JACKSON, Wyo.– Early September was abnormally hot and dry in western Wyoming, but big changes are afoot this week. Moisture from the remnants of Hurricane Fay in the Eastern Pacific will reach our region this week, bringing an extended period of humid conditions as well as cooler temperatures.
Recap September 6-12
Last week started with record heat in Teton County and western Wyoming. The city of Jackson hit 90 degrees for five straight days from Sept. 4-8, and Jackson Hole Airport hit 90 degrees three out of five days.
Unfortunately, the heat wave in the western United States contributed to a significant growth in fires in Idaho and parts of our northwest, resulting in a few days of smoke. The smoke wasn’t as bad as it could have been in Jackson Hole, however, as the air quality was much worse just a little further west from here.
The heatwave finally came to an end last Thursday and over the weekend it started to feel like autumn with cool, crisp mornings and warm but comfortable afternoons. Jackson recorded its first frost of the season Saturday morning, about three weeks later than average, followed by two more frosts on Sunday and Monday morning.
As a cold front swept through last Thursday, showers and thunderstorms developed north of Jackson Hole, with Yellowstone seeing most of the action. Jackson recorded no measurable rain, while Moran recorded 0.09 inches of rain.
High temperatures in Jackson last week ranged from 20°C on September 9 to 30°C on September 6, and low temperatures ranged from 28°F on September 10 and 11 to 50°F on September 8.
Forecast from Tuesday (09/13) to Monday (09/19)
Get ready for a wet pattern! Hurricane Fay in the Eastern Pacific made its way north along the Baja Peninsula, maintaining tropical storm strength not far from the California border. It was the closest a tropical storm has passed in California in 25 years!
While the system eventually weakened into a tropical depression, moisture from this tropical system is heading north this week, aided by a trough of low pressure moving into the western United States.
This pattern will place western Wyoming under a moist southwesterly flow with rain expected daily from at least Tuesday evening through Saturday, and possibly longer. Significant amounts of precipitation can also be expected during this pattern, as well as improved smoke conditions. Precipitation will be a bit lighter farther west in Idaho, but this pattern should still help the fire situation a lot.
Tuesday morning will be dry with highs reaching the mid 70s, but cloud cover will increase through the afternoon. In the late afternoon or early evening, the first of a series of disturbances will arrive with scattered showers and thunderstorms developing.
Showers will continue overnight Tuesday evening and become more widespread Wednesday and Thursday. Occasional thunderstorms can also be expected, particularly during the afternoon hours.
On Friday, precipitation may become more intermittent and lighter than previous days, but it still looks like a wet day overall with additional showers likely. A stronger disturbance should then arrive on Saturday, which could lead to a further increase in shower coverage and intensity.
The amount of water vapor in the atmosphere will be very high by our standards, and for that late in the year, given the tropical source of the humidity. As a result, precipitation rates could be significant at times. Widespread flooding is not expected, but some isolated runoff issues or minor flooding could not be ruled out if heavier downpours develop over an area for an extended period.
Temperatures will also be much cooler in this pattern with highs in the 60s in the valley each day from Wednesday and highs in the 40s at 9,000 feet.
Snow levels will generally range from 11,000-12,000 feet (possibly lower at times) Tuesday evening through Thursday afternoon, before dropping to 10,000-11,000 feet Thursday evening through Saturday. While these snow levels are relatively high, they will be low enough that significant snow accumulations will impact climbing conditions on the higher peaks of the Tetons and Winds.
Beyond Saturday, confidence in the forecast declines significantly. A low pressure system is expected to move across the western United States on Sunday and Monday, but the path of this system and potential impacts for Teton County are highly uncertain.
If the system follows a favorable track, another wave of widespread precipitation accompanied by colder air and lower snow levels will be possible. However, it is also possible for the storm to take a less direct path with warmer air, more intermittent showers (if any) and higher snow levels. Time will tell us…
A choppy pattern will likely continue through the first half of next week, with a lingering chance of rain through at least next Wednesday or Thursday. Rainfall amounts, temperatures, and snow levels are highly uncertain, but we’ll need to at least keep an eye out for possible snow to pass the levels in this pattern.
Towards the end of next week we may see a trend back towards dry sunny conditions, but even that is not yet a given if the low pressure system across the West were to linger/meander for longer provided that.
Alan Smith, meteorologist
Climatology of the city of Jackson from September 13 to 19:
Medium top: 71
Medium Low: 32
High record: 93 (September 18, 1956)
Record low: 13 (September 17, 1936 and September 19, 1988)
Precipitation since October 1: 16.50″ (100% of average)