Spring Flood Risk Frozen in Place

 

March 19, 2015

 

Strong River Ice and Cool Temperatures Freeze Flood Risk in Place

Spring flood potential will remain normal for the remainder of March. However, overall flood potential will increase over time as ice and snow melt is delayed by below normal temperatures.

The Maine River Flow Advisory Commission hosted a conference call today to review current hydrologic conditions. This was a follow-up to the Commission's meeting on March 5.

Governor Paul R. LePage participated in the conference call at the State Emergency Operations Center in Augusta. “We’re monitoring flood potential very closely as the spring progresses,” the Governor said. “We’re encouraged that conditions look stable in the short term, but concerned especially about the potential for ice jam flooding, which can be extremely destructive. I encourage all Mainers, especially those who live and have businesses in flood-prone areas, to stay informed on the potential for flooding.”

Rainfall is the major driver of flooding in Maine. However, snowpack, frozen ground and river ice create additional risk factors at this time of year.

Snow Survey

The Maine Cooperative Snow Survey measured the snowpack at ninety sites this week. Snowpack depth and water content increased slightly across the state.

Water content is in the upper 25% to upper 10% of historical values for the time of year with historically high levels in eastern and coastal Maine.

Snow densities (the relative amount of water in the snow) ranged from .20 to .32 but is increasing slowly. (A snowpack with densities above 0.33 is considered "ripe". A ripe snowpack no longer has the ability to absorb rainfall and would tend to release water during a rain event.)

In Maine, snowmelt alone is usually not sufficient to cause flooding. However, the later we carry significant snowpack into the spring, the greater the likelihood of snowmelt being triggered by warm temperatures and rain and adding to flood risk.
Snow surveys will be conducted weekly until all the snow is gone.

Stream Flow and Ice Conditions

Stream flows are slightly low for the time of year. Rivers and streams generally increase in early March as snowmelt begins. This year, this "turn" in stream flow levels has been delayed due to below normal temperatures. Gradually increasing stream flows in the spring are a major factor in eroding ice cover gradually.

Ice jamming remains a concern as rivers and tributaries still are carrying a significant ice cover, comparable to mid-winter conditions rather than spring. Ice thickness in most locations has remained constant in the last two weeks, and the ice is both thick and strong. If the ice begins to move, the strength and size of the ice chunks could essentially build a dam within minutes, backing up water behind it.

The US Coast Guard reports that ice breaking has been challenging. Next week ice breaking is scheduled on the Penobscot River, with ice breaking on the Kennebec River scheduled for the week of 30th.

Weather Outlook

According to the National Weather Service forecast offices in Gray and Caribou, weather conditions will be quite cold and below normal for the rest of March. No large storms are forecast through the end of March.
This will tend to keep the snow pack in place, or even add water content to it.

Flood Potential

Flood potential across the state remains normal through the end of March but overall potential will increase over the next few weeks due to the lateness of ice and snow melt. The strength of the ice cover in Maine’s rivers creates a vulnerability to ice jam flooding. The probability for major rainstorms and warm temperatures increase as spring progresses.
The River Flow Advisory Commission will convene again in early April to assess conditions across the State. The National Weather Service Forecast offices in Gray and Caribou issue Flood Potential Statements every two weeks during the spring.

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