Fire weather and heat get less attention than tornadoes, but Red Flag Warnings, heat domes, and high wind events are some of the deadliest weather the NWS covers. This page explains the fire weather alert suite and the fuels and humidity terms behind it, plus heat products and the wind advisories that stand alone from thunderstorms.
A Fire Weather Watch is issued when Red Flag conditions are possible within 24–48 hours. Fire managers use this window to pre-position resources and restrict burning. When confidence increases, it upgrades to a Red Flag Warning.
A Red Flag Warning (RFW) is issued when a combination of low relative humidity (< 15–25%), strong winds (> 15–25 mph), and critically dry fuels creates conditions where any fire that starts will spread rapidly and be extremely difficult to control. Common across the western U.S. but can occur anywhere. EAS code: RFW.
The ETO (Particularly Dangerous Situation) for fire weather is a rare upgrade to a Red Flag Warning or Fire Weather Watch, indicating an exceptionally dangerous and potentially historic fire weather event. Criteria vary by region but generally involve critically low RH, very high winds, and very dry fuels — all simultaneously.
Critical fire weather is a combination of conditions — low relative humidity, strong winds, and dry fuels — that allow fires to start easily, spread rapidly, and resist suppression. Offshore/downslope wind events (Santa Ana, Diablo, Chinook) dramatically amplify fire weather risk in the west and are responsible for the most catastrophic California fire outbreaks.
Fuel moisture is the water content of vegetation expressed as a percentage of dry weight. Live fuel moisture (100%+) vs. dead fuel moisture (single-hour, 10-hour, 100-hour fuels). Dead fuel moisture below 10–15% is critically dry. Fuel moisture directly controls ignitability and fire spread rate — lower is more dangerous.
RH recovery refers to how much relative humidity rises overnight as temperatures cool. Poor RH recovery (below 25%) means fuels remain critically dry through the night, failing to rehydrate before the next day's heat and wind. Poor recovery for several consecutive days is a hallmark of the most dangerous fire weather patterns.
The Haines Index (or Lower Atmosphere Severity Index) is a fire weather tool measuring atmospheric stability and dryness in the lower troposphere. It ranges from 2 (very low potential for large fire growth) to 6 (very high potential). A value of 6 indicates hot, dry, unstable air capable of supporting explosive fire behavior.
Offshore flow occurs when winds blow from land toward the ocean. Along the California coast, offshore flow brings the notorious Santa Ana winds — hot, dry, downslope winds from the desert interior. These winds compress and warm as they descend, dramatically lowering relative humidity and creating extreme fire weather conditions.
Spotting occurs when burning embers (firebrands) are lofted by a wildfire's convective column and carried downwind, igniting new fires ahead of the main fire front. In extreme fire weather, embers can travel 1+ miles. Spotting is one of the primary mechanisms for rapid, erratic fire spread and is extremely difficult for crews to anticipate.
A pyrocumulus is a cumulus cloud generated by the intense heat and moisture release of a massive wildfire. At extreme intensity, it becomes a pyroCb (pyrocumulonimbus) — a full thunderstorm driven by fire. PyroCbs produce lightning that ignites new fires, generate their own extreme winds, and can spawn fire whirls (fire tornadoes). Considered one of the most dangerous wildfire phenomena.
A fire whirl (or fire tornado / firenado) is a rotating column of fire and gases produced by intense heating from a wildfire. Most are weak and short-lived, but extreme cases — such as the Carr Fire whirl in 2018 — can reach EF3 intensity. They're distinct from pyroCb tornadoes, which are meteorologically driven.
The Heat Index is the 'feels like' temperature — a combination of air temperature and relative humidity that estimates how hot the human body actually perceives it. High humidity prevents sweat from evaporating (your body's cooling mechanism), raising the apparent temperature dramatically. 90°F at 90% RH feels like ~119°F. Used to trigger heat-related alerts.
A Heat Advisory is issued when heat index values of 100–104°F are expected — dangerous but below Excessive Heat Warning criteria. Limit strenuous outdoor activity, stay hydrated, and use air conditioning or cooling centers.
An Excessive Heat Warning is issued when heat index values will reach 105°F+ for at least 2 hours (thresholds vary by region and acclimatization). Heat is the #1 weather killer in the U.S. annually. Check on elderly neighbors, never leave people or pets in vehicles, and find air-conditioned spaces.
A heat dome is a strong, persistent upper-level ridge that traps hot air beneath it like a lid. Subsiding air in the ridge compresses and warms, further elevating surface temperatures. Heat domes can persist for days to weeks, producing record-breaking heat waves. The 2021 Pacific Northwest heat dome (122°F in Lytton, BC) was a historic example.
A heat burst is a rare nighttime weather event where temperatures spike dramatically (sometimes 20–30°F in minutes) as a collapsing thunderstorm downdraft descends and compresses adiabatically. Associated with gusty, hot, dry winds. Usually lasts minutes to an hour.
A Wind Advisory is issued for sustained non-thunderstorm winds of 25–39 mph and/or gusts 35–57 mph — below High Wind Warning thresholds but still creating hazardous driving conditions for high-profile vehicles, motorcycles, and pedestrians. Secure lightweight outdoor objects.
A High Wind Watch (HWA) is issued 12–48 hours before High Wind Warning conditions are expected. Wind hasn't arrived yet — prepare now and watch for the warning. EAS code: HWA.
A High Wind Warning (HWW) is issued for sustained non-thunderstorm winds of 40+ mph and/or gusts 58+ mph (thresholds vary regionally). Expect downed trees, power outages, and very dangerous driving conditions for high-profile vehicles. Secure outdoor furniture and be prepared for outages. EAS code: HWW.