Weather forecasts are important in dictating our plans for the day, week or even longer. We are reliant on forecasts and timing of potential storms to figure out when we should leave our homes. Simple tasks can turn troublesome such as grocery shopping, a commute to work or a day at the beach due to unexpected rain. Unfortunately, people are quick to criticize (often broadcast) meteorologists — whether storms are in the forecast for the evening but turned out to be a brief shower that cleared up quickly, or sunshine was forecasted but a stray storm passed by unexpectedly. The importance of monitoring forecasts throughout the day is critical for safety and preparation, but also to create a low-stress environment for your plans.
Photo: Thunderstorm structure with emphasis on warm air being the necessary fuel to form clouds and storms (Courtesy of Encyclopedia Britannica).
Convective storms are common types of thunderstorms we are used to that can be capable of producing damaging winds, large hail, or even a tornado. These storms, similarly to rain showers can occur at any time of the day and often dictate potential changes during the day. One of the main ingredients that is necessary for storms to form is heat, usually from the sun (solar heating) that warms the surface to generate the rising air necessary for storms to form. While there are more ingredients involved for storms, heat is the main driver in this situation.
Evening convection is the most common type of convection during summer months since it follows peak heating times (allowing for rising air to fuel storms). Forecasters and storm chasers alike look toward the evening hours to find supercells or lines of convective storms in their region during the early to late summer months. This is the most common type that we see, but that doesn’t mean it’s the only one.
Since evenings are seen as the most common time for storm generation, we may get into the routine of seeing rain in the forecast and expecting it in the evening. The timing of a storm and different atmospheric changes can inhibit the generation of evening storms.
Consider the situation of expecting storms at 5pm (1700) local time, but an earlier line of storms passed by around 2pm (1400) that resulted in significant cooling (due to rain cooled air). Because of the cooling that occurred after the first storm, the expected evening storms may not be nearly as severe, if they occur.
Consider another example: storms have been passing through the area in the early morning and cloud cover has lasted until noon, but storms are still in the forecast for the evening. The cooler air caused by an earlier rain mixed with the lack of solar heating could greatly impact a forecast for storms in the evening. This results in a lack of heat to fuel the storm and thus reduces the severity of the storm.
Multiple factors impact the chance of evening convection aside from lack of solar heating from cloud cover and earlier storms or rain. These are only a few instances where the heat necessary to fuel a storm is taken away. Atmospheric changes can play a factor in an adjusted forecast like a cold frontal passage or cool lake breezes. These factors can actually help a storm form or destroy it. A cold front can pass through a hot and humid air mass that will force the hot air upward and aid storm generation. Similarly, a lake breeze that creates a frontal boundary can meet a warm frontal boundary that helps creating a rising motion. Depending on the strength of these cooler boundaries, they can inhibit the formation of storms by cooling the air in a region where a storm is headed, thus causing the storm to continuously lose heat and energy it needs to strengthen or keep its current strength.
Incoming lines of storms seen on the radar for hours can easily lose energy when coming in contact with a cooler environment. A gust front or outflow boundary is often present with a larger line of storms and act as a cold front. In turn, the storm will move into a cooler environment and quickly lose energy. As you watch the storm move closer to your area and notice it lacks the same energy it had hours before, then there is some environmental factor playing a role.
Overall, forecasts are important during all parts of the day. Keeping a close eye on changes and understanding the multiple factors that go towards a forecast can help you understand why a call for severe weather may not happen as planned. These ideas hopefully expanded your understanding of the expectation of rain throughout a day and provided insight on some of the changes that can occur.
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@2019 Meteorologist Jason Maska