Wednesday, April 06, 2016

A little more about what SPC fire weather forecasts are and are not (3 of 3)

When I started writing about the National Weather Service Storm Prediction Center's (SPC) fire weather outlooks on April 1st, I referred to their "product info". As you read the criteria that the SPC uses to make their fire weather outlooks you will note that relative humidity (RH) at or below regional thresholds is given as one of the criteria. My understanding of RH is that the lower the value, the drier the air is, so a low RH value is one of the factors conducive to the threat of wildfires. Others including  gusty winds, high temperatures, dry fuels, and dry lightning.

I was interested in learning that there are regional variations for relative humidity (RH) thresholds used by the SPC in their fire weather outlooks as depicted in this graphic linked to on the SPC Fire Weather Outlook Product Information page. For example, an RH value of less than 30 percent in New Jersey is one of the factors conducive to the issuance of a critical fire weather outlook for that State. For Florida, the RH threshold increases to less than 35 percent while for much of the southwest (including California) the RH threshold is below 15 percent.

One final and important point about what the SPC does and does not do in regards to fire weather. As I have discussed above, the SPC issues fire weather outlooks. They do not issue fire weather watches or red flag warnings. Rather it is the local NWS offices that issue Fire Weather Watches and Red Flag Warnings. Either the local NWS office or the State will determine the Red Flag Criteria. To see what the Red Flag Criteria is on your location (US only) go to the webpage for your local NWS office ( and look around on their page for fire information you should find information about what the red flag criteria are for your area. The NWS glossary of fire weather terms provides some definitions that you might find helpful:

Fire weather watch - A watch issued when the forecaster feels reasonably confident that red flag conditions will develop in the next 12 to 48 hours.

Red flag criteria - A locally determined set of criteria that expresses environmental and meteorological conditions that would provide for fire starts and rapid, dangerous fire spread.

Red flag warning - A warning issued by fire weather forecasters when red flag criteria are met or expected to be met within 12-24 hours. The warning highlights weather of particular importance to fire behavior and potentially extreme burning conditions or many new fires. It is also used to alert customers to changes in weather that increase the fire danger. Red flag warnings should always be coordinated with the customer. [As I understand it, customer is used here to mean a local agency such as a State Forest Fire Service or a Federal land agency such as the US Forest Service.]


This is all I have for now on fire weather outlooks and forecasting. I am continuing to read. And I hope to be able to share more of this journey of learning about fire weather forecasting some time soon.

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