“Pineapple Express” is another name for a specific and reoccurring type of atmospheric river that transports water vapor out of the tropics near Hawaii, and dumps it over the west coast of North America. The “Pineapple Express” made landfall on Sunday, February 4th, 2024, causing extreme flooding and mudslides. As the week progressed into February 6th, this system moved south, still producing lots of rain, thunderstorms, and even tornado watches for San Diego.
As the system continued throughout the week, there was still lots of strong convection and upward vertical motion, causing storm activity. This convection can be seen in Figure 1, which is the GOES-18 Cloud Top Phase Satellite, taken on February 6th, 2024 at 08:00 UTC. The brighter colors correlate with colder temperatures, and since the troposphere cools while going up in the atmosphere, the colder temperatures correlate to higher cloud tops. Lots of southern California, including the San Diego area, still had high cloud tops and storms present at that time.
Figure 1: GOES-18 Cloud Top Phase Satellite Imagery valid at 6 Feb 2024 08:00 UTC (12 am PST). The brighter colors represent cooler temperatures, and thus, higher cloud tops. Credit: Colorado State University CIRA RAMMB Slider.
There was also tons of moisture in the clouds in southern California. As the system moved down to San Diego, the water vapor turned into precipitation which then poured on the land, causing flooding. This water vapor is seen in Figure 3, which is the GOES-18 Mid-Level Tropospheric Water Vapor Satellite Imagery valid for 6 Feb 2024 at 8 UTC (12 am PST). The white and blue colors are where water vapor is present. We can also see how moist the air was in the atmosphere, specifically in San Diego, in Figure 4. Figure 4 is a sounding analysis where the y-axis is pressure in millibars and the skewed x-axis is the temperature in Celsius. The red line is the air temperature and the green line is the dew point temperature. The closer the dew point temperature line is to the temperature line, the more moisture there is in the air. These two lines are very close to a significant portion of the atmosphere, which highlights how moisturized the air is.
Figure 3: GOES-18 Mid-Level Tropospheric Water Vapor Satellite Imagery valid for 6 Feb 2024 8 UTC (12 am PST). The warmer colors are associated with dry air, and the white and cooler colors are associated with moist air where water vapor is present. Credit: Colorado State Univeristy CIRA RAMMB Slider.
Figure 4: Sounding Analysis for NKX (San Diego) valid for 6 Feb 2024 12:00 UTC. Credit: NOAA/NWS Storm Prediction Center
Although the days following the atmospheric river hitting landfall did not result in significantly as much rain, there were still a few inches and lots of thunderstorms. The warm, moist air, as well as the location of the upper-level trough, caused lots of upward vertical motion and storms. Only a week before, San Diego had just gotten a few inches of rain, so with the additional rain from the “Pineapple Express” and the city not having great flood drainage systems, these additional inches of rain caused serious flooding and mudslides.