There are four regions within the Mountain West, (1) the northern region, (2) the western region, (3) the eastern region and (4) the southern region. How these regions are made and how we define the 14-day extreme precipitation periods can be found here.

Key Information:

  • There are 194 total periods in the Mountain West. Region 2 experienced the most precipitation periods with 67 periods. The other three regions experience about 42 periods each.
  • The Mountain West has two distinct wet seasons. For the western half of the Mountain West, the wet season occurs from November to May, similar to the West Coast wet season. The eastern half the Mountain West has a wet season that occurs from June to October, similar to the Plains.
  • Mountain West periods occurred throughout the entire year, with a minimum for regions 1 and 3 during the beginning of the year.
  • There is a wide range of storm reports for Mountain West periods including Hail, Winter Storm and Flash Flooding.

When do Mountain West Extreme Periods occur?

Seen to the left, 14-Day periods within all Mountain West regions are counted, based on month, from 1915 to 2018.

The Mountain West has two dominant wet seasons: November-May for the western portion and June-October for the eastern portion. The regions within the eastern half (regions (1) and (3)) have a minimum number of periods within the first four months of the year. With no events occurring within region 1 in any January or March from 1915 to 2018.

What are typical storm reports of Mountain West Extreme Periods?

Using NCEI storm reports typical impacts of our 14-day extreme periods, past 1996, can be estimated. Beginning in 1996, 48 event types were recorded into the storm events database. Every report was recorded based on the county of occurrence by a National Weather Service forecast office (WFO) and then passed onto NCEI. Therefore, reports within 14-day extreme periods from 1996-2018 within Idaho, Montana, Wyoming, Nevada, Utah, Colorado, Arizona or New Mexico are counted as Mountain West 14-day extreme periods reports. Definitions of all storm reports can be found within the NWSI, Appendix A.

During the wet season in the western portion of the Mountain West there are many storm reports that are related to frozen precipitation such as “Winter Storm”, average of 66 reports during an extreme period and “Heavy Snow”, average of 49 reports. As well as an average of 56 reports of “High Wind.” During the wet season in the easter portion of the Mountain West the typical storm reports were “Hail”, average 67 during extreme period, and “Thunderstorm Wind”, average 11. Compared to other regions, the Mountain West had much lower average of reports associated with the extreme periods.

Point of contact: Melanie Schroers,