JACKSON, Wyo.– A significant change in the weather pattern will occur Monday evening and Tuesday after several weeks with little precipitation. A significant increase in monsoon humidity will arrive on Tuesday with widespread wetting rains expected as well as potential for heavy local precipitation and excessive runoff over steep/rocky terrain. A similar monsoonal moisture wave is also possible on Friday.
Summary of July 26 to August 1
Last week was hot and dry in northwest Wyoming with only a few isolated thunderstorms on Thursday and Friday that produced little precipitation. The city of Jackson only received 0.01 inches of rain last week, while areas east of Jackson received heavier amounts of rain.
High temperatures over the past week have ranged from 85°F on July 28 to 92°F on August 1. The August 1 high of 92°F was the second 90°F day of summer in Jackson. Low temperatures over the past week ranged from 40°F on July 26 to 46°F on July 29 and 30.
Forecast for Monday evening (8/1) to Monday (8/8)
After a hot day on Monday, towering cumulus clouds begin to appear late Monday afternoon with a few isolated thunderstorms possible before Monday evening. It is a subtle sign of the changes to come.
Smoke has also increased over the past two days, with the main culprit being the Moose Fire near Salmon, Idaho. The smoke will clear Monday evening and Tuesday, then confidence is low for the rest of the week with varying degrees of smoke possible from time to time due to fires in Idaho and California.
On Monday evening, an upper atmospheric disturbance known as “shortwave” will approach from the west/southwest with a heavy influx of monsoonal moisture. It won’t be like some of the monsoon humidity spurts in July that produced only erratic rainfall.
Humidity levels will be abnormally high with this monsoon wave, with precipitable water values (the amount of water vapor in the atmosphere) reaching 200% of average by Tuesday morning. The atmosphere will also become saturated even down to the lower levels of the atmosphere, increasing the potential for widespread wetting rain.
Showers and thunderstorms will develop overnight Monday evening, especially between midnight and sunrise, and increase in coverage during the morning hours of Tuesday. Periods of showers and thunderstorms are then to be expected throughout the day on Tuesday.
Showers and thunderstorms will move west/southwest to east/northeast first Tuesday morning, then west/northwest to east/southeast Tuesday afternoon and in the evening.
Heavy local precipitation will be possible under stronger showers and thunderstorms on Tuesday, particularly in the Tetons as well as the Snake River and Salt River Ranges, although valley areas may also experience heavy precipitation rates. The threat of heavy rain will be highest during the afternoon hours when atmospheric instability is greatest.
Under heavier showers and thunderstorms, excessive runoff and isolated flash flooding will be possible in steep and rocky areas with poor drainage. In other words, Tuesday will not be a good day for hiking and climbing on higher ground due to the high threat of lightning and heavy rain.
For the first time in over a month, high temperatures in the valley are expected to stay below 80ºF on Tuesday due to increased cloud cover. However, Tuesday morning lows will also be much milder than usual (mid to upper 50s) due to increased humidity.
The chance of showers and thunderstorms will persist late into Tuesday evening, then conditions are expected to eventually dry out later Tuesday night.
On Wednesday, drier air will make its way bringing more sunshine with highs in the 80s. However, we may see just enough lingering moisture to bring a slight chance of isolated thunderstorms after -midday. However, any rain should be brief and light.
On Thursday, a slight increase in humidity is expected before a trough of low pressure moving into the Pacific Northwest. This will result in a better chance of isolated thunderstorms on Thursday afternoon, with light to briefly moderate rain possible with any thunderstorm. Thursday’s highs will hit the mid-80s.
As the low pressure trough approaches, another surge of monsoonal moisture is possible on Friday, which could bring widespread thunderstorms with wetting rain, as well as locally heavy rain amid heavier showers and storms. . There is still some uncertainty regarding Friday’s outlook depending on the exact path of the low pressure system, but for now an active day is expected with cooler temperatures.
Over the weekend we could see some lingering humidity on Saturday with a chance of thunderstorms, although the coverage and intensity should be lighter than Friday.
Drier air will then set in on Sunday with only a slim chance of thunderstorms expected then, and we could very well end up with a dry day. On Monday, a slight increase in humidity and risk of thunderstorms is possible.
Temperatures will tend to rise Saturday through Monday with highs reaching the mid to upper 80s.
During the second week of August, another burst of monsoon moisture is possible as the south/southwesterly flow increases between a low pressure trough near the west coast and a high pressure ridge centered just at east of the Rockies.
If this pattern holds, we could see additional chances of thunderstorms with wetting rain. In other words, the rainfall potential looks much better in the first half of August compared to July, which was a very dry month, even by our standards.
Alan Smith, meteorologist
Climatology of the city of Jackson from August 2 to 8:
Medium top: 82
Medium Low: 41
High record: 94 (August 7, 1994 and August 8, 1989)
Record low: 27 (August 8, 1939)
Precipitation since October 1: 12.85″ (88% of average)