of an article I wrote back in the late '90s. While working with the NWS, like anywhere else, I noticed numerous patterns and trends in our weather that would repeat over time. I researched and wrote about many weather events and trends. The following article (originally written in 1997) was about one such pattern.
...SUMMERTIME DRY SPELLS IN SOUTHEAST LOWER MICHIGAN ARE MORE COMMON
THAN YOU MIGHT THINK...
WRITTEN BY: WEATHER HISTORIAN BILL DEEDLER NWS WHITE LAKE MI
THE RECENT DRY SPELL THAT MOST OF THE AREA HAS SEEN THIS JULY IS
FAIRLY COMMON ACROSS SOUTHEAST LOWER MICHIGAN. IN FACT...IT IS RARE
NOT TO HAVE A NOTABLE DRY PERIOD DURING THE SUMMER MONTHS (JUN-AUG).
LOOKING BACK AT THE LAST 30 YEARS (1967-96) CLIMATIC STATISTICS IN
DETROIT SHOWED SOME INTERESTING AND SOMEWHAT SURPRISING DRY SPELL
TRENDS.
FIRST...THERE MUST BE SOME GENERAL DEFINITION OF A "DRY SPELL"
INCLUDING RAINFALL AND DURATION. HERE...CLIMATIC STATISTICS OF THE AREA
SHOULD BE THE GUIDE. THEREFORE...A DRY SPELL IN SUMMER DURING THE PAST
30 YEARS COULD BE DEFINED AS...LIGHT AMOUNTS OF RAIN (.50 OR LESS)
FALLING ON ONLY A FEW OF DAYS IN A PERIOD OF GENERALLY 2 TO 4 WEEKS.
THIS IS OPPOSED TO THE MORE SEVERE "DROUGHT" WHICH IS MORE PRONOUNCED
IN BOTH RAINFALL DEFICIENCY AND DURATION.
IN THE PAST 30 YEARS USING THIS DEFINITION...NEARLY 9 OUT OF 10 TIMES
(OR 87% OF THE TIME) SOUTHEAST LOWER MICHIGAN EXPERIENCED A NOTABLE
DRY SPELL SOMETIME DURING THE SUMMER MONTHS. THUS...IT IS UNUSUAL NOT
TO HAVE A DRY SPELL IN THE SUMMER. THE YEARS THAT SHOWED NO SIGNIFICANT
DRY PERIOD IN THE SUMMER WERE...1972...1977...1987 AND THE COOL SUMMER
OF 1992. IT IS INTERESTING TO NOTE...THESE "WET" SUMMERS WOULD HAVE COME
EXACTLY EVERY 5 YEARS SINCE 1972 IF THE AUGUST OF 1982 HAD NOT HAD A
PRONOUNCE DRY SPELL. SOME MONTHS DID END UP WITH SIGNIFICANTLY LESS
THAN NORMAL RAINFALL BUT DID NOT FALL IN THE DRY SPELL CATEGORY BECAUSE
THE RAINFALL WAS FAIRLY EVENLY DISTRIBUTED.
NEARLY TWO THIRDS OF THE TIME WHEN A DRY SPELL OCCURRED...IT WAS THE
ONLY DRY SPELL THAT SUMMER. THE OTHER THIRD OF THE TIME...WHEN THERE
WAS ONE DRY SPELL...ANOTHER FOLLOWED LATER IN THE SUMMER. ONE OF THE
MORE SURPRISING AND NOTABLE ITEMS TO COME OUT OF THIS BRIEF STUDY WAS
THE SHIFTING OF THE DRY SPELLS OVER THE YEARS. FROM THE MID 1960S
THROUGH THE 70S...IT WAS UNHEARD OF TO HAVE A NOTABLE DRY PERIOD IN
JUNE. AT THE SAME TIME...9 OUT OF THE 13 AUGUSTS (1967-79) HAD ONE OR
MORE DRY SPELLS WITH JULY ABOUT HALF AND HALF. YET IN THE 1980S AND
EARLY 90S...THE REVERSE TOOK PLACE. THE DRY SPELLS SHIFTED BACK INTO
EARLY TO MID SUMMER WITH ONLY TWO YEARS (1982/83) HAVING A DRY SPELL
IN AUGUST. SINCE 1994...DRY SPELLS IN AUGUST HAVE BEGUN TO REAPPEAR.
AUGUST 1996...FOR INSTANCE...WAS THE FOURTH DRIEST ON RECORD IN
DETROIT SINCE 1872 WITH ONLY .43 OF RAIN FALLING (NORMAL 3.43).
LOOKING AT EACH MONTH INDIVIDUALLY...THE DRY PERIODS IN JUNE HAVE
BEEN ALMOST EXCLUSIVELY MID TO LATE MONTH...WHILE IN JULY THEY
OCCURRED AT ALMOST ANYTIME AND IN AUGUST...THE PREFERENCE WAS AGAIN
MID TO LATE MONTH AND FREQUENTLY LASTED INTO SEPTEMBER. WHILE THESE
DRY SPELLS CAN OCCUR AT ANYTIME DURING THE SUMMER...THE CHANCES
GRADUALLY INCREASE AS THE SUMMER EVOLVES. THE BEST CHANCE FOR A DRY
SPELL HISTORICALLY IS FROM MID AUGUST INTO EARLY SEPTEMBER WITH MID
JULY TO EARLY AUGUST A CLOSE SECOND.
ONE OF THE MORE MEMORABLE EXTENDED DRY PERIODS IN RECENT TIMES
OCCURRED DURING THE HOT SUMMER OF 1988. DROUGHT OR NEAR DROUGHT
CONDITIONS DEVELOPED LATE IN THE SPRING AND CONTINUED UNTIL MID JULY.
FROM LATE MAY INTO THE THIRD WEEK OF JUNE ONLY .39 RAIN FELL IN
DETROIT. THEN ANOTHER VERY DRY PERIOD TOOK PLACE FROM LATE JUNE INTO
MID JULY WHEN ONLY .04 FELL. THIS DESERT-LIKE ATMOSPHERE...AS MOST
INHABITANTS OF THE AREA CAN ATTEST...WAS OFTEN ACCOMPANIED BY DESERT-
LIKE TEMPERATURES. MAY OF 1988 WAS THE THIRD DRIEST MAY (.87) ON RECORD
IN DETROIT...WHILE JUNE WAS THE SECOND DRIEST JUNE (.97). THIS ARID
CLIMATE CAME AT THE WORST TIME POSSIBLE...EARLY IN THE GROWING SEASON
...A TIME WHEN CONVECTIVE RAINS NORMALLY BEGIN.
OTHER DRY MONTHS NOTED WERE...JUNE 1984 (1.04)... JULY 1974 (.59)
...JULY 1984 (.95) AND AUGUST 1982 (.72). THE LEAST AMOUNT OF RAIN
EVER RECORDED IN DETROIT DURING A SUMMER MONTH WAS A MEAGER .16
IN AUGUST 1894. THE SECOND HIGHEST AMOUNT OF RAIN IN A SUMMER MONTH
ALSO OCCURRED IN AUGUST...AUGUST 1926 WITH 8.33 INCHES (JULY 1878
WAS THE HIGHEST WITH 8.76). CLEARLY...THE SUMMER MONTHS CAN BE
"FEAST OR FAMINE" WHEN IT COMES TO RAIN.
Dry/Drought map for SE Michigan as of 7/19/11. Note this is a broad scale average and
not fine tuned to a local areas.
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