Wizards of storm forecasting

NOAA Storm Prediction Center applies state-of-the-art techniques to figure out where severe weather may cause trouble for pilots.

Meteorologist Joe Polina from the National Weather Service's New York weather forecast office prepares one of 9 airport terminal forecasts issued daily. These TAFs incorporate convective forecast information issued by the SPC.

Another SPC product that can be of use as a supplement to the convective outlook is the experimental thunderstorm outlook—a graphical product that displays the forecast probability of thunderstorms occurring at a given location within the indicated area.

Such information can be useful when trying to determine just how widespread the storm activity is likely to be in the vicinity of your airport or flight route. However, as with any experimental weather product, it should not be relied on as a primary tool for weather decisionmaking.

Watches and warnings

If you're planning to take to the air in the next hour or so, you might also check out the SPC's severe weather watches. The center is responsible, in collaboration with local NWS offices, for issuing severe thunderstorm and tornado watches, and generally issues them at least an hour ahead of when SPC forecasters feel a severe weather threat will materialize.

While these watches are intended primarily for alerting people on the surface, they can help a pilot verify the convective outlooks and give them a more precise idea about where severe weather is likely to occur. Although the probability of severe weather actually occurring in a watch area is different for each watch—and a watch is not a guarantee that severe weather will occur—a severe weather watch box should generally be treated by pilots as a TFR area. In other words, avoid it if you can.

Severe weather warnings, which are not issued by the SPC, indicate that severe weather conditions are currently present (hail over 1 inch, wind exceeding 50 kts, and/or a tornado—criteria which differ slightly in other countries).

The SPC does display all active warnings on its hazards map, which shades individual counties or coastal zones based on the watch or warning type. Should a tornado or severe thunderstorm warning be issued near your airport of flight corridor, think of it as nature's no-fly zone—there will be forces at work trying to knock you out of the sky. If you are on the ground, seek appropriate shelter.

Extra guidance

Beyond the SPC's operational severe weather products, there are a number of non-operational or experimental products, such as the previously mentioned thunderstorm outlook. These experimental products can be accessed via the "Forecast Tools" link under Research along the left side of the SPC webpage.

On the experimental products page you will find links to upper air and sounding charts, and graphical model output. Two very useful products are the short-range ensemble forecast (SREF) and the Compmap tool.

Outlook showing the probability of thunderstorm winds exceeding 50 kts within 25 miles of a point within the shaded region. The hatched region is reserved for at least a 10% likelihood of winds exceeding 65 kts (the speed at which a storm would be classified as severe).

SREF is a graphical forecast generated by combining the output from 22 different runs of NOAA's weather models. The outputs are processed for "high-impact, mesoscale guidance." In lay terms, these forecasts focus on indicators of severe weather, such as maps of potential energy, instability and moisture. Along the top of the graphic, however, is an "Aviation" tab that lets you display forecasts of convective cloud top heights, and the probabilities that those tops will reach various flight levels.

Compmap is a tool that lets a user create custom weather maps by selecting different products to display. These include satellite images, weather forecast model output, and outlooks. Each variable can be toggled on and off to generate a composite map that can help a pilot better visualize a forecast or briefing.

A final experimental product offered by the SPC is the mesoscale analysis, which acts as a sort of clearing house for graphical weather information. From here you can access surface observations, upper air soundings, satellite and radar imagery, severe weather indices, and dozens of other variables that meteorologists use to make forecasts of adverse and severe weather.

What's more, while you can get an over­view of the entire coterminous US, you can also zoom in to various individual regions and gain greater detail about where adverse weather may be lurking.

While most of the variables require a some expert meteorological knowledge to interpret fully (which should give you a better idea just how much information meteorologists pore over to make your preflight forecast), a look at those with which you're already familiar should provide a pretty good picture of the state of the atmosphere through which you intend to fly.

But taking the time to learn a bit about some of the main severe weather indices can greatly en­hance your use of the mesoscale analysis product. Such indices include convective available potential energy (CAPE), lifted index and K index. Other variables that might be useful are shear, lifted condensation level (LCL) and 850-mb moisture transport.

For example, the K index is a measure of the capability of the atmosphere to initiate convection. A K index above 40 indicates high potential for convection, while values below around 20 mean convection is not very likely. Simil­arly, CAPE is a measure of the amount of energy available for storms to use if they form. Values above around 2500 are good indicators that, if a storm forms, it will have enough energy to become severe.

The lifted index will tell you how unstable the air is. The more negative the index, the more unstable the atmosphere. Values below –4 mean great instability, while values below –7 are extreme, and would mean explosive growth of a developing storm.

If you want to know where the cloud bases are to be found, the LCL is the way to go. The lifted condensation level is the height at which air that has been lifted will become saturated. The base of the lowest cloud deck, and any cumulonimbi, will be somewhere near the LCL.

Shear values (such as the bulk effective shear) are straightforward. They simply measure the difference in winds at different levels to identify regions where the atmosphere might be tilting or rotating the updraft of a storm, thus increasing its potential to become a supercell. High shear values are more indicative of severe weather than low ones.

Likewise, the inflow of moisture at 850 mb provides valuable information as to whether a storm will have a source of fuel (water vapor). High moisture values flowing into an area of convective potential increase the likelihood of storms.

While the SPC products are intended primarily for other meteorologists (and the amount of technical information presented by these products can be overwhelmingly daunting to a pilot), the SPC does provide some help by describing what different terms mean.

Many pages have links to more information. Also, a more detailed description of SPC's products and how to interpret them can be found at spc. noaa.gov/misc/about.html. And even pilots with advanced degrees in meteorology should also remember that while the SPC products do provide a fairly comprehensive weather picture, they are almost all supplementary products, and cannot be substituted for a proper weather briefing.

In addition, most are forecast products, and as such may be in error from time to time, especially when weather conditions are changing rapidly. But the tools offered by the SPC can be of great value to help you visualize the potentially severe conditions the briefer may have warned you about.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.


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