Thursday, July 5, 2018

The Fickle Relationship Between Rain and Fire

Everyone is hoping for a major dousing of rainfall for the Dollar Ridge Fire raging east of Strawberry Reservoir.

Some sprinkles have been noted in the vicinity today, as reported, for example, by Jed Boal of KSL News.

However, sprinkles are not enough and they may do more harm than good because of the fickle relationship that exists between rain and fire.

A best-case scenario for taming the wildfire and reducing (or eliminating) extreme and erratic fire behavior is a situation with high relative humidity, cool temperatures, light wind, and steady precipitation. 

On the other hand, while sprinkles may temporarily increase the relative humidity, they can cause more harm than good by initiating gusty winds or microbursts that lead to erratic or extreme fire behavior. 

That is my concern today.  The morning sounding from the Salt Lake City International Airport shows remarkably dry low levels ripe for the production of strong microbursts. 

Source: SPC
 Meteorologists use a variable, known as Downward Convective Available Potential Energy, or DCAPE, to quantify the amount of energy that may be produced by cold, dense air produced by evaporation.  This morning's sounding had a DCAPE of 860 J/kg, which is quite high.  In other words, conditions are ripe for microburst winds. 

The latest radar shows scattered, but weak cells in the fire region.  These might produce some sprinkles or light rain, but it will be brief and probably accompanied by gusty winds. 

So, if you are praying for rain, be specific.  Sprinkles are not good enough and you could be causing more harm than good.  We need a prolonged period of rainfall or, if that's not not possible, a period of cooler weather with higher relative humidity, weak winds, and no microbursts.  

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