Thunderstorms and a Winter-Like System Approaches the Pacific Northwest

A wet, winter-like frontal system is now approaching the Northwest and will arrive on Friday, but before I discuss it, let's consider the substantial thunderstorm activity that struck the regional mountains on Wednesday.

The weather radar image around 2 PM Wednesday showed a strong thunderstorm over the northeast Olympics and a line of convective showers over the eastern slopes of the Cascades.

The Seattle Space Needle panocam showed the extensive scale of the Olympic Peninsula thunderstorm:

And the Space Needle cam also indicated a line of storms over and east of the Cascades.

The Wednesday thunderstorms were associated with hundreds of lightning strokes, as illustrated by a one-hour sample during the mid-afternoon on Wednesday:

Why is there so much lighting on Wednesday afternoon?    

The air above us was unusually unstable, meaning that a rising parcel of air would tend to accelerate upward if lifted.   A measure of such potential instability is called CAPE (Convective Available Potential Energy), and values of CAPE were unusually high over the region (see plot below).

High for the Northwest, but the folks in the Midwest would laugh.

Convection tends to release at the surface warms, but yesterday the process was aided by increasing westerly winds (from the west), which provided strong upslope flow over the high terrain.  It helps to give the air a kick upwards to get the instability going.

Finally, a wet, winter-like system is now approaching our region, as illustrated by the latest infrared satellite image (below).  Looks like winter!

The total forecast precipitation over the region through Sunday morning is substantial (see below), and should both moisten the surface, add to the water supply, and reduce wildfire potential.