Atmospheric Rivers
Water vapor transport model 4-6-2018 (credit, Center for Western Weather and Water Extremes)
California's winter rainy season is short but atmospheric rivers, commonly called a Pineapple Express, can hit the coast like the rainfall events of 2018. The National Atmospheric and Oceanic Administration (NOAA) released a radar image of the entire weather system stretching from Hawai'i and affecting the length of the entire coastline from California to Baja.
Atmospheric River TPW, April 6, 2018 (credit: NOAA)
According to NOAA: the image shows total precipitable water (TPW) that is being carried to the West Coast. The Agency's scientists used data from the Global Forecast System to run a predictive model for the entire coast. "Precipitable water" is an estimate of how much moisture would be present in the atmosphere if all of the water vapor in the air column above you were to fall as rain. The darker blue shades show where the atmosphere was saturated in moisture, while gray-black areas are regions where the atmosphere was dry.
The atmosphere above the tropical Pacific always carries moisture but a narrow bridge connects the tropics to the West Coast. Such "bridges" become rivers of moisture when the air flows from the region around Hawaii directly towards the West Coast. Rainfall totals varied with the Sierra Nevada mountains receiving additional snow while the charred landscapes created by the Thomas Fire were luckily sparred heavy rains and flooding. Yosemite National Park was closed due to the potential for major floods in the world famous valley.
The University of California has created a video about atmospheric rivers and their importance to the State and the West Coast in general.
An important question remains as to how climate change enhances or reduces these powerful and important weather events. NOAA is the prime research agency work with NASA and the US Geological Service to answer this critical climate change question. The next winter rainfall season along the West Coast begins soon.
WHB