JACKSON, Wyo. — Early September has been unseasonably warm and dry across Western Wyoming, but big changes are on the way this week. Moisture from the remnants of Hurricane Fay in the Eastern Pacific will reach our area this week, resulting in a prolonged stretch of wet conditions along with cooler temperatures.
September 6-12 Recap
Last week started out with record heat across Teton County and Western Wyoming. The Town of Jackson hit 90 degrees on five consecutive days from September 4-8 and the Jackson Hole Airport high 90 degrees on three out of five days.
Unfortunately, the heat wave across the Western U.S. contributed to major fire growth across Idaho and areas to our northwest, resulting in a few smoky days. The smoke was not as bad as it could have been in Jackson Hole, however, as air quality was much worse just a little further west of here.
The heatwave eventually came to an end last Thursday, and over the weekend it started to feel like fall with chilly/crisp mornings and warm but comfortable afternoons. Jackson recorded its first freeze of the season on Saturday morning, which is about three weeks later than average, followed by two more freezes on Sunday and Monday mornings.
As a cold front moved through last Thursday, showers and thunderstorms developed north of Jackson Hole, with Yellowstone seeing most of the action. Jackson did not record any measurable rainfall, while Moran recorded 0.09 inches of rain.
High temperatures in Jackson last week ranged from 69ºF on September 9 to 92ºF on September 6, and low temperatures ranged from 28ºF on September 10 and 11 to 50ºF on September 8.
Forecast for Tuesday (9/13) to Monday (9/19)
Get ready for a wet pattern! Hurricane Fay in the Eastern Pacific worked its way northward along the Baja Peninsula, maintaining tropical storm strength not too far from the California border. It was the closest a tropical storm had passed to California in 25 years!
While the system eventually weakened into a tropical depression, moisture from this tropical system is working its way northward this week, aided by a trough of low pressure setting up across the Western U.S.
This setup will place Western Wyoming under a moist southwest flow with rain expected each day from at least Tuesday evening through Saturday, and possibly longer. Significant rainfall amounts can be expected during this pattern as well, along with improving smoke conditions. Rainfall will be a bit lighter further west in Idaho, but this pattern should still help out a lot with the fire situation.
Tuesday morning will start out dry with highs reaching the mid 70s, but cloud cover will be on the increase during the afternoon. By late afternoon or early evening, the first in a series of disturbances will arrive with scattered showers and thunderstorms developing.
Showers will continue through the overnight hours on Tuesday night and will become more widespread on Wednesday and Thursday. Occasional thunderstorms can be expected as well, especially during the afternoon hours.
On Friday, rainfall may become more intermittent and lighter compared to prior days, but it still looks like a wet day overall with additional showers likely. A stronger disturbance is then expected to arrive on Saturday, which could result in another uptick in shower coverage and intensity.
The amount of water vapor in the atmosphere will be very high by our standards, and for this late in the year, given the tropical source of the moisture. As a result, rainfall rates could be heavy at times. Widespread flooding is not expected, but some isolated runoff problems or minor stream flooding couldn’t be ruled out if heavier showers develop over an area for a prolonged period of time.
Temperatures will also be much cooler in this pattern with highs in the 60s in the valley each day from Wednesday on and highs in the 40s at 9,000 feet.
Snow levels will generally range from 11,000-12,000 feet (possibly lower at times) from Tuesday night through Thursday afternoon, before dropping to 10,000-11,000 feet from Thursday night through Saturday. While these snow levels are relatively high, they will be low enough for significant snow accumulations to impact climbing conditions over the higher peaks of the Tetons and Winds.
Once we get beyond Saturday, confidence in the forecast decreases substantially. A low pressure system is expected to move into the Western U.S. on Sunday and Monday, but the track of this system and potential impacts for Teton County are highly uncertain.
If the system takes a favorable track, another round of widespread precipitation along with colder air and lower snow levels will be possible. However, it’s also possible that the storm could take a less direct path with warmer air, more intermittent (if any) showers, and higher snow levels. Time will tell…
An unsettled pattern will likely continue through the first half of next week, with rain chances persisting through at least next Wednesday or Thursday. Rain amounts, temperatures, and snow levels are highly uncertain, but we’ll need to at least keep an eye out for possible snow down to pass levels in this pattern.
Toward the end of next week, we may see a trend back toward dry and sunny conditions, but even that isn’t a given yet if the low pressure system across the West were to lingers/meander for longer than currently expected.
Alan Smith, Meteorologist
Town of Jackson Climatology for September 13-19:
Average High: 71
Average Low: 32
Record High: 93 (September 18, 1956)
Record Low: 13 (September 17, 1936 and September 19, 1988)
Precipitation Since Oct 1st: 16.50″ (100% of Average)