While people along and east of the Rockies seem to have thunderstorms almost every other day this time of year, folks along the West Coast never see our level of exciting weather. Putting aside the ongoing drought, why doesn't the West Coast ever seem to see thunderstorms?
Saying that the West Coast never sees thunderstorms is an overstatement, of course. The top image of this post shows a thunderstorm over the Bay Area in September 2001. The West Coast just doesn't see anywhere near the amount of storms seen east of the Rocky Mountains.
To highlight how few thunderstorms occur along the West Coast every year, we can look at climate data taken from some of the region's largest cities. The records keep count of the number of days that featured thunderstorms each year, along with the average number of thunderstorm days seen over the thirty year period between 1981 and 2010.
Thunderstorm data over the past couple of years is somewhat skewed due to the ongoing drought, but not by much. For example, Los Angeles normally sees three to four thunderstorm days per year, and the city saw one day with thunder in 2013, preceded by three days with thunder in 2012. Seattle and Portland see a few more days with storms, averaging between six and seven every year, with Seattle recording nine days with thunder last year.
And then we look east of the Rockies. Cities in the central and eastern United States see storms on a regular basis. Chicago reported thunder on 38 days last year, or almost 10.5% of 2013. Washington D.C. averages around 32 storm days each year, with New York following behind with around 25 days.
Why doesn't the West Coast see many thunderstorms? It comes down to two major factors: cold water and dry air.
1) The Pacific Ocean Is Cold
It's not really news that the eastern Pacific Ocean is cold, especially to people who have swum in it before. Meteorologists make a big deal out of the fact that cold water kills hurricanes, and the Pacific Ocean's temperatures cause pretty much the same phenomenon.
Water is able to slowly heat or cool the layer of air above it through conduction. When water temperatures are warm, it warms the air; cool water cools the air. Thunderstorms need warm, unstable air to form, so they thrive when they move over warmer waters. This is why hurricanes are able to strengthen so quickly when they move over the Gulf of Mexico, for example.
The cold water of the Pacific creates a very stable airmass along the coast and inland, killing any spontaneous convection. In other words, for thunderstorms to form along the West Coast, they need a strong forcing mechanism such as a cold front to help the air rise fast enough to create a storm.
2) The Air Is Usually Dry
A second factor directly related to the stable airmass that sets up thanks to the Pacific is that the air is usually too dry for hefty thunderstorm activity to form. When I talk about dry air, I'm not talking about the relative humidity, which is a crappy measure of moisture in the air, but rather the dew point. The dew point is the temperature to which the air needs to cool in order achieve full saturation, or 100% relative humidity. The lower the dew point, the drier the air.
Dew points lower than 55°F are generally considered to be comfortable, while readings above 60°F start to feel muggy. Dew point readings in the eastern two-thirds of the United States routinely reach 65-70° or higher, creating that signature soupy summertime atmosphere that's ripe for rocky weather.
Take a look at the average dew points during the summer months for some major cities around the country:
Cities along the West Coast typically see lower dew points than their counterparts to the east, keeping the air on the drier side and joining forces with the stable air to kill just about any chance of thunderstorm activity outside of a powerful storm system moving in from the west.
Even just plain old rain would be a welcome sight across the drought-stricken areas of the western United States, where almost 30% of the region is experiencing extreme or exception drought conditions.
[Images via AP and NOAA / charts by the author]