM PST SATURDAY FOR PZZ470.
SMALL CRAFT ADVISORY FOR WINDS UNTIL 7 PM PST SATURDAY FOR
PZZ475.
&&
$$
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