The drought currently affecting Maryland began in July, 1998 with monthly precipitation 2 to 3 inches below normal in many locations throughout the state. The remaining months of 1998 also saw below normal rainfall at most stations, with deficits of several inches recorded monthly for many stations. January and March of 1999 brought above normal precipitation to the state, while in February and April precipitation remained close to normal. May, June, and July of 1999 were extremely dry for most areas of Maryland, causing severe drought conditions throughout the state. Throughout Maryland, precipitation totals from July, 1998 through July, 1999 range from 10 to 21 inches below normal for this period. Such extreme departures from normal place the drought of 1998-99 among the 3 worst droughts to affect Maryland this century.
During the 1960's, MD also suffered from severe drought. Lasting from 1963 until September of 1966 when heavy rains brought relief to much of the state, the drought of the 1960's created 13-month precipitation totals far below normal. Precipitation totals from May, 1964 through May, 1965 show that areas in Maryland were 10 to 18 inches below normal. During the August, 1965 through August, 1966 period, regions of Maryland were 10 to 19 inches below normal. However, while the current drought has so far lasted only a little over a year, the 1960's drought lasted for nearly 4 years. For the 4-year period from January, 1963 through December, 1966, precipitation was between 26 and 38 inches below below normal.
While the 1960's drought was the longest drought to plague Maryland this century, no 13-month period in its duration could match the severity of the 1930-31 drought. The 13-month period from January, 1930 to January, 1931 was the driest on record, with precipitation totaling from 15 to 26 inches below normal for stations throughout Maryland.
During the month of August, 1999, most Maryland stations recorded above normal rainfall due to several large storms which moved through the area. While this rain helped to bring 14-month precipitation totals from July, 1998 through August, 1999 throughout Maryland closer to normal than were the July, 1998 through July, 1999 13-month totals, the drought continued with departures for the 14-month period between 8 and 19 inches below normal. A map of the Palmer Drought Index after the August rains shows that areas of Maryland remained classified as areas of moderate or extreme drought. 14-month totals from two of the most severe periods in the 1960's drought show that for most areas in Maryland, the 1998-99 drought is still comparable in severity to the 1960's drought. During the 14-month period from May, 1964 through June, 1965, precipitation totals were between 12 and 20 inches below normal. The period from July, 1965 through August, 1966 also recorded precipitation totals between 12 and 20 inches below normal. The 1930's drought remains more severe than either the 1998-99 or the 1960's drought, with precipitation totals for the period from January, 1930 through February, 1931 between 17 to 26 inches for locations throughout Maryland.
While it may not be as lengthy as the 1960's drought or as severe as the 1930-31 drought, the 1998-99 drought caused a number of problems for Maryland residents and businesses. For the first time in its history, Maryland declared a statewide drought emergency and implemented mandatory water restrictions. A map of the Palmer Drought Index during one of the most severe periods of the drought so far shows that Maryland was one of the areas most dramatically affected by this drought. For a current Palmer Drought Index Map, click here. For more information on the 1998-1999 drought from the National Climatic Data Center, click here.
The month of September, 1999 was one of the wettest on record for many stations throughout Maryland due to the rains of Hurricanes Floyd and Dennis, as well as a number of other storms. This month's unusually high precipitation totals helped to bring this year's drought to an end for many areas of Maryland. Precipitation totals for 1999 are now as much as 13 inches above normal in some places, while totals from July, 1998 through September, 1999 range between 11 inches below to 4 inches above normal. A map of the Palmer Drought Index after the end of September shows that much of Maryland is now in the midst of an unusual moist spell, while only Garrett county remains in extreme drought.
A study of summer rainfall totals from 1997 through 1999 shows that seasonal rainfall totals have generally been below normal throughout the state for each of the three most recent summers. In each of the three years, the majority of the stations had summer rainfall totals below normal, with some stations as much as 4 to 5 inches below normal. In 1997, only 3 stations out of 14 recorded summer precipitation totals slightly above normal. In 1998 and 1999, only 1 station in each year recorded above normal rainfall.
