“Cry me a(n) Atmospheric River” over Southwest California and the Baja

Meteorologists are still in amazement over the heavy rainfall event the beginning of the workweek.

From before sunrise Monday and for the next 24 hours, a very significant “atmospheric river” event brought heavy rain to the western 2/3rds of San Diego County, the Inland Empire, the Riverside County Mountains, and Coachella Valley. Heavy rain extended south of the border with flooding reported in Tijuana and as far south as Ensenada.

Rainfall in San Diego County approached 9 inches at Palomar Mountain, near 6 inches in the Mt. Laguna area, and ranged from 2 to 4 inches across much of the coast and inland valley areas. Riverside County saw totals to around 4 inches in the mountains, 2 to 3 inches Inland Empire. The Palm Springs area even saw rains around 1 inch, an uncommon event.

Down south into the Baja, rainfall amounts were likely in the 2 to 3 inch range.

See CNRFC graphic for US rainfall totals.


Rainfall totals from the Monday February 27 to Tuesday February 28 Rain event (From NOAA/NWS/CNRFC)

This is an example of an event researchers have termed an Atmospheric River. While the term is descriptive, it is overused by the media. Forecasters look for a persistent (more than 24 hours), wide, deep “ribbon” of high atmospheric moisture , especially at the lower levels of the atmosphere. This ribbon of moisture is pulled from the tropics by a approaching weather system and lifted, producing abundant widespread heavy precipitation.

This ribbon can be seen in computer weather forecasts. Shown below is the Global Ensemble Forecast System (GEFS) forecast for water vapor transport for later Monday morning into the afternoon, 2/27. The area in green outlined by red shows where large amounts of tropical moisture are being transported into southern California.


GEFS Model Output valid 1800 UTC (1000 PST) Monday, February 27

Unfortunately this event was not forecast by the computer models well in advance. While some similar events this winter in northern California were evident to forecasters 3 to 5 days (or more) in advance, this event only became evident about 12 hours before it occurred, and was more focused on far southern San Diego County and the Baja.