[MTHOLLYSVRWX] Public Information Statement : PHI@ 11/13/2014 4:30:10 AM

[svrwx at wx3i.net]

NOUS41 KPHI 130926
PNSPHI
PAZ054-055-060>062-070-071-101>106-131415-

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
426 AM EST THU NOV 13 2014

THE WEEK OF NOVEMBER 9TH THROUGH 14TH IS WINTER WEATHER AWARENESS
WEEK IN PENNSYLVANIA.

EACH DAY WE WILL FEATURE A DIFFERENT EDUCATIONAL MESSAGE CONCERNING 
THE DANGERS AND SAFETY CONCERNS ASSOCIATED WITH WINTER WEATHER. 

TODAY'S MESSAGE...
WINTER FLOODING

ONE OF OUR DEADLIEST WINTER WEATHER HAZARDS IS FLOODING. 

IN PENNSYLVANIA ONE USUALLY ASSOCIATES SNOW...ICE...AND BITING COLD 
WITH WINTER. BUT SOMETIMES NATURE THROWS A CURVE AT US WITH 
UNSEASONABLE WARMTH...AND WITH IT...RAIN.

A NUMBER OF DIFFERENT FACTORS WORK TOGETHER TO PRODUCE FLOODS IN 
WINTER.

WHEN UNSEASONABLE WARMTH COMES TO THE REGION IT WILL OFTEN MELT 
MUCH...IF NOT ALL OF THE SNOW ON THE GROUND IN THE LOWER ELEVATIONS.
THE MELTING SNOW WILL SATURATE THE GROUND AND ALSO BEGIN TO SWELL 
THE RIVERS.

OFTEN THE WARMER AIR WILL RISE OVER THE COLDER AIR TRAPPED IN THE 
REGION. AS THIS AIR RISES IT RESULTS IN CLOUDS AND RAIN...WHICH WILL 
COMBINE WITH THE MELTING SNOW TO INCREASE FLOWS IN THE RIVERS. WHEN 
THIS MELTING SNOW COMBINES WITH HEAVY RAINS IT CAN PUT ENOUGH WATER 
INTO THE RIVERS TO SEND THEM OVER THEIR BANKS. 

SOME OF OUR WORST WINTER FLOODS ARE CAUSED BY INTENSE CYCLONES THAT 
TRACK FROM THE OHIO VALLEY NORTHEAST UP THE SAINT LAWRENCE VALLEY 
INTO CANADA. THESE STORMS BRING A LOT OF WARM AND MOIST AIR INTO THE 
REGION FROM THE GULF OF MEXICO AND THE ATLANTIC...BRINGING BOTH MILD 
TEMPERATURES AND HEAVY RAIN TO THE REGION. 

IN JANUARY 1996...UNSEASONABLY WARM AIR AND HEAVY RAIN COMBINED TO 
MELT A DEEP SNOWPACK IN PENNSYLVANIA. THE COMBINATION OF MELTING 
SNOW AND HEAVY RAIN LED TO THE WORST FLOODING IN PENNSYLVANIA SINCE 
HURRICANE AGNES IN 1972. 

ANOTHER WINTER PROBLEM IS FLOODING CAUSED BY ICE JAMS ON THE RIVERS. 

AS RIVER FLOWS INCREASE...WATER LEVELS RISE. SINCE ICE THAT COVERS 
THE RIVERS IS LIGHTER THAN WATER IT WILL TEND TO FLOAT. UNDER THE 
PRESSURE IT WILL OFTEN BREAK INTO HUGE SLABS. THESE SLABS WILL THEN 
MOVE DOWNSTREAM IN THE CURRENT UNTIL THEY RUN INTO AN OBSTRUCTION 
SUCH AS A BEND...ISLAND...OR WIDE SHALLOW AREA. WHEN THIS HAPPENS 
THE ICE WILL OFTEN STOP AND PILE UP INTO A JAM. WHEN THE FLOW OF THE 
RIVER IS BLOCKED BY AN ICE JAM...THE WATER CAN OVERFLOW THE RIVER 
BANKS IN LESS THAN AN HOUR AS IT TRIES TO GET AROUND THE ICE. AS THE 
WATER RISES...THE PRESSURE CAN BREAK THE JAM AND RELEASE A SUDDEN 
SURGE OF WATER AND ICE DOWN THE RIVER. 

WHILE ICE JAMS OFTEN FORM IN THE SAME SPOTS YEAR AFTER YEAR IT IS 
NEARLY IMPOSSIBLE TO PREDICT EXACTLY WHEN OR WHERE A JAM WILL 
FORM...OR WHEN ONE WILL BREAK. SOMETIMES A JAM THAT FORMS IN EARLY 
WINTER WILL STAY PUT MOST OF THE WINTER.

SEVERE FLOODING OF ROADS AND BUILT UP AREAS CAN ALSO OCCUR WHEN 
MOUNDS OF PLOWED SNOW AND ICE BLOCK AND PLUG UP THE GRATES AND STORM 
DRAINS...SO THE WATER FROM THE PAVEMENT HAS NOWHERE TO GO. THIS 
STANDING WATER CAN CAUSE DANGEROUS BLACK ICE IF IT FREEZES. 

$$









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