SPC Severe Weather Outlooks
The Storm Prediction Center (SPC) issues daily severe weather outlooks for the contiguous US. These give advance warning of days with elevated severe weather potential — often well before any NWS watches or warnings are issued.
LMKwhen monitors three types of SPC outlooks: categorical convective outlooks, probabilistic outlooks, and fire weather outlooks.
Categorical convective outlooks
Each forecast day carries a single categorical risk level from the table below.
| Rank | Label | Meaning |
|---|---|---|
| 1 | Thunderstorm (TSTM) | General thunderstorm activity possible; not necessarily severe |
| 2 | Marginal (MRGL) | Isolated severe storms possible |
| 3 | Slight (SLGT) | Scattered severe storms likely |
| 4 | Enhanced (ENH) | Numerous severe storms likely |
| 5 | Moderate (MDT) | Widespread severe storms, some intense |
| 6 | High (HIGH) | Significant severe weather outbreak expected |
When creating a categorical alert rule, you set a minimum rank threshold. For example, a threshold of Slight (3) or higher alerts whenever your location falls within SLGT, ENH, MDT, or HIGH risk areas.
Outlook days
| Day | Covers | Typical issuance times (UTC) |
|---|---|---|
| Day 1 | Current convective day | ~01Z, 06Z, 13Z, 16–17Z, 20Z |
| Day 2 | Tomorrow | ~06Z, ~17–18Z |
| Day 3 | Day after tomorrow | ~07–08Z |
Each issuance supersedes the previous one for the same day. When a new issuance arrives, LMKwhen updates the alert in place if the risk level is unchanged (reconfirmed), or resolves and re-fires if it changes (upgraded or downgraded).
Simultaneous Day 1 alerts during the 06Z transition
The 06Z issuance begins a new convective day. Between 06Z and the natural 12Z expiry of the previous Day 1, two Day 1 alerts may be active simultaneously for the same location:
- The outgoing Day 1 (covering the current day, expiring at 12Z)
- The incoming Day 1 (covering the next convective day)
This matches how SPC itself displays the outlooks during that window. Both alerts resolve independently — the outgoing one at its 12Z expiry, the incoming one when the next 06Z cycle arrives.
Probabilistic outlooks
Probabilistic outlooks express the likelihood of a specific hazard occurring within 25 miles of a point.
| Product | Days available |
|---|---|
| Tornado probability | Day 1, Day 2 |
| Hail probability | Day 1, Day 2 |
| Wind probability | Day 1, Day 2 |
| Severe (any) | Day 3 |
Threshold examples: alert when tornado probability ≥ 5%, or when hail probability ≥ 15%.
Probabilistic outlooks update on the same schedule as their corresponding categorical day outlook.
Fire weather outlooks
SPC also issues fire weather outlooks for areas with elevated wildfire risk due to dry, windy conditions.
| Rank | Label | Meaning |
|---|---|---|
| 1 | Elevated | Conditions favorable for fire spread |
| 2 | Critical | Critical fire weather conditions expected |
| 3 | Extreme Critical | Extreme fire weather danger |
| Day | Typical issuance times (UTC) |
|---|---|
| Day 1 | ~06Z, ~19–20Z |
| Day 2 | ~06Z |
Fire weather rules work the same way as categorical convective rules — set a minimum rank threshold and LMKwhen alerts when your location falls within a matching outlook area.
Alert lifecycle
All SPC outlook alerts follow the same lifecycle:
- Issued — your location enters a risk area that meets your threshold
- Reconfirmed — a later issuance covers the same area at the same level
- Upgraded / Downgraded — risk level changes; old alert resolves, new one fires
- Resolved — the outlook expires at its natural end time (typically 12Z for Day 1, rolling over each convective day)
If you have send all-clear enabled on the rule, a resolved notification is sent when the alert closes.
Use case
SPC outlooks are ideal for planning ahead. A Moderate categorical risk on tomorrow's Day 2 outlook is a strong signal to adjust schedules, pre-position resources, or notify tenants — often 12–36 hours before any NWS watches or warnings are issued.