Summer WX Patterns

We’re rapidly heading into the summer months, and that means weather systems are driven directly by solar heating rather than the whims of jet streams and different air masses. Those can still play a factor, but they take more of a back seat to what we see from June through early September. Summertime weather is, in many ways, more readable than the complex frontal systems of winter and spring, but only to the pilot who has taken the time to learn its nuances and behavior.

Thermal-driven convection, mesoscale convective systems, Gulf moisture surges, and the subtle but consequential effects of density altitude all follow recognizable patterns with identifiable precursors. A pilot who understands all the ins and outs of summer weather around the country carries a decision-making advantage that no preflight briefing can replicate.

In this article you’ll get the perspective of an experienced aviation forecaster with 35 years at the desk. In the same way that I can learn valuable things about flying if I were to ride along with some of the old-timers here, I will be able to convey to you some of my own experience and hopefully you’ll be able to pick up on useful facts for your planning and weather navigation tactics. Aircraft performance issues, engine management techniques, and specific procedural guidance are outside my area of expertise so I recommend consulting your POH, a CFI, or your favorite piloting forum for your type of aircraft.

Density Altitude

One of the biggest problems by far is excessive density altitude. A 90° day in Cleveland or Memphis already bumps the density altitude to 2,000 feet, resulting in less lift and longer takeoff runs. Take that 90° day up into a mountain airfield in the Appalachians and that spikes to 6,000 feet, and in the Rockies at a place like Santa Fe you’re looking at 10,000 feet and up. Each 0.10″ altimeter setting loss raises that by another 100 feet, and values of 29.50″ are indeed common in the interior during summer.

Adding to the problem is humidity. Moist air is actually less dense than dry air at the same temperature. Water vapor, that is, hydrogen and oxygen, displaces heavier nitrogen and oxygen air molecules, meaning that a hot, humid day in Memphis or Atlanta carries a density altitude beyond what the thermometer alone suggests.

When an upper-level ridge combines with a Gulf moisture surge at the surface, pilots across the entire Southeast and lower Midwest face a compounding effect: extreme heat, high dew points, and low pressure all pushing density altitude in the same upward direction simultaneously. Early morning departures, when temperatures are at their lowest, offer the most favorable density altitudes and shorter takeoff runs.

Haze

Of course, haze is an unwelcome factor during the summer months. The main driver of haze is sulfate and nitrate particles from industrial sources, along with volatile organic compounds from forested regions, the Great Smoky Mountains being a classic example. Saharan Dust that crosses the Atlantic especially in July and August is a very occasional visitor to haziness the southeast states. But in recent years, smoke from U.S. and Canadian wildfires and Central American land clearing has been an enormous contributor to reduced visibility and can spread thousands of miles. All of this is exacerbated by the seasonal slowdown in wind flow, especially in the Mid-Atlantic and Southeast states where Bermuda High surface ridging slows the wind further.

Speaking of wildfire smoke, you can often find hotspots on sites like skyvector.com since TFRs usually appear within hours of a fire being worked by firefighting resources. Typically these are out to 5 nm and up to 3,000 ft AGL, but can be larger. Visibilities are often restricted downwind of these fires, especially at altitudes of 5,000 to 15,000 ft AGL.

Except in unusual cases, haze is usually absent in Class A airspace. It tops out at roughly 7,000 feet on average across the southeast half of the country, but sometimes can reach up to 15,000 feet in the Mid-Atlantic region during stagnant patterns. Haze can be enough of an issue to result in spatial disorientation at night, especially over water where reference points can disappear, effectively rendering IFR conditions. The nighttime JFK Jr. crash in July 1999 off the coast of Massachusetts is a classic example, where airports reported 5 to 7 miles in haze. At least one pilot bound for Martha’s Vineyard canceled his takeoff due to the his assessment of the visibility. 

Thunderstorms

You will see thunderstorms in the summer in all of the Lower 48 and southern Canada during the summer. If you venture south of the border they’re there too, including Cuba, the Caribbean, and Mexico, all of which have summer thunderstorm maximas.

An important rule of storm safety is recognizing the life cycle that every storm follows. There is Stage 1, the cumulus stage, where it is dominated by updrafts with solidly defined towers. At this time, rain, graupel, and snow is beginning to form within the cloud at around 15,000 to 25,000 feet MSL. This leads to Stage 2, the mature stage, where updrafts are paired with a rainy downdraft. This transition can happen very quickly in the southern states, with Stage 1 going to 60 dBZ (Level 5) radar returns in less than 20 minutes.

