Public Information Statement
Issued by NWS Seattle/Tacoma, WA

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NOUS46 KSEW 312018
PNSSEW

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE SEATTLE WA
120 PM PST TUE MAR 31 2015

...WHAT IS UNSTABLE AIR?...

SHOWERS AND THE THREAT OF THUNDERSTORMS ARE BACK IN THE FORECAST
THIS WEEK AND WILL LIKELY BE IN THE FORECAST LATER THIS SPRING.
WEATHER FORECASTERS USE THE TERM UNSTABLE AIR TO HELP DESCRIBE -THE
WHY- BEHIND THE SHOWERY UNSETTLED WEATHER. BUT WHAT IS UNSTABLE AIR?
WHAT DOES IT MEAN?

THE PRIMARY WEATHER INGREDIENTS FOR UNSTABLE AIR IN THE LOWER
ATMOSPHERE ARE HEAT AND MOISTURE. AN UNSTABLE AIR MASS USUALLY IS
WARMER AND HAS MORE MOISTURE CLOSE TO THE SURFACE THAN THE AIR ABOVE
IT. LIKE LETTING A HELIUM BALLOON GO...IF A PARCEL OF AIR NEAR
SURFACE RISES AND KEEPS GOING...THAT IS UNSTABLE AIR AND THE AIR
MASS IS CONSIDERED CONVECTIVE.

DURING OUR SPRING SEASON...THE LONGER DAYS AND HIGHER SUN ANGLE
RESULT IN WARMER TEMPERATURES IN THE LOWLANDS OF WESTERN WASHINGTON.
YET WE STILL GET COOLER AIR ALOFT ROLLING INLAND FROM THE GULF OF
ALASKA. THIS WEATHER PATTERN OFTEN RESULTS IN UNSTABLE AIR. IF
MOISTURE IS PRESENT AND IT OFTEN IS...CLOUDS...SHOWERS AND EVEN
THUNDERSTORMS DEVELOP.

ON THE CALENDAR...OUR WESTERN WASHINGTON CONVECTIVE SEASON USUALLY
BEGINS AROUND MARCH 1ST AND TENDS TO PEAK IN APRIL. IF YOU ENCOUNTER
RAIN AND THEN SUNSHINE...AND IT REPEATS SEEMINGLY AGAIN AND AGAIN...
THOSE ARE USUALLY SHOWERS ASSOCIATED WITH AN UNSTABLE AIR MASS. THAT
FACT IS WHY THEY ARE CALLED SPRING SHOWERS.

BUEHNER




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