An atmospheric river is a long and narrow region in the atmosphere that transports water vapor from the tropics. “Pineapple Express” is the name of a recurring atmospheric river that brings moisture from the tropics around Hawaii to the west coast of North America. On February 4th, 2024, the “Pineapple Express” powered a storm that brought life-threatening wind and flooding. We can look at mid-level water vapor imagery to visually understand the advection of water vapor into southern California. By looking at the grayish-white colors in the imagery below, you can see the narrow river of water vapor that impacted southern California at 6:20 PM local time on February 4th.
Fig. 1: Mid-level tropospheric water vapor satellite imagery GOES-18 valid at 2:20:22 UTC February 5th, 2024, which is 6:20 PM on February 4th PST. The warmer colors represent dryer regions with less water vapor. The cooler colors represent regions with more moisture in blue.
Credit: Colorado State University CIRA RAMMB Slider Page
Below, there is a video of radar imagery that shows California from 11 AM PST on February 3rd to 1 PM PST on February 5th. Radar technology, based on ground stations, emits microwave radiation and measures the power of this radiation scattered by precipitation particles back to the radar. This information can be used to measure precipitation location, movement, and intensity. This method, though highly effective over land, has its limitations over open water due to the lack of radar coverage. In the radar video, the green color signifies lighter precipitation, and the red and deep orange colors signify more intense precipitation. It is clear in Fig. 2 that California was impacted by precipitation for a long period of time. This is almost a two-day loop, and some areas were constantly getting rain.
One thing to note about this video is that the radar imagery does show some “clutter” on the imagery. For example, west of San Jose over the ocean, there are some light blue colors that are not moving or changing in intensity. This is sea clutter, which occurs whenever the radar picks up energy from waves or sea spray, and it was most likely so prominent on the radar at this time because of the high wind speeds that created larger waves and more sea spray. It is important to understand that this is just clutter and not part of the storm, and you may notice some other parts of the video that have low reflectivity (blue colors) and seem to have some of the same characteristics as the sea clutter, just on a smaller scale. This could be ground clutter.
Fig. 2: Radar Imagery from a NEXRAD Level II Radar Station in San Francisco, CA. The imagery is valid from 19 UTC February 3rd, 2024, to 21 UTC February 5th, 2024, which is 11 AM PST February 3rd, 2024, to 1 PM PST February 5th, 2024. The scale is on the right-hand side. The gray and blue colors represent less intense precipitation, and the orange and red colors represent more intense precipitation.
Credit: NWS Bay Area Twitter
When an atmospheric river makes landfall, the water vapor releases into the form of precipitation, so this low-pressure system powered by the “Pineapple Express” brought extreme flooding and wind to California. Most regions in California are not used to the amount of precipitation that was brought by this storm. For example, Los Angeles receives, on average, around 12 inches of rain per year. Below is an image of the observed precipitation from February 4th at 4 AM PST to February 5th at 4 AM PST. Los Angeles received 6.06 inches, which is half of their yearly average in only 24 hours. Between Santa Maria, CA, and Los Angeles, CA, some locations observed around 15 inches of precipitation.
Fig. 3: Observed precipitation in inches from February 4th at 4 AM PST to February 5th at 4 AM PST. The purple colors exhibit the highest amounts, and the light green colors exhibit the lowest amounts.
Credit: National Weather Service Sacramento, California River Forecast Center
This storm brought devastation to California. The event led to the destruction of many properties and, sadly, claimed lives. The rain created landslides and life-threatening flooding, and there were strong winds that created the danger of falling trees and debris. That being said, the forecasting for this event did help prepare communities. Meteorologists and governing officials were able to warn people in advance, and there were some mandatory evacuations that were ordered.