Much needed rain in California

An upper-level low pressure system is slowly moving across California and generating scattered areas of showers and thunderstorms in parts of the state.  Rainfall overall has not been heavy or widespread, but any rain is beneficial after a very dry winter.  Showers have been most extensive over southern California, the Sierra Nevada, and parts of the Central Valley.  Downtown Los Angeles received over 2/3 of an inch of rain on Monday - this is actually double the normal amount of rain they receive during the entire month of May.  One storm up in Lassen County produced 65 mph winds.  Localized heavy rain also fell in the Sacramento region, with estimates of over an inch in some areas as the thunderstorms are moving very slowly through the region.  Much of the Bay Area, however, has seen little if any rain out of this system.

Counter-clockwise flow around the upper-level low has resulted in warm, offshore winds in the Pacific Northwest.  Similar to the Bay Area, the marine layer offshore the Pacific Northwest coast plays a huge role in determining temperatures.  Surface winds there are typically onshore, which results in relatively cool temperatures and occasional low clouds and fog, especially along the coast.  However, when offshore flow develops, the Pacific Northwest can really heat up, even out to the coast.  Seattle reached 87º on Monday, smashing the old record high of 79º for the date.  Seattle was also warmer than almost all other areas of the country on Monday, including all of Florida, Hawaii, and Texas - only Phoenix was as warm as Seattle today.  Quite a rare and impressive feat. 

This upper-level low is now gradually moving east and will move into Nevada on Tuesday.  The system will weaken as it crosses the Rockies but will help to generate thunderstorms, possibly severe, in the southern Plains states by Wednesday and Thursday.  The set-up will not be ideal since the upper-level support with this system will be weakening, but the combination of increasingly humid air moving northwestward from the Gulf of Mexico and considerably drier air moving eastward out of New Mexico will set the stage for intense thunderstorms in western Texas, Oklahoma, and Kansas.  The cooler temperatures aloft associated with the weakening upper-level low will help to destabilize the atmosphere and promote convection.

The upper-level low that brought record cold to the South is still spinning over the region.  The system is now pulling in moisture off the Atlantic and Gulf, resulting in very heavy rain in the southern Appalachians.  The image on the right shows precipitation from yesterday (using ground observations and radar estimates).  Some areas in western North Carolina received several inches of rain, resulting in flooding concerns.  The upper low is gradually shifting northeastward, putting Virginia and the Mid-Atlantic states at risk for heavy rain today.  The east facing slopes of the Appalachians will be at greatest risk as east-southeasterly upslope flow will locally enhance rainfall rates.