Spring Flood Potential Above Average throughout Northern and Eastern Maine

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FGUS71 KCAR 310841
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MEC003-009-019-021-029-020845-WINTER/SPRING FLOOD POTENTIAL OUTLOOK
NATIONAL WEATHER SERVICE CARIBOU ME
441 AM EDT Fri Mar 31 2017

...SPRING FLOOD POTENTIAL OUTLOOK FOR NORTHERN... CENTRAL...AND
DOWNEAST MAINE...

This is the seventh spring flood potential outlook for 2017,
issued by the National Weather Service in Caribou, Maine. This
outlook is for northern, central, and Downeast Maine for the two-
week period from March 30 to April 13, 2017.

The spring flood potential for open water flooding is above
normal for all of northern, central, and Downeast Maine. The
potential for flooding due to ice jams is near to slightly above
normal across the entire region.

...CLIMATOLOGICAL GUIDANCE...

Colder than normal temperatures have been the norm through much of
March, with many locations across the state reporting monthly
average temperatures 4 to 6 degrees below normal. This cold has
allowed the river ice and snowpack to not only persist, but to
even grow in some instances. The weather pattern has been rather
active, though precipitation and snowfall amounts haven`t been
overly significant over the past two weeks.

This active pattern is expected to persist over the next several
days. The storm system that will affect the region over the
upcoming weekend isn`t expected to bring much in the way of
precipitation to the eastern half of the state, but forecast
models are indicating a series of low pressure systems will move
through the northeastern United States over the next couple of
weeks. At this time it is hard to say what impacts these systems
will have on our area; precipitation types and amounts will depend
heavily on what track each low pressure system takes, and the
long range models are showing little agreement on these sorts of
details. There are some hints that we may be breaking out of this
cold spell with a return to near normal temperatures, but there
could still be cold spells, especially if any of the storm systems
track to our south. Regardless, an active pattern means that we
could either hold on - or even add - to our snowpack or, if
warmer conditions do take hold, see several rain on snow events.

The official 6 to 14 day outlook from the Climate Prediction
Center for April 5 to 13 supports the above scenario, calling for
wetter than normal conditions, particularly over the next 10
days, with no strong signals for either warmer or colder than
normal temperatures.

...OBSERVED SNOW DEPTH AND WATER EQUIVALENTS...

As mentioned above, the snowpack has been very persistent the
past couple of weeks and, in fact, has even grown a bit over the
last few days. While portions of the Downeast coast and the Bangor
region are down to just a few inches of snow, much of the state is
still blanketed with 18 to 28 inches of the white stuff. In
particular, Aroostook County down into the Central Highlands have
the deepest snowpack, with a few locations reporting more than 30
inches. This lessens a bit as one heads toward the coast; the
Lincoln area down toward Bangor has 12 to 18 inches of snow. Most
areas have a near to above normal snowpack for late March. The
exception is the aforementioned portions of the Downeast coast
where the pack is 6 inches or less.

Perhaps more importantly, the snowpack holds a very significant
amount of water. The snow water equivalent, or the amount of
water held within the snowpack, is running 7 to 11 inches in much
of northern and central portions of our forecast area, with some
locations still reporting a foot or more of SWE. These numbers are
above normal, particularly in the Central Highlands where the most
water lies. The water content lessens as one heads toward the
coast; interior Downeast and the Bangor region generally have 4 to
7 inches of water, while the immediate coast has 2 to 5 inches of
water locked in the snowpack.

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

Outside of far eastern Maine which is wetter than normal, near
surface soil moisture states across the region are generally near
normal. This drying out make sense given the cooler conditions and
reduced runoff. The latest Palmer Drought Severity Index, which
looks at soil moisture conditions in the longer range, is also near
normal across the entire state. Meanwhile, groundwater monitoring
wells, courtesy of the USGS, are variable around the region. The
Saint John Valley is showing above normal groundwater, while
portions of the Central Highlands and far Downeast Maine are below
normal. Wells across the remainder of our region are indicating
near normal groundwater conditions.

As has been noted in previous outlooks, the ground remains
unfrozen under the deep snowpack. This usually helps to slow
surface runoff because moisture is allowed to percolate through
the soil rather than run across it. However, given the very moist
near surface soils, the ground may not be able to readily absorb
runoff, especially if it occurs rapidly or in great amounts.

...RIVER AND ICE CONDITIONS...

River flows have been steady or decreasing over the past two
weeks as the colder weather and lack of significant rainfall has
slowed runoff. Some of the waterways in the Penobscot,
Piscataquis, and Saint Croix basins have showed some recent minor
rises as temperatures have been warm enough to allow for some
modest snowmelt, but the overall trend across the state has been
to hold steady or fall. In general, river flows are near to above
normal for late March.

River ice remains well entrenched across the northern and central
waterways. The Aroostook, Saint John, and Allagash Rivers are all
still mostly ice covered with a goodly amount of snow on top of
the ice. Some of the smaller streams still have some open
stretches. This holds true for the upper reaches of the
Piscataquis and Penobscot Rivers, as well. General ice states are
unknown, but with warming temperatures and some possible runoff,
it would be expected the ice should gradually weaken as we head
through early April. Downeast and coastal waterways, including
the lower stretches of the Penobscot, likely have little ice. No
additional ice strengthening or growth is expected from here on
out.

...IN CONCLUSION...

Based on the above information, the spring flood potential for
open water flooding is above normal across all of northern,
central, and Downeast Maine. Many rivers and streams are running
near to above normal, and soil moisture is likewise near to above
normal. There is plentiful water available in the snowpack, in
some cases nearly a foot or more. With an active weather pattern
expected and a likely turn toward warmer conditions, a rain on
snow and/or significant runoff event becomes more likely as we
head through early April.

The potential for flooding due to ice jams is near to slightly
above normal for those waterways that still have significant ice,
basically all of interior Maine. River ice has remained
substantial in coverage through March and has even grown a bit
since the warm spell in late February. No additional strengthening
is expected, but it will likely take another 2 or 3 weeks before
we see significant weakening and/or movement of the ice,
especially where substantial snow covers the river ice. Any
significant rain or snowmelt events during this time will have the
potential to cause ice movement and  ice jam flooding. The threat
is highest on the northern waterways, including the Aroostook,
Allagash, and Saint John Rivers, as well as in the upper
Penobscot and Piscataquis basins. This is where the ice is
thickest and covered with substantial snow. Overall this is not
an abnormal scenario heading into April, but with the anticipated
active pattern, a near to slightly above normal threat seems
reasonable.

It is important to remember that very heavy rainfall can bring
flooding at any time of the year, even in areas that don`t have a
significant snowpack.

The next spring flood potential outlook will be issued by NWS
Caribou on Thursday, April 13, 2017.

$$

Hastings