Yesterday's RFW Bust

This was originally posted to my twitter account:

A thread: So the Red Flag Warning for western SD yesterday was a bust. Why? Answer: smoke. See all of the grey stuff in the satellite image? That is incredibly thick smoke moving over central/western SD yesterday late afternoon.

Smoke limits solar heating at the surface which limits convection, or the vertical movement of air. So why did this matter? Drier air existed higher up in the atmosphere. As did stronger winds.

If you limit sunlight, you limit surface heating, thus limiting convection, thus limiting the mixing down of drier air that has higher momentum ("stronger winds").

And we can see this in last night's radiosonde from Rapid City. Our observed mixing height was roughly 1.4 km (up to 750 mb, horizontal black line) with a surface temperature of 86 F. Winds peaked at 10 kts in that layer.

If we would've reached 92 F or so, our mixing height would've reached 3.8 km (up to 580 mb, horizonal black line). Winds up there are at 30 kts. And these could have mixed to the surface as gusts ~25-35 mph.

So the smoke limited surface heating by 5-7 degrees F, which limited vertical mixing by over 2 km! Thus we didn't get the stronger winds mixing down to the surface (nor the drier air shown by lower dew points up there).

That leads me to predictability. Should we have known this? Well the HRRR smoke model (0600 MDT forecast run) showed dense smoke over the area yesterday afternoon (shown at 1600 MDT).

So yesterday early morning, we all had a good idea there would be a lot of smoke in the air. We also know that smoke blocks sunlight, in turn reducing convection. But to what extent? What if there was a break in the smoke for a few hours?

Meteorologists typically hedge their bets towards worst case scenarios. The potential existed for a RFW day BUT we knew smoke was coming. What does one do? Let folks know that critical conditions may develop and that's what was done.