Forecasting
SPC mesoscale discussions
Mesoscale Discussions are the intermediate step between an SPC outlook and a watch box. Here is how to read one like an SPC forecaster.
What an MD is
An MD is a short-term (0-6 hour) discussion of a specific weather concern. SPC issues them ahead of watches, or in place of watches when the threat is less clear.
MDs cover: convective initiation timing, severe thunderstorm/tornado concerns, hail and wind timing, mesoscale features (drylines, cold fronts, outflow boundaries), heavy rain/flash flood potential.
The structure of an MD
- Header: MD number, WFOs affected, valid time.
- Area: Latitude/longitude bounds of concern.
- Concerning: One-line summary — e.g., "Severe potential" or "Watch likely."
- Discussion: The paragraph explaining current setup, expected trends, and forecast concerns.
- Probability of watch: Percent chance a watch will be issued.
The key phrases
"Watch likely"
A tornado or severe thunderstorm watch is expected within 1-2 hours.
"Watch possible"
Watch may be issued if trends continue.
"Watch not currently anticipated"
MD is FYI — SPC is monitoring but not planning a watch.
"Discrete supercells"
Individual isolated supercells expected. Tornado risk higher.
"Line segment"
Squall line or QLCS. Wind + brief spin-ups more likely.
"Cluster"
Multiple storms merging. Hail + wind main risks.
"Rapidly intensifying"
Storm is strengthening quickly. Warnings likely.
"Weakening/dissipating"
Storm losing structure. Warning expiration coming.
"Downstream"
Applies to areas ahead of current storm.
"Upstream"
Applies to areas behind current storm.
The convective parameters they cite
- MLCAPE — Mixed Layer CAPE. Instability.
- MUCAPE — Most Unstable CAPE. Buoyancy of any parcel.
- 0-6km bulk shear — Wind change over the depth of the storm. >40 kt favors supercells.
- 0-1km SRH — Storm-Relative Helicity in the near-ground layer. Tornado potential.
- LCL — Lifted Condensation Level. Low LCL = tornado favored.
- STP — Significant Tornado Parameter.
- SCP — Supercell Composite Parameter.
- CIN — Convective Inhibition. Cap that needs to break.
Reading between the lines
Experienced SPC-watchers learn to read subtext. Some tells:
- "Watch likely" + high probability = expect it in the next hour.
- "Considerable uncertainty remains" = forecaster is genuinely unsure.
- "Trends to be closely monitored" = they're watching a specific parameter.
- "Cell mergers" = storms may consolidate; watch for tornado.
- "Elevated" storms = above cold air, less tornado risk.
- When a specific supercell is named ("supercell near ADM") = they're watching it minute by minute.
How MDs become watches
Not every MD leads to a watch. Rough sequence:
- MD issued 1-3 hours before initiation.
- If storms fire and organize, MD gets an updated watch probability.
- When probability >70%, watch coordination with WFOs begins.
- Watch issued.
- MDs continue during the watch to track evolution.
- MDs after the peak often discuss dissipation timing.