Cliff Mass Weather Blog: Why Are There So Many Thunderstorms?

Hundreds of lightning strikes occurred across the region over the weekend, with some residents enjoying more than an inch of rain as a series of thunderstorms rolled in.

There was even some pretty big hail.

Some of you have asked me why this happened. What unusual conditions combined to create such extraordinary baby boomers?

To start, below are two regional lightning maps for the 24 hours ending at 1am Sunday and today (Monday).

The first phase is a key one, with major lightning strikes above and west of the Cascades. Eastern Western Australia also gets a piece of the action.

Lightning distribution on Sunday was mainly limited to western WA and the nearby Cascades.

Total rainfall from this event was significant for August, with rainfall ranging from half an inch to an inch (see rainfall totals for the past two days below)

Some locations directly in the path of strong thunderstorms saw 1-3 inches of rain.

Take a look at the rainfall totals west of the Washington Cascades (below). One location in Bellevue, linked to a strong thunderstorm moving up the eastern side of Puget Sound, received over 3 inches!

Why are there so many strong thunderstorms on the weekend?

Thunderstorms are associated with instability vertically. The phases when air seeps through like hot cereal in a pan on your stove. Or in a lava lamp (see below). Another name for this turbulent atmospheric situation is convection.

To create atmospheric instability, a necessary condition is temperature decreases rapidly with altitude

Another essential thing is a power source–upward movement–to help create instability.

Finally, the relatively humid atmosphere is very conducive to the instability and development of thunderstorms.

To make the temperature drop rapidly with altitude, you need to warm the air at low altitudes, or cool the air at high altitudes, or DO BOTH.

Warm air near the surface is good for instability, But that’s a problem in western Washington because the cool ocean air near the surface makes turbulence much more difficult.

And most of our low-level air comes from the ocean!

Cooling at altitude (e.g. 5-20 thousand feet) can help create large temperature variations with altitude (also known as rate of decrease(By the way, many of our thunderstorms and convective activity are related to cold air from Alaska or Siberia moving across the relatively warmer Pacific Ocean.

The satellite image below shows you such a situation, with popcorn-like clouds signaling instability and possibly even thunderstorms.

We need lift!

Even though the temperature drops rapidly with altitude, it is important to have some upward vertical movement to help free instability. Think of the potential instability from large temperature changes with altitude as the explosive and the lift as the igniter.

Upward motion or lift is usually stronger at east of upper-level lows or troughs (areas of low pressure aloft), as shown in the diagram below. Importantly, upper-level lows and troughs are often associated with cold air aloft, so they twice as good in promoting instability.

So what’s special this weekend?

The upper-level weather map (500 hPa, about 18,000 ft) at 11 p.m. Saturday is shown below (heights — like pressure — are shown as solid lines, wind vectors are indicated, and colors are temperatures at that level). A strong upper-level low lies off the coast of Oregon, providing upward motion over western Oregon and Washington.

Look closely and you’ll see cooler air over western Oregon and Washington (blue, green, and pale yellow). Cooling at high altitude increases temperature variation with altitude… good for instability.

This top pattern is very unusual, as shown in the graphic below at the same time. This image shows the pattern at the top level (solid lines), with the abnormality from normal shown in color. This low is very irregular (purple in the middle).

But there is more to it than that.…… Let’s see what’s happening at the lower levels.

During the day (11am) low pressure to the south, leading to low level southeasterly winds bringing warm air over western WA at low level. The map at 850 hPa (about 5000ft) shows easterly winds and warm conditions over western WA. Helping to develop instability!

In the figure below, sPressure at each level is shown by solid lines, temperature at low levels (around 2500 ft) is indicated by color (red and orange indicate warmer temperatures), and surface winds are indicated by shown by wind vectors (at 11 p.m. Saturday). Temperatures are relatively warm in Puget Sound, western Washington, with minimal onshore winds.

As noted above, cold air aloft, warm air near the surface, and lift from an approaching turbulence are all good for business… if you like thunderstorms. All of these factors (and more!) are present on Saturday afternoon.