Fire Weather on Tuesday, March 31, 2020

I expect elevated fire danger tomorrow, March 31, 2020 across much of southwestern SD due to the combination of warm temperatures, low RHs, and gusty winds. The one thing going for the area is that it did get good moisture late last week but it has been dry since Saturday. These aren't going to be absolutely critical conditions but I do think fires will easily spread if ignited.

Here are the forecast graphics from the Rapid City NWS office:

These show forecast maximum temperature, minimum relative humidity, and maximum winds gusts for the 31st. Southwestern SD is definitely drier than the surrounding areas, hence my thinking of higher fire danger there. But I also think it is going to be drier and windier than what these images show, especially over Fall River, eastern Custer, eastern Pennington, Oglala Lakota, Bennett, Mellette, Jackson, and Todd Counties.

Below is a forecast skew-t/log P diagram for 43.0 N and 103.5 W, roughly near Ardmore, SD. The top image is from the 12Z GFS run from this morning forecasting the thermodynamic profile for 18Z tomorrow. The bottom image is the same except for it is from the HRRR weather model. The profiles are very similar to each other, except for that the HRRR keep dew points lower and temperature higher across the area. This would bring lower RHs than what is forecast currently by the NWS (which lean towards the GFS dew point solution). Lately, the HRRR has been more accurate in depicting dew point than the GFS. The other thing to note in both forecast images is that there is a relatively deep mixing layer with strong unidirectional winds extending 6-7 kft above the surface. Deep mixing may very well lead to winds gusts in excess of 30 mph over the area.

The caveat with forecasting for a deep mixing layer is cloud cover. Both soundings show midlevel moisture which may be realized in the form of altostratus clouds which could kill daytime heating and prevent deep mixing from occurring. If this happens, conditions will not be as warm, dry, or windy as would be if the clouds were not present. I also see some conditional instability in the soundings so don't rule out a thunderstorm either. Lightning can ignite fires!

Let's take a look at the HDW index as well for that same area.

You can see that the HDW, for all ensemble members, is above the 90th percentile. So this is historically a rather hot, dry, and windy day. But the HDW is based upon the GFS ensembles which may suffer from the same issues that the GFS does--too moist in the lower levels. It may be drier than what this graph represents.

It is a tough forecast but I am hedging my bets towards warmer, drier, and windier than what is currently in the NWS forecast products. Stay aware out there and don't be surprise if you see critical fire weather conditions on the ground.

Look for a strong cold front to push through Wednesday with accumulating snow across the state on Thursday and Friday which will effectively end this period of higher fire danger.