Thunder was heard all over the Northwest on Saturday, including many areas without a drop of rain.
Take a look at the lightning strike map for Saturday. Quite a few lightning strokes over the western slopes of the Cascades and Rockies....and a number of boomers moved over southeast Washington.
Yesterday morning one could see the cumulus convection start to pop up over the Cascades, and by early afternoon (2 PM for the Seattle Panocam shown below) there was fairly deep convection (the meteorological term for cumulus clouds) with prominent cirrus anvils.
I was taking a walk with Steve Pool, the retired KOMO TV meteorologist about this time, and we were admiring the huge cumulus over the mountains.
The cumulonimbus producing the lightning had enough precipitation that the weather radar "lit up" over the region, something illustrated by the regional weather radar around 2 PM (see below). Many of the showers formed initially over terrain and then drifted off.
For example, some of the convection over the Cascades drifted west during the afternoon, with one getting as far as Bellevue.
Take a look at the PanoCam shot looking northeast from Seattle around 6:30 PM. You can see a shaft of rain reaching the surface! This cell produced thunder, which was heard all over Seattle.
And there was something else. An upper-level trough of low pressure was over us (see the 500 hPa--about 18,0000 ft--upper-level map below), and such features produce upward motion that encourages cumulus convection.
The cumulus convection was very apparent in the visible satellite imagery, as illustrated by the image around 2 PM (see below). I put in an arrow to indicate one of the cumulus cells.
How much rain fell during these showers? The answer is below (the rainfall for the entire day is shown). Much of the Cascades and eastern Washington got some light rain (.01 to .1 inch), with a few lucky regions getting half an inch or more. The showers will help keep the soil moisture up in eastern WA.
Now you may ask: why did we get these showers and why were the mountain slopes favored? Good question
There are two main reasons. The air over us yesterday was colder than normal (surprise, surprise), while the sun has become powerful, strongly warming the surface.
The combination of warm air near the surface and cool air aloft produces a large decline in temperature with height (called a lapse rate).....and large lapse rates foster atmosphere convection, where the atmosphere starts to percolate.
A sign of the potential for instability is something called Convective Available Potential Energy (CAPE), and as shown by the figure below, the values were relatively high for our region (as high as 500!)
But why were many of the thunderstorms initiated over the mountains? Because mountains provide additional upward motion that helps give the upward moving air a jump start!
Today the thunderstorms will be absent from the region--so enjoy.
And better enjoy this weekend. Memorial Day weekend is going to be memorable...but NOT for nice weather.
