Happy Independence Day! I hope you have a safe and enjoyable holiday.
The weather is not cooperating for part of the day. It will rain for about four or five hours today and then be cloudy and muggy for fireworks Thursday night. Severe thunderstorms are not expected today and the threat for severe weather over the next seven days is very low.
The strong winds aloft that are sometimes needed for spring severe storms are forecast to stretch across the northern United States. Look at the graphic below. See the blue and green shaded areas? Those are zones where the winds aloft are blowing at more than 60 miles an hour. That can add the extra oomph to a storm to turn it into a tornado or large hail producer. Between now and next Tuesday, the jet stream (the red arrows) and the associated strong winds aloft are forecast to stretch from the pacific northwest across the northern tier of the U.S. into the New England.
Lower severe weather threats aren’t surprising. It’s summer and the chance for tornadoes will be much lower in July and August.
The image below uses data collected from USTornadoes.com to illustrate the greater risk of summer tornadoes, (areas shaded in orange) shifts to the high plains and great lakes during the month of July. Missouri and Kansas still have some risk but on average Kansas sees seven tornadoes in July while Missouri only sees three tornadoes.
The tornado risk drops even more in August. The greater tornado threat is smaller and even farther north during a month when the jet stream reaches its weakest point during the year. Kansas averages roughly three tornadoes every year and Missouri often experiences only one or fewer tornadoes in August.
But this time of year, another severe weather emerges and it’s difficult to predict. I’m talking about a weather phenomenon known as “microbursts.”
What’s a microburst? Here is the National Weather Service’s definition of a microburst.
“A microburst is a localized column of sinking air (downdraft) within a thunderstorm and is usually less than or equal to 2.5 miles in diameter. Microbursts can cause extensive damage at the surface, and in some instances, can be life-threatening.”
Check out the explainer graphic below.
How do microbursts form? There are two types of microbursts. Dry microbursts and wet microbursts. The graphic above shows you how a dry microburst forms. Look at the arrows pointing toward the cloud. See how it says dry air. Dry air is drawn into the thunderstorm. This causes the raindrops inside the storm to evaporate. This process is called evaporative cooling.
See the label at the top of the storm? Since cooler air is heavier than warm air, the cool air begins to fall. If conditions are right, the colder blob of air will accelerate. When it hits the ground, the air is forced outward. Most microbursts have winds just over 60 miles per hour but in extreme conditions winds can exceed 150 miles per hour and spread out and cause damage over an area up to two miles.
A wet microburst is similar but comes with a sudden and violent torrential downpour. Most microburst damage, when observed from the air, appears to have a starburst pattern while tornado damage looks more like a swirl. It’s hard to predict when and where microbursts will occur but we’re at the time of the year when they become more common. Why? Because the winds aloft are weak, thunderstorms can move very slowly. A slow-moving storm has time to build up a lot of evaporative cooling and more potential to collapse. We can often see collapsing thunderstorms on radar. They are strong radar echoes, one minute and seemingly gone the next.
Hopefully the rain on the 4th of July will exit just as quick. Have a happy and safe Independence Day!