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: 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.