As we move into spring, we also enter severe weather season in the NWS Boise forecast area. Research conducted locally on severe weather reports in our area from 1955 to 2005 yields important information about the onset of each of the major severe weather threats in our area: tornadoes; hail; wind; and flooding. NOTE: Hail is defined as 1″ or greater, wind is defined as 58+mph.
This first image shows time as a circle, with Jan 1 at the top and June 30 at the bottom, and time progressing clockwise. The four different colored lines indicate the relative frequency with which separate types of severe weather occur. The numbers on the concentric circles indicate the total number of occurrences of a given type of severe weather in a moving 19-day window (to smooth the raw data a bit and make it more presentable). As you can see below, wind (red) and hail (purple) reports ramp up rapidly in April, with flooding (green) increasing in May. Tornadoes (blue), while minimal in number, show a narrow peak in late April and a longer peak in from late May into early July. The hail (purple) maximum is in late June. Wind has two significant peaks in activity, with one in late June and another from late July to early August. The late June peak is probably associated with stronger late- Spring weather systems, while the second peak is associated with hot (relatively) dry days when evaporation of rain produces downbursts.
The second image contains the same data presently differently. The height of the peak for each day is the total number of reports (within the moving 19-day window) of all four phenomena. This is a good way to view the period with the highest overall threat of severe weather. Here we can see that the period of maximum severe weather shows the same basic two peaks as the wind (red) reports. This makes sense because wind is our most common form of severe weather. Clearly, June is our “busiest” month, with late July into August a close second.
One type of “severe” weather not touched on in this study is fire weather. Experience indicates that the storms that lead to the second wind maximum often start fires via lightning and then may lead to rapid spread due to the high winds.
Finally, here’s a map showing the location of severe weather reports from roughly 1955 to 2010. The brownish dots are wind, the green are hail, and the purple are tornadoes (Note: this map was originally created for a different study and therefore the colors do not match the graphs above).