How Dry Air Fueled Julia Creek's Powerful 146 km/h Storm Winds

Author: Chris Nitsopoulos

Posted on: Monday, November 11th, 2024, 9:01:36 AM

Earlier this afternoon, Julia Creek experienced a fierce storm with wind gusts reaching up to 146 km/h—a rare and intense event for this small town. But this wasn’t just an ordinary storm. A unique combination of atmospheric conditions, with dry air playing a starring role, created the perfect setup for these powerful winds. Let’s unpack exactly how this storm developed and why dry air was such a crucial ingredient in intensifying those gusts.

Setting the Stage: A Storm Fueled by Heat and Moisture

At 3PM on the day of the storm, the surface temperature in Julia Creek soared to 38.8°C, while the dew point, a measure of moisture in the air, was 18.6°C. This hot, humid air at ground level provided the energy needed to kick-start a thunderstorm. When the warm, moist air began to rise, it quickly formed towering storm clouds, with plenty of energy to drive powerful updrafts—currents of air moving upwards within the storm. 

We can see the storms here formed in an area of moderate instability shown by the blue shading (image 1 taken from our Weather IQ Forecast video this morning) and near a surface boundary line shown by the arrows. 

In Image two from this morning's member forecast video we see the forecast radar showing localised convective cells popping up around 3:00PM this afternoon

and finally we see an interesting and dynamic mix of moist and dry air through the mid levels of the atmosphere in Image 3 from our Weather Centre and it's this particular feature of the atmosphere that gives us a clue as to why the winds from this storm got so intense.

While heat and moisture at the surface helped fuel the storm, the instability present and the boundary line helped develop the lift for the storm, the presence of dry air above and near the ground added an entirely new layer of intensity to this event, setting the stage for a potent combination of updrafts and, more importantly, strong downdrafts.

 

The Crucial Role of Dry Air Aloft

The sounding for Julia Creek at 3:00PM from our Weather Centre gives us some clues as to what was about to take place: 

A few kilometers above the surface, a layer of dry air sat between the storm clouds and the ground. This dry air played a crucial role in enhancing the storm’s downdrafts—the powerful columns of air that descend from the storm clouds toward the ground. Here’s how it works:

1. Evaporative Cooling: As rain and hail fall through this dry layer, they begin to evaporate. Evaporation is a cooling process, and as raindrops evaporate, they cool the surrounding air. This cooling makes the air denser and heavier, causing it to accelerate downward. Think of it like dropping a heavy weight from a height—the colder and denser the air becomes, the faster and harder it falls.

2. Acceleration of Downdrafts: With each metre the downdraft falls through this dry layer, it cools and accelerates further. By the time it approaches the ground, it’s moving at high speeds, bringing that momentum with it. When this fast-moving, cool air hits the surface, it spreads out quickly, creating strong, straight-line wind gusts. This is why dry air aloft is often associated with damaging wind events in storms—it essentially supercharges the downdraft.

 

Dry Air Near the Surface: Adding Fuel to the Fire

Not only was there dry air above, but the crucial point here was that there was some really dry air near the surface. This near surface-level dryness played a critical role in intensifying the winds even further:

• Continued Evaporation: As the downdraft approached the ground, it encountered the dry air near the surface, causing any remaining precipitation to continue evaporating. This added evaporation meant even more cooling, making the air denser and accelerating the downdraft as it made its final descent.

• Less Resistance: Dry air near the ground doesn’t provide as much frictional resistance to the downdraft as moist air would. This means that the descending air keeps more of its momentum, allowing it to spread out as powerful gusts upon hitting the ground.

 

The Perfect Recipe for Intense Winds

In Julia Creek’s storm, these layers of dry air both above and near the surface combined with the hot, humid conditions at ground level to create an ideal setup for damaging winds. The storm’s intense updrafts lifted warm, moist air high into the atmosphere, building energy and forming towering storm clouds. Then, as rain and hail fell, they interacted with the dry air aloft, triggering rapid cooling and accelerating the downdrafts.

When these downdrafts met the dry air near the surface, they intensified even further, creating sudden and severe wind gusts that spread out as the downdrafts hit the ground. This is why Julia Creek experienced such a powerful burst of wind during the storm—the unique combination of heat, moisture, and, most importantly, dry air supercharged the storm’s downdrafts, resulting in the intense 146 km/h gusts that took many by surprise.

 

Why Understanding Dry Air Matters

Meteorologists watch for these kinds of setups because dry air can be a potent enhancer of severe weather. When dry air exists both aloft and near the surface, it often signals a higher likelihood of strong downdrafts and intense wind gusts. For communities like Julia Creek, understanding these factors allows forecasters to predict when storms might produce severe winds and issue warnings in advance.

So, the next time you hear about dry air in a weather forecast, remember that it’s not just a passive player. In the case of Julia Creek’s storm, dry air was the driving force behind those powerful gusts, transforming a hot, stormy day into a memorable weather event.

 

What charts does Weather IQ provide our users with that can help them identify these potential threats? 

