TERMINOLOGY

Relative Frequency of Strikes

The use of enhanced terminology within forecast products may help emphasize the lightning danger. For example, when other threats associated with thunderstorms (tornadoes, damaging winds, large hail, flooding) are expected to be minimal, the term  "LIGHTNING STORMS" should be used rather than "THUNDERSTORMS" to highlight the main threat. Here's an example from a Hazardous Weather Outlook.

The threat for injuries/deaths and property damage likely increases as the amount of cloud-to-ground strikes increase.  For this  reason, it is suggested that quantitative terms are used to indicate the expected frequency or current frequency of strikes. This will also ensure that our lightning terminology is consistent from one forecast shift to the next. ***Preliminary research*** indicates that the following thresholds should be used:

OCCASIONAL:     1-2    CG strikes per minute
FREQUENT:         3-11  CG strikes per minute
EXCESSIVE:        12+    CG strikes per minute

Rather than using "instantaneous" 1-minute rates however, it is better to use the above frequencies averaged over a 7-minute period to confirm persistence (note: the NLDN PC will default to a 7-minute display cycle). You may use the NLDN PC to zoom down to storm scale and observe the number of CG strikes during the last 7 minutes. If the rates reach the following  preliminary thresholds, you should include the appropriate wording in short-term forecasts or severe weather statements.

OCCASIONAL:     1-20    CG strikes per 7-minutes
FREQUENT:         21-83  CG strikes per 7-minutes
EXCESSIVE:         84+     CG strikes per 7-minutes

***As we gain further knowledge regarding flash rates, these thresholds may be adjusted.***

Anvil Lightning versus "Bolt from the Blue"

In order to avoid confusion between anvil lightning and "bolts from the blue", note the following local definitions:

Anvil Lightning: Any bolt which develops in the ANVIL and travels generally straight down to strike the ground.

"Bolt from the blue": A bolt which originates within the UPDRAFT of the storm, typically 2/3rds of the way up, travels HORIZONTALLY for many miles, then strikes the ground. Here's a great example of a "bolt from the blue" and the locally written Severe Storm Conference preprint which inititially defined the "bolt from the blue".

Both types are cloud-to-ground lightning and are very dangerous. The basic difference between the two is that ANVIL lightning is discharged from the ANVIL portion of the storm, while a "bolt from the blue" is discharged from the UPDRAFT portion (side) of the storm. A person alert to his environment will see clouds above him/her with anvil lightning while a bolt from the blue will "appear" to originate from "clear air". It is the "bolts from the blue" and anvil lightning which likely kill people locally, as both typically occur far from the cell core (rain area).