A Mesoscale Discussion, or MD, is a short-fuse SPC product that describes rapidly evolving conditions over a specific region, typically issued in the hour or two before a watch or in response to an ongoing severe weather episode. It is the bridge between the longer-range convective outlook and the eventual watch or warning.
What an MD contains
An MD has a graphical component and a text component. The graphic shows an outlined polygon over the area of concern. The text summarizes the current mesoanalysis, the ingredients coming together, the forecaster's expectation for the next few hours, and if applicable, an estimate of the probability that a watch will be issued and when.
MDs cover more than severe thunderstorms. SPC issues them for winter weather setups and heavy rain episodes as well. In severe weather context, the most operationally important MDs are the pre-watch discussions and the in-watch updates issued as an episode evolves.
How forecasters and chasers use MDs
NWS forecast offices monitor MDs to get advance signal on where watches will land, which shapes their own severe messaging and staffing. Chasers use them to refine targets during the last hour of pre-event repositioning, because an MD often names the specific boundary or feature the forecaster is watching.
An in-progress MD issued after storms have fired is also useful. It updates the operational read on whether the current environment favors tornado, wind, or hail as the dominant mode, and how long the threat window is expected to remain open.
What an MD is not
An MD is not a watch or a warning. It carries no protective-action guidance, no NOAA Weather Radio activation, and no wireless alert. It is a professional forecast discussion, not a public safety product. Members of the public who see one referenced in coverage should treat it as advance context.
An MD is also focused and short-lived. Its polygon can be narrower than the surrounding categorical outlook, and its reasoning may be superseded within an hour if conditions shift. Read the most recent one, not the one from three hours ago.