Watches and Warnings for California

NWS offices issue Fire Weather Watches and Red Flag Warnings for critical fire weather patterns that contribute to extreme fire danger and/or fire behavior.

A Fire Weather Watch is used to alert agencies to the high potential for development of a Red Flag event in the 12-72 hour time frame. The Watch may be issued for all or selected portions of a fire weather zone or zones. A watch may be issued in the first 12 hour time period only for an expected dry thunderstorm event.

Criteria for Red Flag Warnings/Fire Weather Watches

Dry Lightning - A lightning event that is not accompanied by enough precipitation to significantly wet fuels that have been identified as critically dry. Significant precipitation is defined as ranging from .05 inches for grass or brush fuels to .15 inches for closed-canopy timber/heavy fuels.

Watches and warnings will be issued when dry lightning is expected to be widespread. Isolated events or events of short duration (i.e., events which start dry but become wet within an hour or two) do not need warnings but will be headlined in the forecast.

Wind and Humidity - Wind and humidity criteria are geared toward those situations which may result in rapid spread of wildfires. Because topography and vegetation play a big role in this, several sets of criteria are used across California. Where possible, issuance criteria have been meshed with those used in adjacent states to meet the needs of agencies whose jurisdictions cross state lines. Criteria are listed in the Wind/Humidity Table.

Red Flag warning/fire weather watches in discussions and headlines - In the discussion portion of the Fire Weather Planning Forecast (FWF), NWS offices will mention critical weather patterns that might lead to conditions approaching or exceeding Red Flag criteria through the extended forecast. This will assist fire agencies in their allocating and moving resources in anticipation of increased fire activity. Fire Weather Watches and Red Flag Warnings will be headlined in spot forecasts, the fire weather narrative, and appropriate zone sections within the fire weather planning forecast. The headline will be in the same format as on the RFW product itself.

Collaboration with agencies - Fire Weather Watches and Red Flag Warnings normally will be issued only after conferring with the affected agencies or a representative subset of affected agencies, to include the Redding and Riverside Predictive Services Units. This will allow for input on fuel conditions and local concerns. However, the ultimate responsibility for the issuance of a watch/warning rests with the NWS forecaster.

Red Flag warning/fire weather watch access - If issuance of a Red Flag Warning or Fire Weather Watch requires an update of the general forecast, the NWS office will verbally notify the Redding and Riverside Predictive Services Units as soon as possible. During non-duty hours for the PSUs, contact the GACC Coordinator on Duty (COD).

Wind/Humidity Table Wind/RH RFW Decision Matrix for Northern California West of the Sierra Crest

Area Description

NWS Fire Weather Zones

Criteria

Southern California desert area excluding the Colorado River Valley

226-228, 230, 232, 260262

Relative Humidity ≤ 15% and wind gusts GTE 35 mph for 3 hours or more

Colorado River Valley

229,231

Relative Humidity ≤ 15%, with sustained winds (20 foot) ≥ 20 mph and/or frequent gusts ≥ 35 mph for 3 hours or more

Antelope Valley and SE Kern County Deserts

298, 299, 259

Relative Humidity ≤ 15% and sustained (20-foot) winds ≥ 25 mph for a duration of 8 hours or more

Southern California from mountains westward

234-258, 288-297

Either Relative Humidity ≤15%, with sustained winds ≥ 25 mph and/or frequent gusts ≥ 35 mph (duration of 6 hours or more)

Or Relative Humidity ≤ 10% (duration of 10 hours or more) regardless of wind

Northern California East of Cascade/Sierra Crest and Western Great Basin including the Modoc Plateau

214, 270-273, 278, 284, 285

Tahoe Management Basin: Three hours of wind gusts ≥ 30 mph and Relative Humidity ≤ 20%

Rest: Three hours of wind gusts ≥ 30 mph and Relative Humidity ≤15%.

Northern California West of the Cascade/Sierra Crest

201-213, 215-221, 263269, 274-277, 280-282

See matrix below

 

Matrix assumes daytime 10-hour fuel moisture ≤ 6% (measured at 1300), annual grasses are cured, and that no wetting rain (greater than 0.10 inch) has fallen in the last 24 hours.

Sustained 20-foot Wind Speed (Note: the wind event should be expected to last at least 8 hours). W indicates that the forecaster should consider a warning.

Relative Humidity

Sustained Wind 6-11 mph

Sustained Wind 12-20 mph

Sustained Wind 21-29 mph

Sustained Wind 30+ mph

Daytime Minimum RH 29-42% and/or Nighttime Maximum RH 60-80%

 

 

 

W

Daytime Minimum RH 19-28% and/or Nighttime Maximum RH 46-60%

 

 

W

W

Daytime Minimum RH 9-18% and/or Nighttime Maximum RH 31-45%

 

W

W

W

Daytime Minimum RH < 9% and/or Nighttime Maximum RH < 31%

W

W

W

W