Thursday, April 5, 2018

What Happens to Atmospheric Rivers during Inland Penetration?

As we have discussed the last couple of posts, a major atmospheric river event will bring significant precipitation, primarily in the form of rain except in the highest elevations, to California tonight through Saturday, but what will happen to this system as it moves inland to Utah?

All weather systems are substantially modified as they approach and move across the complex terrain of the western United States.  In the case of atmospheric rivers, their landfall frequency during the cool season (November to April) is greatest along the Pacific Northwest coast and declines as one moves southward along the California coast.  This reflects the more active storm track over the Pacific Northwest.  
Source: Rutz et al. (2014)
The frequency of atmospheric rivers declines precipitously, however, as one moves inland.  Protrusions of higher frequency exist over the Columbia Basin and Idaho Panhandle and the Snake River Plain.  In contrast, a minimum in atmospheric river frequency exists east of the southern "high" Sierra.  Although the atmospheric river frequency is low over the southwest, the decline as one moves inland is small, indicating that atmospheric rivers making landfall on the southern California or Baja California coast frequently penetrate well inland.  

Another perspective on this is provided if we launch a low-level trajectory at the coast whenever an atmospheric river makes landfall and examine the characteristics of those trajectories that remain within an atmospheric river all the way into the western U.S. interior.  This allows us to see preferred pathways for atmospheric river penetration into th western interior and there are two of them.  The first is across the Pacific Northwest coast, Columbia Basin, Idaho Panhandle, and northern Montana.  The second is across Baja California into Arizona.  If you look carefully, you can see that the Snake River Plain is also a preferred corridor for atmospheric river penetration, although it is weak.  In contrast, it is atmospheric rivers rarely survive transit across the southern high Sierra.  

Source: Rutz et al. 2015
Nearly all atmospheric rivers weaken with inland penetration.  If you want one to survive deep into the interior, it's best to have very large integrated vapor transport values at the coast and for the atmospheric river pathway to avoid the southern high Sierra.  The preferred pathways for atmospheric river penetration are located in regions where the topography is relatively modest, which results in less precipitation and water vapor depletion.

These effects can be readily seen in forecasts for the next three days.  The GFS forecast total integrated vapor transport during that 72 hour period is strongest in a narrow corridor over the eastern Pacific, but then declines rapidly near the coast and as one moves inland.  The effects of the southern high Sierra are somewhat subdued in this forecast, but the GFS poorly resolves that high-mountain barrier.  

Source: CW3E
Total precipitation produced by the GFS is greatest in the coastal mountains of northern California, the northern Sierra Nevada, and the southern Sierra Nevada.  It is not coincidental that this is also where the total IVT for the period declines precipitously.  Water vapor in the atmospheric river is being converted to rainfall and depleted. 

Source: Tropical Tidbits
This weakens the atmospheric river as it moves inland.  Nevertheless, it survives in a weakened state and we will see some "leftovers" from this event, starting with a little appetizer in the form of low- and mid-elevation rain showers and upper elevation snow showers tonight.  the warm and cold fronts affect things Friday night and Saturday.  If time permits, I'll take a look at those tomorrow.  


  1. If you get time for a post tomorrow, you will have at least one reader, me.

    Meantime, now that the NAM12 has come into range, this looks to be a truly miserable Saturday. Baldy (11,000 ft) temp maxs at 36 F, from 11am to 3pm, qpf at Alta-Collins (or thereabouts) increases from 0.6 inches 4am to 1.34 inches at 7pm, but snowfall stays at 4 inches.

    3-quarter inches of rain on 4 inches of snow.

    Cold front progged to cross Saturday afternoon with Baldy temps falling to 9 F Sunday morning, unfortunately after all the moisture is gone.

    Sunday could be the worst skiing of the season. Saturday's warm-up puts wet roller balls everywhere which get rained on and then freeze in place.

    The stuff character is made of.

    Curious to see if the NAM12 verifies.

    On the optimistic side, the GFS has more moisture with the cold front so we get maybe 8 inches on top of the frozen roller balls.

  2. For April a saturated 700-mb temp of +4 C, well represented in this moisture plume, is really high. A dry 4 C at 700 mb is not very high this time of year, but 4 C at saturation is a different story.

  3. As a follow up, I see that the source regions of most of this moisture appear to have near normal sea surface temps (although some adjacent areas are a bit above average). But even so, the sea surface temps out in those areas are in the upper 60s and 70s F or about 20-26 C.