"Dry thunderstorms" is a phrase
used quite often in the wildland fire fighting community. It is usually
meant to describe thunderstorms that produce little or no precipitation
at the surface, as shown in the photo above. The "drier" the
thunderstorm, when combined with dry vegetation (or fuel), the more efficient
it is in
terms
of fire
ignitions
by cloud to ground lightning.
Thunderstorms can be considered dry if they don’t
produce any rain at the surface. However, for the fire weather community,
a "dry thunderstorm" may be used to describe a storm producing
very little rainfall, such that is not effective in checking fire spread
after lightning ignition. The second scenario is more common, though
there are rare instances when thunderstorms produce no sensible
rainfall. In the purest
sense, “dry” thunderstorms produce rain just below the cloud
base but due to a very dry atmosphere below the cloud base, the rain
evaporates at some point between the cloud base and the ground.
Meteorologically speaking, this is called virga. In the fire fighting
world, “dry” thunderstorms
produce measurable precipitation but due to varying fuel types measurable
rain (.01”) is not sufficient to moisten the fuel bed effectively.
As an example, this video depicts a "dry
thunderstorm" for which short
duration heavy rain reached the ground, but barely moistened the
exceptionally dry soil
(photo following
the event)
.
Dry Thunderstorm in Gila
duration: 16 seconds
In order to moisten all layers
of vegetation, rainfall amounts
need
to
penetrate
the aerial
canopy
(or
the highest
vegetation
layer)
and
reach the surface. This is referred to as a "wetting
rain".
For fire weather considerations, dry versus wet thunderstorms is a
rolling scale, dependent on the type of surface fuel bed and antecedent
weather
conditions.
If
it
has
been
hot and dry for a long period, rainfall amounts need to be larger in
order to moisten the fuel bed and lessen the chances of lightning fire
ignition. If the surface fuel bed lies below a closed forest canopy,
more rainfall is required to penetrate the canopy and reach the surface
fuel. For example, in most closed canopy forests of New Mexico, rainfall
less than a tenth of an inch defines a “dry” thunderstorm.
Within grassland areas where a forest canopy is not present, lesser rainfall
is needed from a thunderstorm in order for it to produce wetting rain.
A “dry” thunderstorm could ruin a picnic, but could ignite
many wildfires as well.
Fire weather meteorologists also use the phrase “mixture of wet
and dry” to describe thunderstorms which produce a diverse rainfall
pattern. This means that the main rain shaft accompanying the thunderstorm
produces wetting rain but the footprint of this wetting rain is small,
while much lighter amounts are found around the periphery of the cell.
Lightning can strike outside of the wetting rain footprint. On May 24th,
2009 one such thunderstorm cell affected the greater Albuquerque metro
area between 5 to 6 pm. The cell formed to the west of Rio Rancho and
progressed southeastward across Corrales and the far Northeast Heights
of Albuquerque.
City Net observers and Albuquerque area observation sites (see
rainfall table) measured 0.25 to 0.45 inches of rainfall
within the main rain shaft.
Pea sized hail was also reported with this storm. Rain amounts outside
of the main rain core were less than a tenth of an inch. Typically
in New Mexico, when a person cannot see through a rain shaft high precipitation
totals generally result. This is especially true when surface humidity
is greater than 30% and the main rain shaft is persistent. Under
these situations a quarter to three quarters of an inch of rain can fall
in
a half hour. Photos taken during the event show a rainshaft
completely
obscuring the horizon.
|
A
10 second video shows cloud to ground
lightning outside of the main rain shaft (look to the right of the
heaviest rain shaft when viewing the video).
Lightning Outside Rain Shaft
Look to the right at about 3 seconds into the video
duration: 8 seconds
The one hour precipitation
total as estimated by the radar shows a long
but
narrow
wetting rain
footprint. Wetting rain is depicted by the blue and green
areas on the graphic.
It is not known whether a fire was started from this particular
cell but under the right fuel conditions a fire could have resulted because
lightning occurred outside of the wetting rain footprint.
Other factors that affect fire ignitions due to thunderstorms
include storm motion, the duration of any rainfall which occurs and antecedent
conditions immediately following the passing thunderstorm. Storms that
possess a large wetting rain footprint but move quickly (20 to 30 mph)
can still start fires, in part because rainfall duration in any one location
can be very brief and moisture may not effectively penetrate the surface
soil or fuel bed. Storms are generally more effective at igniting fires
if atmospheric conditions immediately following the cell passage become
drier and breezier and the fuel bed is especially dry preceding the storm.
“Dry” thunderstorms are most common in New
Mexico during May and June, and are most likely to start fires
during the month of
June. However, if the preceding winter was dry then followed by
very little rainfall during the early spring period, the period mid-April
through
May
is also
susceptible
to
dry
thunderstorm effects, particularly fire ignitions. A few episodes of “dry” thunderstorms
can also affect the fall period.
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www.srh.weather.gov 
National
Weather Service Weather Forecast Office
