Why do Atmospheric Rivers Occur?

A significant atmospheric river event will occur tomorrow (Sunday) and Monday, with heavy precipitation in the mountains and South Sound.

The latest UW WRF model forecast predicts up to 10 inches in the Cascades, with the southern WA Cascades being hit the hardest (see below).   In contrast, a profound rain shadow will extend from the NE Olympics to Bellingham. 

It is now clear that there will be some serious flooding in some locations. For example, the NOAA/NWS River Forecast Center is forecasting major flooding on the Snoqualmie River near Carnation.   

Want to see something spectacular?   Go to Snoqualmie Falls on Monday.  If you can get there.😁

Atmospheric rivers in our region are associated with plumes of warm, moist air from the south, southwest, and west.

NOAA weather satellites can sense the water vapor, which is very evident in the water vapor image from this morning (below).  A massive plume of water vapor is moving our way.

 

Numerical weather prediction models, like the UW WRF model, can skillfully forecast such water vapor plumes, which release massive amounts of water as the air is forced to rise by our region's substantial terrain.

Meteorologists favorite diagnostic for atmospheric rivers is integrated water vapor transport (IVT), which is the amount of water vapor times the wind speed, summed up in the vertical (that is where the integrated comes from).   This quantity is more closely related to the potential for regional rainfall.

The prediction of IVT for Sunday morning is shown below, with the arrows showing both the direction and magnitude of the moisture transport   Large values, but not record-breaking by any means.  

Such moisture plumes are generally warm as well.   In fact, to get large values they HAVE to be warm since the amount of moisture air can hold depends on temperature (warm air can potentially hold more water vapor than cold air).

Atmospheric rivers form in our region when strong southwesterly atmospheric wind currents occur, currents that move large values of water vapor out of the subtropics.

The atmospheric flow pattern that does this is usually characterized by a strong low-pressure area in the Gulf of Alaska and higher pressure west of southern California.

Not surprisingly, this is exactly the pattern forecast for Sunday morning at 10 AM for a level around 5000 ft  (below).  With high pressure to the south and low pressure to the northwest, a strong pressure/height change is created, which produces a strong southwesterly flow. 

The interesting thing about atmospheric rivers is that they are like regular rivers in another way...not only does water come into the river at the beginning, but there is a convergence of water vapor from the sides along the way, not unlike the small streams that drain into a river over its entire path.