The forecast-to-warning chain
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Six acronyms issue nearly everything you will act on, and they do not do the same job. Knowing which office owns which product tells you who your report is for and when it is worth making.
In this lesson you will
- Describe the roles of SPC, WFOs, WPC, NHC, and RFCs, and what NSSL is (research, not operations).
- Order products from days out to minutes out: outlook, mesoscale discussion, watch, warning.
- Interpret SPC categorical risk and what the probabilities behind each category mean.
- Distinguish watch vs. warning, and who issues each (SPC vs. WFO).
- Identify your WFO and its County Warning Area.
Who does what
- SPC
- Storm Prediction Center. National severe convective outlooks, mesoscale discussions, and watches. Works in hours to days, across the country.
- WFO
- Weather Forecast Office. 122 of them, each responsible for a County Warning Area. Issues the warnings, and is the office your report reaches.
- WPC
- Weather Prediction Center. National precipitation and excessive-rainfall guidance, which is where flooding starts before an RFC or a WFO acts on it.
- NHC
- National Hurricane Center. Tropical cyclone forecasts and the products that go with them.
- RFC
- River Forecast Centers. Hydrologic forecasting, feeding the river and flood products a WFO issues.
- NSSL
- National Severe Storms Laboratory. Research, not operations. NSSL builds and tests the techniques. It does not issue warnings and does not take your report.
From days out to minutes out
- Convective outlook. SPC, days ahead. Where severe weather is plausible and roughly how likely.
- Mesoscale Discussion. SPC, hours ahead and much smaller. This is the signal that a watch is likely, and it is worth watching before your phone goes off.
- Watch. SPC, in coordination with the WFOs it covers. Conditions are favourable over an area for the next several hours.
- Warning. The WFO, minutes ahead, on a drawn polygon. Something is happening or is imminent for the people inside that shape.
Counties are added to and cleared from a watch by the WFOs, not by SPC. That is why a watch can end for your county while it is still running two counties over. A PDS watch, particularly dangerous situation, is the wording SPC uses when the expected outcome is much worse than a routine watch. Treat it as a cue to be in position early.
Reading the SPC categorical scale
| Category | Abbreviation | Level |
|---|---|---|
| General thunderstorms | TSTM | none |
| Marginal | MRGL | 1 |
| Slight | SLGT | 2 |
| Enhanced | ENH | 3 |
| Moderate | MDT | 4 |
| High | HIGH | 5 |
Each probability is the chance of that hazard within 25 miles of a point.
The category is whichever of the three hazards, tornado, wind and hail, reaches furthest, and the thresholds are not the same for each: the tornado probability that earns a category is lower than the wind or hail probability that earns the same one. The category summarizes the three maps. It does not replace reading them.
TSTM is the odd one out. It is the general thunderstorm contour, roughly a one-in-ten chance of any thunderstorm at all, and it carries no severe level, which is why the column above has no number against it. A day drawn entirely in TSTM is not a quiet severe day. It is a day with no severe risk area on it.
Since 3 March 2026 the conversion from probability to category also folds in Conditional Intensity Groups, CIG1 to CIG3, which say how intense the storms are expected to be if they happen. They replace the older hatched area, which could only say yes or no to significant severe. A modest coverage probability paired with a high intensity group now reaches the same category as a much higher probability on its own, so the conversion is a table rather than a line. The categories themselves did not change.
HIGH is reserved for a high-end tornado threat or a widespread, high-end wind event of the derecho kind. Hail alone never produces a HIGH risk, whatever the size. If you are looking at a HIGH, somebody is expecting either tornadoes or a wind swath.
The scale also coarsens with lead time. Day 2 works by hazard the way day 1 does; day 3 collapses to a single any-severe probability plus intensity; days 4 to 8 offer only 15% and 30%, roughly SLGT and ENH. That is worth remembering the next time somebody treats a day-6 outlook as a plan.
The categorical outlook is drawn as a selectable layer on the [WxAlerts live map](/), so the day's risk areas sit over the radar you are already watching.
Find your office
Every report you make goes to one specific WFO, and every warning you act on came from one. Learn which office, learn the shape of its County Warning Area, and learn where its boundaries fall relative to the roads you drive. A storm crossing a CWA boundary is handled by two offices, and a report on either side goes to a different desk.
Check yourself
A tornado watch is in effect, and then a tornado warning is issued for your town. Who issued each?
- SPC issued the watch; your local WFO issued the warning
- Your local WFO issued both
- SPC issued both
- NSSL issued the watch; your local WFO issued the warning
Hint
One of these four organizations does research and issues nothing at all.
Answer
SPC issued the watch; your local WFO issued the warning
Watches come from SPC, in coordination with the WFOs they cover, and the WFOs add and clear their own counties. Warnings are the local office's, on a polygon. NSSL is a research laboratory and issues no operational products.