CW3E Event Summary: 18-20 February 2024

27 February 2024

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Low-Pressure System and Atmospheric River Produce Heavy Rain and Snow in CA

  • An atmospheric river (AR) associated with a slow-moving area of low pressure brought widespread precipitation to California during 18–20 Feb.

The AR:

  • A deepening mid-level trough off the US West Coast interacted with a remnant plume of subtropical moisture, leading to an AR landfall over California on 18 Feb.
  • AR1–2 conditions (based on the Ralph et al. 2019 AR Scale) were observed in coastal Northern and Central California.
  • As the eastward progression of the trough stalled and the synoptic-scale flow became more southerly, the AR briefly re-intensified over Southern California, prolonging precipitation over the Transverse Ranges.


  • The heaviest precipitation occurred in the western Transverse Ranges, with more than 10 inches in some locations.
  • At least 1–3 feet of snow fell in the Sierra Nevada, with higher amounts in the vicinity of Lassen Peak.
  • Heavy rain falling on moist soils caused minor riverine flooding in the Sacramento Valley.
  • Flooding and mudslides closed portions of US-101, SR-1, SR-33, and SR-150 in Ventura and Los Angeles Counties.
  • Portions of coastal Southern California have received more than 75% of their normal total annual precipitation during the first 3 weeks of February.
  • Unusually cool and wet conditions during the month of February have facilitated a dramatic improvement in snowpack conditions throughout the state.

Click images to see loops of West-WRF IVT/IWV analyses and forecasts

Valid: 0000 UTC 18 February – 0000 UTC 21 February 2024












Summary provided by C. Castellano, S. Bartlett, P. Iniguez, and S. Roj; 27 Feb 2024

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