Flood Potential Outlook
Issued by NWS Portland, ME

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Winter/Spring Flood Potential Outlook
National Weather Service Gray ME
148 PM EDT Thu Apr 11 2024

...SPRING FLOOD POTENTIAL OUTLOOK FOR NEW HAMPSHIRE AND
WESTERN MAINE...

The flood potential outlook for New Hampshire and Western Maine
is above normal. Ground conditions are exceptionally wet across
the region with limited runoff storage. Melt combined with
saturated antecedent conditions sets up ideal conditions for
flooding, particularly in the mountains. The risk will remain
elevated until the remaining snow has melted and spring green-up
has begun.

This is the eighth in a series of regularly scheduled flood potential
outlooks that are issued during the winter and spring seasons
highlighting the flood potential during the next two-week period.
This issuance represents the flood risk between April 11 through
April 25th for New Hampshire and western Maine.

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

The snow depth and snow water equivalents were well below normal
for all but the higher elevations and wooded areas north of the
foothills. Elsewhere, the ground was mostly bare. Snow depths of 6
to 12+ inches above 1500 feet with snow water values ranged from
1.00 to 3.00+ inches...locally 4.00 to 7.00+ inches at high
elevation regions of the White Mountains.

The snowpack was nearing isothermal conditions, indicative of a
ripe snowpack. An ongoing rain on snow event is expected to ripen
and melt a significant amount of remaining snow. Looking ahead the
forecasts favors gradual melt the next two weeks with limited
opportunities for growth.

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

Exceptionally moist ground conditions, high streamflows, and high
reservoir and lake levels suggest very limited storage for storm
runoff. Soils in areas without snow have thawed with water release
showing up in groundwater wells. The groundwater levels remain
much above normal with additional recharge over the last few weeks
as thawing increased. Streamflow levels courtesy of the USGS were
running above to well above normal due to a recent rain on snow
event.

Runoff storage capacity remained below normal for the time of
year with high lake levels for most of the region. Kennebec River
Basin storage in Maine was 84 percent full which was 110 percent
above normal. The nearby Androscoggin River storage was 72.5
percent full which was 116 percent above normal. Lake
Winnipesaukee...New Hampshire`s largest lake...increased to 0.8
feet above normal as of 09 April 2024. The lake is at its full
level despite almost continuous releases during February.

...RIVER AND ICE CONDITIONS...

The ice threat has ended for the season.

...CLIMATOLOGY AND TEMPERATURE AND PRECIPITATION OUTLOOKS...

El Nino has continued to weaken, although its influence has
remained. The active subtropical jet stream has continued to
advance moisture laden low systems into the Northeast. The result
has been a winter with well below normal snowfall and well above
normal temperatures and precipitation. This active trend looks to
continue for a while longer, but with some shifting in the low
track pattern.

The Northern Atlantic Oscillation (NAO) which has been negative
and has been promoting blocking and troughing is forecast to turn
more neutral along with the Arctic Oscillation (AO). Meanwhile the
Pacific North American (PNA) is forecast to become more negative
which would promote more troughing in the western CONUS. This
setup would tend to favor weak ridging across the southeastern
U.S. and mean troughing from the central into the western CONUS.
Overall, this setup favors a southwesterly flow of milder air into
the Northeast with a low track generally favored to across the
Great Lakes. The official National Weather Service 6 to 14 day
Outlook for 16-24 April 2024 leans likely above normal
temperatures and near normal to above normal precipitation.

...IN CONCLUSION...

Based on the above meteorological and hydrological information,
the short term flood risk above normal. Groundwater levels are
well above normal and storage capacity for runoff is limited. The
potential for rapid runoff remains elevated until the seasonal
green-up is underway. The weather pattern over the next couple of
weeks looks to transition into more spring-like conditions with
above average temperatures and increased precipitation
opportunities. The addition of snowmelt and presence of ground
frost in the mountains makes the risk higher in those areas. 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.

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. The next Spring Flood Outlook will
be issued on Thursday, April 25th.

$$

Jamison


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