Surface radar reflectivities reach their peak during Stage 2. Sometimes the downdraft can be invisible and catch pilots unaware, especially in the western states, but the cloud mass will usually be comprised of either diffuse or tufts of virga directly above these invisible downbursts. 

Finally, Stage 3 is the dissipation stage, where the cumulus cloud elements are gone or stratified and you’re left mostly with the downdraft and virga. This is accompanied by a gradual reduction in lightning activity and precipitation. The sky often has a dirty, disorganized look, but that’s a good sign.

Identifying a nearby storm visually and determining what stage it is in is what us forecasters do when amending the forecast or briefing an aircrew, and is also a technique used by storm chasers to figure out where the situation is headed. A sky that is filled with cumulus towers and dark solid bases is in Stage 1 and will likely deteriorate, while a fibrous appearance, flattened clouds, and virga hanging in the sky is more suggestive of Stage 3.

If you have a radar system on board, you do have an advantage, but there’s no getting around the physics of what a thunderstorm can do to a light airframe with limited power margins. Furthermore, certain processes like updraft cores and embedded microbursts are often not detectable by radar since these areas can contain air that’s devoid of rain drops and ice, meaning radar won’t detect anything. These are two notorious areas of severe turbulence. The takeaway here is that radar should be be treated as a tool for avoidance, not penetration.

The 20-mile rule of thumb is sound advice and will keep you from getting walled in by storm clusters. If you find that is happening, don’t waste time pulling up a sectional and finding a place to set down and wait it out for an hour. But the best way of avoiding storms is simply planning your flying activities as early as possible in the day close to the so-called “convective minimum” around dawn. In the mountains, things go downhill quick, so plan to be on the ground by late morning.

Occasionally large complexes of thunderstorms will develop and persist well into the night and into the following day. During the summer this is very common across the Midwest and the Northern Plains. These are referred to as mesoscale convective complexes (MCCs) and are important enough where the U.S. corn industry is dependent on them. They happen when summertime storms are augmented by nearby jet stream dynamics which allow for the development of a low-level jet that sets up a conveyor belt of tropical moisture northward. You can avoid them by paying attention to the Storm Prediction Center Convective Outlook and watching for large, elliptical areas of Slight or Enhanced Risks in the north central or northeast states.

Sometimes when these systems are strong enough they can lay down dangerous corridors of afternoon or evening hail and tornadoes, especially in places like Minneapolis, Chicago, Indianapolis, and Cleveland, so don’t discount severe weather just because it’s not spring. Summer is peak severe weather season from the Dakotas to the Great Lakes and New England.

When up in the air, your eyes should be your first indicator about storm activity, with datalink radar images second. Remember data can be 15 minutes old by the time it appears on your screen. At nighttime, lean on your datalink products, increase your safety margins, and don’t be afraid to get help from ATC as they likely have access to all the NEXRAD and TDWR displays.

Fog Events

Radiational fog events are common during the summer, especially in the southern and eastern states. This can be a problem because summer is usually when pilots aren’t thinking about fog. The favored setup for radiational fog is high soil moisture, recent soaking rains the previous day, clear skies at night, and light winds. If one of these is missing, the setup is likely to fail.

The short summer nights work against fog, since radiational cooling takes some time to set up. The temperature-dew point spread can be large during summer days; in other words relative humidity is low, and this also offsets fog development time. But the things you need to watch for are recent frontal passage, where rains have occurred. Cooler, drier air is being dragged out against this terrain, enhancing clearing.

There’s also advection fog and stratus, which is an important phenomenon due to the cold Pacific waters off of the West Coast. Here, sea surface temperatures are in the 50s, producing a sharp inversion and forming a “marine layer.” This usually rolls in or strengthens during the night, especially at places like San Francisco and Oregon coast airports, and are intensified when there are hot temperatures in the interior, especially in midsummer. If you’re flying cross-country and arriving at night, be aware that coastal airfields which were 10SM CLR in the afternoon may have deteriorated to LIFR by mid-evening. It’s good to have an inland alternate firmly in mind before departing, because an airfield that has gone downhill will likely only get worse.

Coastal stratus usually doesn’t go much more than 50 to 100 nautical miles inland, blocked largely by the coastal ranges, so many alternates will be accessible if your reserve fuel is in order.

