JACKSON, Wyo. — An unseasonably wet pattern last week has bumped August rainfall totals to 200-400% of average across Teton County (through August 23). Taking the average precipitation totals across all reporting Applied Climate Information System (ACIS) stations in Teton County, this has been the wettest August since 2014 and the second wettest since 1968.
Given the hot and dry early summer period and worsening drought conditions, this late summer boost in precipitation has been extremely welcome.
August started off an active note with impressive precipitation totals during the first week of the month, thanks in part to a surge in moisture associated with the North American Monsoon that extended northward into the Northern Rockies. The second week of August was much hotter and drier, and fire danger started to creep back up after a temporary reprieve.
Then, last week, we transitioned to a cool and wet pattern more reminiscent of fall as a strong and slow-moving low pressure system set up just west of Jackson Hole. A persistent southerly flow produced widespread moderate to heavy rain showers that added up to 1.43 inches in Jackson from Tuesday night through Friday morning.
Also, high temperatures last Wednesday and Thursday were only in the 50s in Jackson, a good 25-30 degrees below average. Snow fell across the higher elevations of the Tetons as well with the colder air in place.
A second faster-moving storm system arrived on Saturday, producing an additional 0.70 inches of rain in Jackson.
Check out the one-week rain totals from August 17-24.
To compare this year’s August rainfall to that of past August’s, we have taken the average rainfall among multiple ACIS weather stations in Teton County for both this year and prior years dating back as far as 1904. There are still seven days left in August, of course, but at this point, there is minimal potential for any measurable rainfall for the remainder of the month.
To accurately assess precipitation totals in the area, it’s helpful to evaluate data from multiple weather stations in the area since rain totals vary substantially by station, especially during summertime convective rains, where there can be highly localized variations in rainfall beyond our typical terrain effects.
Also, over the past 100+ years of data, there are numerous years in which weather stations in Teton County have missing monthly data, so by taking the average rainfall of multiple stations, it helps to even out the data if one or more stations are missing data for a particular month/year.
In this case, we examine the seven ACIS weather stations in Teton County where historical data is available: Jackson, Moose, Moran, Alta, Snake River (at the south entrance of Yellowstone), Old Faithful and Yellowstone Lake.
Most years have at least five stations with reported August precipitation totals dating back to 1932. In 2021, reports are currently not available for Old Faithful or Yellowstone Lake, so they are not included in this year’s precipitation report. If data for these stations does become available, it could change this year’s assessment slightly, but probably not by much.
The average rainfall for Teton County weather stations that are reporting in August 2021 so far is 3.46 inches, which is well above the long-term average of 1.39 inches or the 30-year average of 1.37 inches. This puts August 2021 in second place during the last 50 years.
August 2014 was the wettest of the past 50 years with an average of 3.97 inches of precipitation. Prior to 2014, you have to go all the way back to 1941 to find a higher monthly precipitation average, when an average of 4.53 inches of precipitation was recorded across five stations.
Wettest Augusts on record for Teton County (averaged across multiple locations):
- 1941 – 4.53 inches (5 out of 7 reporting stations)
- 2014 – 3.97 inches (7 out of 7 reporting stations)
- 1968 – 3.84 inches (5 out of 7 reporting stations)
- 1912 – 3.67 inches (4 out of 7 reporting stations)
- 1951 – 3.47 inches (5 out of 7 reporting stations)
- 2021 – 3.46 inches (5 out of 7 reporting stations)
Interestingly enough, August of 2020 was tied for the fifth driest August on record in Teton County with average rainfall of 0.31 inches, whereas August 2021 is the sixth wettest on record so far.
Let’s take a look at some of the individual stations and where August 2021 ranks, just for fun. Keep in mind that individual stations do have some missing years, so there could be some omissions in the data for “wettest Augusts” as a result.
Wettest recorded Augusts in Jackson (dating back to 1905):
- 1945 – 3.80 inches
- 2021 – 3.49 inches
- 1983 – 3.27 inches
- 1941 – 3.27 inches
- 1951 – 3.22 inches
Wettest recorded Augusts in Moose (dating back to 1958):
- 1978 – 4.18 inches
- 2021 – 3.95 inches
- 1983 – 3.24 inches
- 1971 – 2.86 inches
- 1968 – 2.80 inches
Wettest recorded Augusts in Moran (dating back to 1911):
- 1941 – 5.14 inches
- 2014 – 3.64 inches
- 1968 – 3.56 inches
- 1945 – 2.98 inches
- 2021 – 2.90 inches
Wettest recorded Augusts in Alta (dating back to 1909):
- 1968 – 4.87 inches
- 2014 – 4.74 inches
- 2021 – 4.26 inches
- 1951 – 3.86 inches
- 1912 – 3.38 inches
Wettest recorded Augusts at the Snake River near the YNP South Entrance (dating back to 1906):
- 2014 – 5.67 inches
- 1941 – 5.42 inches
- 1951 – 3.56 inches
- 2004 – 3.54 inches
- 1945 – 3.23 inches
Impressive rainfall this month has also helped to reduce the fire danger substantially, and at a time of year when average temperatures are also on the decline and daylight hours are shortening.
With no major warm spells expected through early September, we may very well get through the season without any significant wildfires.
Fuel moisture levels have risen well above the 10% critical level to the 16-20% range for 1,000-hour fuels (medium to large trees) and around 15% for 100-hour fuels (grass and shrubs). This is great news!
Now the smoke on the other hand… While there aren’t any ways to quantify smoke data in Jackson Hole due to a lack of historical air quality data, this certainly seems to have been one of the smokiest summers in recent memory, even though all of the smoke has come from distant fires.
Regardless, the recent moisture has helped our local situation substantially. We are currently in a severe drought in Teton County, and this recent wet pattern won’t erase the drought, though it will be interesting to see if there is any small improvement when the next drought report is released later this week.
For the remainder of August and early September, a drier pattern is expected but temperatures are projected to be below average this week and near average next week. It seems that the worst of the summer “heat” (multi-day stretches of 85+ temps for Jackson) is probably behind us at this point with fall right around the corner.
Alan Smith, Meteorologist