Flooding No Longer a Question, the Question is Where

STATE COLLEGE – reports the monster snowstorms during the winter of 2009-10 have resulted in a tremendous snow pack over much of the Northeast and the northern and central Plains.

It appears the stormy pattern will continue for the next several weeks. As a result, there is no longer a question of whether or not there will be flooding. The question is when and where the flooding will be.

The flooding situation through the spring will depend on the track and strength of upcoming storms.

The area that currently has more than a foot of snow on the ground is immense. However, the depth of the snow, though important, is not the only factor when considering flooding. A significant portion of the snow over West Virginia, southwestern Pennsylvania, New York state and western and northern New England is holding from 4 to 8 inches of water.
The situation over the northern Plains is not any better. Precipitation during the winter has been three times more than average over a large part of the area. A large part of that fell in the form of snow and remains locked up in the snow pack.
Minor Flooding Imminent
Even the expected controlled thaw without rainfall this weekend will be enough to cause urban and farm field flooding, as well as impressive rises on streams and rivers in the Northeast and much of the Midwest.
Nighttime freezing cycles during thaw will slow the meltdown to some extent.
Unfortunately, there appears to be a rainstorm coming during the first half of the next week from the central Plains to the Northeast.
Added rainfall from this storm on top of surging stream and river levels could be enough to cause minor flooding along some of the major rivers in the Northeast alone, including the Connecticut, Delaware, Mohawk, Monongahela, Susquehanna and their tributaries.
The amount of rain that falls from the storm holds the key. A few tenths of an inch with temperatures in the 30s and 40s will translate to few problems. An inch or two of rain with warm winds would be a much more serious threat.
A return of cold air and longer periods of freezing temperatures will slow the runoff during mid- and late-March.
Potential for Major Flooding Later
Winter is not over yet, according to Long Range Expert Joe Bastardi. He expects a resurgence of cold air, following the thaw fueling up to several additional snowstorms in several weeks over the Central and Northern states.

New snow and/or remaining snow will raise flooding concerns all over again.

As spring and thaw progress, the frozen, saturated ground in many areas will release its water, while rainfall and snow melt further add to the runoff.
With the ongoing potential for snowstorms and lingering cold air in the spring, comes an increasing risk of the “rapid” meltdown. One such event occurred in January 1996 over the interior Northeast, as several feet of snow vanished in hours during a heavy rain with warm, moist air.
As if flooding concerns in the North were not enough, heavy precipitation this winter has left the ground saturated in many areas of the South.
Mike Smith of WeatherData Services, Inc., an AccuWeather company, stated that the saturated ground, combined with Bastardi’s expected above-average rainfall in March on the southern Plains and in April over the southern Plains and the Southeast, puts many of the southern rivers at risk for flooding as well.
Given all of the snow and the rain from this winter over the eastern two-thirds of the nation, while the winter of 2009-10 has become known as the winter of monster storms, the spring of 2010 could be remembered for the spring of monster floods.

Planning and preparing for the major flooding potential later this spring should be high on the list of priorities for communities along rivers, including the Ohio, Mississippi, Missouri, Red (of the Dakotas) and many others, including those mentioned earlier.

A Glimmer of Hope
While flooding worries loom for a large part of the nation, Bastardi expects a lower-than-average number of severe thunderstorms (and tornadoes) through March and perhaps the first part of April. He added a diminishing El Nino and colder-than-average Gulf of Mexico waters should shut off some of the fuel for the violent thunderstorms.
Joe pointed out that while the risk of flooding is high in the North this spring, there was a winter with excessive snowfall that was followed by no serious flooding problems: the winter/spring of 1978 in the Northeast.
Story by Senior Expert Meteorologist Alex Sosnowski
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