EF1 tornado, Gig Harbor, Washington, January 18, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
915 AM PST SUN JAN 18 2015

.SYNOPSIS...MOIST WESTERLY FLOW WILL PRODUCE SCATTERED SHOWERS OVER
WESTERN WASHINGTON THIS MORNING. SHOWER ACTIVITY WILL DIMINISH LATER
TODAY AS AN UPPER LEVEL RIDGE STARTS TO BUILD OFFSHORE. THE RIDGE
WILL SHIFT OVER THE REGION TUESDAY AND WEDNESDAY FOR A PERIOD OF DRY
WEATHER. THE RIDGE WILL WEAKEN ENOUGH LATE IN THE WEEK TO ALLOW A
COUPLE OF WEAK WEATHER SYSTEMS TO BRING SOME RAIN INLAND.

&&

.SHORT TERM...RADAR SHOWS SCATTERED SHOWERS...MOSTLY MOUNTAINS AND
COAST. SATELLITE IMAGERY SHOWS CLOUDS IN THOSE AREAS BUT SOME
CLEARING AS WELL. TREND TODAY WILL BE TOWARD FEWER SHOWERS AND SOME
AFTERNOON SUN. HIGHS WILL BE 45 TO 50.

HEIGHTS WILL BUILD OVER THE AREA OVER THE NEXT FEW DAYS AS AN UPPER
RIDGE SHARPENS OFFSHORE TUESDAY THEN SHIFTS INLAND WEDNESDAY. THIS
WILL BRING MORE CLEARING WITH MOSTLY SUNNY SKIES TUESDAY AND
WEDNESDAY AFTERNOONS. LIGHT OFFSHORE SURFACE FLOW ALONG WITH LONG
NIGHTS WILL BRING PATCHY FOG TUESDAY MORNING...INCREASING TO AREAS
OF FOG WEDNESDAY MORNING. BUT MOST AREAS SHOULD CLEAR IN THE
AFTERNOONS. HIGHS BOTH DAYS WILL REMAIN MAINLY IN THE 45 TO 50
RANGE. ANYWHERE THAT FOG PERSISTS WILL PROBABLY STAY IN THE LOW TO
MID 40S...AND THE COAST WILL BE THE WARMEST PLACE WITH HIGHS IN THE
LOW TO MID 50S. BURKE

.LONG TERM...PREVIOUS DISCUSSION...THE UPPER RIDGE WEAKENS ON
THURSDAY AS A PACIFIC FRONTAL SYSTEM BRUSHES WRN WA. LATEST 00Z
MODEL RUNS KEEP THE BULK OF MOISTURE TO THE N OVER B.C. BUT SOME
PAST RUNS HAVE BROUGHT THE TAIL END OF OF THIS SYSTEM ACROSS WRN WA.
WILL LEAVE THE CHANCE OF RAIN IN THE FORECAST WITH SOME UNCERTAINTY
AS TO HOW FAR S SOME LIGHT PRECIPITATION MIGHT EXTEND. A TRAILING
SYSTEM ARRIVES FRIDAY AND APPEARS A BIT STRONGER WITH A BETTER SHOT
OF BRINGING SOME LIGHT RAIN TO THE AREA. AGAIN...THIS SYSTEM SHOULD
WEAKEN AND NOT BE A BIG RAIN PRODUCER BUT IT DOES HAVE A BETTER
CHANCE OF BRINGING MEASURABLE RAIN INTO WRN WA.

MOST LONG RANGE GLOBAL MODELS SHOW THE RIDGE STRENGTHENING OVER THE
WEEKEND. THERE WILL BE SHORT WAVE SYSTEMS RIDING OVER THE TOP OF THE
RIDGE WITH SOME WARM ADVECTION RAIN PUSHING INTO B.C. AND POSSIBLY
BRUSHING WA. RIGHT NOW...SATURDAY LOOKS TO HAVE THE BEST CHANCE OF
SOME RAIN BUT DETERMINING EXACTLY WHERE THE PRECIPITATION CUT OFF
LINE WILL BE IS NOT POSSIBLE IN THIS PATTERN THIS FAR OUT. THE
CURRENT FORECAST HAS A CHANCE OF RAIN INDICATED SATURDAY/SATURDAY
NIGHT...THEN DRIER SUNDAY. THIS LOOKS TO BE A GOOD A GUESS AS ANY SO
NO CHANGES PLANNED. HIGH TEMPERATURES DO APPEAR TO WARM UP A BIT
WITH HIGHS EXPECTED TO REACH THE 50S. MERCER

