Drop Zone Weather Forecasting

We’ve all been there—sitting on the ground for hours, glaring at the billowing wind sock as powerful gusts threaten to free the packing tent from its earthly tethers. The weather can seem fickle, so how is a safety-conscious skydiver—one who prefers to arrive and depart the DZ in the same unbroken condition—to know whether to place a bet on better conditions later in the day or to admit that Mother Nature is holding the trump card and fold early?

Following the steps below will help you place that winning bet. While reading this article won’t turn you into the next Jim Cantori, it will provide you with a few meteorological tricks of the trade so you can put together a fairly accurate same- or next-day forecast for skydiving.

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Step 1: Check the Local Forecast
You can use any reliable source for your forecast information. The National Weather Service (weather.gov) and Weather Underground (wunderground.com) are both easy-to-use web resources that conveniently compile METAR (Meteorological Aviation Report) data, local forecasts, regional radar and regional satellite images on one page for one-stop weather shopping.

First, take a look at the current conditions and short-term forecast for your area. Pay particular attention to projected high and low temperatures, the winds and cloud cover. Notice whether those are expected to change much over the next couple of days. If the forecast is fairly consistent, your location is probably under the influence of high pressure (associated with generally fair weather) or is transitioning between high- and low-pressure systems. If you see partly cloudy, mostly cloudy or cloudy skies in the forecast and notice a significant change in the expected temperatures and/or winds over the next several days, a low-pressure storm system is likely on the way, and your forecast will be more complicated.

Keep in mind that the farther into the future you look, the less accurate any forecast is going to be.

Step 2: Identify the Major Meteorological Players
Once you know what the professionals are predicting for your region, it’s time to determine which meteorological forces are controlling those skies. To do this, check a national surface weather map that displays areas of high and low pressure, frontal boundaries and radar or satellite images.

Surface frontal boundaries (cold fronts, warm fronts, occluded fronts and stationary fronts) are responsible for the majority of significant weather changes we experience, so note any fronts in your area. Upper-level atmospheric features can also have strong influences on the weather. These features will not show up on surface maps, but look on a satellite image and you’ll be able to see whether they’re creating clouds and precipitation. As you study a national satellite image, you should notice that the pattern of cloud cover corresponds fairly well to the presence of low- and high-pressure systems, with clouds found near low-pressure systems and generally clear skies found near high-pressure systems.

If your area was free and clear of nearby fronts but you see cloud cover over your region, the clouds are likely due to an upper-level feature or topographical/geographical effects (more on this a bit later).

Step 3: Determine What to Expect

Cold Front in Your Area:
If a cold front is in your area, local surface winds will typically be out of the south or southeast and will likely strengthen and become gustier as the cold front approaches. High, thin cirrus and scattered cumulus clouds can precede the front by a couple of days but will quickly give way to rain and storm clouds as the cold front nears. Heavy showers and strong-to-severe thunderstorms often form along and ahead of the front. As these storms progress across the country, they frequently merge together to form a thin line of storms, known as a “squall line,” just ahead of the frontal boundary.

You can monitor the progress of clouds and precipitation associated with an advancing cold front using local radar and satellite image loops. Behind the cold front, expect clearing skies and a rapid end to precipitation, along with a sharp shift in wind direction. Strong north-to-northwesterly winds will push in colder and drier air, and you’ll likely see fair weather for several days after the front passes.

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Also pay attention to the winds-aloft forecast. As a front passes through, the winds aloft will be slower to switch direction than those on the ground. The winds and temperatures aloft are given in forecast blocks valid for several hours; however, if a front passes during that forecast period, it is entirely possible for winds to start shifting direction before the next forecast period begins. If the direction and speed of the winds aloft change dramatically from the current forecast period to the next, expect winds aloft to be in flux throughout both time periods. Assuming you’re skydiving after the clouds clear, prepare to adjust the spot continuously as the winds swing around.

Warm Front in Your Area:
Like cold fronts, warm fronts also bring clouds, a change in wind direction and, often, precipitation. However, the weather changes associated with warm fronts are generally less abrupt and less severe than those associated with cold fronts, and because warm fronts move more slowly than cold fronts, the clouds and rain tend to stick around longer.

When a warm front is in your area, expect cloud cover to gradually increase over the course of a day or two. Clouds thicken and cloud bases lower. A broad swath of light-to-moderate rain is common, and rain can last a day or more, lingering even after the front has passed. During the summer, thunderstorms are sometimes embedded within the broader area of rain both ahead of and behind the front. However, the intensity and coverage of the rain depends largely on the availability of moisture, so warm fronts do not guarantee extended periods of rain. Monitor local satellite and radar trends to determine when clouds and rain will move in and when they will finally depart.

Behind a warm front, temperatures usually increase and moderate easterly winds shift to a more south or southwesterly direction. This wind shift can be subtle, though, and the temperature increase might not be immediately noticeable. You should again monitor the winds-aloft forecast for shifts in the upper-level winds behind a warm front, but don’t expect the abrupt shift associated with cold-frontal passage.

Stationary or Occluded Front in Your Area:
A stationary front forms when an advancing cold front or warm front loses steam and stalls out. The weather associated with a stationary front is similar to that associated with a warm front—broad areas of cloud cover and light-to-moderate rain; however, stationary fronts can also act as a focus for scattered thunderstorms during the summer. Unfortunately for skydiving operations, stationary fronts often stick around a given region for days.

