Flood Potential Outlook
Issued by NWS Caribou, ME

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WINTER/SPRING FLOOD POTENTIAL OUTLOOK
NATIONAL WEATHER SERVICE CARIBOU ME
149 PM EDT Thu Mar 16 2017


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

This is the sixth 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 16 to March 30, 2017.

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

...CLIMATOLOGICAL GUIDANCE...

Below normal temperatures dominated the first half of March with
much of the region averaging 5 to 7 degrees colder than normal.
Precipitation averages were a little more variable; northern Maine
saw above normal precipitation while central and southern
locations were near to below normal for early March. This
precipitation generally occurred over 2 or 3 separate events with
short breaks in between. The most recent snow event brought
widespread 10 to 20 inches to the region, with northwestern areas
receiving the highest amounts. Most sites were reporting below
normal snow totals for the month prior to this storm. Now a
majority of locations are several inches above normal for mid
March and a foot or more above normal for the snow season as a
whole.

We don`t anticipate this overall cold and active pattern to alter
much going into the latter half of March. General upper-level
troughiness will persist over the next two weeks, keeping
temperatures generally near to below normal. Any warm-ups will be
brief. There will be the threat for precipitation, likely in the
form of snow, every few days, though these events should be
fairly minor through at least the next 7 days. A low is expected
to develop and strengthen off the East Coast this upcoming
weekend, but the model trends have been to keep it further
offshore, with little to no snowfall expected in our area. This
system will continue to be watched closely for any signs of a
westward shift, which would potentially bring significant snow
Downeast. Beyond this system, long range models are hinting at a
potentially stronger system around March 24 or 25, but it is too
far into the future to speak with any certainty.

Thus, with a fairly persistent trough across the northeast along
with several possible precipitation events, we anticipate below
normal temperatures and near normal precipitation over the next
two weeks. The official 6 to 14 day outlook from the Climate
Prediction Center for March 21 to March 29 supports this analysis.

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

The snowpack grew a fair amount after the most recent storm, with
most locations seeing 10 to 20 inches of snow. The snow depth is
now above normal across just about our entire region, particularly
the Central Highlands. The only exception is the immediate coast,
where the snow cover is a bit below normal.

The immediate coast is reporting 10 inches or less of snow cover
on the ground. This increases quickly as one heads inland; 1 to 2
feet is common across interior Downeast and the Bangor region.
Traveling northward into the Central Highlands and Aroostook
County, the snowpack is now 20 to 30 inches deep. The highest
terrain of the Central Highlands is covered with more than 3 feet
of snow; Chimney Pond`s latest observation indicated the snow
depth was estimated to be around 7 feet deep.

The snow water equivalent, or the amount of water contained in the
snowpack, is above normal across the Central Highlands, where 6 to
10 inches of water lies in the snow. The highest elevations have a
foot or more of SWE. This decreases across Aroostook County and
interior Downeast, where SWEs of 4 to 7 inches are common, with
locally higher amounts in excess of 8 inches. This is near to a
bit above normal for mid March. Further south, Bangor down to the
coast has 2 to 4 inches of water locked in the pack, with only an
inch or 2 along the coast.

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

Near surface soil moisture states remain above normal, even
though runoff has slowed substantially during the colder
conditions. The latest Palmer Drought Severity Index, which looks
at longer-term soil conditions on the range of weeks to months, is
indicating near normal moisture conditions across the state.
Meanwhile, groundwater monitoring wells, courtesy of the USGS,
are mainly near to above normal. The exception continues to be in
the Central Highlands, which still have not caught up from last
year`s drought.

It should be noted that in areas where there has been a deep
snowpack through the winter, the ground is not frozen. This
usually helps to slow surface runoff because moisture is allowed
to percolate through the soil rather than run across it. However,
as noted above, near surface soils are moist, so the ground may
not be able to readily absorb runoff, especially if it occurs rapidly
or in great amounts.

...RIVER AND ICE CONDITIONS...

The recent spate of colder weather has allowed river flows to
subside as runoff receded. Flows are now generally near normal,
though southern streams are likely on the low side while northern
and central ones are higher. This trend will continue over the
next several days as no significant runoff events, either from
snowmelt or liquid precipitation, are anticipated. The
strengthening March sun should allow for some snowmelt, but this
will be minor.

The cold weather also allowed river ice to thicken and become
reestablished on some of the smaller streams that opened up in
late February. The Saint John River had some ice break up and
movement in the Allagash region during the February mild spell, as
well, but the river should be frozen again and any ice jams
locked in place. Although the ice cover may have strengthened a
bit with the colder temperatures, it is likely still weaker than
normal since hard black ice was never really established this
season. The snow that now lies on top of the ice will help to
protect the ice from the sunshine, but with the bitterly cold
temperatures (hopefully) at an end, little to no additional ice
growth is expected to occur.

...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, though it`s closer to normal along
the coast. This threat is mainly in the longer term as no
significant runoff events are anticipated in the near future.
However, the longer this deep snowpack and its substantial amount
of water lingers into late March or early April, the greater the
chance of flooding from sudden snowmelt and/or heavy rainfall.
With a cold and active weather pattern expected to continue
through the latter half of March, the snowpack is not expected to
diminish overly much. In fact, it may even grow if we do get some
more significant snow events. Therefore, an above normal flood
threat seems reasonable.

The potential for flooding due to ice jams is near to slightly
above normal across the entire region. Many of the mainstem rivers
are still mostly ice covered, and in fact, the ice has thickened
and become better established over the past couple of weeks. The
ice is still mostly comprised of gray or white ice, which is
weaker than the black ice that we normally see, especially on the
northern rivers. While weaker ice is more easily broken up, it is
less prone to prolonged jamming and/or significant flooding. As
with the snowpack, however, the longer that the river ice lingers,
the greater the potential threat of ice jams and flooding. It
would be ideal if the ice was allowed to simply sit and rot in
place. However, given the anticipated cold temperatures, the ice
will likely rot more slowly than what we might normally see. And
with an active weather pattern, the longer the ice sits, the
better the chances that a heavy runoff event will cause the rivers
to rise quickly and thereby "force" the ice to move before it is
in a substantially weakened state. With several positive and
negative factors, the ice jam threat is deemed to be near to
slightly above normal.

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, March 30, 2017.

$$

Hastings



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