CINDY HOWELL
18th Air Support Operations Group, U.S. Air Force
Cairns Army Airfield
Fort Rucker, Alabama
What do you think of when you hear “spring” or “summer?” Family vacations? Beach trips? BBQs? Swimming? Mowing the lawn? Sounds like fun — except for the part about mowing the lawn! To most meteorologists, however, spring and summer conjure up something very different. Air mass thunderstorms, sea breezes, outflow boundaries, microbursts, severe weather, squall lines, hurricane season — all that fun stuff that makes aviation a potential nightmare. Understanding these hazards and preparing for them are the keys to staying safe.
Thunderstorms
All thunderstorms imply severe icing, severe turbulence, lightning, heavy precipitation, hail and low-level wind shear (LLWS). It says so right there in the very fine print of Block 22 of your DD 175-1, Flight Weather Briefing. All thunderstorms have the potential to produce a microburst, and all thunderstorms have the potential to turn severe with little or no notice.
To understand the basic dynamics of a thunderstorm, just remember M-I-L. Moisture. Instability. Lift. Start with some moisture, add a pinch of instability and a dash of lift. That’s the basic recipe for a thunderstorm. Throw in a sea breeze, an outflow boundary, merging cells or other interaction, and a run-of-the-mill air-mass thunderstorm can quickly turn into something far more dangerous.
Of all the tools at our disposal, radar is the single-most important in monitoring the development and progression of thunderstorms. Using radar imagery, we can track the location and movement of precipitation; identify thunderstorms versus showers; pinpoint any areas of hail or rotation; and interpret whether the storms are building or weakening.
Radar imagery can be deceiving if you don’t know what you’re looking at. Seeing a red blob on radar imagery does not necessarily mean there’s a thunderstorm present. The red could mean a thunderstorm, but it could also mean large raindrops or many raindrops. The interpretation of red also depends on which product you are viewing (BR or CR).
BR, or base reflectivity, looks at a specific layer of a storm. Depending on how far away the storm is from the radar and the elevation/tilt, you can know exactly how high you are looking. This is very helpful in determining the severity of a storm. CR, or composite reflectivity, takes all layers of a storm and plots them as a single composite image. Because of this, CR products tend to look “doom and gloom,” while BR products usually give a more realistic depiction of a storm. Most importantly, you can use the lowest elevation/tilt of BR to identify those pesky outflow boundaries. You can’t always do so using CR products.
Sea breeze
The sea breeze forms along coastlines due to the differential heating of land and water. By day, the land heats up more quickly than the water, which creates a pressure gradient and an onshore flow. By night, the process reverses. This same effect can occur with any large body of water (lake, river, etc.). The sea breeze is evident on satellite and radar imagery.
Outflow boundaries
An outflow boundary, or gust front, marks the dissipating stage of a thunderstorm. You’d think that’s good news, but it’s not! Across an outflow boundary, you’ll notice a wind shift, gusty winds, LLWS and turbulence. There’s even a difference in the temperature and dew point. It is essentially a very small-scale cold front! It is along these boundaries that new thunderstorms tend to form. When outflow boundaries converge with each other or with other storm cells, things can go downhill quickly. It’s important to note that outflow boundaries are not precipitation. Since most websites and apps (including ForeFlight) filter non-precipitation returns, outflow boundaries can be challenging to identify unless you know how to find them.
So how do you find them? First, know where to look. Use a data source that does not filter outflow boundaries. The National Oceanic and Atmospheric (NOAA)/National Weather Service (NWS) and College of DuPage are excellent sources. Be sure you’re looking at a BR product, lowest elevation/tilt (typically 0.5 degrees). Finally, look for the classic curved or arc-shaped return that is moving away from the parent storm.
Microbursts
As noted earlier, every thunderstorm has the potential to produce a microburst. A microburst is a sudden, violent downdraft of wind in a thunderstorm that is less than 2.5 miles in scale. The winds rush down and out, radially, in all directions. The damage from a microburst can be even worse than that of a tornado, and microbursts present a grave danger to aviation, particularly to rotary-wing pilots.
Air-mass thunderstorms are prevalent during the summer months. In and of themselves, they are usually manageable and easy to pick around. However, certain interactions can quickly wreak havoc on aviation. We constantly monitor the radar for outflow boundaries, merging cells, sea breeze and other interactions that tend to ramp up convective activity. Thunderstorms hazards exist up to 20 miles outside of the core of a thunderstorm. The best way to avoid these hazards is to steer clear of thunderstorms.
Squall lines
Squall lines occur frequently during the springtime. A squall line looks nasty and that’s because it is nasty. Even nastier is the bow echo. When you see a line of thunderstorms begin to bow out, that’s an area you want to avoid. A bow echo is usually indicative of damaging straight-line winds.
Hurricanes
Hurricane season runs from 1 June through 30 November. Hurricanes form over the warm ocean waters and can even impact areas well inland. Impacts vary by location. Coastal locations will see the strongest winds along with storm surge. Winds weaken as a tropical cyclone moves inland, but inland locations can still see hurricane-force winds, heavy rain, flooding and tornadoes as the rain bands rotate around the center of circulation.
Hurricane season 2022 was about normal. There were 14 named storms (normal), eight hurricanes (above normal) and two major hurricanes (below normal). What do the tropics have in store for 2023? It’s too soon to speculate, but the experts will weigh in soon. The Colorado State University team usually releases its first guess in April, while NOAA makes its initial outlook in early May, so mark your calendars!
A final word
We are all impacted by weather in our daily lives, but probably no one more so than aviators. Mother Nature can quickly turn a routine mission into a life-threatening situation. Ensuring you have the most current weather brief, understanding these hazards and knowing how to mitigate them is critical to keeping your aircrew safe. Be informed, ask questions and stay safe!