Life is full of ironies and contradictions, and our weather situation of the past month is a wonderful example.
A series of atmospheric rivers have moved into the Northwest this month, bringing above-normal precipitation on the western slopes of the Olympics and Cascades.
Ironically, the SAME conditions that produce bountiful precipitation on the western slopes of our mountains cause drying over the downwind and lee slopes, resulting in profound rainshadows. Huge precipitation contrasts reign.
But to understand why this is true, you must understand the nature of atmospheric rivers.
As I have mentioned in many blogs, atmospheric rivers are associated with plumes of large amounts of moisture from the tropics and subtropics that project into the midlatitudes. For example, a well-defined tongue of moisture will be heading into our region this afternoon and overnight, as shown by a plot of such moisture at 10 PM tonight.
This is a plot of integrated water vapor (IWV), which is the total water vapor in a vertical column from the surface into the stratosphere.
But moisture is not enough to produce heavy precipitation. You need to lift the air to saturation and to continually bring in new moisture.
That takes wind. Wind can bring in more moisture. And strong winds pushing up mountain slopes produce the upward motion that converts water vapor into precipitation.
Thus, meteorologists who need to evaluate the precipitation-producing potential of atmospheric rivers also plot the product of water vapor and wind, a quantity known as integrated water vapor transport or IVT for short.
Below is the plot for 1 PM today, with blue colors indicating the largest values. Wow...a potent atmospheric river is approaching, one that is moving huge amounts of water vapor towards our region (note the arrows indicate the direction and magnitude of this water vapor intrusion).
Strong atmospheric rivers in our region are generally associated with strong southwesterly flow (winds from the southwest). A NOAA coastal radar wind/temperature profiler at Astoria shows the increasing winds today, which are increasingly from the southwest (see below, yellow and red colors indicating stronger winds). The profiler is a type of weather radar that can provide wind speed, temperature, and more above a location.
The strong southwesterly winds will produce massive precipitation on the western sides of our regional terrain. But as this strong flow descends the northeast side of the Olympics and the eastern sides of the other region terrain, the air will sink, producing warming and declining relative humidity. Such sinking air is unfavorable to the production of precipitation, resulting in a rainshadow.
Consider the forecast precipitation totals over the next 48 hours (below). Precipitation ranges from 8-10 inches over "favored" locations on the upper windward slopes to virtually nothing near Port Townsend on the NE side of the Olympics. Heavy upslope precipitation on one side, little precipitation on the lee side.
Let me put it another way: there will be a roughly 100 to 1 ratio between precipitation on the western slopes (10 inches) compared to the downslope/rain shadow area near Port Townsend (.1 inch). A very small area may have a 500 to 1 ratio.
And we have not even mentioned the dry conditions in the sinking zone east of the Cascades.
The strongest atmospheric rivers often have the strongest rainshadows, with their powerful winds being a key contributor.
And there is another story regarding atmospheric river events: a huge contrast in wind speed and direction that occurs as they interact with our terrain. But that will wait until a future blog.
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