&&

.HYDROLOGY...THE FLOOD WARNING ON THE SKOKOMISH RIVER HAS ENDED.
FLOODING IS LIKELY OVER THE NEXT SEVEN DAYS. BURKE

&&

.AVIATION...MODERATE WESTERLY FLOW ALOFT TODAY WILL BECOME LIGHT NW
TONIGHT. A MOIST AND UNSTABLE AIR MASS TODAY WILL GRADUALLY BECOME
DRIER AND MORE STABLE TONIGHT AND TUE...WITH SOME LOW-LEVEL MOISTURE
REMAINING TRAPPED NEAR THE SURFACE OVER THE LOWLANDS ON TUE.

WEAKENING LOW-LEVEL ONSHORE FLOW TODAY WILL HELP SCATTERED SHOWERS
PERSIST AND REMAIN MOST HEAVILY FOCUSED OVER THE MTNS. STRATUS OVER
CENTRAL AND SOUTH PUGET SOUND WILL LIFT A BIT AND SCATTER OUT SOME
THIS AFTN. AS THE AIR MASS STABILIZES TONIGHT...SHOWERS WILL
DWINDLE...WITH NO SHOWER ACTIVITY EXPECTED TUE. ON TUE MORNING...
WITH A SHALLOWER MOIST LAYER NEAR THE SFC...WOULD EXPECT SOME
LOWLAND FOG DEVELOPMENT. HOWEVER...SCT-BKN CLOUDS AROUND 150-200 MAY
INHIBIT FOG DEVELOPMENT AND KEEP FOG CONFINED TO MORE FOG-PRONE
LOCATIONS SUCH AS KOLM.

KSEA...EXPECT BKN CLOUDS BELOW 020 THIS MORNING TO LIFT AND SCATTER
A BIT THIS AFTN...WITH THE LOWEST CLOUD DECK LIFTING INTO THE
025-035 RANGE BY SUNSET. A PASSING SHOWER OR TWO ARE POSSIBLE AT
ALMOST ANY TIME THROUGH ABOUT 06Z THIS EVNG...BUT ANY SHOWERS WILL
BE BRIEF. OVERNIGHT...SURFACE-BASED MOISTURE BECOMES MORE
SHALLOW...WHICH WOULD NORMALLY RAISE CONCERNS FOR FOG DEVELOPMENT
AROUND SUNRISE. HOWEVER...EXPECTING BKN150 AT THE TERMINAL ARND
SUNRISE TUE...WHICH SHOULD INHIBIT FOG DEVELOPMENT. MAIN QUESTION
WILL BE HOW MUCH ANY FOG DEVELOPMENT IS OFFSET. IN THE TAFS...WILL
NOT REDUCE VSBY ON TUE MRNG BELOW 6SM...BUT IT BEARS WATCHING. WILL
HEDGE THE BET WITH BCFG.    HANER

&&

.MARINE...SURFACE HIGH PRESSURE CENTERED OFF THE OREGON COAST THIS
MORNING WILL TRANSITION TO BEING CENTERED OVER SOUTHERN B.C. EAST OF
THE CASCADES AND COAST RANGE TONIGHT. THEREFORE OVER WRN
WA...SOUTHERLY ONSHORE FLOW WILL CONTINUE TO EASE TODAY...WITH
GRADIENTS BECOMING WEAKLY OFFSHORE TONIGHT.

SEAS OVER THE COASTAL WATERS ARE 11 TO 12 FT THIS MORNING AND WILL
SLOWLY SUBSIDE TO LESS THAN 10 FT THIS EVENING. SMALL CRAFT
ADVISORIES FOR HAZARDOUS SEAS WILL REMAIN IN EFFECT OVER THE COASTAL
WATERS AND AT THE WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA UNTIL
SEAS SUBSIDE.

WEAK OR DISSIPATING FRONTS AROUND THU AND FRI SHOULD DISRUPT THE
OFFSHORE FLOW.      HANER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...
PZ...SMALL CRAFT ADVISORY COASTAL WATERS AND WEST ENTRANCE TO THE
      STRAIT OF JUAN DE FUCA FOR HAZARDOUS SEAS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML