Flood Potential Outlook
Issued by NWS Burlington, VT

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Winter/Spring Flood Potential Outlook
National Weather Service Burlington VT
1131 AM EDT Thu Apr 12 2018

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

This is the eighth 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 snow melt and break up of river ice across
central and northern Vermont and northern New York.

...Overview...

The potential for open water flooding due to snowmelt and/or
rainfall is above normal for all of northern and central Vermont,
and the Adirondacks of northern New York. The threat is near
normal for the Saint Lawrence Valley. The threat is predicated on
the near term threat of heavier precipitation and snowmelt early
next week (Sunday-Monday, April 15-16, 2018). Beyond this period
the open water flood threat is near normal.

The potential for flooding due to ice jams has passed for the
season. Any remaining ice on area rivers is largely rotten, and
confined to areas outside primary channels.

...Snow Depths and Water Equivalent...

In New York State, snow depths and snow water equivalents have
generally decreased during late March and early April, with mainly
bare ground now being observed in the Saint Lawrence and
Champlain Valleys. However, some pockets of significant snowpack
remain across the higher elevations of the Adirondacks. In these
areas snow depths range from 2 to 6 inches in the mountain
valleys, 6 to 10 inches in the 1000-2000 foot range, and from 1
to in excess of 3+ feet above 2000 feet in the High Peaks. In the
lower elevations, the highest concentration of snowpack remains
on more shaded, north-facing slopes. Snow water equivalents range
from nil across the Saint Lawrence and Champlain Valleys, 1 to 3
inches across the lower elevations of the Adirondacks, and 3 to 5+
inches in the High Peaks region above 2500 feet.

In Vermont, snow depths are nil in the Champlain Valley.
Elsewhere, values range from nil to 6 inches below 1000 feet, 6
to 12 inches from 1000-2000 feet, and 1 to 3+ feet above 2000
feet. Similar to northern New York, the highest concentration of
snowpack remains on more shaded, north-facing slopes across lower
elevations. Snow water equivalents are mostly near normal, but
rise to above normal across higher elevations in east and
northeastern portions of the state. Values range from nil in the
Champlain Valley to 2 inches or less in elevations below 1000 feet
elsewhere. Above 1000 feet values generally range from 2 to 4
inches, with locally higher amounts from 3 to in excess of 5+
inches on the higher peaks above 2500 feet.

...River and Soil Conditions...

Soil moisture conditions are near normal across most of northern
New York and central and northern Vermont as cool weather has
allowed a slower snowmelt and reduced evaporation in snow-free
areas.

Taking a look at groundwater monitoring wells across the region,
courtesy of the USGS, we see that groundwater is near to below
normal in Vermont and much of northern New York. Slight recharge
is occurring and is still available in some areas due to leftover
snowpack, primarily across elevated terrain.

River flows across the area are averaging below normal by early
April standards with precipitation generally running below normal
since the middle of March. This has allowed most rivers and
streams to gradually decline over the past few weeks. Runoff from
snowmelt has helped modify the decline somewhat in areas that have
remaining snow cover.

...Weather Outlook...

A large and impactful weather system will affect the region from
April 14 through 17, bringing a wide variety of conditions
including the potential for heavier precipitation and snowmelt.
Current ensemble river forecasts for our area show substantial
rises during the 15th and 16th with some rivers nearing or
exceeding flood. While the amounts and timing of any heavy
precipitation are still uncertain, the potential for flooding is
certainly higher during this period, which will be the time frame
of most concern over the next two weeks.

Beyond this time frame, a continuation of our current pattern
looks to hold in place, with blocked downstream flow across the
North Atlantic into northeastern Canada and a negative North
Atlantic Oscillation. This setup favors cooler air filtering down
into the northeast from southern and eastern Canada and periodic
weather systems impacting the region with showery weather.

The official National Weather Service 6 to 14 day outlook for
April 17 through 25 matches this thinking, calling for cooler and
near to slightly wetter than normal conditions during the period.

...Summary...

Based on the above information, the threat for open water
flooding due to snowmelt and runoff is above normal for all areas
except the Saint Lawrence Valley. The threat is mainly focused in
the near term as significant low pressure affects the area from
April 14-17. Outside of this time frame the overall threat is near
normal.

The threat for ice jam flooding has passed for the season as
rivers no longer have significant ice cover.

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,
April 26, 2018, contingent on the weather and hydrologic
conditions at that time.

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

$$

JMG



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