NOAA SPC GREFS

GribStream Code: grefs

SPC GREFS probability guidance

Calibrated tornado, hail and damaging-wind probabilities combining atmospheric conditions from GEFS with storm characteristics from REFS.

Parallel guidance: These datasets are available in GribStream using NOAA’s parallel feed. NOAA plans to make SPC_POST v3 operational on or after 14 October 2026, beginning with the 12 UTC cycle. Severe weather may delay the transition. Read the implementation notice.

Cycles and probability periods

Runs at 00, 06, 12 and 18 UTC provide four-hour probability windows ending every hour from +4 through +60 hours, plus remainder-of-convective-day guidance. Select TORPROB, HAILPROB or WINDPROB at surface.

Each REFS run can be paired with GEFS from the same cycle or from six hours earlier. Choose the exact GEFS lag 0h or GEFS lag 6h selector; these are separate guidance versions, not individual ensemble members. The two versions may become available at different times. Remainder-of-day periods vary in length; each selector gives the exact duration.

What does GEFS input lag mean? For a REFS run initialized at 12 UTC, GEFS lag 0h uses GEFS initialized at 12 UTC; GEFS lag 6h uses GEFS initialized at 06 UTC. Both use the same 12 UTC REFS run. At 00 UTC, the six-hour-lagged GEFS input is from 18 UTC on the previous day. The lag describes how old the GEFS input is. It does not change the forecast period or indicate a publication delay. A smaller lag does not necessarily mean a more accurate forecast.

Explore the SPC probability dashboard.

Querying probabilities

Values are percentages from 0 to 100. Select the exact name, level and info combination from the inventory. The ensemble information has already been combined into calibrated probabilities, so you do not select individual members.

{"name":"TORPROB","level":"surface","info":"4 hour probability; GEFS lag 0h; prob >0","alias":"tornado_pct"}

Use this variable in a POST /api/v2/grefs/timeseries request, or /api/v2/grefs/runs to compare initialization times. forecasted_time is the interval end; subtract the selector’s duration to obtain its start. The text prob >0 is source GRIB metadata, not a user-selected probability threshold. Cycle times indicate when a forecast run was initialized; the data becomes available later.

Scheduled retirement of older SPC feeds

At NOAA’s SPC_POST v3 implementation, scheduled on or after 14 October 2026 at 12 UTC, the following HREF-based SPC products stop receiving new runs. If NOAA postpones the transition because of severe weather, retirement is postponed too. The replacements use different inputs and calibration, so applications need to select the new datasets and variables.

• spctor4hr, spchail4hr, spcwind4hr: migrate to GREFS four-hour TORPROB, HAILPROB and WINDPROB selectors.
• spctstm1hr, spctstm4hr: migrate to REFSCT TSTM, selecting one-hour or four-hour periods.
• spcltg4hr: no lightning-density replacement among these three datasets.

GEFSCT adds daily thunder guidance; it does not replace lightning density. This notice concerns SPC’s derived guidance; retirement dates for the underlying models require their own notices.

Archive and older datasets

GribStream retains runs initialized within the last two days, matching the existing SPC feeds. This limits how far back you can query runs, not how far ahead they forecast: each retained GEFSCT run still includes all eight daily periods. The older SPC datasets keep their identifiers, and their data expires under the same two-day retention policy. Query GREFS separately; its values are not appended to the older series.

Source: NOAA / Storm Prediction Center. Guidance is public-domain NWS information subject to the NWS disclaimer. GribStream is independent of NOAA. These probability fields are forecast guidance, not official warnings.

Related guidance: GREFS severe hazards · REFSCT short-range thunder · GEFSCT daily thunder.

Dataset details

• Archive window: Last 2 days

Parameters