|Fig 1 -- US Warnings, watches and advisories as of 1918Z, Feb 19, 2011.|
During my time in Oklahoma, I saw a lot of red flag warnings come and go for the state, particularly on the high plains to the west. But I never figured out just what exactly that unique warning meant. I knew it had something to do with fire danger, but what conditions were needed to merit a red flag warning?
It turns out there are three criteria, all of which make sense in the context of fire danger:
- Relative humidity at the surface that is less than or equal to 15%.
- "20 foot winds" of 20 mph or more and/or gusts to 35 mph.
- National Fire Danger Rating System danger level of "high" or greater.
Surprisingly, very few people seem to generate relative humidity maps at the surface. As meteorologists we tend to be far more concerned with dewpoints at the surface as opposed to relative humidity. Usually this is because relative humidity is "relative" to the temperature (hence its name). Thus it makes a somewhat poor variable for actually looking at how much moisture is in the air.
With that said, I had to go to Intellicast to find a relative humidity map at the surface for today:
|Fig 2 -- Surface relative humidity as of 2:00 PM EST. From Intellicast.|
Let's check this morning's sounding from El Paso:
|Fig 3 -- This morning's 12Z sounding from El Paso, TX (EPZ) on Feb. 19, 2011. From the HOOT website.|
So we have reason to believe that as things are warming up today the relative humidity will drop and the surface layer relative humidity will fall. So we can extrapolate that the first criterion will be satisfied in west Texas if it hasn't been already.
What about the next one--the winds at "20 feet" need to be at least 20 mph or gusting to 35 mph. How do we check that?
There's an odd disconnect between this particular criterion and what we actually measure. In typical weather stations, winds are measured at 2 meters and 10 meters above the ground--or, at about 6.5 feet and 33 feet. The meteorological community likes to keep things in the international standard units, using meters and hectopascals and degrees Celsius. But there are those occasional areas where we have to defer to the public's use of Imperial Units here in the US. Hence at the surface, we use miles per hour and degrees Fahrenheit and whatnot. I'm guessing this "feet" specification comes from the fact that these red flag warnings are designed for widespread public use, so 20 feet is easier to understand. (But why not 30 feet? Why not the 6.5 feet? These are things I need to look up...)
So here are the wind observations as of early this afternoon:
|Fig 4 -- Surface observations for the CONUS at 19Z, Feb. 19, 2011. From the HOOT website.|
It's somewhat of a hard sell in both regions based on these observations--also note these are 10 m or 33 foot winds, which tend to be slightly higher than what we'd see at 20 feet. In west Texas there is a big hole in this observation map--to the east in west central Texas there are somewhat weaker winds around 10 knots. However, we do see winds in the 20 knot range throughout central New Mexico. On the El Paso sounding above we also saw winds at 20 knots out of the southwest a little bit above the surface. With vertical mixing going on, gusts to or above 20 knots are definitely possible. So we can assume that the winds will meet the criterion in west Texas.
In Virginia and North Carolina it's a bit harder to tell. There are some 20 knot wind observations in the foothills of the Applachians. Also note that the general flow is out of the west--downslope out of the mountains. This could contribute to some adiabatic warming which would drop the RH values even further--just a side note there. But anyhow, through much of the central part of these states, winds are only showing sustained values at 10 knots. Let's check this morning's sounding from Greensboro, NC.
|Fig 5 -- Sounding from 12Z this morning at Greensboro, NC (GSO) from Feb. 19, 2011. From the HOOT website.|
So what about the final condition? The National Fire Danger Rating System describes itself as a bunch of models that try to forecast fire danger not only due to the weather, but also due to topography and the current quality of the fuel material on the surface. For instance, lots of dry vegetation in the area would increase the fire danger and so on. They have a somewhat rudimentary, but still very educational website here that details what they do and how they do it. They also publish these fire danger maps based on current observations and on model forecasts. Here is their forecast map for today:
|Fig 6 -- Forecast fire danger map from the NFDRS for Feb. 19, 2011.|
There's a look at what goes into a red flag warning. Red flag warnings usually result in burning bans or cautionary statements that any sort of wildfire that starts will have a strong chance of escalating quickly. Be cautious about any sort of open flames when under a red flag warning...
Looking at a the potential for a strong storm later next week in the Pacific Northwest and with warmer temperatures in the central US, all that snow from the blizzards is going to start to melt. Flooding concerns are about to get huge--so start paying attention if you live in a flood prone area...