Flood Potential Outlook
Issued by NWS Burlington, VT

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Winter/Spring Flood Potential Outlook
National Weather Service Burlington VT
341 PM EST Thu Mar 7 2019

...Winter/Spring Flood Potential Outlook...

This is the fifth flood outlook for the 2019 winter/spring
season. Flood outlooks are issued bi-weekly by the National
Weather Service in Burlington, Vermont to summarize the flood
potential due to snowmelt and break up of river ice across central
and northern Vermont and northern New York.

...Overview...

The threat of open water flooding due to snowmelt and rainfall is
above normal across most of eastern, north central and
northeastern Vermont as well as the Adirondack Mountains of New
York. The threat is near normal in the Saint Lawrence and
Champlain Valleys. Much of the threat exists toward the latter
half of the two week period. The threat of flooding due to ice
jams is above normal across the entire region, especially in week
two of the outlook period with numerous ice jams remaining frozen
in place.

...Snow Depths and Water Equivalent...

A decent snowpack still exists across most of the North Country.
Snow depths are generally near to above normal for most of the
higher terrain of the Adirondack and Green Mountains. Values are
near normal for lower elevations of the Saint Lawrence, Champlain
and lower Connecticut River Valleys. The deepest snowpack exists
across the higher terrain of both the Green and Adirondack
Mountains where depths of 4 to 6 feet are common above 3000 ft. A
current snow depth of 108 inches is being reported on top of
Mount Mansfield and similar values are likely on the Adirondack
High Peak summits. Depths decrease to 15 to 35 inches in mid-
slope regions from 1500-2500 feet. The St Lawrence and Champlain
Valleys are generally covered with less than a foot of snow, as
depths average 6 to 10 inches with some slight variability.

Snow water equivalent, or the amount of water contained in the
snowpack is above normal for this time of year in elevated terrain
above 1500 feet. Elsewhere values are closer to early March
seasonal levels. This includes the lower elevations of the Saint
Lawrence, Champlain and lower Connecticut River Valleys where
values of 1 to 4 inches are common. Values increase markedly
across the mid-slope terrain from 1500-2500 feet however. In this
zone equivalents range from 3 to 7 inches in the Adirondacks and
from 5 to 9 inches in Vermont. Even higher values are present in
the high terrain of the Adirondacks and Greens and elevations
above 3000 feet likely contain more than 10 inches of water in the
pack. The highest positive departures exist across elevated
terrain of eastern and northeastern Vermont where values of +150%
are being observed.

...River and Soil Conditions...

An overview of mean river flows across the region show values have
decreased slightly over the last two weeks and are largely near
to slightly above normal, falling generally within the standard
25th to 75th percentile range as we enter the second week of
March. This includes Lake Champlain, which is running about 1 foot
higher above its normal early March level - a decrease from the 2
foot positive departure observed in late February. Ice cover
remains variable across the area with more solid cover existing
across northern watersheds and more variability observed further
south. However, numerous ice jams remain in place with more
notable locations including the Missisquoi River in Richford and
the Stevens Branch in Barre. Additional jams remain in place on
the Winooski River at Richmond and near Middlesex, the East Branch
of the Ausable River in Upper Jay and Keene, and the Oswegatchie
River near Ogdensburg. Where solid ice cover still exists, it is
estimated to be 8 to 16 inches thick.

The active precipitation pattern has kept antecedent soil
moisture states quite high across our region. In fact, there
remains little to no ground frost in middle and higher elevations
where a deep snowpack was built up quite early last November and
December. Recent snow core data suggest surface layers remain very
moist to wet in these areas. Ground frost is more prevalent in
lower elevations and the broader Champlain and St. Lawrence
Valleys where periodic thaws and snowmelt have allowed winter cold
to penetrate into the near surface soil layers.

...Weather Outlook...

The weather pattern has trended more stable over the past 2 weeks
with modified polar airmasses bringing seasonably cold weather
and only light, scattered precipitation mainly in the form of
snow showers. However, consensus among more reliable weather
models show the pattern trending more active through the middle of
March. Two discrete larger-scale systems appear on track to
affect the area, the first occurring during March 10-11 and a
potentially more impactful event occurring by March 14-16. The
first system will likely bring a period of light snows or rains to
the region along with milder temperatures. Excessive warmth is
not expected with this system however, and in a large sense the
threat of rapid snowmelt, river rises and/or river ice breakup
remains minimal at this time. Looking further out confidence
continues to grow that a larger-scale warm-up and associated
rainfall will affect the region during the second event from March
14-16. While still too far out to hone in on specifics, this
would be a window during which potential sharper river rises due
to snowmelt and/or ice break-up would be possible.

The official National Weather Service 6-14 day outlook from 11-19
March 2019 generally agrees with this analysis indicating broadly
above normal temperatures and chances for above normal
precipitation.

...Summary...

Based on the above information, the threat for open water
flooding due to runoff from snowmelt and rainfall is near normal
in the Champlain and Saint Lawrence River Valleys. It is above
normal elsewhere. Snow depths and the water equivalent contained
in the pack are near to slightly above normal, with highest
departures across eastern and northeastern Vermont. River flows
are near to slightly above normal, and antecedent soil moisture
states are above normal at most reporting sites. While the overall
concern for rapid snowmelt and flooding is rather low in the
immediate near term (through March 13) confidence is growing that
the potential for significant thawing and snowmelt may occur by
week two of the outlook period (March 14-16).

The threat for flooding due to ice jams is above normal across
the entire region with focus mainly on the latter half of the
outlook period from 13 March onward. Although there is a slight
chance of some ice movement in the near term, the jams that are
currently in place will likely remain there until mid-March.
Thereafter, concern increases as larger scale thawing conditions
and potential rainfall may act to increase river flows by week two
of the outlook period.

It is important to note that heavy rainfall can result in
flooding at any time of the year, even in areas that have little
or no snow on the ground.

The next Winter/Spring Flood Outlook will be issued on Thursday,
March 21, 2019.

Access current weather conditions and forecasts on our web site
at www.weather.gov/btv.

$$

JMG/WFO BTV


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