A study of summer rainfall totals from 1970 through 1999 shows that summer rainfall totals have generally been below normal throughout the state 18 to 20 out of 30 years, and above normal only 10 to 12 out of 30 years. Of 15 stations, 13 recorded more summers with below normal rainfall than above normal rainfall and 2 recorded an equal number of summers with below normal rainfall and above normal rainfall.
In order to end the drought, several large rain events were needed. These events can be caused by hurricanes and tropical storms, as well as by powerful frontal systems. A study of rain events greater than 3 inches occurring in the months of August, September, and October at 3 Maryland stations over the 50-year period from 1948 to 1997 showed that these events were most frequent at the Eastern Shore station selected.
Salisbury Airport, located in Wicomico County, was found to have had 35 events greater than 3 inches in 24 hours over the 50 years of record examined. Of these 35 events, 4 were related to hurricanes. The largest 24-hour rainfall was 5.44 inches on Septemebr 12, 1960 due to Hurricane Donna.
BWI Airport in Anne Arundel County had 12 events in the same period. Here we have considered events spanning more than one day since more detailed precipitation data are avilable for this station. Of these 12, 5 were related to hurricanes or tropical storms. The largest event to occur at BWI in this period was on August 12 and 13, of 1955 when 8.49 inches of rain fell from 3 am on August 12 until 7 am on August 13.
Hancock in Washington County, the station selected in Western Maryland, had only 3 24-hour rain events greater than 3 inches in the 50 years examined. Only one of these events was associated with a hurricane. On September 1, 1952, Hurricane Able caused a rainfall of 4.3 inches.
On September 4, 1999 through September 6, 1999, some areas of Maryland received heavy rains caused by the remnants of Hurricane Dennis. Storm totals for stations throughout Maryland show 5 stations with storm totals greater than 3 inches, and 3 stations with storm totals greater than 4 inches. 4 of the 5 stations with storm totals greater than 3 inches were located in Montgomery County.
Beginning on September 15, 1999 and ending on September 16, 1999, many areas of Maryland received heavy rains from Hurricane Floyd. Storm totals for stations throughout Maryland show that except for stations in Garrett County, all stations recevied at least 2 inches of rain. 6 stations had a storm total greater than 7 inches and 4 of these stations had a storm total greater than 9 inches. The stations recording the largest precipitatiotn totals are all located near the Chesapeake Bay. Following the heavy rains of Dennis and Floyd, a map of the Palmer Drought Index shows that the majority of Maryland is no longer considered to be suffering from drought and now has near normal conditions. Alleghany, and Washington are now considered areas of moderate drought, while Garrett county remains in a severe drought. For more information on Hurricane Floyd from the National Climatic Data Center, click here.
The current dry conditions, as well as the need for significant rainfall to end the current drought, can also be examined by looking at the 200 and 500 mb pressure surface maps of wet and dry months. By comparing these maps with an average map, it can be determined what kinds of variations in normal flow patterns cause Maryland to have a particularly wet or dry month. Here we used the monthly mean analyses from the NCEP/NCAR Reanalysis Project.
July, 1966 was one of the driest periods in the 1960's drought. The 200 and 500 mb contour maps of this month show a pronounced ridge in the Midwest United States and a trough just off the East Coast. A similar ridge-trough formation is evident on 200 and 500 mb maps for July, averaged from 1979 to 1998. The ridge and trough are much more pronounced, however, on the maps of July, 1966.
July, 1998, the beginning of the 1998-99 drought, was also an extremely dry month. 200 and 500 mb maps for this month show a ridge-trough shape much like that of July, 1966, another very dry month. The pattern for July, 1998 is also more pronounced than on the average July maps as was the case for July, 1966.
One extremely wet summer month was August, 1967 in which several large storms. The 200 and 500 mb maps for this month show a ridge and trough similar in shape to those found in maps of the dry July months, except that both the ridge and trough are shifted to the west. The trough is no longer positioned off the East Coast, but is located over the Ohio Valley. Comparing the maps of this wet August month to the average maps for August reveals that the ridge-trough pattern was much more pronounced for August, 1967 than average. The August average ridge and trough are also positioned similarly to those for the July average, with the trough along the East Coast. Therefore for August, 1967 the ridge and trough were considerably farther west than usual for August.
