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Public Information Statement
Issued by NWS Boston, MA

000
NOUS41 KBOX 031534
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CTZ002>004-MAZ002>024-026-NHZ011-012-015-RIZ001>008-031945-

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE TAUNTON MA
1134 AM EDT FRI MAY 3 2013

...SEVERE WEATHER PREPAREDNESS WEEK CONTINUES - LOW-TOPPED
THUNDERSTORMS...

THE NATIONAL WEATHER SERVICE IN TAUNTON HAS DECLARED THE WEEK OF
APRIL 29 TO MAY 3 AS SEVERE WEATHER PREPAREDNESS WEEK IN SOUTHERN
NEW ENGLAND. THE FOLLOWING IS THE FIFTH IN A SERIES OF FIVE
INFORMATIONAL STATEMENTS.

WHICH WOULD PRODUCE MORE SEVERE WEATHER...A THUNDERSTORM WITH A
TOP OF 30,000 FEET OR ONE WITH A TOP OF 50,000 FEET? THE ANSWER
IS THAT IT DEPENDS ON HOW HIGH THE TROPOPAUSE LEVEL IS FOR THAT
DAY.

MOST WEATHER OCCURS IN THE TROPOSPHERE...THE LOWEST LAYER OF THE
ATMOSPHERE. WHEN A THUNDERSTORM DEVELOPS...IT GROWS TALLER AND
EVENTUALLY REACHES THE TROPOPAUSE...THE INTERFACE BETWEEN THE
TROPOSPHERE AND THE VERY STABLE STRATOSPHERE. BECAUSE THE
STRATOSPHERE IS STABLE...IT CAUSES THE TOP OF THE THUNDERSTORM
TO FLATTEN OUT INTO WHAT IS TERMED AN ANVIL...WHICH IS ROUGHLY
AT THE TROPOPAUSE LEVEL.

IN THE HEART OF SUMMER...THE WARMER CONDITIONS LEAD TO A HIGHER
TROPOPAUSE LEVEL. IN THE SPRING...THE TROPOPAUSE CAN BE MUCH
LOWER. IF THE TROPOPAUSE LEVEL IS 30,000 FEET ON A PARTICULAR
DAY...IT MAY ONLY TAKE A 33,000 FOOT STORM TO BECOME SEVERE. ON
A DAY WHEN THE TROPOPAUSE IS 50,000 FEET...A 52,000 FOOT STORM
COULD BECOME SEVERE.

THUS...WHAT DETERMINES THE SEVERITY OF A STORM IS HOW HIGH ITS
CLOUD TOP IS...RELATIVE TO THE TROPOPAUSE LEVEL FOR THAT DAY. A
30,000 FOOT LOW-TOPPED THUNDERSTORM IN NEW ENGLAND CAN PACK JUST
AS MUCH PUNCH AS A 50,000 FOOT STORM...IF THE TROPOPAUSE LEVEL
IS LOW.

WEATHER BALLOONS ARE LAUNCHED AT 93 SITES ACROSS NORTH AMERICA...
TWICE PER DAY. THESE BALLOONS REPORT BACK TO THE WEATHER OFFICE
WHAT THE TROPOPAUSE LEVEL IS. A SIMPLE VISUAL CLUE AS TO WHETHER
A STORM MAY BE SEVERE IS TO LOOK FOR A CLOUD TOP THAT MUSHROOMS
UP ABOVE THE REST OF THE ANVIL LEVEL...ACTUALLY OVERSHOOTING
INTO THE STRATOSPHERE FOR SEVERAL MINUTES.  THIS OVERSHOOTING
TOP IS INDICATIVE OF A VERY STRONG UPDRAFT AND IT IS YOUR VISUAL
CLUE THAT A STORM MAY BE SEVERE...PRODUCING LARGE HAIL AND
DAMAGING WINDS.

$$
FIELD

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