Thunder rumbled across the Northwest on Saturday, with many areas without a drop of rain.
Take a look at the lightning strike map for Saturday. Quite a few lightning bolts hit the western slopes of the Cascades and Rockies….and some moved over southeast Washington.
Yesterday morning, convection could be seen starting to appear over the Cascades, and by early afternoon (2 p.m. for the Seattle Panocam shown below) there was already fairly deep convection (a gas term represents cumulus clouds) with prominent cirrus grooves.
I’m walking with Steve Pool, retired meteorologist of KOMO TV this timeand we’re admiring the giant mound in the mountains.
The lightning-producing plumes had enough precipitation for weather radar to “light up” the area, something illustrated by regional weather radar around 2 p.m. (see below). Many showers form initially over the terrain and then drift away.
For example, some convection over the Cascades drifted westward in the afternoon, with some convection as far as Bellevue.
Convection is very evident in the visible satellite image, as illustrated by the image around 2 p.m. (see below). I put an arrow to indicate one of the tick cells.
How much rain falls during these showers? The answer is below (precipitation for the whole day is shown). Much of the Cascades and eastern Washington saw light rain (0.01 to 0.1 inches), with some lucky areas getting half an inch or more. Showers will help retain soil moisture in eastern WA.
Now you might ask: why do we have these showers and why are the slopes in favor? Good question
There are two main reasons. The air above us yesterday was colder than usual (surprise, surprise), while the sun has grown strong, warming the surface strongly.
The combination of warm air near the surface and cool air overhead produces a large temperature drop with altitude (called error rate) ….. and the large lapse rate promotes atmospheric convection, where the atmosphere begins to seep in.
One indication of the possibility of instability is what is known as the Convective Available Potential Energy (CAPE) and as shown in the figure below, the values are relatively high for our region (high up to 500!)
And there is something else. A low-pressure trough in the upper stage of the low has passed over us (see map 500 hPa – about 18,0000 ft – map of upper strata below), and such features create an encouraging upward movement convection.
But why do so many thunderstorms start in the mountains? Because the mountain provides additional upward movement, the upward movement of the air begins to take a leap!
Thunderstorms will be absent in the area today – so enjoy.
And better enjoy this weekend. The Memorial Day weekend will be memorable…but NOT because of the nice weather.
