Heat in Washington, April 19, 2016

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
347 PM PDT TUE APR 19 2016

.SYNOPSIS...HEAT-PRODUCING UPPER LEVEL HIGH PRESSURE WILL EXIT TO
THE EAST ON WEDNESDAY...WITH COOLING ONSHORE FLOW SPREADING INLAND
ON WEDNESDAY AFTERNOON. A WAVE OF LOW PRESSURE WILL MOVE UP FROM
THE SOUTHWEST ON WEDNESDAY EVENING...BRINGING A CHANCE OF SHOWERS
AND THUNDERSTORMS. ADDITIONAL WEATHER SYSTEMS SHOULD AFFECT THE
REGION WITH A GOOD CHANCE OF SHOWERS AT TIMES INTO THE FIRST PART
OF NEXT WEEK.

&&

.SHORT TERM...ANOTHER AFTERNOON OF RECORD-SHATTERING HIGH
TEMPS...EXCEPT ALONG THE COASTAL BEACHES WHERE COOLING SEABREEZES
KICKED IN THIS AFTN. EXPECT THE MARINE STRATUS ALONG THE ORE COAST
THIS AFTN TO SPREAD NORTH TONIGHT...WITH SOME FOG AND MARINE
CLOUDS INTO THE LOWER CHEHALIS VALLEY AND GRAYS HARBOR AT SUNRISE
ON WED. ELSEWHERE...WED WILL STILL START OFF WITH THE WARM AIR
MASS STILL IN PLACE. FALLING UPPER HEIGHTS ON WED AFTN WILL
REALLY ALLOW MARINE AIR TO PUSH INLAND...CUTTING OFF RISING TEMPS
A FEW HOURS EARLIER THAN TODAY OVER THE INTERIOR. HOWEVER...THIS
MAY NOT OCCUR BEFORE YET ANOTHER ROUND OF RECORD HIGH TEMPS IN
THE SEATTLE AREA BEFORE THE MARINE PUSH STARTS COOLING THINGS
LATER IN THE AFTN.

LOOKING LIKE A PARTICULARLY FAVORABLE SETUP FOR THUNDERSTORMS IN
THE PUGET SOUND LOWLANDS ON WED EVNG. A NEGATIVELY TILTED
SHORTWAVE --- CURRENTLY OFF THE NRN CAL COAST --- WILL LIFT N-NE
THROUGH WRN WA WED NGT. IN ADVANCE OF THE WAVE...THE AIR MASS WILL
BECOME QUITE UNSTABLE...WITH AFTERNOON THUNDERSTORMS FIRING OFF
OVER THE CENTRAL CASCADES AND POSSIBLY THE OLYMPICS. SE FLOW ALOFT
WILL THEN WANT TO TAKE STORMS OUT ACROSS THE WRN WA LOWLANDS...
PERHAPS ALL THE WAY TO PUGET SOUND. SOME SPEED AND DIRECTIONAL
SHEAR IN THE MID-LEVEL WIND PROFILE WILL FAVOR LOOSE ORGANIZATION
OF THE STORMS...WITH MULTI-CELL CLUSTERS AND SHORT LINES OF
THUNDERSTORMS POSSIBLE.

THE SHORTWAVE WILL EXIT INTO B.C. ON THU...WITH MAINLY A FEW
LINGERING SHOWERS OVER THE MTNS. THE MARINE LAYER WILL DEEPEN
FURTHER...WITH NO MORE RECORD HIGHS EXPECTED THU.

A NEGATIVELY TILTED UPPER LOW CENTER WILL MOVE EASTWARD ACROSS NRN
CAL OR FAR SRN ORE ON FRI AND FRI NGT. THIS WILL CAUSE THE FLOW
ALOFT OVER WRN WA TO BECOME SE AGAIN ON FRI AFTN...WITH ANOTHER
SHOT AT AFTN THUNDERSTORMS OVER THE SRN WA CASCADES MOVING NW
INTO THE SW INTERIOR OR PUGET SOUND LOWLANDS.		HANER

.LONG TERM...WILL BE BETWEEN UPPER LEVEL TROUGHS ON SAT...BUT
SHOULD STILL BE ENOUGH MOISTURE AND INSTABILITY FOR CONVECTIVE
SHOWERS TO DEVELOP IN THE AFTN OVER THE MTNS.

