Mesoscale Discussion 0166
Issued by: NWS Storm Prediction Center
Time: 09:57 AM CST, Sat Mar 07, 2026

Areas Affected: Southern Ohio, much of Kentucky, western and middle Tennessee, northern Mississippi, northwestern Alabama.

Severe Potential: Severe weather watch possible.

Summary:
Thunderstorm activity is intensifying and may lead to strong to severe wind gusts as it spreads eastward through 1-3 PM CST (2-4 PM EST). A severe weather watch may be needed soon.

Discussion:
Stronger surface pressure rises (3-5 mb) are moving northeast from northeastern Arkansas into Missouri. This is linked to a low-amplitude jet streak forecast to affect Kentucky and Tennessee by late morning. Winds in the 850-700 mb layer are expected to be 30-50 knots. With ongoing heating and moisture advection, conditions are favorable for thunderstorms to strengthen and possibly produce severe wind gusts later in the day.

 

I am learning new words again, from Google:

Mesoscale refers to atmospheric phenomena spanning roughly to, operating between microscale (small, local) and synoptic-scale (large, continental) systems. Key examples include thunderstorms, squall lines, land-sea breezes, and urban heat islands. These systems are studied using high-resolution observational data, remote sensing, and numerical modeling (e.g., WRF model) to predict localized, high-impact weather. 

A Mesoscale Discussion (MD) is a technical bulletin issued by the Storm Prediction Center (SPC) in Norman, Oklahoma. Its primary purpose is to provide short-term guidance on organized, hazardous weather occurring or expected to develop within the next few hours.

While a "General Forecast" covers days, and a "Warning" covers minutes, an MD fills the gap by focusing on the mesoscale—weather systems that are larger than a single thunderstorm but smaller than a cross-country cold front (typically 10 to several hundred miles wide).

Key Features of an MD
1. The "Why" and the "When"
An MD is essentially the SPC’s internal "thinking" made public. It explains the atmospheric ingredients—like instability, wind shear, and moisture—that are currently coming together. It usually addresses:

Severe Thunderstorm or Tornado Potential: Often issued to indicate that a Watch is likely or to explain why one is not being issued.

Winter Weather: Discussing heavy snow rates (e.g., 1–2 inches per hour) or significant freezing rain.

Heavy Rainfall: Analyzing the potential for flash flooding.

2. The Map and "Polygon"
Each MD includes a map with a highlighted area (a polygon). This isn't a warning, but rather a "heads up" for meteorologists and emergency managers in that specific region to stay weather-aware.

3. Probability of Watch Issuance
One of the most critical parts of an MD is the Probability of Watch Issuance. The SPC will give a percentage (e.g., 5%, 40%, 80%) indicating how likely they are to issue a formal Tornado or Severe Thunderstorm Watch for the highlighted area.