Weather Blog

A smorgasbord of thunderstorms topics

With thunderstorms back in the news, I figured it'd be a good idea to dig this out of the blog archives for today:

1) What's does a "Severe Thunderstorm Warning" mean? We've haven't had any of these in the Seattle area yet Monday, but they are possible and just in case they are issued, it simply means a thunderstorm meeting severe criteria is occurring or imminent.

1a) What's the criteria?

Officially, the National Weather Service declares that a thunderstorm is severe if the storm produces strong winds in excess of 58 mph or if there is hail that is 3/4 of an inch or larger. (Technically, it's also "severe" if it produces a tornado, but then it would be a Tornado Warning and not a Severe Thunderstorm Warning.)

Note that the frequency of lightning nor the intensity of the rain is a factor. You can have very frequent lightning and torrential downpours, and it still might not meet the criteria.

The trigger here for the warnings of the past 24 hours has been the size of hail. However, since severe weather is rare here, the local NWS office can use lower criteria to issue the warnings -- most reports have had hail at about 1/2" in diameter -- still very large for this area.

2) What's the difference between a Severe Thunderstorm Watch and a Severe Thunderstorm warning?

A "Watch" means conditions are possible. A "Warning" means conditions are occurring or imminent. I can't ever remember a Severe Thunderstorm Watch for Western Washington -- those are issued by the Severe Storms Laboratory in Norman, Oklahoma while warnings are issued by the local weather service office. They might be sticking to official criteria and thus, could explain why no watch is in effect now as hail is not expected to reach 3/4 of an inch in diameter.

3) What Causes Lightning?

It's still not quite fully understood and is incredibly complex, but one main theory goes that as water droplets move about inside some clouds, they can build up an electric charge -- much like how if you wear socks on a carpet, you build up a charge. The negatively charged particles tend to conglomerate at the bottom of a cloud, while the positively charged ones are at the top.

As the storm intensifies and more negative charges build at the base, it can actually push the negatively charged particles on the Earth's surface further into the ground (since similar charges repel each other) leaving mainly positive charges along the surface. Then, a conductive path will be generated between the negative cloud and positive Earth, allowing a huge electrical current to flow between the two charges. That's what we see as lightning.

4) Why are strong thunderstorms rare around here?

To get strong thunderstorms, you need to get unstable air. Explaining unstable air without using really crazy charts is difficult, but in a nutshell, unstable air means that atmospheric conditions are such that it is easier for parcels of warm air to rise higher into the sky, which is more conducive to cloud and storm development. 

Unstable air can occur when you have cold air moving in the upper layers -- that is our most common unstable air occurrence.  That's typical when a storm moves through in the spring or fall and chilly air moves in behind it, and is the trigger for those days after a solid rain where it rains for 20 minutes, then we get blazing sunshine.

But today, it's a warm, humid air mass moving through that has us unstable -- and this is more typical of storms in the Midwest and East Coast.

It's rare here because it's difficult to get both warm and humid here. Either we have a cool, marine flow off the ocean -- which is actually very stable because that cool air keeps the temperatures difference small.

When we get hot, it's a dry east wind and we don't get much of any moisture to even begin to form any cells.

But this week, it's been a southerly flow.  That allows us to tap into some warm, tropical moisture to the south. We don't get the cool, stabilizing influence off the ocean, and we don't get the drying effect of an east wind.   The south wind will also then use the mountains in Oregon and Washington as a ramp, helping launch that air skyward and trigger or enhance thunderstorms.

A southerly wind doesn't mean widespread storms -- note that we've generally been in this pattern all week. But starting last night we had an area of low pressure draw closer to the coast, which added additional lift and more moisture and basically added tons of fuel to the fire.

I'll add more here as I come up with them or you, the reader, ask them :)