Spring Flood Risks: NOAA Finds Low Threat For Western Washington

SEATTLE —The National Weather Service has released its flood outlook for spring and early summer, finding a relatively low risk for Western Washington, despite the above-normal snowpack.

This season's projections largely fall in the normal range for the Evergreen State, apart from a slightly elevated threat for a few areas east of the Cascades.

Forecasters said floods resulting from mountain runoff are generally unlikely, even during the peak months for snowpack melt between April and June. According to the National Weather Service, rivers west of the Cascades usually record their highest flows during winter, resulting from heavy rain and other winter weather. Historically, almost all major floods have occurred between November and March.

This year, Washington's snowpack sits well above 100 percent in most regions, bolstered by winter storms earlier in the year. However, forecasters note runoff from melting snow has a relatively small impact on local rivers, even in years with considerable snowfall followed by unusually high spring temperatures.

However, there are a few factors that can lend to exceptions on more flood-prone waterways.

The National Weather Service writes:

"While flood-producing rainfall is not common after March, heavy, or even moderate rain in spring, while rivers are swollen with snowmelt runoff, occasionally will drive the most flood-prone rivers above minor flood stage. Typically these are rivers such as the Skokomish and Snoqualmie Rivers. Heavy rain in summer, when Ross Lake is full, can also cause the Skagit River to flood. While these floods are minor compared to the winter events, they sometimes cause substantial damage to farm crops since the flood plains are often in use during the spring and summer."

The situation is a bit different east of the Cascades, where many rivers reach peak flow from late spring to early summer and tend to crest between mid-May and mid-July, forecasters said.

"As a general rule the larger the snowpack is at the end of the season, the higher the crests will be. When spring snowmelt flooding does occur, it is usually the combination of a much greater than normal snowpack that experiences a rapid warm up of hot temperatures. Flooding during the snowmelt season can occur anywhere when heavy rain falls in a river basin if the rain is intense enough. Typically this can occur with thunderstorms. This is especially true if the rain falls during a time when the streams are swollen with snowmelt runoff."

Read more about the hydrologic outlook and find forecasts for specific rivers on the NOAA website.