JACKSON, Wyo. – We have seen a quick transition to spring and early summer weather conditions during May, following a winter season that wouldn’t quit until late April. A warm and moist southwest flow pattern has set up this week and will continue through the end of May, resulting in daily rounds of showers and thunderstorms across the Teton-Yellowstone region.
Cold Winter Followed by Above-Average Warmth in May
Following six consecutive months of below-average temperatures, we have suddenly flipped into a warmer-than-average regime during May. So far, temperatures are 3ºF warmer than average this month and we have already exceeded 70ºF on 10 days this month. The long-term average high in Jackson during the month of May is 64ºF.
So far in May 2023, the average high is 66.4ºF. Compare this to April 2023 when the average high for the month was only 46.7ºF. In other words, daytime temperatures have been 20 degrees warmer in May compared to April!
The warmth we have experienced this month has certainly made for some enjoyable outdoor conditions following a cold winter. But believe it or not, mountain snowpack is actually below average for late May following our recent spell of warm temperatures and a lack of any new high-elevation snow that can periodically delay the melting process.
The snow depth at Jackson Hole’s Rendezvous Bowl Plot at 9,580 feet is 69 inches as of May 24, compared to a 15-year average of 88 inches. The snow depth at the Grand Targhee Snotel at 9,260 feet is 76 inches, compared to an average of 86 inches.
In other words, despite a deep winter for snowfall, mid to high-elevation trails in the Tetons are not expected to melt out later than usual this year.
The uptick in snowmelt in recent weeks has led to a rise in streams across Teton County. The Gros Ventre River and Pacific Creek are both near flood stage, and the National Weather Service has issued Flood Advisories for both of these rivers due to continued snowmelt and forecast rain from showers and thunderstorms in the near future.
The past few days aside, May has also been a drier month so far with below-average precipitation. As of May 24, the town weather station in Jackson had only received 0.66 inches of rain for the month. Average rainfall for the month is 2.20 inches.
We should make up some ground over the final week of the month with rain in the forecast nearly every day, though it will be hit or miss in nature.
The Tetons typically receive late-season snowfall from time to time in May, but it has been a rarity this May. The Rendezvous Bowl Plot averages 32 inches of snow during the month of May (15 years of data), but has only received 2 inches so far this month. This is on track to easily be the “least snowy” May in the past 15 years.
Daily Rounds of Thunderstorms This Week and Over Memorial Day Weekend
A trough of low pressure has set up over the West Coast and will exhibit little movement over the next week or more as a strong ridge of high pressure just east of the Continental Divide will act as a barrier to the trough moving inland.
The result will be a consistent southwest flow of moisture into Western Wyoming, aided by a series of embedded “disturbances” in the upper atmosphere. Temperatures will also be relatively warm with no significant cold fronts expected, resulting in higher levels of instability, or thunderstorm “fuel.”
Scattered to numerous showers and thunderstorms can be expected each day from Wednesday through Friday at a minimum, and will likely continue on a daily basis through the holiday weekend as well. Thunderstorm chances will be highest in the afternoon and evening hours with only a slight chance of overnight or early morning activity.
Strong thunderstorms will also be possible at times, which will be capable of producing frequent cloud-to-ground lightning, locally heavy rain, small to medium-sized hail (up to marble size) and gusty winds. Storm motions will generally be from southwest to northeast.
Rainfall amounts are always hit-or-miss in warm season thunderstorm patterns, but atmospheric moisture levels will be high for our area in this pattern, which should result in some nice downpours from time to time with most areas of Teton County and NW Wyoming seeing respectable rainfall amounts add up over time.
High temperatures over the next week will generally range from the mid 60s to low 70s in the Jackson Hole Valley each day, and this will largely depend on the extent of cloud cover during the peak heating hours. Overnight lows will be milder than average given the abundance of moisture in place, and will generally range from the upper 30s to low 40s.
Lightning Safety
We are just at the start of lightning season in Jackson Hole, which historically covers the period from May through September, while June, July, and August are the most active months.
Late May is setting up to be a more active period for lightning than usual this year, and with the holiday weekend coming up, it’s an important to re-familiarize yourself with lightning safety.
The number one rule of thumb regarding lightning safety is that no place outdoors is completely safe from lightning. And if you can hear thunder or see lightning, then you are close enough for lightning to be a concern. Hence the slogan, “When thunder roars, head indoors.”
Given that we are in an outdoor paradise, it’s not always possible to quickly retreat to an indoor environment (including your car) when lightning is present or imminent. But there are ways to reduce your exposure. The graphic below, produced by the National Weather Service in Salt Lake City, is a good overview of types of terrain that are more or less susceptible to lightning danger than others.
What About the Canadian Wildfire Smoke?
Late last week, we saw an unwelcome and unusually early appearance of wildfire smoke. The smoke originated from large wildfires in Northern Alberta. This region experienced below-average snowfall this winter and has also experienced record early-season heat during the month of May. As a result, fire activity has been much more severe than usual.
We saw a period of north/northeast winds late last week and over the weekend, which transported smoke from these fires into Wyoming. Fortunately, winds have shifted back to a more typical southwesterly direction, which has helped to scour out the smoke with a return to clean air. Smoke is not anticipated to return in the near future, either.
Northern Alberta did see some beneficial rain early this week, which should help with their fire situation, at least in the near term. But it’s unknown how persistent or severe fire activity will be in this region heading into the summer season.
Extended Outlook
As we head into early June, the stubborn pattern featuring a ridge of high pressure nearby or just to our east and a trough of low pressure near the West Coast is projected to continue, keeping a southwest flow in place.
While slight fluctuations in these large features could alter our weather patterns from time to time, in general, shower and thunderstorm chances are expected to persist on a frequent basis while temperatures are also expected to be on the warmer side of average.
Alan Smith, Meteorologist
