A stormy forecast

Picture8

Main Threats

Thursday Night-Friday: The possibility of strong to severe thunderstorms. Damaging winds will be the main threat.

Summary

For today, we will see mostly cloudy conditions. We can’t rule out an isolated shower or sprinkle through noon. This morning cloudiness is the result of an old boundary that is moving through. Behind it, we will notice the humidity increasing and that will help generate an isolated shower or storm this afternoon/evening.

On Thursday, we may stay dry the better part of the day but showers and storms look certain for Thursday night. Some of those storms could be strong. The same continues for Friday, with more disturbances riding along the slow-moving cold front. Any storm that develops on Friday will have the potential to produce strong, gusty winds.

The Storm Prediction Center has placed all of the plateau in the marginal risk for severe storms for Thursday night, and again for the whole day Friday.

The unsettled weather continues for the weekend, with scattered/numerous showers and storms for Saturday. Sunday may be a bit drier, but it will continue to be unsettled.

Discussion

The system that will be moving in on us tomorrow night produced some severe storms across Oklahoma yesterday. Chasers were on a storm west of the Norman/Moore yesterday afternoon. It showed rotation on radar but never enough to produce a tornado. WeatherTAP’s RadarLab HD+ showed the storm rotating, while chasers were live-streamed from the base of the storm. It’s always cools to see that line up. That bright area you see behind that dark cloud is the rear-flank downdraft. It results from winds aloft striking the storm. Most of the wind goes around the storm, while some of the wind shoots down the back side of the storm. Imagine a storm being like a rock in a stream, with the winds of the atmosphere being the water. Most of the water goes around the stone, right?

The wind that descends the rear of the storm is the rear flank downdraft. The downward motion of the air causes some clearing. We think this downdraft plays a crucial role in determining which storms produce tornadoes, and which ones do not.

I circled the area of rotation that was showing up behind the storm. The picture to the right is of that area of rotation. Notice it just lowered, but no rotation ever extended to the ground. Thankfully, a tornado warning was never issued, nor was it needed. A severe t-storm warning was sufficient.

Picture9

 

We could see similar severe storms tomorrow night and Friday. Like Oklahoma, our environment will not support tornadoes. You can never rule them out 100% when you have a chance for strong storms, but they are certainly not the main threat.

Tropics

We now have subtropical depression five out in the Atlantic. The only folks concerned about this system are the fishes in the sea (ha). It is expected to become a named storm sometime today. If named, it will become Ernesto.

083830_5day_cone_no_line_and_wind

Records

Today’s record involves the dewpoint. As most of you know, we often measure the amount of moisture in the air by telling folks what the relative humidity is. You know that if the relative humidity is 100% it’s raining or snowing outside. But, dewpoint is a much better way to measure that moisture. The dewpoint is the temperature required for it to be raining/snowing. In other words, if my air temperature is 75 degrees and my dewpoint is 65 degrees, I know we are only 10 degrees away from a relative humidity of 100%. If the temperature should drop to 65, it will be precipitating outside.

The difference between the air temperature and the dewpoint will tell you how bad the humidity is. The closer the dewpoint is to the temperature, the more humid it is. Take our example above and change the dewpoint to 35 degrees. We would then be 40 degrees from precipitation and the air would be very dry.

Imagine how unbelievably humid it must have been in Nashville on this day in 1995. That is when the highest dewpoint temperature ever recorded for Nashville took place. The dewpoint temperature topped out at 82 degrees. That means the humidity would have been 100% at 82 degrees. And the air temperature can NEVER go below the dewpoint. Never. The air temperature must always cool to the dewpoint.

That also means that even if it had rained that day in Nashville, the temperature could not have dropped below 82 degrees. That’s pretty crazy, right?

We use the dewpoint all the time in severe weather forecasting. The rule of thumb is that you need dewpoints in the 50s to have strong storms. Dewpoints need to be in the 60s or better for severe storms. That’s not etched in stone, but it’s a good rule of thumb.

Always keep in mind that warm, moist air is the fuel for storms. The more warm and moist the air is, the more fuel that is available for storms.

The next record comes from a place and time where low dewpoints had led to very dry conditions.

I noticed a remarkable record from Idaho this morning. On this day in 1967, a lightning strike ignited a forest fire. Winds with the thunderstorms were 50 mph, which spread the fire like, well, wildfire (ha). In just 12 hours, 56,000 acres were torched. The heat of the fire produced fire whirls (aka fire tornadoes) that had winds of 300 mph! That’s right, 300 mph. Trees were flung like matchsticks when these fire whirls would move through the forest. I honestly don’t think I’ve ever heard of such a violent wildfire. Wow.

The 50 mph winds were likely created by the rainfall from the storms not being able to reach the ground. Sometimes, it’s moist enough in the atmosphere to support thunderstorms, but the air here at the surface is too dry for rain to make it to the ground before evaporating (it’s called virga). Evaporation is a cooling process and that cooler air pools up under the clouds. Eventually, the cool air becomes too heavy and, with the help of gravity, rushes down to the ground, causing very gusty winds.

Thankfully, our wet summer weather has us worrying about anything BUT wildfires. I think we can all be grateful for that.

You all have a great day!

 

 

Leave a Reply