Unseasonably High Waves Recorded Across Southern Lake Michigan

PUBLIC INFORMATION STATEMENT...UPDATED
NATIONAL WEATHER SERVICE CHICAGO IL
406 AM CDT WED JUL 24 2013 /506 AM EDT WED JUL 24 2013/

...UNSEASONABLY HIGH WAVES RECORDED ACROSS SOUTHERN LAKE
MICHIGAN ON TUESDAY AFTERNOON AND NIGHT...

THE STRONG COLD FRONT WHICH HAD MOVED ACROSS THE REGION EARLY ON
TUESDAY BROUGHT A MORE TYPICAL AUTUMN PATTERN THAN THAT OF JULY
ACROSS LAKE MICHIGAN. PERSISTENT STRONG NORTHERLY WINDS DOWN THE
FETCH OF THE LAKE LED TO NEAR RECORD LARGE WAVES FOR JULY. AT THE
SOUTHERN LAKE MICHIGAN MID LAKE BUOY WAVE HEIGHTS AT THE HOURLY
OBSERVATIONS PEAKED AT 7.9 FT...LEAVING JULY 23RD 2013 RANKED IN
5TH AS JULY DAY WITH THE LARGEST WAVE HEIGHTS EVER RECORDED AT THE
SOUTH BUOY. HERE ARE THE TOP 5 JULY WAVE HEIGHTS BY DAY AT THE
SOUTH MID LAKE BUOY...

1) JULY 21 1981     9.2 FT
2) JULY 15 1987     8.9 FT
3) JULY 20 2007     8.5 FT
4) JULY 19 1996     8.1 FT
5) JULY 23 2013     7.9 FT

THE LAKE MICHIGAN SOUTHERN BUOY HAS A FAIRLY INTACT PERIOD OF
RECORD DATING BACK TO 1981 OFFERING THE OPPORTUNITY TO SEE HOW
REMARKABLE THESE WAVE HEIGHTS ARE FOR SUMMERTIME.

THE BUOY OFF THE SHORE OF MICHIGAN CITY INDIANA SAW CONSIDERABLY
LARGER WAVES...WITH WAVES PEAKING THERE AT 16.1 FT JUST AFTER 10
PM CDT TUESDAY EVENING. THAT BUOY RECORD WAVES OF 10 FT OR GREATER
FOR AROUND 6 HOURS...WHICH IS FAIRLY IMPRESSIVE FOR FALL OR WINTER
AND OUT RIGHT REMARKABLE FOR JULY.

SEVERAL FACTORS WERE LIKELY AT PLAY RESULTING IN THE MUCH LARGER
WAVES AT THE MICHIGAN CITY BUOY...WHICH ARE LIKELY MORE
REPRESENTATIVE OF THE WAVES EXPERIENCED ALONG THE INDIANA SHORE
AND EVEN THE CHICAGO SHORE TO A LARGE EXTENT. FIRST...THE FETCH OR
DISTANCE OVER THE LAKE THAT THE WINDS BLOW RESULTS IN LARGER WAVES
AT THE END OF THE FETCH AS WAVES BUILD OVER DISTANCES. THE OTHER
FACTOR WAS LIKELY THE WATER TEMPERATURE AND RESULTANT INSTABILITY.
THE WATER TEMPERATURES AT THE SOUTH BUOY WERE 3 TO 5 DEGREES
COLDER THAN ALONG THE SHORE...YET AIR TEMPERATURES WERE NEARLY
IDENTICAL. THIS RESULTED IN SUBSTANTIALLY STRONGER INSTABILITY
NEAR THE SHORE WHICH ALLOWED FOR STRONGER WINDS AND LARGER WAVES
THAN OBSERVED AT THE MID LAKE BUOY.

PRIOR TO TUESDAY...THERE HAD ONLY BEEN TEN OTHER HOURLY OBSERVATIONS
DURING THE MONTH OF JULY WITH WAVE HEIGHTS AT OR ABOVE 7.9 FT AT
THE SOUTH BUOY. LOOKING AT METEOROLOGICAL SUMMER /JUNE-
AUGUST/...THERE HAD ONLY BEEN 78 HOURLY OBSERVATIONS OF WAVE
HEIGHTS OF 7.9 FT OR HIGHER. THAT IS ONLY 0.001 PERCENT OF ALL
SUMMERTIME OBSERVATIONS AT THAT BUOY!

ITS INTERESTING TO NOTE THAT NORTH WINDS SUCH AS SEEN ON TUESDAY
ARE ALMOST ALWAYS THE CULPRIT TO HIGH WAVES OVER LAKE MICHIGAN EVEN
IN THE SUMMERTIME. IN ALL THE SUMMERTIME INSTANCES OF WAVES AT OR
ABOVE 7.9 FT AT THE SOUTH BUOY...THE WIND DIRECTION WAS
NORTHWEST...NORTH...OR NORTHEAST ON ALL BUT TWO OF THE HOURLY
OBSERVATIONS.

$$

MTF/IZZI


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