San Francisco Bay Area Faces Flood Concern due to Atmospheric River (Credit: San Jose Mercury News)

DISCUSSION: A major storm is expected to hit overnight tonight through Thursday night across the San Francisco Bay Area. The storm is expected to bring heavy rains in mountainous regions especially across the North Bay as well as the Santa Cruz Mountains. Flood concerns are very high in the North Bay due to the amount of precipitation being forecasted. In addition, the South Bay’s Coyote Creek will also be monitored due to the creek flooding in February which caused heavy damage in parts of San Jose.  
The North Bay is expected to receive between 2 and 3 inches of rain overnight from the storm.  The event is bringing flood concerns along the Napa and Russian Rivers especially in cities such as Napa, Santa Rosa, Petaluma, and Rohnert Park. These floods along the rivers are a common occurrence during heavy rain events in the Bay Area. In addition to concern of river flooding, flooding is also a concern especially in the areas that were devastated by the multiple wildfires that were burning almost a month ago. These areas are suspect to flooding due to soil conditions as well as a lack of plant life to take some of the water. The National Weather Service office in Monterey has issued a Flash Flood watch for much of the North Bay due to the concerns especially in the burnt areas.
In addition, the South Bay’s Coyote Creek will be monitored during the situation especially in San Jose. However, the South Bay is not expected to receive as much rain in the flooding area where forecast rainfall is expected to between 1 and 2 inches in the Hamilton Range mountains and an inch or less forecasted in the valleys. However, those living along Coyote Creek are bracing for a possible flood such as placing sandbags just in case more rain occurs than expected.
The storm is expected to bring heavy rain due to an Atmospheric River. An Atmospheric River is an event where moisture is drawn in from the warmer parts of the Pacific especially near Hawaii and makes its way to California when there is a deep trough in the Pacific Northwest.  In addition, vertical velocity with respect to pressure are forecasted to be at about 0.1 Pa/s (pascals per second) or higher in an upward motion. This upward motion makes the atmosphere more saturated at a higher level to the point of complete saturation. At which point, the atmosphere is unable to hold any more water which is then released as rain. 
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©2017 Meteorologist JP Kalb