JACKSON, Wyo. – Moisture from both the North American Monsoon and post-tropical storm Hilary impacted Northwest Wyoming over the past week with frequent rainfall, cloud cover and more humid than usual conditions.
The next surge of monsoonal moisture will arrive late this week, and believe it or not, this surge will also involve the remnants of a tropical storm, but from the Gulf of Mexico. Tropical Storm Harold recently made landfall in Southeast Texas, and the remnants are becoming absorbed into the monsoonal circulation across the Southwest U.S.
Recent Weather Conditions and Post-Tropical Storm Hilary Recap
Following a rare (for this summer) one-week stretch of dry weather, we reverted back to a wet pattern late last week as a surge of monsoonal moisture arrived, resulting in showers and thunderstorms from Thursday (August 17) through Saturday (August 19).
The remnants of Hurricane Hilary then became a factor in the days to follow.
Hilary became a major hurricane in the Eastern Pacific, just offshore of the Baja Peninsula. As it moved northward, it quickly weakened upon hitting cooler waters. However, the storm maintained significant forward momentum, making landfall in Northwest Mexico as a tropical storm, while maintaining tropical storm strength as it moved into Southern California.
It’s not unusual for the remnants of tropical storms in the Pacific to reach the Western U.S. and often this moisture becomes entrained into the North American Monsoon circulation.
However, it is rare for tropical cyclones to reach Southern California at tropical storm strength, which by definition means that maximum sustained surface winds (not including gusts) range from 39-73 mph.
The storm resulted in heavy rainfall across the mountainous and desert regions of Southern California and Nevada, while even the San Diego and LA metro areas received 2 to 4 inches of rain – a rarity for this region in August.
As the storm moved northward, widespread heavy rain also fell across much of Idaho, Eastern Oregon, and Northwest Montana, putting a significant damper on fire season in these areas.
Jackson Hole and Northwest Wyoming caught the eastern fringe of moisture from this system, with frequent rain showers and thunderstorms but a lack of significant heavy rain and flooding issues experienced in other areas.
Significant cloud cover also resulted in cooler-than-average high temperatures and warmer-than-average low temperatures – a common theme this month.
Rainfall was most widespread on Sunday. Then on Monday, temperatures warmed up a bit and we saw more scattered afternoon thunderstorms that favored the higher elevations and western slopes of the Tetons.
One unusual element of Monday in Jackson Hole was the humidity. Dewpoint temperatures (the temperature at which the air must be cooled to become saturated with water vapor) rose to 61ºF at the Jackson Hole Airport on Monday, which is unusually moist for our area.
During infrequent past instances when the dewpoint has hit 60ºF in Jackson, the air temperature is usually not much warmer than this. However, the air temperature on Monday rose into the upper 70s on Monday, which combined with the high dewpoint value, resulted in a warm and humid feeling to the air – maybe not by Midwest or Eastern standards, but certainly by local standards.
Rainfall totals over the previous week (August 17-23) ranged from a half-inch to an inch in the Jackson Hole Valley and Teton Valley, and one to two inches in Star Valley and Yellowstone. Sunday (August 20) was the highest rainfall day for most areas.
Rainfall for the month of August as a whole has been very impressive. The rain gauge at my house in Jackson just topped 3 inches for the month, while the official climate station in Jackson is at 2.76 inches as of August 23.
If the official town station reaches 3 inches before the end of the month, this would be the third year in a row in which August rainfall would have exceeded 3 inches. An impressive streak considering that average August rainfall is only 1.23 inches.
Here is a list of monthly rainfall totals across the Teton/Yellowstone region so far in August 2023:
- 4.43″ – Afton
- 4.32″ – Grant Village/Yellowstone
- 3.71″ – West Yellowstone
- 3.70″ – Alta
- 3.69″ – Star Valley Ranch
- 3.57″ – Kelly
- 3.52″ – Yellowstone Lake
- 3.45″ – Driggs
- 3.26″ – Jackson Lake/Moran
- 3.26″ – Afton
- 3.02″ – Jackson (Downtown)
- 2.76″ – Jackson (Visitor Center)
Upcoming Weather Pattern
We will see a little bit of a reprieve in the weather pattern during the middle part of this week as deeper moisture retreats further south.
However, lingering low level moisture from our recent pattern combined with solar heating and instability will result in a chance of afternoon showers and thunderstorms on Wednesday. Coverage will be more isolated to scattered in nature with brief moderate to heavy rain possible.
Thursday may be the one day we truly dry out with mostly sunny to partly cloudy skies and highs in the upper 70s in the valley. A stray afternoon thunderstorm couldn’t entirely be ruled out, but the threat is low.
On Friday, the next surge of monsoonal moisture will arrive from the south. This surge will be aided by the remnants of Tropical Storm Harold, and an area of low pressure associated with Harold will also work its way toward the Utah/Wyoming/Colorado border region.
There is still a bit of uncertainty with regard to Friday’s forecast, however. Moisture will be increase, but heavy cloud cover and limited instability could potentially inhibit shower and thunderstorms development around the Tetons and Yellowstone, with better chances further south from Star Valley to the Wind River Range.
Also, it’s questionable whether or not the area of low pressure associated with the remnants of Hilary will make it far enough north to act as a trigger for showers and thunderstorms.
For now, it would still be a good idea to plan on rain and lightning potential if you have outdoor plans in the Tetons and Yellowstone. But there is some “bust” potential too.
On Saturday and Sunday, lingering moisture along with the potential for warmer temperatures and greater instability will result in a good chance of afternoon thunderstorms both days. Stronger storms will be capable of producing frequent lightning and heavy downpours.
Highs are projected to reach the upper 70s on both Saturday and Sunday.
Here is a look at projected rainfall over the next week.
On Monday and Tuesday, high pressure will strengthen over the Central Rockies. This will result in a warmer and drier pattern with more abundant sunshine along with little to no thunderstorm potential. Highs will warm up into the 80s in the valley.
Extended Outlook
Heading into the final few days of August and into Labor Day weekend, the pattern is uncertain due to poor agreement among longer-range weather models.
We may see some weaker moisture intrustions from time to time which could result in thunderstorm chances, but overall, moisture levels and rainfall coverage/amounts will almost certainly be less compared to recent weeks.
Temperatures may cool slightly around the middle of next week but then are projected to rebound to above-average levels heading into early September. For perspective, average highs in Jackson are in the upper 70s during the last week of August and mid 70s during the first week of September.
Alan Smith, Meteorologist