An occluded front occurs when a fast-moving cold front catches up to and overtakes a slow-moving warm front. Ahead of the occluded front, the weather is similar to that associated with a warm front, while along and behind the occlusion, the weather is similar to that associated with a cold front.

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Under a High-Pressure System:
When an area of high pressure parks over your region, consider yourself graced by the skydiving weather gods. Strong high-pressure systems suppress cloud formation and bring relatively light winds at the surface. However, an exception most jumpers have probably noticed is that on otherwise ideal skydiving days, surface winds tend to be lighter near sunrise and sunset but gain in strength, turbulence and gustiness during the heat of the day. This is part of the diurnal (day-to-night) cycle, and it is most noticeable when the sky is mostly clear and the sun is fairly strong—as is often the case under the influence of summertime high pressure.

Under such conditions, vertical thermals form, lifting air from the surface and replacing it with air from aloft. Since winds aloft are generally stronger than those at the surface, these thermals “mix down” the stronger winds. A common indication of vertical thermals (and a bumpy canopy descent in general) is the development of fair-weather cumulus clouds during the afternoon. However, these clouds will form only when the air is moist, so while they are an indication that vertical mixing is occurring, their absence on clear, dry days does not guarantee that mixing is not occurring.

When the heat of the day finally subsides near sunset, so too do the thermals. Surface winds quickly diminish, and the sunset load is graced with beautiful views and easy landings.

Between High- and Low-Pressure Systems:
Between high- and low-pressure systems, conditions can be somewhat variable. If a high- and a low-pressure system are close together, winds on the ground and aloft can be quite strong, and unlike strong winds created by diurnal heating, winds generated by the squeeze play between high- and low-pressure systems tend to persist throughout the day and evening.

If winds are blowing off of a large body of water (e.g.: the famous “lake effect” experienced in regions south of Lake Erie and similar conditions that frequently plague coastal locations), they tend to bring moisture to the region, which can result in scattered to widespread cloud cover. Since a number of factors affect whether and how clouds form in this scenario, your best bet to predict cloud cover associated with this type of weather pattern is to monitor a satellite image loop.

Topographical or Geographical Effects:
Forecasting the weather for a coastal or mountainous DZ requires taking terrain and geographical features into account.

In addition to the influence of low- and high-pressure systems, skydivers at drop zones near the coast have to consider the changing surface winds associated with the daily resurgence of the sea breeze. Sea breezes tend to develop shortly before noon and strengthen until the hottest point of the day (usually around 3 to 4 p.m. at the height of summer) before weakening near sunset. A sea breeze always blows onshore (from the sea to the shore), but its strength can change dramatically throughout the day—from calm in the morning to upwards of 15 or 20 mph during the afternoon.

Since sea breezes typically do not extend more than a couple thousand feet into the atmosphere, the formation of a strong sea breeze can also lead to the phenomenon that skydivers call a “dog leg” and meteorologists term “vertical wind shear.” This usually occurs right at most jumpers’ opening altitudes. If winds aloft are blowing in any direction other than onshore, the formation of a sea breeze will cause winds at the surface to be different than those aloft. Such conditions can make it quite tricky to spot for the aircraft, since freefall and canopy drift can be in dramatically different directions.

Sea breezes can also enhance cloud development and trigger shower and thunderstorm activity. A field of puffy cumulous clouds develops almost daily at most coastal drop zones during the summer, and it’s fairly common to see a broken line of showers and thunderstorms form parallel to the coast on such afternoons. This line marches inland as the day progresses, pushed ahead of the strengthening sea breeze, and once the line of showers moves inland of a given location, the rain chances are pretty much over for the afternoon.

In mountainous regions near water, cloud (and possibly rain) formation is likely whenever the prevailing wind is blowing onshore. As this wind is forced by topography to rise, it cools and condenses to form clouds and, if enough moisture is present, rain. With a strong onshore prevailing wind, clouds and rain can form even under the influence of a moderately strong high-pressure system.

Wrapping It Up
There you have it—three fairly simple steps to transform a skydiver into a proficient amateur forecaster. To form a general idea of what weather to expect, check the local forecast, surface map, winds aloft and radar and satellite images before you head out to the DZ. Then, if you’re lucky enough to jump at a DZ with wireless internet access (or where “3G” means something other than your pull altitude), follow the progress of any clouds or rain throughout the day using local radar and satellite images.

Feel like a rock star when you show up at the DZ just as the clouds are clearing or when you predict to the hour when the winds will die down in the late afternoon. Of course, you can’t jump if you’re not at the drop zone, so don’t put too much faith in a forecast! Sometimes the weather gods smile unexpectedly.

About the Author
FEATURE20116-30Megan Walker-Radtke, B-34746, is an atmospheric and environmental scientist as well as a passionate science educator and communicator who is always looking for new opportunities to share her enthusiasm for the natural world with others. Although currently residing in sunny Florida, she grew up chasing storms in the heart of Oklahoma’s Tornado Alley and still wants to be Helen Hunt’s character from the movie Twister when she grows up.

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Michal
Tue, 05/26/2015 - 02:57

I don't understand why not just look at an hourly or daily forecast (depending on when you want to jump) on, e.g.: wunderground.com?

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