Duration of Weather Hazards – Reasonable Expectations
June 26th, 2016
When looking at various weather hazards we can set reasonable expectations for their impacts — by remembering how often duration and intensity are inversely related. The most fearsome and hazardous events tend to strike quickly and cover a relatively small area. The less damaging but more widespread events can take some time to pass by. This post considers different types of hazardous weather, contrasting and comparing these events by their duration and size of area affected. (Click here to see a good overview of the warnings, watches, and advisories issued by the National Weather Service.) For the purpose of this explanation, I divided them into three groups, but would be interested to know if you might have classified them differently.
Top Level Weather Hazards
The top level of weather hazards truly deserve an urgent response as a direct hit from any of them is often deadly. In much of the northern Unites States, spring and summer characteristically produce these most extreme and dangerous weather events — in the form of tornadoes, flash floods and cloud to ground lightning. In recent years, warnings for tornadoes and flash flooding have become more accurate and focused on more precise areas. Tornadoes and flash floods tend to strike very limited areas – usually a tiny fraction of a county, no more than a few square miles. The National Weather Service, which is the issuer of public warnings, has a challenge of striking the delicate balance between avoiding false alarms and giving a sufficient lead time to allow life-saving action. The extreme fury of rising water or destructive winds can reach full force within seconds so it is good for you to plan in advance what you will do when a warning is issued.
Public warnings are not issued for lightning, but you want to be aware of how to protect yourself from the danger! It can be difficult to judge the lightning risk posed by a thunderstorm simply by its appearance on radar. The amount of lightning in a storm or the proportion of dangerous strikes which go from cloud to ground doesn’t always correlate with the intensity of rainfall. Lightning tends to become more frequent in the mid and late summer. Lightning increases not because of summer’s hotter temperatures, but due to lighter winds aloft. This happens as the summer jet stream weakens and shifts northward. The diminished “wind shear” results in a lower proportion of severe storms in mid to late summer, but allows storm tops to rise higher in the atmosphere. Lightning is caused by separation between positive and negative charge within a thunderstorm. The taller the storm, the greater the separation of charge – resulting in more lightning.
Damaging Weather – Frequently Troublesome
The next level of damaging weather events are less often deadly, but are frequently troublesome, with the potential to cause property damage. Injury or loss of life can still occur in cases where humans are caught unprotected. These moderately impactful weather hazards would include severe thunderstorms (bringing both hail and wind hazards), and significant snow events. It seems that in recent years, the NWS is quicker to issue warnings or advisories for this class of hazards. False alarms can be a bit more common for severe thunderstorms and significant snow, because their effects can vary considerably across the warned area. These hazards can cover several counties and conditions supporting their occurrence may persist for several hours, but usually for no more than 12 to 18 hours.
Other Hazards – Low Impact for Broad Areas
The weather hazards producing relatively low impact for broad areas have the longest duration — ranging from many hours to on and off for several days. These hazards are caused by features which are often more apparent on surface weather maps. This group of hazards can cover large sections of states or even several states. By taking proper precautions or changing plans, people affected by these hazards are rarely imperiled personally. These include temperature extremes, strong winds, and air quality issues.
Extremes of cold temperature occasionally require wind chill advisories or warnings in the winter. The corresponding events of hot summer weather prompt heat advisories and excessive heat warnings. Strong winds which are not associated with thunderstorms but rather with pressure gradients between large high and low pressure systems are most often covered by wind advisories, (and less commonly require high wind warnings when gusts over 58 miles per hour are expected). Unlike others on this list, dense fog advisories often are not issued until the fog has started to form. If sensitive to fog you’ll be well-served by learning how to foresee the types of conditions favorable for its development. We’re also seeing air quality alerts issued more frequently – especially in years like 2015 and 2016 with significant wildfire activity in Canada and in the higher elevations of the western US. These may be issued by the Pollution Control entity for your state rather than by NWS forecasters. Because these are widespread and relatively long-lasting events they can be forecast several days in advance. It’s not at all common for advisories to be issued days in advance, but you might hear forecasters talking about their potential as much as 5 to 7 days ahead of time.
Considering the time and area scale of weather events can give a good idea of how long the threat could last. This will also help to set your expectations for how much area will be covered by the threat. It’s fortunate that the most intense and dangerous weather conditions tend to pass quickly and affect tiny areas. Other less dangerous but tiresome hazards can require more patience as we wait for the event to alleviate.