South San Francisco, CA

Weather Underground Forecast for Monday, May 20, 2013.

Heavy rains and severe thunderstorm activity will continue for areas of the Plains into the Midwest on Monday as a broad, nearly stationary low sits over the Northern Plains. As the system lingers, ample moisture and energy will stream northward from the Gulf of Mexico, while an associated cold front extends from the low into Iowa and back across the Central and Southern Plains. The sharpening cold front combined with a dry line in the Southern Plains and the moisture laden conditions of the region will allow for multiple clusters of thunderstorms to develop across the Plains and into the Mid-Mississippi Valley. Portions of Oklahoma, southeastern Kansas, extreme, northwestern Arkansas, an southwestern Missouri are at moderate risk of severe thunderstorm activity as environmental conditions remain favorable for strong tornadoes, damaging wind gusts, and very large hail. Areas surrounding this moderate risk region from the parts of the southwestern Texas into the Upper Great Lakes are at slight risk of severe thunderstorm development with hail and isolated tornadoes.

Meanwhile, areas of heavy rain and widely scattered showers will persist near the low and it's associated warm front in portions of the north-central U.S. Continued precipitation in these already saturate areas combined with rapid runoff will continue flood concerns from parts of North Dakota into northwestern Wisconsin.

Elsewhere, scattered showers and thunderstorms are expected in the Mid-Atlantic and southeastern corner of the nation as a weak upper trough of low pressure drops southward over the region. Locally heavy rain amounts of up to 2 inches may result in areas of minor flooding, especially in poor drainage areas. To the north, additional showers will be possible in the Northeast.