Classifications of Location, Maximum Probable Extent, Probability of Future Events, and Overall Significance are pulled directly from FEMA and are outlined below. Use the quick links at the left to navigate to specific sections quickly.
Location
Location refers to the geographic area affected.
Negligible: Less than 10 percent of planning area or isolated single-point occurrences
Limited: 10 to 25 percent of the planning area or limited single-point occurrences10 to 25 percent of the planning area or limited single-point occurrences
Significant: 25 to 75 percent of planning area or frequent single-point occurrences
Extensive: 75 to 100 percent of planning area or consistent single-point occurrences
Maximum Probable Extent
Maximum Probable Extent refers to the magnitude/strength of an event based on historic events or future probability.
Weak: Limited classification on scientific scale, slow speed of onset or short duration of event, resulting in little
to no damage.
Moderate: Moderate classification on scientific scale, moderate speed of onset or moderate duration of event,
resulting in some damage and loss of services for days
Severe: Severe classification on scientific scale, fast speed of onset or long duration of event, resulting in
devastating damage and loss of services for weeks or months
Extreme: Extreme classification on scientific scale, immediate onset or extended duration of event, resulting in
catastrophic damage and uninhabitable conditions
In addition to the above defined categories, some hazards have their own scale/index. Hazard specific scales are defined below.
| Hazard | Scale/Index | Weak | Moderate | Severe | Extreme |
|---|---|---|---|---|---|
| Drought | Palmer Drought Severity Index | -1.99 to +1.99 | -2.00 to -2.99 | -3.00 to -3.99 | -4.00 and below |
| Earthquake | Modified Mercalli Scale | I to IV | V to VII | VII | IX to XII |
| Richter Magnitude | 2, 3 | 4, 5 | 6 | 7, 8 | |
| Hurricane Wind | Saffir-Simpson Hurricane Wind Scale | 1 | 2 | 3 | 4, 5 |
| Tornado | Fujita Tornado Damage Scale | F0 | F1, F2 | F3 | F4, F5 |
Probability of Future Events
Unlikely: Less than 1 percent probability of occurrence in the next year or a recurrence interval of greater than
every 100 years.
Occasional: 1 to 10 percent probability of occurrence in the next year or a recurrence interval of 11 to 100
years.
Likely: 10 to 90 percent probability of occurrence in the next year or a recurrence interval of 1 to 10 years
Highly Likely: 90 to 100 percent probability of occurrence in the next year or a recurrence interval of less than
1 year.
Overall Significance
Low: Two or more criteria fall in lower classifications or the event has a minimal impact on the planning area.
This rating is sometimes used for hazards with a minimal or unknown record of occurrences or for hazards
with minimal mitigation potential.
Medium: The criteria fall mostly in the middle ranges of classifications and the event’s impacts on the planning
area are noticeable but not devastating. This rating is sometimes used for hazards with a high extent rating but
very low probability rating.
High: The criteria consistently fall in the high classifications and the event is likely/highly likely to occur with
severe strength over a significant to extensive portion of the planning area.
Sources
FEMA Local Mitigation Planning Handbook: https://www.fema.gov/sites/default/files/2020-06/fema-local-mitigation-planning-handbook_03-2013.pdf
Cumulative meteorological drought and wet conditions: http://ncdc.noaa.gov/
Earthquake intensity and effect on population and structures: http://earthquake.usgs.gov
Earthquake magnitude as a logarithmic scale, measured by a seismograph: http://earthquake.usgs.gov
Hurricane rating based on sustained wind speed: http://nhc.noaa.gov
Tornado rating based on wind speed and associated damage: http://spc.noaa.gov