The forecast-to-warning chain

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 RFCRiver Forecast CenterA regional office that forecasts river levels and flooding, feeding the flood products a local office issues. or a WFOWeather Forecast OfficeA local National Weather Service office. It issues the warnings for its own area, and it is the office your report reaches. acts on it.

NHC

National Hurricane Center. Tropical cycloneThe general term for a tropical depression, tropical storm or hurricane. Their outer rainbands produce small, shallow supercells that are a recognised tornado threat well away from the centre. 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. NSSLNational Severe Storms LaboratoryA research laboratory, not an operational office. It develops the techniques forecasters use, but issues no products and takes no reports. builds and tests the techniques. It does not issue warnings and does not take your report.

From days out to minutes out

1. Convective outlookThe SPC product issued days ahead showing where severe weather is plausible and roughly how likely, drawn as categorical risk areas.. SPCStorm Prediction CenterThe national office in Norman, Oklahoma that issues severe weather outlooks, mesoscale discussions and watches. It does not issue warnings., days ahead. Where severe weather is plausible and roughly how likely.
2. Mesoscale DiscussionA short SPC product covering a small area a few hours ahead, usually explaining what forecasters are watching and whether a watch is coming.. 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.
3. Watch. SPC, in coordination with the WFOs it covers. Conditions are favourable over an area for the next several hours.
4. Warning. The WFO, minutes ahead, on a drawn polygonThe shape a warning is drawn on, replacing whole-county warnings in 2007. Being inside the shape is what matters, not which county you are in.. 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 PDSParticularly Dangerous SituationWording reserved for the rare watch, and occasionally the warning, where long-lived strong or violent tornadoes, or an extreme wind event, are expected. It is not a change of tone; treat it as an instruction to be in position early. 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

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 summarises 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 GroupsConditional Intensity GroupHow intense SPC expects storms to be if they happen, on a scale of CIG1 to CIG3, separate from how likely they are. Since March 2026 it feeds into which risk category an area is given., CIG1 to CIG3, which say how intense the storms are expected to be if they happen. They replace the older hatched areaThe older cross-hatched shading on an SPC outlook, marking where significant severe weather was possible. It could only say yes or no, and conditional intensity groups replaced it in March 2026., which could only say yes or no to significant severeThe upper tier of severe: hail of two inches or more, or wind of 65 knots or more. It is also the range where radar loses almost all skill at sizing hail.. 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.

The scale also coarsens with lead timeHow long between a warning being issued and the severe weather arriving. Quoted two ways: counting only the warnings that beat the event, or averaging in the events nobody warned at all, which roughly halves it.. 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.

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 CWACounty Warning AreaThe counties and zones one forecast office is responsible for forecasting and warning. Storms crossing the boundary are handled by two different offices. boundary is handled by two offices, and a report on either side goes to a different desk.