Guest blog by Alexandre Ramos, Instituto Dom Luiz, Portugal

An Atmospheric River (AR) is a long, narrow, and transient corridor of strong horizontal water vapour transport that is typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone. The water vapour in ARs is supplied by tropical and/or extratropical moisture sources. ARs frequently lead to heavy precipitation where they are forced upward — for example, by mountains or by ascent in the warm conveyor belt. Horizontal water vapour transport in the mid-latitudes occurs primarily in ARs and is concentrated in the lower troposphere (Ralph et al., 2018). Impacts of ARs are usually associated not only with heavy precipitation and floods but also with wind extremes.

Example of ARs crossing the North Atlantic Ocean (MIMIC Total Precipitable Water)

ARs frequently contribute to compound events, in particular when heavy precipitation occurs together or in close succession with other natural hazards. Common compound events associated with ARs are Rain-on-snow events, Rain after fire, and sequences of ARs:

  1. Rain on snow: A rain-on-snow flood occurred in the Bernese Alps, Switzerland, on October 10th 2011 and caused significant damage due to the peak flood. The atmospheric drivers of this rain-on-snow flood were (i) sustained snowfall followed by (ii) the passage of an AR bringing warm and moist air towards the Alps. As a result, intensive rainfall (average of 100 mm/day) was accompanied by a temperature increase that shifted the 0° line from 1500 to 3200 m a.s.l. in 24 h with a maximum increase of 9 K in 9 h (Rössler et al., 2014).
  2. Rain on post-fire regions: The Thomas Fire burned 114 078 ha in Santa Barbara and Ventura counties, southern California, USA, during December 2017–January 2018. On January 9th 2018, high-intensity rainfall occurred over the Thomas Fire burnt area in the mountains above the communities of Montecito and Carpinteria, initiating multiple devastating debris flows. The highest rainfall intensities occurred with the passage of an AR where orographic enhancement played a role in intensifying precipitation (Oakley et al., 2018). A climatological study for “The Transverse Ranges” of southern California, USA, showed that, from a compilation of 93 post-fire debris flow events during 1980–2014 triggered by 19 precipitation events, ARs are a dominant feature, observed in 13 out of the 19 events (Oakley et al., 2017).
  3. Sequences of ARs: Families of ARs (Fish et al., 2019) usually occur in association with mesoscale frontal waves or back-to-back storms that occur within a short duration of time (e.g., <5–7 days). Whiles an AR may only produce beneficial and marginal high-impact weather, a pair or series is capable of producing more severe impacts (e.g., flooding, landslides) owing to preconditioning of hydrological conditions. An example of this was the 2016–2017 winter season in California where the events of January and February 2017 resulted in year-to-date precipitation totals greater than 150–200% above normal for many Northern California locations. One of the most recognized impacts, largely covered by the media, was Lake Oroville’s Dam Stress.


Fish, M.A., Wilson, A.M., Ralph F.M. (2019) Atmospheric River Families: Definition and Associated Synoptic Conditions. J. Hydromet., in review.

Oakley, N. S., Lancaster, J. T., Kaplan, M. L., and Ralph, F. M. (2017) Synoptic conditions associated with cool season post-fire debris flows in the Transverse Ranges of southern California, Nat. Hazards, 88, 327–354.

Oakley, N. S., Cannon, F., Munroe, R., Lancaster, J. T., Gomberg, D., and Ralph, F. M. (2018) Brief communication: Meteorological and climatological conditions associated with the 9 January 2018 post-fire debris flows in Montecito and Carpinteria, California, USA, Nat. Hazards Earth Syst. Sci., 18, 3037-3043.

Ralph, F.M., Dettinger, M.D, Cairns, M.M. , Galarneau, T.J.  and Eylander, J. (2018) Defining “Atmospheric River”: How the Glossary of Meteorology Helped Resolve a Debate. BAMS, 99, 837-839.

Rössler, O., Froidevaux, P., Börst, U., Rickli, R., Martius, O., and Weingartner, R. (2014) Retrospective analysis of a nonforecasted rain-on-snow flood in the Alps – a matter of model limitations or unpredictable nature?, Hydrol. Earth Syst. Sci., 18, 2265-2285.