Flood Potential Outlook
Issued by NWS Burlington, VT

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Winter/Spring Flood Potential Outlook
National Weather Service Burlington VT
1228 PM EDT Thu Apr 16 2020

...Spring Flood Potential Outlook...

This is the eighth and final flood outlook for the 2020
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. This outlook
is valid for the two week period of April 16 to April 30, 2020.

...Overview...

The threat for open water flooding is near normal across the
hydrologic service area for the whole of the two week outlook
period. The threat for flooding due to ice jams has ended for all
basins across Vermont and northern New York.

...Snow Depths and Water Equivalent...

With consistent milder temperatures during the first half of April
and modest rainfall events on the 9th and the 13th, snow cover is
now largely confined to higher elevations, mainly above 2000-2500
feet. Lower elevations, including the Saint Lawrence, Champlain
and Connecticut River Valleys are void of any significant snow.
Recent analysis from cooperative snow observers and National
Operational Hydrologic Remote Sensing Center (NOHRSC) data show
snow depths in general range from 2 to 6 inches in elevations
between 2000 and 2500 feet. This is mainly across north central
and northeastern Vermont as well as portions of the northern
Adirondacks. Snow water equivalents in this zone generally average
an inch or less. Large variability also exists within this
elevational band with shaded northerly slopes showing totals near
the higher end of this range with sunlit southerly slopes having
significantly less. Some bare ground is noted at a few sites.
Above 2500 feet more significant snow cover and snow water
equivalents remain, especially near summit levels above 3500 feet.
However these amounts cover such a small portion of area
watersheds the threat posed to future flooding is basically
negligible.

...River and Soil Conditions...

Current river flows are averaging near normal across most of
northern New York and Vermont which is supported by seven-day
average streamflow data from the United States Geological Survey.
Values on the higher end of this normal range are mostly
attributed to runoff from lingering high elevation snowmelt,
warming temperatures and regular episodes of light to moderate
precipitation.

River ice is non-existent, with any lingering ice in far northern
watersheds having flushed out within the past two weeks.

Near surface soil moisture states are averaging normal to wetter
than normal depending on location. This is due to frequent bouts
of precipitation during the first half of April along with areas
of high elevation snowmelt. Across south central Vermont soil
moisture states lean more closely to seasonal normals with less
precipitation being observed over the past month. Recent rainfall
events over the past month in this area have helped alleviate
longer term dryness.

Groundwater levels mostly fall within the normal to above normal
range across most of northern New York and Vermont as we head
into the second half of April. The only exceptions are in the St.
Lawrence Valley and portions of south central Vermont where values
are slightly below normal. The Groundwater monitoring is courtesy
of the United States Geological Survey (USGS).

...Weather Outlook...

As we head deeper into spring, the weather pattern has shifted
decidedly from the one that was present much of the winter. This
will result in a rather defined cooldown across the area. The
North Atlantic Oscillation (NAO), which was largely positive this
past winter, abruptly switched phase to negative in late March and
may stay neutral to negative as we head through the remainder of
April. With a negative NAO pattern, an upper level blocking
pattern frequently sets up in the vicinity of Greenland which
serves to shunt cold air from eastern Canada south into the
northeastern United States. This results in a below normal
temperature pattern. So far in April, that has not held true as
temperatures have generally averaged 1 to 3 degrees above normal.
That trend is expected to change through the remainder of the
month, however, as NOAA`s Climate Prediction Center (CPC) calls
for temperatures to average below normal right through the end of
April.

As we head through the remainder of April there are also signs
that the Pacific North America (PNA) pattern could turn positive.
With a positive PNA, upper level troughing is favored across the
eastern United States which can lead to coastal storm development.
The challenging forecast problem through the end of April will be
whether any of these coastal storms ride far enough north to
affect all of New England or if they are suppressed to the south
by the downstream blocking pattern associated with the negative
NAO. Currently, medium range numerical weather model guidance
would favor southern and eastern New England for the heaviest
precipitation through the end of the month although the CPC does
call for precipitation to average above normal across all of New
England and New York state right through the end of April.

...Summary...

In summary the winter/spring flood potential is near normal for
entire NWS Burlington Hydrologic Service Area for the remainder of
April 2020. While near-surface ground moisture states are largely
above normal, most streamflows are near normal across the area
with snow cover largely gone below 2500 feet. River ice cover is
also gone for the season. While somewhat cooler than normal
temperatures are forecast on average over the next few weeks no
large-scale heavy precipitation events appear on the horizon at
this point.

It is important to remember 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.

This is the last Winter/Spring Flood Outlook for 2020. Outlooks
will resume in January 2021.

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

$$

JMG


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