The storm system we mentioned in our last post a few days ago has trended toward a later arrival and is now on track to impact the Tetons & Western WY Sunday-Tuesday. Fri-Sat is looking drier with mild temps & gusty winds. The Sun-Tue system will bring off-and-on showers with snow levels dipping as low as 8,500 ft. This system doesn't look quite as wet compared to the last one.
Short Term Forecast
Current Conditions (as of Thursday PM):
A cold front moved through on Wednesday night, and we are experiencing cooler temperatures, partly cloudy skies, and breezy winds at times. Highs reached the 60s in the valleys, as opposed to 70s in prior days. Still, a pretty nice day out overall.
Here is a view from the top of Targhee on Thursday afternoon with mid to high-level clouds hovering over the Tetons. There is also still snow left over from last week's system above 10,500 feet on more shaded aspects.
Forecast for Friday to Saturday:
The end of the week is looking nice for outdoor activities after all. We will see partly cloudy skies on both days, with more cloud cover on Saturday compared to Friday. Winds will also be gusty at times out of the south/southwest, especially on Saturday.
Highs in the valleys will reach the upper 60s on Friday and low 70s on Saturday despite the gradual uptick in cloud cover. Morning lows will fall to near freezing in the valleys.
There is a slight chance of showers developing by late Saturday afternoon and evening, but this activity will most likely hold off until late Saturday night, if not Sunday.
Forecast for Sunday to Tuesday:
Cooler and wetter conditions will take hold during this period with off-and-on showers possible at all hours of each of these three days.
The culprit will be a large area of low pressure moving into the Great Basin. The exact location and track of the low pressure center will influence just how much rain and snow we receive, and there are still some model discrepancies. But confidence is high that conditions will turn cool and wet at a minimum.
The jet stream will nose its way into Western Wyoming from the south on Sunday when shower activity begins to pick up. The valleys (especially the JH side) could see more rainfall during this period since the rain shadow influence of the Tetons will be lost with winds blowing from the south.
On Monday, colder air will arrive and winds will shift to more of a west/southwest direction before turning northwest on Tuesday. This will result in more orographic-enhanced precipitation that will favor the west side and upper elevation east side of the Tetons.
Snow levels will range from 9,500-10,500 feet on Sunday, before falling to 8,500-9,500 feet late Sunday night through Tuesday as colder air arrives. Snow levels could occasionally dip lower than 8,500 feet but little to no accumulations are expected below that.
The rain/snow amount forecast is tricky with this storm since we are still a few days away, and because of the wobbly movement of this low pressure system. Slight variations in the storm track will have an impact on who sees the heaviest precipitation.
I think the most likely scenario is that we will end up with 0.50 to 1.25 inches of total precipitation (rain and liquid-equivalent snow) in the Tetons from Sunday through Tuesday, but totals could be higher or lower if the storm track ends up shifting compared to current projections.
Valley totals will most likely end up in the 0.25 to 0.75-inch range.
The higher elevations of the Tetons should pick up a few inches of snow with this system, with most of the accumulation occurring above 9,500 feet. Light accumulations are also possible down to 8,500 feet, but this zone looks to be right on the edge of the rain/snow line.
In addition to the Tetons, the higher terrain of Yellowstone will pick up a few inches of snow, as will the Salt River, Wyoming, Wind River, Gros Ventre, Absaroka, Beartooth, and Bighorn Ranges.
The below snowfall map assumes a 10:1 snow-liquid ratio (1" of liquid = 10" of snow). In reality, the ratios will probably be a little bit lower than this (7 or 8:1 perhaps) given the warmer airmass compared to what we see in the winter months.
High temperatures in the valleys will reach the upper 50s on Sunday, mid-50s on Monday, and upper 40s/low 50s on Tuesday. Temperatures will quickly drop as you go up in elevation.
Gusty winds can be expected on Sunday with higher gusts in the 30-40 mph range in the mountains and 20-30 mph range in the valleys. Winds should decrease a bit on Monday but then may pick up again on Tuesday, at least across the higher terrain.
There is a decent chance we will see some thunderstorms on Sunday afternoon in the warmer sector of this system while winds are out of the south. Thunderstorms are unlikely on Monday and Tuesday once the cooler airmass takes hold.
We should see a drying trend on Wednesday (October 4) and Thursday (October 5) on the backside of this system with just a slight chance of light/lingering showers. Temperatures will start to rebound from Tuesday's chilly values, but will still be on the cool side with highs in the upper 50s/low 60s.
Confidence is low in the pattern late next week, as some models are hinting at a weak system arriving from the northwest, while others are keeping this system north and east of the Tetons with drier conditions locally. We'll get a better idea of this next week.
Starting on Sunday (October 8), the European and GFS Models are hinting that we may head into a warmer and drier period with high pressure taking control of the pattern. The Canadian Model is not on board with this, however, which further adds to the uncertainty of the longer-range outlook.
Thanks so much for reading! Next update on Monday (October 2).