Summer Monsoon

We often associate summer in the Desert Southwest with clear blue skies and relentless 105-degree temperatures, but that’s the kind of thing more common to May and early June. The Southwest Monsoon, when it hits, turns that idea upside down and brings dangerous and fast changing weather conditions that can leave VFR pilots caught in clouds and disoriented over the mountains. This has resulted in a not insignificant number of summertime crashes throughout the Four Corners region that are scattered across the NTSB archives. Knowing how the monsoon behaves and where it comes from will give you a better understanding of what to expect.

The Southwest Monsoon forms in response to the normal seasonal reversal of ocean versus land pressure gradient, where the oceans take on high atmospheric pressure (often about a 30.40″ ALSTG) while the land masses decrease (to about 29.60 to 29.80″ in the interior). This results in a pressure gradient that orients the wind field from ocean to land. The heating is very intense in the southwest quarter of the country. Over the higher plateau areas, the heating can exceed the ISA (International Standard Atmosphere) temperatures by well over 28° C (50° F), adding tremendously to the density altitude.

The result is an enhanced pressure gradient superimposed on the large scale land/sea pressure gradient, which brings oceanic air from the Eastern Pacific northward through the Gulf of California into the Desert Southwest. A second route is from the Gulf of America, and this passes through northeast Mexico and much of southwest Texas.

First to get the monsoon is New Mexico in June, though this year we have seen an early monsoon-like pattern due to a series of stalled fronts. Eastern Arizona and the Mogollon Rim sees the monsoon next, then by mid July we see the monsoon in the rest of Arizona, including the big cities. By August it reaches California and Nevada, then there is a gradual decline in storm activity through September as heating diminishes.

The monsoon has a strong diurnal cycle. Mornings are almost always VFR with scattered to broken mid and high layers. By midday, cumulus rapidly form on the mountains, favoring the southern sunlit slopes. These grow into thunderstorms, in the process moving out into the valleys in a direction that follows the mean tropospheric flow. Storm outflow radiating away in different directions triggers new secondary storms that can join together to form clusters. This activity diminishes after dark but can persist into the night if strong low-level winds continue to feed the storms with moisture.

Tropical Cyclones

As we get into July and August, for any flying in the southeast third of the country it’s important to have an awareness of any storms brewing in the tropics. Landfalling hurricanes, tropical storms, and tropical depressions are easily avoidable with proper planning, but their paths after landfall are difficult to forecast and it is important to not cut the forecasts close. Extensive heavy precipitation and large fields of IFR and MVFR conditions can reach far inland, often into the Mississippi and Ohio River regions and even the Great Lakes.

As a new forecaster one of the things that surprised me was the sheer amount of tropical moisture that these systems carry. We learned to it was safer to overforecast the impacts because the rich moisture and high precipitation efficiencies had a tremendous effect on weather downstream.

Tropical cyclones are deserving of an article of their own, so watch for that in the months ahead. For the time being, the National Hurricane Center should be your one-stop shop for figuring out what is on the way.

Forecast Discussion

Finally, don’t forget the NWS Area Forecast Discussion. If your preflight briefing calls for VFR and light winds all the way, you don’t need to worry about this. But otherwise, the forecast discussion is an enormously helpful tool because it’s written by very experienced, technically competent forecasters in your local area. If you’ve never seen this before it can be a bit intimidating, but stick with it and check it regularly. I guarantee you’ll learn a lot. Every forecast discussion has an Aviation Weather Discussion where you can get extra information that you won’t get in a typical phone weather briefing.

These discussions only focus on areas within the office’s area of responsibility, typically an area about 200 by 200 miles wide, so it’s good to understand the territory when using these products. Do a search for “NWS forecast offices.” One of the first links is a map showing the office’s area. Each Area Forecast Discussion serves this area only. And if you want to really dig into a cross-country route, you can make a list of all the areas along the route and read each Area Forecast Discussion’s aviation weather section one by one. With five minutes of quick reading I guarantee you’ll have a comprehensive understanding of almost anything that could possibly occur on that route.

Pireps

Finally, don’t forget to consider filing a PIREP. They are severely underfiled during the summer, to the point where they are often a topic of complaints. ATC sector controllers are sometimes too busy to type them in, but filing via Flight Service or WX-BRIEF guarantees they’ll be entered. The key items your fellow pilots want to see are the bases and tops of cumulus and towering cumulus clouds, ride quality, and in-flight visibility. If there’s any wind shear on the approach or departure, that’s important too. Negative PIREPs are important as well and are used to help shape AIRMETs and SIGMETs.