Wx Watch: Mesoscale discussions

Severe weather, under a microscope

May 1, 2013

Mesoscale discussion

Most of us are diligent about studying convective sigmets, convective outlooks, and Nexrad radar imagery before our flights in the warmer months. They’re great for a broad-brush view of any convective situations, but there is another source of information that delves into greater detail, and provides analyses of situations that could be factors in your decision making.

Mesoscale discussions (MCD) are issued by the Storm Prediction Center, and they’re generated whenever forecasters see a combination of threatening elements with the potential to create dangerous weather. In other words, they’re short-term “nowcasts,” and often are valid at the time of issuance. Compared with convective outlooks, which give up to three-day (and sometimes out to eight days!) predictions of areas that may experience thunderstorms, mesoscale discussions are hair-trigger assessments of smaller-scale events based on near-real-time evidence. MCDs also are issued for winter weather, localized high winds, and heavy precipitation situations.

You can find MCDs by going to the Aviation Digital Data Service (ADDS) website and clicking on the Convection tab at the top of the homepage. Under the Severe Weather Products heading on the page, click on the Mesoscale Discussion link—it’s right above the link to Convective Outlooks.

Sigmets and convective sigmets draw most of our attention, but remember that these advisories are valid for large-scale areas. How large? Try 3,000 square miles. Mesoscale discussions, on the other hand, can be very specific in predicting the locations of any threats. They sometimes are focused on areas no larger than a few counties. (The American Meteorological Society defines mesoscale phenomena as those that have horizontal scales ranging from a few to several hundred kilometers.)

That sort of granularity can be much more helpful than a blanket warning, which amplifies the information in sigmets—and it’s too bad we can’t access MCDs through datalink weather providers. Sure, sigmets, convective sigmets, and Nexrad radar imagery are the mainstays of our datalink information stream, but when you’re facing thunderstorms and other forms of meteorological mayhem, the more information the merrier.

When a day becomes convective, you can expect a flurry of MCDs as storm systems travel, change their nature, and affect more and more geography. To see what I mean, check the MCD archives for past storm days by clicking on the MCD page’s “retrieve MCDs” button and type in the date that interests you.

Each posting comes with a chart outlining the threat area and a brief text explanation of the how, why, and magnitude for each event. There’s a short summary, plus a rating of the probability of the event’s requiring a weather watch or warning. Yes, there’s likely to be a lot of meteorological jargon, so be prepared—but don’t dwell on it. The plain-language summary tells just about everything you need to know.

Your ground school curriculum probably didn’t mention MCDs, but they’re yet another asset in our weather-information arsenal. What did we do before the Internet, when this sort of tactical analysis was much more difficult to find?

Email tom.horne@aopa.org.

Thomas A. Horne

Thomas A. Horne | AOPA Pilot Editor at Large, AOPA

AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.