DOI: 10.3886/e233242v1
title: Climate.gov Data Snapshots: Historic Probability of Severe Weather

description: <b>Q: Where is severe weather likely at this time of year?
</b><b>
 </b><br><b>A: </b>Shading
 on each map reflects how often severe weather occurred within 25 miles 
during a 30-year base period. The darker the shading, the higher the 
number of severe weather reports near that date. For this map, severe 
weather encompasses tornadoes, thunderstorm winds over 58 miles per 
hour, and hail larger than three-quarters of an inch in diameter.

 
 <br><b>Q: How were these maps produced?
</b><b>
 </b><br><b>A: </b>For
 each day of the year, scientists plotted reports of severe weather from
 1982 to 2011 on a gridded map. To reveal the long-term patterns of 
these events, they applied mathematical filters to smooth the counts in 
time and space. Keep in mind that severe weather is possible at any 
location on any day of the year.

 
 <br><b>Q: What do the colors mean?</b><b>
 </b><br><b>A:</b> Shaded
 areas show the historical probability of severe weather occurring 
within 25 miles. Meteorologists estimated these probabilities from 
severe weather reports submitted from 1982-2011. For each day of the 
year, scientists plotted reports of severe events onto a map marked with
 grid cells 50 miles on a side. For each grid cell, they counted the 
number of years with at least one report, and divided by the total 
number of years. To reveal the long-term patterns suggested by this 
relatively small dataset, they used statistical methods to smooth the 
data. For instance, to smooth clusters of events in time, a mathematical
 filter replaced the value in every grid cell with a 15-day average. 
Another filter extended report locations over a 25-mile-wide circle to 
indicate the probability that the event could have occurred at other 
points within that area.

 
 <br><b>Q: Why do these data matter?</b><b>
 </b><br><b>A:</b> Knowing
 when and where severe weather tends to occur through the year promotes 
preparedness. Residents who are alert to the possibility of severe 
weather are better able to respond in ways that keep them safe. These 
data can also help emergency response personnel plan for when and where 
their services may be necessary.

 
 <br><b>Q: How did you produce these snapshots?</b><b>
 </b><br><b>A:</b> Data
 Snapshots are derivatives of existing data products: to meet the needs 
of a broad audience, we present the source data in a simplified visual 
style. NOAA's National Weather Service Storm Prediction Center produced 
the Severe Weather Climatology files. To produce our images, we obtained
 the climatology data as a numpy array, and ran a set of scripts to 
display the mapped areas on our base maps with a custom color bar. <br><br>
 
 
 <b>Additional information</b>
 Data for 
these images represents an update and extension of work first put forth 
by Dr. Harold Brooks of the National Severe Storms Laboratory.

 
 <br><b>References</b> <br>Brooks, H. E., C. A. Doswell, III, and M. P. Kay, (2003) Climatological estimates of local daily tornado probability, Wea. Forecasting, 18, 626-640.<br><br>Source: https://www.climate.gov/maps-data/data-snapshots/data-source/historic-probability-severe-weather
<br><br><br>This upload includes two additional files:<br>* Historic Probability of Severe Weather _NOAA Climate.gov.pdf is a screenshot of the main Climate.gov site for these snapshots (https://www.climate.gov/maps-data/data-snapshots/data-source/historic-probability-severe-weather
)<br>* Cimate_gov_ Data Snapshots.pdf is a screenshot of the data download page for the full-resolution files.<br><br><br>
description: One image for each day of the year.<br><br>Data is divided in to folders by image resolution -- full size (zip), small (png) and large (png). Occasionally there may be one image missing in a particular size but available in another size.<br><br>Data downloaded from climate.gov 6/17/2025.