Stop Calling Everything an Atmospheric River

The phrase atmospheric river is a relatively new one.  The phrase "tropospheric river" was first used in a paper by Reginald Newell and coauthors in 1992 to describe narrow "filaments" of strong water vapor flux or transport in the atomphsere.  Then in a 1994 paper, Newell and Yong Zhu termed these features atmospheric rivers, a phrase that has caught on and is now widely used not only amongst scientists, but also with the general public.  Hereafter, I will abbreviate atmospheric river to AR for convenience.

In Utah, however, the phrase is being used to describe many situations that do no reach AR criteria, including today.  

ATMOSPHERIC RIVER: Another subtropical train of moisture will move in today. Brunt of the valley rain will be This afternoon into Wednesday morning. Rain totals look healthy and mountain snow totals will be around 1-2 FEET for the Cottonwoods. #utwx pic.twitter.com/XciepWzobZ

— Matthew Johnson (@KSL_Matt) February 20, 2024

ARs are defined and categorized using a variable called integrated vapor transport or IVT.  Think of it as a measure of how much water vapor is moving through a one meter wide curtain that extends from the Earth's surface to very high altitudes every second.  The larger the value, the more water vapor is moving through this curtain.  

Scientists have developed a categorization scale for ARs to help describe their strengths and impacts.  In this scale, the minimum IVT needed for an AR is 250 kg/m/s.  That would be the equivalent of 250 kg of water vapor moving through that curtain every second.  The scale uses both IVT and its duration to categorize atmospheric rivers.  At 250 kg/m/s, an AR would be categorized as either category 1 or category 2, the latter if it were to persist for more than 48 hours.  Impacts are expected to be primarily beneficial (e.g., soaking rains, rain or snow for water resources, etc.) but potentially become hazardous if long duration.  

Source: Ralph et al. (2019)

ARs can have much higher IVTs, in some cases exceeding 1250 kg/m/s.  At these higher levels, ARs are more hazardous, especially if they are long duration.  

ARs reaching northern Utah are much weaker than when they made landfall on the Pacific coast.  This is because they loose a good deal of water vapor as they move inland and produce precipitation, especially over mountain ranges like the Cascades or Sierra Nevada.  Most ARs we observe here are weak, category 1 ARs.  They primarily have beneficial impacts, but they can produce hazards related to flooding and avalanches.

On weather.utah.edu, our four-panel "synoptic" diagnostic includes an analyisis of IVT at lower right.  The GFS forecast for this morning showed weak AR1-level IVT of ≥ 250 kg/m/s on the SoCal coast and near Las Vegas, but elsewhere, IVT was below such levels.  Over northern Utah, in many areas we were below 100 kg/m/s. 

By 0000 UTC 21 Feb (5 PM MST Tuesday) we are still below 250 kg/m/s.  

What we are seeing are the remnants of what was a weak AR on the California coast (it may have reached category 2 for a time while it was moving across central California), but it is no longer an AR. 

My concern about calling these weak features ARs is that it will condition the public to not take them seriously.  Ideally, we should not be using the phrase AR to describe today's weather in northern Utah.  It just doesn't meet the criterion.