A SHARP UPPER TROUGH AXIS WILL MOVE INTO THE PAC NW ON
SUNDAY...POSSIBLY PINCHING OFF AS A SLOW-MOVING CLOSED UPPER LOW.
THIS WILL PUSH AN EVEN COOLER AIR MASS INTO WRN WA...WITH SNOW
LEVELS FALLING TO BETWEEN 3000 AND 4000 FEET AT TIMES FROM SUNDAY
THROUGH TUE AM. SHOWERS WILL BE COMMON ON SUNDAY AND
MONDAY...ESPECIALLY WHERE LOW-LEVEL ONSHORE FLOW PROVIDES
OROGRAPHIC LIFT OVER THE MTNS OR A PUGET SOUND CONVERGENCE ZONE
UPON WHICH TO FOCUS. LOOKS LIKE A RELATIVE LULL NEXT TUE AS THE
UPPER LOW EXITS...BUT ENOUGH LINGERING MOISTURE AND INSTABILITY
WILL PROVIDE A SMALLER CHANCE OF SHOWERS.		HANER

&&

.AVIATION...HIGH PRESSURE ALOFT WILL PERSIST OVER THE INTERIOR PAC
NW THROUGH WEDNESDAY AFTERNOON. A WEAK TROUGH WILL LIFT NORTH ACROSS
WESTERN WA WEDNESDAY NIGHT. LIGHT/VARIABLE FLOW ALOFT...BECOMING
LIGHT E/SE FLOW ALOFT LATER TONIGHT/WEDNESDAY. AIR MASS INITIALLY
DRY AND STABLE...EXCEPT MOIST IN THE LOWER LEVELS NEAR THE COAST...
WEST PORTION OF THE LOWER CHEHALIS AND WESTERN STRAIT OF JUAN DE
FUCA WITH AREAS OF MORNING FOG AND IFR STRATUS. VFR WITH ONLY THIN
HIGH CLOUDS OVER THE INTERIOR UNTIL LATER WEDNESDAY AFTERNOON.

INCREASING MOISTURE AT ALL LEVELS AND BECOMING UNSTABLE ACROSS
WESTERN WA LATE WEDNESDAY AFTERNOON AND EVENING AS A MARINE PUSH
DEVELOPS COINCIDING WITH THE ARRIVING UPPER TROUGH. THERE WILL BE A
FEW SHOWERS AND A SLIGHT RISK OF TSTMS/LIGHTNING...ONLY AROUND 15
PERCENT...AT ANY GIVEN TERMINAL WEDNESDAY EVENING. THE SOUTHWESTERLY
MARINE PUSH SHOULD BRING STRATUS W/ CIGS BELOW 3K FT INTO MUCH OF
THE INTERIOR LATER WEDNESDAY NIGHT AND THURSDAY MORNING.

KSEA...NORTHWEST 5-9 KT BECOMING NORTH EARLY THIS EVENING...THEN N-
NE 3-7 KT BY 06Z TONIGHT. ONLY FEW-SCT HIGH CLOUDS ABOVE 20K FT AT
TIMES. MULTIPLE CLOUD LAYERS DEVELOPING WEDNESDAY EVENING WITH A FEW
SHOWERS AND A 15 PERCENT CHANCE OF LIGHTNING WITHIN THE VICINITY OF
THE KSEA TERMINAL. A SOUTHWESTERLY MARINE PUSH COULD BRING LOWER
STRATUS CIGS BELOW 3K FT TO THE TERMINAL LATE WEDNESDAY
NIGHT/THURSDAY MORNING. GIVEN THE UNSTABLE AIR MASS...FORECAST
STRATUS CIGS IN THE TAF ARE HIGHLY UNCERTAIN BEYOND WEDNESDAY
AFTERNOON. DTM

&&

.MARINE...THERMALLY INDUCED LOW PRESSURE NEAR THE COAST WILL SHIFT
INLAND OVER THE INTERIOR WATERS BY WEDNESDAY AFTERNOON. ONSHORE FLOW
STRENGTHENS ALONG THE COAST...THEN A SOUTHWESTERLY MARINE PUSH
DEVELOPS OVER THE INLAND WATERS BY LATE WEDNESDAY AFTERNOON AND
EVENING. WESTERLY FLOW WILL ALSO DEVELOP IN THE STRAIT WITH WINDS OF
20 TO 30 KNOTS. ONSHORE GRADIENTS COULD BE STRONG ENOUGH FOR GALE
FORCE WINDS TO 35 KNOTS IN THIS PATTERN. A GALE WATCH REMAINS IN
EFFECT FOR THE CENTRAL/EASTERN STRAIT WEDNESDAY NIGHT.

VARYING DEGREES OF ONSHORE FLOW WILL CONTINUE THURSDAY THROUGH THE
WEEKEND. SMALL CRAFT ADVISORY WESTERLIES IN THE CENTRAL AND EASTERN
STRAIT ARE POSSIBLE EACH DAY DURING THE EVENING HOURS. DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WATCH FROM WEDNESDAY AFTERNOON THROUGH LATE WEDNESDAY NIGHT
     FOR CENTRAL U.S. WATERS STRAIT OF JUAN DE FUCA-EAST
     ENTRANCE U.S. WATERS STRAIT OF JUAN DE FUCA.

&&

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