Public Information Statement
Issued by NWS Boston, MA

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000 NOUS41 KBOX 011250 PNSBOX CTZ002>004-MAZ002>024-026-RIZ001>008-021300- PUBLIC INFORMATION STATEMENT NATIONAL WEATHER SERVICE TAUNTON MA 850 AM EDT FRI MAY 1 2015 ...SEVERE WEATHER PREPAREDNESS WEEK CONTINUES -- DOWNBURST WINDS FROM SEVERE THUNDERSTORMS CAN BE POWERFUL... THE NATIONAL WEATHER SERVICE HAS DECLARED THE WEEK OF APRIL 27 TO MAY 1 AS SEVERE WEATHER PREPAREDNESS WEEK FOR SOUTHERN NEW ENGLAND. THE FOLLOWING IS THE FIFTH IN A SERIES OF FIVE INFORMATIONAL STATEMENTS ABOUT SEVERE WEATHER. WHILE NOT AS NOTORIOUS...OR PERHAPS AS SPECTACULAR TO WITNESS AS A TORNADO...STRAIGHT-LINE WINDS ARE RESPONSIBLE FOR MOST THUNDERSTORM WIND DAMAGE...ESPECIALLY ACROSS SOUTHERN NEW ENGLAND. A DOWNBURST IS A STRONG...RELATIVELY SMALL...AREA OF RAPIDLY DESCENDING AIR BENEATH A THUNDERSTORM. IT CAN RESULT FROM STRONGER WINDS ALOFT BEING TRANSPORTED DOWNWARD TO THE SURFACE... OR IT CAN RESULT AS AIR WITHIN THE DOWNBURST IS COOLED SIGNIFICANTLY AS RAIN EVAPORATES INTO INITIALLY DRIER AIR. THIS COOL...THUS DENSE...AIR SINKS RAPIDLY TO THE SURFACE. A DOWNBURST IS DIFFERENTIATED FROM COMMON THUNDERSTORM WINDS BECAUSE THE DOWNBURST WINDS HAVE THE POTENTIAL TO CAUSE DAMAGE NEAR THE GROUND. SURFACE DAMAGE PATTERNS HAVE SHOWN THAT WHETHER THE WINDS ARE STRAIGHT OR EVEN A LITTLE BIT CURVED...THEY TEND TO SPREAD OUT...OR DIVERGE...CONSIDERABLY AS THEY REACH THE SURFACE. CONVERSELY...DAMAGE PATTERNS RESULTING FROM A TORNADO GENERALLY CONVERGE TOWARD A NARROW CENTRAL TRACK. INTENSE DOWNBURSTS CAN BE PHENOMENAL. SPEEDS HAVE BEEN CLOCKED AS HIGH AS 175 MPH NEAR MOREHEAD CITY NORTH CAROLINA AND AT 158 MPH AT ANDREWS AIR FORCE BASE IN MARYLAND. CLOSER TO HOME...104 MPH DOWNBURST WINDS WERE MEASURED AT BOTH WORCESTER MASSACHUSETTS ON MAY 31 1998 AND WHITMAN MASSACHUSETTS ON MAY 21 1996. STRONG DOWNBURSTS WILL DEFINITELY CAUSE ROARING SOUNDS AND PEOPLE MAY OFTEN REFER TO A SOUND LIKE A FREIGHT TRAIN...TERMS TYPICALLY ASSOCIATED WITH TORNADOES. ALTHOUGH DOWNBURSTS ARE NOT TORNADOES...THEY CAN CAUSE DAMAGE EQUIVALENT TO THAT OF A SMALL TO MEDIUM TORNADO. AFTER ALL...WIND IS WIND. DOWNBURSTS ARE CLASSIFIED AS EITHER MACROBURSTS OR MICROBURSTS... DEPENDING ON THE AREAL EXTENT OF THE DAMAGING WIND SWATH. A MACROBURST/S DAMAGE EXTENDS HORIZONTALLY FOR MORE THAN 2.5 MILES. A MICROBURST IS A SMALL DOWNBURST WITH ITS DAMAGING WINDS EXTENDING 2.5 MILES OR LESS. THE SMALL HORIZONTAL SCALE AND SHORT TIME SPAN OF A MICROBURST MAKES IT PARTICULARLY HAZARDOUS TO AVIATION. THE NATIONAL WEATHER SERVICE ISSUES SEVERE THUNDERSTORM WARNINGS FOR THUNDERSTORMS THAT ARE EXPECTED TO PRODUCE DAMAGING WIND GUSTS OF 58 MPH OR GREATER...OR HAIL THAT IS ONE INCH OR GREATER IN DIAMETER. $$ FOR THE LATEST UPDATES...PLEASE VISIT OUR WEBPAGE AT WWW.WEATHER.GOV/BOSTON YOU CAN FOLLOW US ON FACEBOOK AT WWW.FACEBOOK.COM/NWSBOSTON YOU CAN FOLLOW US ON TWITTER AT @NWSBOSTON

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