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Flood Potential Outlook
Issued by NWS Portland, ME
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Winter/Spring Flood Potential Outlook
National Weather Service Gray ME
749 AM EST Thu Feb 29 2024

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

The flood potential outlook for New Hampshire and Western Maine is Below
Normal. The potential for flooding due to ice jams is also below normal.

This is the fifth 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 March 1
through March 14th for New Hampshire and western Maine.

The short term flood risk through March is normal in the
mountains given some remaining snow and continuous melt. The ice
movement and above normal flows increases the risk of ice jams
flooding over the next two weeks. From the foothills southward the
risk is below normal given meager snowpack combined with limited
opportunities for significant runoff events over the next two
weeks.

The long term flood risk is below normal. Runoff storage is low
given high streamflows and saturated soils. However, the meager
snowpack would need to show growth in March to increase the
overall spring thaw flood risk to above normal levels.

...OBSERVED SNOW DEPTH AND WATER EQUIVALENT…

The snow depth and snow water equivalent was well below normal for
late February. The two-week change in snow conditions has been
nondescript apart from the mountains where on and off snow showers
accumulated several inches. Elsewhere, the snowpack compressed
and melted.

In New Hampshire the snow depths ranged from little if any snow in the
southern and coastal locations, up to 6 inches north of the Lakes
Region. From the White Mountains to the Canadian border the snow
was more robust averaging 1 to 2 feet, upwards of 3 feet in the
mountains. The stored water in the snowpack was generally less
than 1 inch in the lower elevations, and 2 to 4 inches in the
north country, and upwards of 6 inches above 2000 feet.

In Western Maine the snowpack was limited to the foothills northward.
In the foothills and valleys the snow was mainly prevalent in the
wooded areas and up to 12 inches deep. The stored water ranged
form 0.5 to 3 inches. From the Mountains to the Canadian Border
the snow was more continuous and ranged from 1 to 2 feet deep,
highest above 2000 feet. The stored water ranged from 3 to 6
inches with locally higher amounts.

Recent rain on snow event caused melt and ripening even in the
mountains with densities around 30% or higher. The snow
temperatures were warming and primed to melt over the next two
weeks. Looking ahead the forecast favors gradual melt with
limited opportunities for growth.

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

Frost Depth ranged from 5 inches in the mountains to 12 inches
across the region. Areas without a snowpack observed topsoil
thawing. Given the deep frost depth this period of topsoil thawing
will only result in limited amounts of water release. The
groundwater remains well above normal for the region with the
expectation that southern areas will start to see additional
recharge over the next few weeks as thawing increases. Streamflow
levels courtesy of the USGS were running 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. The
Androscoggin and Kennebec reservoirs have approximately 15-36%
less capacity than normal, but are nearing guide curves. Storage
remains high in the Connecticut and Merrimack watersheds where
USACE reservoirs had only used 0-3% capacity. Lake
Winnipesaukee...New Hampshire`s largest lake…recently dropped from
maximum recorded levels to near seasonal normals due to continued
drawdowns in February. Sebago Lake in Maine likewise dropped to
seasonal normal levels following high levels much of winter.

...RIVER AND ICE CONDITIONS...

River ice coverage was below normal for the time of year. Recent
rain and melting snow event increased river flows to above normal,
breaking up some river ice and moving it downstream. The
temperature forecast favors thermal melt over the next two weeks
as streamflow remains high due to melting snow.

…CLIMATOLOGY AND  TEMPERATURE AND PRECIPITATION OUTLOOKS…

Very warm conditions have existed since December. By the end of
February, the winter-to-date temperatures were running in the top
5 warmest with temperatures 4 to 8 degrees above normal. The
winter was defined by the limited number of cold Arctic air
intrusion events resulting in a paltry snowpack. Precipitation was
heavy in December and the first half of January, but lessened in
February. The dry conditions at a key period in winter resulted in
limited snow growth. By the beginning of March the snowpack was
running well below normal with little to no snow south of the
foothills.

The weather pattern over the next couple of weeks offers only
limited precipitation chances. Temperatures over the weekend will
start a warming trend which will favor a gradual melt over the
next 7 to 14 days. Looking ahead the Climate Prediction Center
continues favoring above normal temperatures and above normal
precipitation through late March. The North Atlantic Oscillation
is expected to turn negative mid-month, which could favor another
cool down keeping some opportunities for snow.

...IN CONCLUSION...

Based on the above meteorological and hydrological information,
short term flood risk is normal across western Maine and New
Hampshire mountains, and below normal elsewhere. The long term
flood risk is below normal due to a lack of deep snowpack or
significant river ice. The meager snowpack, increased sun angle,
and above normal temperatures favor an early spring thaw.

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, March 14th.

$$

$$
National Weather Service Weather Forecast Office
