Spring Flood Potential Above Normal as Snow Will Stay for a While

 

March 5, 2015

 

Dramatically deep snows, coupled with a weather pattern that will keep snow and ice in place in the short term, are creating elevated flood potential in the State.

The Maine River Flow Advisory Commission, meeting today in Augusta, reviewed information on current hydrologic conditions across the state, as well as short-term weather forecasts.

“The amount of snow on the ground does NOT mean flooding is inevitable,“ said Bob Lent of the USGS, co-chair of the Commission. “It does mean we have risk factors in place that could become critical when temperatures warm and we begin to see rain instead of snow.” Rainfall is the driver of flooding events. The longer snow lingers on the ground into the spring, the greater the risk of a warm-up and rain event, which could combine with snowmelt to cause flooding.

According to the National Weather Service, snow depths are dramatically above normal for this time of year in many central and coastal areas. Some examples include Portland, with 550% of the normal depth of snow; Bangor with 700% and Machias at 950% of normal.

Snow surveys conducted March 2nd and 3rd showed snowpack water content ranging from 5 to 8 inches across the state, with the largest amounts (7 to 8 inches) in a swath from the midcoast to downeast coast. These levels put water content in the highest 10% of the last 10 years in this coastal area, in the normal range in the headwaters of the Kennebec, Androscoggin and Penobscot Rivers and in the lowest 25% of the last 10 years in the far north.

This pattern, with higher water content along the coast, is the opposite of what is usually seen at this time of year. Accordingly, the potential for small stream and urban flooding is elevated closer to the coast, while the flood potential for mainstem rivers is more in the normal range.

Snow densities are so far low, meaning the snow has the capacity to absorb some amount of light rainfall. Snow survey maps are available at http://maine.gov/rfac/rfac_snow.shtml

The National Weather Service noted that for the next two weeks, temperatures will be in the normal range and some fast-moving systems could bring light precipitation. This will tend to keep the existing snow pack in place, possibly increasing water content slightly. Beyond this period, there is increased potential for warmer temperatures and more precipitation, risk factors for flooding.

River ice is in the normal range in most areas of the State, but expected weather conditions will tend to keep ice in place. The US Coast Guard is working closely with the USGS, National Weather Service, MEMA and other operational partners to monitor river ice conditions and plan for continued ice-breaking on the Penobscot and potential breaking on the Kennebec. Along other rivers, areas prone to ice jams will be monitored closely as the spring progresses.

Reservoir storages in the headwaters of Maine’s large rivers are being drawn down to prepare for spring rains and runoff. River basin managers report that they are on track to achieve their drawdown targets.

The most important factor influencing flooding is rainfall, but even a normal snowpack adds risk. When water in the snowpack is released suddenly, by rainfall and warm temperatures, runoff amounts are increased dramatically. A rain event along the coast with a high coastal Maine snowpack could cause serious small stream and road flooding,

Significant ice in rivers and streams presents an additional risk that cannot be forecast. Ice can jam, and water behind the jam can rise several feet in just a few minutes. When an ice jam lets go, water levels downstream rise suddenly.

The best case scenario at this time of year is days above freezing and cooler nights, to melt snow and ice gradually. However, with continued cool temperatures over the next two weeks, any melting will likely be minimal. The later that snow and ice persist into the spring, the greater risk they pose.

The annual meeting of the Commission represents the beginning of “flood season” in Maine, when all parties focus close attention to risk factors for flooding, and to temperature and precipitation forecasts. The following steps are recommended:

  • Check your flood insurance coverage if your home or business is in a flood-prone area. Most home and business owner’s policies do not cover flood damages. And there is 30-day waiting period before a new policy goes into effect.
  • If you live or work in a flood-prone area, pay close attention to conditions in the next several weeks. Be aware that any sudden warm-up or significant rainstorm in the late winter and early spring carries the risk of flooding.
  • If you live along a river or stream, watch for signs of ice jams. Signs include ice piling up and appearing to block the channel, coupled with rising water levels behind the jam. Report any suspected ice jams to local officials or the National Weather Service.

Commission members will stay in close communication throughout the spring season. They will likely meet again to assess risk later in the month. Snow surveys will be conducted each week from now until the snow cover is gone.

The River Flow Advisory Commission meets annually in late winter to share information, examine potential for spring flooding and to renew operational protocols. The Commission is composed of state, federal and industry representatives with an interest in hydrologic issues.

The full report of the March 5 meeting will be available online at http://www.maine.gov/rfac by close of business on March 6.