While we provide our users with video assessments of storm potential on a daily basis across QLD which identified this region as being storm prone this afternoon, we also provide users the tools to see these threats for themselves. Some of the most important charts we encourage our users to look at when assessing the threat of extreme downdraft winds like these are: 

1. The 850-500hPa relative humidity chart
2. A Skew T or aerological diagram or what we commonly refer to as a sounding

The 850-500hPa RH chart

The 850-500 hPa relative humidity (RH) chart shows the amount of moisture in the mid-levels of the atmosphere (roughly between 1,500 and 5,500 meters). This chart is essential for several reasons:

• Identifying Dry Air Layers and boundaries: The 850-500 hPa RH chart helps meteorologists and keen storm watchers see where dry air layers are located in the mid-levels and if there are any sharp boundaries between areas of mid level dry air and mid level moist air which are hugely important In storm setups (especially when looking at severe parameters like wind), a layer of dry air around these levels can lead to enhanced downdrafts as falling precipitation evaporates and cools the air, making it denser and accelerating its descent. Knowing that a dry layer exists between 850-500 hPa when other storm ingredients are present can signal a higher potential for severe wind gusts due to strong downdrafts. We look for values between around 45-70% on these charts and we look for areas where these values are present next to areas where these values are not present to find these pockets of severe potential.

• Assessing Instability: High humidity at the 850-500 hPa level often supports sustained convection and very heavy rainfall rates, while dry air at these levels can destabilize the atmosphere more abruptly with height. The dry air can create a sharp gradient with the humid air below, intensifying rising motions and adding energy to the storm. This is especially important in hot surface conditions, as seen in Julia Creek, where dry air above moist air at the surface enhances the potential for strong convective updrafts and downdrafts.

• Forecasting Precipitation Type and Intensity: The 850-500 hPa RH chart can help determine whether storms will primarily produce rain (values above 70%) or if they may also lead to microbursts or downbursts (localized, intense wind events). If the chart shows drier air at these levels (with values between 45% and 70%), it increases the likelihood of microbursts, as the dry air will promote evaporative cooling in downdrafts. This is critical in forecasting where and when strong wind events might occur during storms. This also becomes a critical chart for assessing whether there will be any storms at all with values of 40% or less likely to result in no significant deep convection as the air is too dry to support the depth of cloud necessary for storm development. 

 

Skew-T diagrams (or soundings) are detailed graphical representations of atmospheric temperature, humidity, and wind profiles at various altitudes. In storm forecasting, especially for setups involving dry air and downdrafts, Skew-T diagrams are indispensable for the following reasons:

1. Detailed Vertical Profile of Temperature and Humidity: A Skew-T diagram shows temperature and dew point at various pressure levels, offering a precise look at how moisture is distributed vertically. For the Julia Creek setup, the Skew-T allows meteorologists to pinpoint where dry and moist layers exist. In this case, the dry air layers at both mid and near-surface levels were crucial for enhancing downdrafts through evaporative cooling. A Skew-T diagram reveals these dry/moist transitions clearly, which is essential for understanding potential downdraft strength.

2. Assessing Stability and CAPE (Convective Available Potential Energy): Skew-T diagrams help measure CAPE, an indicator of the atmosphere’s instability. High CAPE values suggest strong potential for rising air, which can intensify storms. In Julia Creek’s sounding, CAPE was high, indicating strong updrafts, but the Skew-T also showed how dry air in mid-levels would allow for intense downdrafts. Combining high CAPE with dry air aloft signals that storms will likely produce both strong updrafts and downdrafts, leading to severe winds.

3. Diagnosing Wind Shear and Downdraft Potential: Skew-T diagrams show wind direction and speed at different altitudes, giving insights into vertical wind shear. Wind shear helps organize storms and can influence whether downdrafts will be linear or rotate. In Julia Creek’s case, the vertical wind profile indicated moderate shear, which wasn’t sufficient to produce rotation but supported organized, straight-line wind events. Skew-T diagrams also allow meteorologists to assess dry air’s influence on downdrafts, helping them anticipate the likelihood of damaging gusts.

 

Bringing It Together

In Julia Creek’s case, the 850-500 hPa RH chart and the Skew-T diagram provided complementary information:

• The 850-500 hPa RH chart helped identify mid-level dry air/moist air boundaries and levels, a crucial factor for strong downdrafts.
• The Skew-T diagram provided a detailed look at temperature, humidity, wind, and stability at different levels, confirming the setup for severe downdrafts due to evaporative cooling from dry air layers (both in the mid level and low level troposphere).

These tools together allow meteorologists to understand the dynamics of storm development, specifically in setups where dry air can enhance downdrafts. Using these charts, forecasters can better predict the potential for intense wind gusts, like those experienced in Julia Creek, by recognizing how dry air interacts with storm processes to produce damaging surface winds.

 

Did you know that Weather IQ provides all these charts and daily video assessments of storm threats during the Wet Season for Queenslanders? Check out our offerings below and become a member today. https://weatheriq.com.au/sign-up