Flood Potential Outlook
Issued by NWS Portland, ME

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Winter/Spring Flood Potential Outlook
National Weather Service Gray ME
1124 AM EDT Thu Mar 28 2024

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

The flood potential outlook for New Hampshire and Western
Maine is above normal. This is the seventh 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 28 through April 11 for
New Hampshire and western Maine.

...OBSERVED SNOW DEPTH AND WATER EQUIVALENT…

The snow depth and snow water equivalents were below normal for
all areas apart from the White Mountains where snowpack was near
normal. The two-week change in snow conditions was notable growth
due to a significant storm on March 23. This single event brought
widespread 1.75 to 2.75 inches of additional snow water
equivalent to most of the region. The event was complex with snow,
rain, sleet, and freezing rain. For the mountains and foothills
new snowfall was 1 to 2 feet deep, lesser to the coast where ice
played a more significant role. The rainfall event ongoing at the
time of this issuance was ripening the snowpack with the
expectation that there would be a notable reduction in depth, both
from compaction and melt.

Far southeastern sections of New Hampshire reported bare ground.
Across the remainder of southern New Hampshire...snow depths were
mostly under 6 inches now with up to a foot above about 1000 feet.
Across central and northern New Hampshire...snow depths averaged
6 to 12 inches in the valleys but do increase to between 1 and 3
feet above 2000 feet with isolated higher depths near treeline in
the White Mountains. For example...43 inches of snow was observed
on the ground on 27 March at Carter Notch at an elevation of 3317
feet. Snow depths were near normal across portions of southern
and central New Hampshire for the end of March but were generally
below normal across northern Coos County near the Canadian
border.

Across southern New Hampshire...between a Trace and 2 inches of
water was available for melt runoff with higher amounts limited
to areas above 1000 feet. Across the remainder of central and
northern New Hampshire...1 to 4 inches of water was held in the
lower elevation snowpack but those amounts do increase to between
4 and 8 inches above 1500 feet with even higher amounts in the
highest elevations of the White Mountains. As of 26 March
2024...Moose Falls...near the border with Quebec Province
Canada...reported 8.6 inches of water in the 30 inch snowpack. And
at 4373 feet in the White Mountains...Gray Knob reported more
than a foot of water in the 49 inch snowpack. Snow water
equivalents have recovered to near normal across a lot of central
and northern New Hampshire, but remain a bit below normal across
southern portions of the Granite State.

In Western Maine the snowpack had rebounded with the recent snow
event, but depths remained generally below normal. From the
Mountains to the Canadian Border the snow was more continuous and
ranged from 1 to 2 feet deep, highest above 2000 feet. Some of
the higher peaks in western Maine had over half a foot of water
in the high elevation snowpack. The stored water ranged from 3 to
6 inches with locally higher amounts. The foothills ranged from 1
to 2 inches of stored water with depths around 6 to 10 inches. The
ice storm along the coast had depths of only a few inches, but
stored water averaging around an inch.

The snow condition was compressed with high densities. The ongoing
rain on snow event was causing 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 forecasts favors gradual melt with the
potential for additional snow the first week in April. Thereafter
there will be limited opportunities for growth.

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

After what was an abnormally dry February...the return to
excessive wetness for most in March combined with snowmelt
dramatically increased moisture states. Much of southern and
Western Maine had thawed prior to the recent snowfall event
including in the Rangeley area. Frost depths were reduced in mid
March with most lower elevations thawing. In the mountains depths
ranged from 5 inches in the mountains to 12 inches across the
region. The groundwater remained well 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 well below normal for the time
of year with high lake levels for most of the region. Water supply
reservoirs across the region continue to average above to much
above normal pool levels. Kennebec River Basin storage was 80
percent full as of 25 March 2024 which was 123.2 percent above
normal. The nearby Androscoggin River storage was 70.7 percent
full which was 128.8 percent above normal. 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…showed 0.5 feet above normal levels with continued
drawdowns ongoing.

...RIVER AND ICE CONDITIONS...

The ice threat has ended for the season.

…CLIMATOLOGY AND  TEMPERATURE AND PRECIPITATION OUTLOOKS…

Temperature and precipitation in March continued the theme over
the winter of being above normal. Precipitation for the month to
date was running in the top 5 wettest, with Portland and Augusta
approaching the record. The month started off with above normal
temperatures, but in the mountains conditions still supported
snowpack growth. By the last week temperatures dropped below
normal allowing a significant snowfall event occurred on the
23rd. With the warm temperatures during March 2024...most long
term climate sites have reported below normal snowfall.

Although El Nino has begun to weaken...we are still seeing its
influence manifested in very intense southern stream storm systems
that gather Gulf of Mexico moisture and then try to head up the east
coast, as was the event unfolding at the time of this issuance.
After that...it looks like early April 2024 will be characterized
by a negative North Atlantic Oscillation (NAO) Index. This should
promote a lot of upper level blocking downstream of New England
in the North Atlantic which will likely deflect cold air from
Canada southward into New England and New York State. At the same
time...medium range climate indices call for the Pacific North
America (PNA) Index to remain neutral to slightly positive. A
positive PNA favors upper-level troughing across the southeastern
United States. The combination of a negative NAO and somewhat
positive PNA does increase the odds of east coast storminess. It
remains to be seen where exactly the upper level blocking sets up
and whether it will keep the coastal systems south and east of New
England or allow them far enough north to impact the local area.
Also...with near to below normal temperatures expected in early
April...we may not be done with snow for this Winter...especially
across the mountain regions.

The official National Weather Service 6 to 10 day Outlook for 1-5
April 2024 seems to agree with this overall theme as it leans
towards below normal temperatures and precipitation leaning above
normal. Looking further out, the temperature outlooks favor
warming towards mid April with near normal precipitation chances.


...IN CONCLUSION...

In summary, the active weather pattern combined with residual
snowpack and below normal storage supports an above normal risk of
flooding through mid April. The potential for rapid runoff
remains elevated until the seasonal green-up is underway. We do
not anticipate large scale water supply issues heading into
Spring. 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 particularly before green-up has begun.


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 11th.

$$


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