When the Storm Prediction Center paints a risk area over your town, these are the words behind the colors. This page covers the SPC’s categorical outlooks from Marginal to High Risk, conditional intensity groups, and the discussion products forecasters issue around them. Coverage Experts in the Xtreme Weather Discord (XWD) reference these constantly during severe weather, and the parameters behind them live on the Instability & CAPE and Wind Shear & Composite Indices pages.
The Storm Prediction Center (SPC) is the NWS national center in Norman, OK responsible for severe thunderstorm, tornado, and fire weather forecasting across the contiguous U.S. They issue Convective Outlooks, Mesoscale Discussions, and Watches.
SPC Categorical Outlooks rank severe weather probability from lowest to highest: TSTM → MRGL (1) → SLGT (2) → ENH (3) → MDT (4) → HIGH (5). As of March 2026, these are paired with Conditional Intensity Groups (CIG1–CIG3) to communicate how strong storms could be if they form.
Marginal Risk (MRGL / Cat 1) is the lowest tier of SPC categorical outlook. Isolated severe storms expected with limited duration and coverage. Tornado probability typically <2%.
Slight Risk (SLGT / Cat 2) indicates scattered severe storms with short-lived/varying intensity. The most common severe weather day — tornado probability typically 2–5%.
Enhanced Risk (ENH / Cat 3) means numerous severe storms with greater coverage and intensity. Significant severe weather becomes more likely — often paired with a CIG1 or higher intensity tag.
Moderate Risk (MDT / Cat 4) is the second-highest tier — widespread severe storms likely, including strong tornadoes and significant wind/hail. MDT days produce most major outbreaks short of historic levels.
High Risk (Cat 5) is the rarest and most extreme SPC outlook tier — issued ~3 times per year on average. Indicates a major outbreak of long-tracked, violent tornadoes and/or a widespread derecho. Treat as a generational severe weather day.
Conditional Intensity Groups (CIG1–CIG3) are SPC's intensity overlay introduced in March 2026, replacing the old "Significant Severe" hatched area. They run parallel to the categorical risk (MRGL → HIGH), which still measures probability. CIGs answer the second question forecasters get most: "how strong could it be?"
Hail outlooks only use CIG1 and CIG2.
CIG1 (Conditional Intensity Group 1) is the lowest of SPC's three intensity tiers. Indicates a baseline significant severe threat if storms develop:
Replaced the old 30% "Significant Severe" hatched area for ENH/MDT outlooks.
CIG2 (Conditional Intensity Group 2) is the middle SPC intensity tier:
Indicates a meaningfully higher ceiling than CIG1, even if probability stays the same.
CIG3 (Conditional Intensity Group 3) is SPC's highest intensity tier — reserved for the most extreme conditional environments:
CIG3 paired with a HIGH risk = generational outbreak setup.
A Mesoscale Discussion (MCD) is a short-fuse SPC product highlighting an area where severe weather is becoming likely in the next 1–3 hours. MCDs often precede watch issuance and contain the forecaster's reasoning about the setup.
SPC Mesoanalysis is the Storm Prediction Center's near-real-time gridded analysis of severe weather parameters — CAPE, CIN, SRH, STP, SCP, hodographs, boundaries, and more — updated hourly. The single most commonly used operational tool for analyzing the current severe weather environment. Freely available at spc.noaa.gov.
WoFS (Warn-on-Forecast System) is an experimental NOAA/NSSL prediction system that runs a high-resolution convection-allowing model every 30 minutes, producing probability-of-rotation forecasts for the next 0-6 hours. The goal is to shift severe weather warnings from observation-based (Warn-on-Detection) to forecast-based, ultimately extending tornado lead times from minutes to ~30+ minutes.
ProbSevere is an ML-based NOAA tool that provides real-time probabilities of a storm producing severe weather (tornado, large hail, damaging wind) within the next 30–60 minutes. It ingests radar, satellite, and lightning data. Forecasters use ProbSevere to support SVS decisions and prioritize storm-tracking attention.