Flood Potential Outlook
Issued by NWS Burlington, VT

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Winter/Spring Flood Potential Outlook
National Weather Service Burlington VT
440 PM EST Thu Feb 15 2018

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

This is the fourth flood outlook for the 2018 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 is near to
above normal, though the threat for localized flooding due to ice
jams remains above normal.

...Snow Depths and Water Equivalent...

Snow depths and water equivalents have increased across Northern
New York and Vermont over the last two weeks. Snow depths remain
near to below normal for mid-February. Snow depths in the
Adirondacks range from 1 to 2 feet, and increase to near 3 feet
above 2500 feet. In the Saint Lawrence valley snow depths are
between 6 to 12 inches. In the deeper valleys in Vermont,
including the Champlain valley, snow depths were between 6 to 12
inches, lower in some spots. Across the remainder of Vermont 1 to
2 feet snow depths are common, with 30 inches of snow observed in
Granby Vermont. These values in Vermont are near to below normal.

Snow water equivalents, or the amount of water in the snowpack
was also below normal across most of the region. Snow water
equivalents in the Adirondacks range from 2 to 4 inches in the
valleys to between 4 and 8 inches in the higher terrain. Around 1
to 3 inches of water equivalent was observed in the Saint
Lawrence valley. In the Champlain valley in Vermont, there was
about 1 to 3 inches of water in the snow. These amounts increase
to between 3 and 6 inches across the remainder of the state with
localized amounts of 7 to 8 inches in the highest terrain in
Vermont. These values are also below seasonal normals for the
middle of February.

...River and Soil Conditions...

Streamflows and ground moisture levels were still near to above
normal across the area. The latest Palmer Drought Severity Index
from 10 February 2018, which looks at moisture indices over weeks
to months shows virtually all of northern New York and Vermont as
unusually moist to very moist. Lake Champlain readings are much
closer to normal February levels after receding some over the past
2 weeks.

River flows across the North country are running mostly above
normal for mid-February. River ice remains an extreme concern
across Northern New York and Northern Vermont. Many of the ice
jams that formed in mid-January remain frozen in place. Across
the area, 10 to 20 inch ice thicknesses are common.

The most serious ice jam situations include:

In Vermont...

The Missisquoi River in the village of Swanton.

The Winooski River near Middlesex.

The Lamoille River near Johnson.

In New York...

The Great Chazy River near Perry Mills.

The Ausable River in Au Sable Forks.

The St. Regis River near Akwesasne.

These ice jams remain a serious situation going forward toward the
breakup season and we could see some ice movement over the next 2
weeks.

...Weather Outlook...

Over the next week to 10 days, we are seeing strong signals for a
pattern shift. There will be a shift west in the storm track,
bringing mild air to much of Northern New York and Vermont. The
shift in the storm track west would favor heavier precipitation
migrating inland from the coast towards the Great Lakes.
Therefore, apart from a quick hitting snow event across the region
this upcoming weekend, a warmer and wetter than normal pattern
will emerge going into late February. Above normal temperatures
and above normal precipitation are forecasted.

...Summary...

Based on the above information, the threat for open water flooding
due to snowmelt and runoff is above normal. River flows are at or
above normal, and antecedent moisture states are wet. There is
fairly high confidence for a significant warm up with rain next
week across the region, snow will be melting, and increasing river
flows are anticipated. The determining factor will be amount of
rainfall that we have. All of these parameters point to a higher
than normal flood potential through late February. The flood
potential is closer to normal across Eastern and Southern Vermont,
due to uncertainty on how much warm air and rainfall actually gets
East of the Greens.

The potential for flooding due to ice jams remains above normal
and could potentially become serious later this winter or spring
across Northern New York and Northern Vermont. In these areas,
serious ice jams continue to clog many rivers and although
flooding has subsided, the warmer temperatures and rainfall
forecast over the next week to 10 days suggests ice could begin to
move. Until the ice moves out, any large scale thaw or runoff
event could pose a serious flood potential.

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 1, 2018.

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

$$

Neiles



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