Typically, when one hears the word "thunderstorm" they think of rain in addition to lightning. But sometimes, you may hear meteorologists or wildland firefighters refer to "dry thunderstorms". Can a thunderstorm really occur without rain? Let's take a look...

What is the Definition of a Dry Thunderstorm?

According to the U.S. National Weather Service, a dry thunderstorm is one that produces less than a tenth of an inch (0.1") of rainfall underneath the core of a thunderstorm. A dry thunderstorm may simply be one that produces only brief light rain, but in some cases, dry thunderstorms can produce little to no rain and only lightning and gusty winds.

What Causes Dry Thunderstorms to Occur?

Dry thunderstorms are most common when the air temperature is hot and the dewpoint temperature and relative humidity are low.

The greater the difference between the air temperature and the dewpoint temperature (typically a difference of 40ºF or more) during the warmest part of the afternoon, the more likely a thunderstorm will be "dry".

A typical dry thunderstorm setup involves moisture arriving in the mid-levels of the atmosphere, steep lapse rates (rate of temperature decrease with height) that contribute to instability, and dry air in the lower levels of the atmosphere.

When And Where Do Dry Thunderstorms Occur In North America?

Given that hot and dry air in the lower levels of the atmosphere are typical for dry thunderstorm setups, it should come as no surprise that dry thunderstorms are most common in Western North America and quite rare across Eastern North America.

Thunderstorm season across Western North America typically lasts from May to September (but storms can occasionally happen outside of this period) and both wet and dry thunderstorms are possible across all regions of the west during these warmer months.

However, some regions are more susceptible to dry thunderstorms than others depending on the time of year. Keep in mind that this is just a general climatological timeline and can vary from year to year. 

Southwest and Southern Rockies:

The Southwest U.S. and Western Colorado are more likely to experience dry thunderstorms early in the season in May, June, and early July when temperatures are heating up but moisture from the North American Monsoon has not become established yet.

Once the monsoon arrives, wet thunderstorms are more common than dry thunderstorms across this region, but the tail end of monsoon season in September may feature a higher percentage of dry thunderstorms as moisture begins to decrease.

California, Northwest, and Northern Rockies:

Dry thunderstorms are more likely across the Northern Rockies, Great Basin, Sierra Nevada Range, and Pacific Northwest from July through September.

The reason is that 1) the jet stream typically shifts northward into Canada during mid/late summer, and 2) these areas are not influenced by the monsoon as consistently compared to areas further south, and often see only weak surges of monsoon moisture – just enough to result in dry thunderstorms but not enough for wet thunderstorms.

Western Canada:

Western Canada has a less defined dry thunderstorm season as both wet and dry thunderstorms are common across the BC Interior and Canadian Rockies from June to August with higher rainfall (from a climatological standpoint) the further east you go away from the coast.

Thunderstorms in general are less common across the BC Coast Range – but similar to the Cascades, when thunderstorms do occur they are more likely to be of the dry variety during the heat of the summer in July and August.

It should also be noted that all areas of Western North America can see a mix of wet and dry thunderstorms at times, depending on the setup.

Impacts of Dry Thunderstorms:

While we usually think of wet thunderstorms as being the most impactful from a weather and outdoor recreation perspective, dry thunderstorms can produce significant impacts as well – some of which are similar and some of which are different compared to wet thunderstorms.

Cloud-to-Ground Lightning:

Rain or no rain, lightning is always one of the greatest thunderstorm hazards from an outdoor recreation perspective. 

Lightning frequency can range from low to high with dry thunderstorms. While the "ceiling" in terms of lightning strike frequency is typically higher with wet thunderstorms, dry thunderstorms can also produce a high frequency of cloud-to-ground lightning strikes and should not be taken lightly. 

Strong Winds:

Dry thunderstorms typically produce locally strong wind gusts. This is because much of the rain falling from the base of a thunderstorm (in the form of a downdraft) evaporates once it reaches the dry air in the lower levels of the atmosphere.

The evaporation of moisture causes the falling air to cool at a faster rate and become more dense, and the speed of the downdraft accelerates as a result. Once this strong downdraft reaches the surface, it spreads outward quickly in the form of strong straight-line winds.

Occasionally, straight-line winds can cause severe wind damage to vegetation and property.

In desert areas and large valleys/basins with dry soils, strong outflow winds associated with dry thunderstorms can also cause dust storms.

Wildfires:

While lightning and gusty winds pose more immediate and localized threats to outdoor recreation, the number one hazard of dry thunderstorms from a public safety perspective is wildfire.

Dry thunderstorms occur when relative humidity is low and often occur when temperatures are hot – both of which are factors that lead to high fire danger.

When cloud-to-ground lightning strikes occur with little to no rain, they are much more likely to ignite new wildfires than if moderate to heavy rain is occurring.

Also, strong outflow winds that occur with dry thunderstorms can cause new or existing fires to quickly spread.

Changes in prevailing wind direction that may occur with thunderstorm outflow winds (ex. winds were blowing from the west before the thunderstorm, but strong winds from the east occurred during/after the storm) can also lead to erratic fire behavior and change the direction a fire is moving.

Of course, fuels must already be dry and primed to burn prior to a dry thunderstorm for there to be a significant fire risk. But if fuels are sufficiently dry and dry thunderstorms are in the forecast, then this could lead to a significant fire risk.

Storm Coverage of Dry Versus Wet Thunderstorms:

Typically, wet thunderstorms are more numerous in coverage and result in higher percentage thunderstorm chances in a forecast. This is simply because more moisture means more "fuel" for thunderstorms and a higher likelihood of occurrence.

Dry thunderstorms are often more isolated in coverage, but this is not always the case. More robust shortwaves with an unstable atmosphere, sufficient moisture in the mid-levels of the atmosphere, and dry air in the low levels of the atmosphere can result in widespread dry thunderstorms.

Widespread dry thunderstorm outbreaks have been well-known contributors to large wildfires in the past. The August 2020 dry lightning event in California and the resulting large wildfire outbreak is a prime example.

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Alan Smith