Flood Potential Outlook
Issued by NWS Portland, ME

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Winter/Spring Flood Potential Outlook
National Weather Service Gray ME
340 PM EDT Thu Apr 12 2018


...WINTER/SPRING FLOOD POTENTIAL OUTLOOK...

The flood potential is above normal for western Maine with the
exception of southwest Maine where it is normal. The flood
potential is above normal for northern New Hampshire and near
normal for southern New Hampshire.

There is no longer a threat for ice jam flooding this season.

This is the eighth in a series of regularly scheduled flood
potential outlooks that are issued during the winter and spring
seasons. These outlooks will be issued every two weeks until the
end of the snow melt season, and will assess the potential for
flooding based on a number of factors.

...CLIMATOLOGICAL GUIDANCE...

March ended with near to somewhat above normal temperatures and
precipitation that was below normal. Temperatures for April are
averaging 4 to 6 degrees below normal with below normal
precipitation.

Medium to long range models are indicating a continuation of
colder temperatures. Although it has been fairly dry for the last
few weeks it does appear that the dry trend will end.

The official National Weather Service 6 to 10 day forecast calls
for below normal temperatures and about normal precipitation. The
8 to 14 day forecast calls for below normal temperatures with
above normal precipitation.

Strong low pressure will move into Maine and New Hampshire early
next week. This storm has the potential to produce 2 to 4 inches
of rain and strong winds.

...OBSERVED SNOW DEPTH AND WATER EQUIVALENT...

...NEW HAMPSHIRE...

The ground is now bare in southern New Hampshire for the most
part. The only exception to this is in Monadnock region where
snow depth of 1 to 6 inches exist above 1000 feet in elevation.

Farther north in the Pemigewasset and Saco River basins snow
depth ranges from little if any in the valleys up to 1 to 2 feet
above 1500 feet. A snow measurement taken by the Corps of
Engineers near Cannon Mountain at an elevation of 1950 feet had a
snow depth of 23 inches.

From the white Mountains north to the Canadian border snow depth
ranges from little if any in the lower elevations up to 2 to 3
feet near the Canadian border. A snow measurement taken by Great
River Hydro at Moose Falls had a snow depth of 31 inches.

Snow water equivalent of 1 to 1.5 inches exist in the higher
terrain of the Monadnocks. From the Upper Pemigewasset and Saco
River basin north to the canadian border snow water equivalent of
less than 1 inch exist in the lower elevations up to 4 to 8 inches
in the higher elevations. Near Cannon Mountain snow water
equivalent was 8.4 inches and at Moose falls 8.7 inches.

Snow water equivalent is below normal in southern New Hampshire
but increases to near normal in the upper Pemigewasset and Saco
River basins to a bit above normal near the Canadian border.

...WESTERN MAINE...

The ground is bare within about 20 miles of the coast. Snow depth
of 1 to 12 inches exist north of a line from Fryeburg to Augusta.
From the foothills north to the Canadian border snow depth ranges
from 12 to 30 inches.

Snow water equivalent ranges from little if any within 20 miles
of the coast up to 3 inches in the foothills. North of the
foothills to the Canadian border snow water equivalent ranges from
3 to 8 inches. A measurement taken by Brookfield

Snow water equivalent is below normal in southern and coastal
locations and near normal from the foothills to the Canadian
border

...SOIL MOISTURE AND WATER SUPPLY CONDITIONS...

Soil moisture anomaly maps from April 11 indicate normal moisture
conditions in western Maine and New Hampshire.

The long term Palmer Drought Severity Index from April 7
indicates normal moisture conditions across New Hampshire and
Maine.

Reservoirs in the Androscoggin River basin are 45.9 percent full
which is 6.9 percent above normal.

All reservoirs in the Kennebec River basin are above normal for
the time of year.

Groundwater levels courtesy of the USGS show that monitoring
wells in western Maine are below normal in southern areas and near
normal in northern locations. In New Hampshire groundwater is
mostly below normal for the time of year.

Since the snow has melted away in southern areas, groundwater
will likely continue to drop unless we get increased
precipitation.

...RIVER AND ICE CONDITIONS...

River flows are now mostly below normal due to the below normal
precipitation over the last few weeks. In addition the snow in
southern areas is now gone. The mountain snowpack will help to
increase mainstem river flows as it melts over the next 2 to 4
weeks.

There is no longer any ice cover on rivers in western Maine and
New Hampshire. The threat of ice jam flooding has ended for the
season.

...IN CONCLUSION...

Based on the above information the flood potential is above
normal for all of western Maine and central and northern New
Hampshire. This is due mainly to a normal snowpack in the
headwaters that has not yet begun to melt. Normally we would be
seeing a steady increase in river flow this time of year as the
mountain snowpack melts. However rivers continue to fall due to
colder than normal temperatures and below normal precipitation.
However heavy rain is expected early next week and this should
start the spring melt. The farther we proceed into the spring
season the greater the risk for a rapid warm-up combined with
heavy rain.

The threat for ice jam flooding is over for the season.

It is important to note that major flooding does not occur from
snowmelt alone. Rainfall, how much and in how short a period of
time is the most important factor in determining the severity of
flooding.

Another Winter/Spring Flood Potential Outlook will be issued
April 26.

$$
TFH



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