The National Weather Service in Taunton, MA assumed its Hydrologic Service
Area (HSA) in November 1994, extending from the lower Connecticut River Valley eastward to
coastal Massachusetts. This study presents a climatology of all flood and flash flood
events during the period 1994-2000. Events are separated into two distinct categories.
These categories are Flood/Flash Flood, which includes flooding that occurs within twelve
hours of a heavy rain event, but not strictly limited to true flash flooding associated
with short duration heavy rainfall; and River Flood, which is defined as all flooding
associated with the larger main stem rivers and their largest tributaries. Flood/Flash
Flood events are calculated on a county by county basis, while main stem river flooding is
calculated on a forecast point specific basis for 34 forecast locations on 19 major rivers
in the region.
2. HYDROLOGIC SERVICE AREA
The National Weather Service Forecast Office, in Taunton, MA, has a very
complex HSA. Geographically, it extends from the eastern slopes of the Berkshires and
Monadnocks of western Massachusetts and southwest New Hampshire, to the coastal plain of
southeast New England including Cape Cod, Marthas Vineyard and Nantucket Island.
Small stream response times range from a matter of hours in the hilly terrain of the
northwest to days along the coastal tributaries. The HSA includes the Connecticut and
Merrimack Rivers, two of the largest river valleys in New England. In addition, the HSA
includes three highly urbanized, hydrologically fast responding metropolitan regions
including the Hartford, CT, Providence, RI and Boston, MA metropolitan areas, all with
their own unique modes of urban flood control.
3. DATA SOURCES
The National Climatic Data Center (NCDC) publishes a monthly publication
entitled Storm Data. This publication contains a chronological listing by state of
occurrences of storms and unusual weather phenomena. Included in the publication are
reports pertaining to these categories of Flood/Flash Flood and River Flood events. The
database which is used for the Storm Data publication draws upon a variety of
sources to determine the occurrence of flooding. These include, trained NWS Spotters,
local Amateur Radio Spotters, newspaper clippings and United States Geological Survey
(USGS) stream gaging stations for location where official flood stages have been
For this study, a Flood/Flash Flood event was defined as the occurrence of
flooding associated with heavy rainfall, melting snow, or a combination of events leading
to the inundation of normally dry areas. This included both inundation along known
waterways, including some gaged small streams, as well as significant urban flooding.
River Flooding is defined as flooding which occurs along a major river or large tributary
at a known gaged and forecasted location in the HSA. Official flood stages have been
determined for each location and are used as the reference point at which flooding begins
along a corresponding reach of the river.
For Flood/Flash Flood events, any occurrence
of flooding or the first observed rise above flood stage at a gaged small stream location
was considered one event for the county. Multiple occurrences of observed flooding from
the same city or along the same small stream were not included as a new event. For River
Flood events, the first observed rise above flood stage at the forecast point along the
river was considered one event. Multiple crests were ignored unless a corresponding fall
below flood stage occurred.
5. FLOOD/FLASH FLOOD CLIMATOLOGY
The following subsections provide a summary of yearly and monthly
Flood/Flash Flood distributions. The HSA includes the counties of Hartford, Tolland and
Windham in northern Connecticut, all of Massachusetts, with the exception of Berkshire
County, all of Rhode Island and Cheshire and Hillsborough counties of southwest New
5.1 Yearly Distribution
Flood/Flash Flood episodes varied considerably from year to year. Note in
Figure 1, the dramatic periods of quiet versus extremely active years. There were 154
Flood/Flash Flood events, from 1994 through 2000. Two major flood episodes affected the
HSA. These included the devastating floods of October 1996, resulting from a developing
coastal storm tapping tropical moisture from offshore Hurricane Lili. An equally
devastating urban flood event occurred in June 1998 and was associated with a slow moving
frontal boundary in which training of convection was the catalyst.
Figure 1. Yearly distribution of Flood/Flash Flood events for 1994-2000.
5.2 Monthly Distribution
Flood/Flash Flood can occur any month of the year, as
evident in Figure 2. Clearly, the October 1996 and June 1998 events stand out
dramatically. Events are distributed throughout the year. Maxima occur in the later spring
and early summer related to convective activity, and in late summer and early fall,
related to the effects of tropical cyclones (Vallee, 2000).
Figure 2. Monthly distribution of Flood/Flash Flood events for 1994-2000.
5.3 Hourly Distribution
Figure 3 provides the hourly distribution of events,
divided into 6 hour periods. A maxima of activity does appear to occur in the late
afternoon through the overnight hours.
Figure 3. Time distribution of Flood/Flash Flood events for 1994-2000.
|6. RIVER FLOOD CLIMATOLOGY
The following section provides a summary of yearly and monthly distribution of River
Flooding. The HSA includes the following major rivers:
In the Connecticut Basin: Connecticut, Chicopee, Westfield,
In the Merrimack Basin: Merrimack, Piscataquog, Souhegan,
Nashua, North Nashua,
Squannacook, Assabet, Sudbury,
Concord, and Shawsheen
In southeast New England: Shetucket, Blackstone, Charles and
6.1 Yearly Distribution
River Flood episodes vary considerably from year to year. There were 100 River Flood
events from 1994 through 2000. Note in Figure 4, the dramatic periods of quiet versus
extremely active years as was the case for Flood/Flash Flood events. The 1996 year was
hydrologically active, with record breaking winter snows followed by multiple River Flood
episodes through late spring. This was followed by Tropical Storms Bertha and Edouard, and
ended with the devastating floods in October.
Figure 4. Yearly distribution of River Flood events
6.2 Monthly Distribution
River Flood events can occur in any month of the year, as
evident in Figure 5, but there are two more distinct periods of activity. These periods
are related to the late winter/spring snowmelt season and the late summer and early fall
associated with the affects of nearby tropical cyclones. A third maxima is also noted in
the early winter, associated with extratropical cyclones producing heavy liquid
Figure 5. Monthly distribution of River Flood events for
6.3 Hourly Distribution
Figure 6 provides the hourly distribution of River Flood
events. The hour of occurrence is defined as that time at which a forecast point location
exceeds its official flood stage. River Flood events appear to be rather evenly
distributed in time.
Figure 6. Hourly distribution of River Flood events from 1994-2000.
After reviewing Flood/Flash Flood events and River
Flood events since 1994, several trends were noted for both categories. The frequency of
Flood/Flash Flood and River Flood events vary considerably from year to year. Flood/Flash
Flood events tend to occur most frequently in the late spring and early summer, with a
second maxima in the late summer and early fall. River Flood events also show maxima in
these time periods. Flood/Flash Flood events were more common during the late afternoon
and evening time frame, while River Flood events showed little preference in time of
National Climatic Data Center (NCDC), 1994-2000: Storm
Data, Asheville, North Carolina.
Vallee, D. R., 2000: A Centennial Review of Land
Falling Tropical Cyclones in Southern New England. Preprints, Twenty-forth Conference on
Hurricanes and Tropical Meteorology (Fort Lauderdale), American Meteorological Society,