Whenever we have thunderstorms that are not caused strictly by afternoon heating,  there is usually another root cause. Most commonly, thunderstorms that form overnight or in the early morning hours in the Rockies are a result of a low pressure center or cold front passing overhead, or monsoon moisture.

This morning, Tuesday July 17th, 2018, Jackson Hole experienced some early morning thunderstorm activity that was due to an upper level Trof of Low-pressure moving across Eastern Idaho and Northwestern Wyoming.

There were no fronts or obvious surface Low-pressure centers marked on the Surface map over eastern Idaho or far western Wyoming. (See map below).

The Trof was barely detectable on the upper level weather maps, that is, it was not a big, obvious wave. However, it did have two things going for it that the computer models did predict very well: 1) High level moisture. 2) Positive Vorticity Advection or PVA. (PVA is a technical term for an area of spin that causes upward motion. In other words, it creates more instability in the atmosphere. Sometimes referred to as a “short-wave”.)

The PVA shown was most likely responsible for perking up these early morning thunderstorms, which produced moderate amounts of lightning and brief heavy rain in a very concentrated area.

Last map below shows the predicted high level moisture in the early morning hours on July 17th. Relative Humidity greater than 90% over Teton Co. Idaho & Wyoming, which is where the Vorticity was headed and where the line of thunderstorms developed.

Finally, the photo below is from the Teton Pass webcam, showing storm as it was passing over Jackson Hole.

Post by: meteorologist Jim Woodmencey
17-JULY-2018

In this week’s post I will talk a little bit about how to prepare yourself each day to avoid being surprised by thunderstorms. And if for some reason you do get caught out during a thunderstorm, some tips on what to do.

Weather Check

Your first order of business each day, should be to check the weather forecast. That will to give you some idea about what to expect. In particular, check the probability of precipitation, and also if any thunderstorms are mentioned in that forecast.

On the Jackson Hole Forecast page of mountainweather.com you will see daily weather icons, as well as in the textual description for each day, showing the percent chance of precipitation. Below that text, “Precipitation Probability” is also listed for the mountains, which often times will be a little higher than for the valley.

A “Lightning Activity” forecast is given each day, listed as: None, Low, Moderate, or High. The activity rating will depend on the likelihood of thunderstorms and how strong those thunderstorms are expected to be. Further down the page this info for the mountains is displayed in a graph format.

A high probability of precipitation, greater than 60-percent, in combination with moderate or high lightning activity, should perk-up your interest and be cause for concern that day.

That said, even a low chance of thunderstorms or lightning activity warrants some vigilance of your surroundings. That is, keep an eye on the sky for building cumulus clouds.

Activity Check

The next thing you should do, whenever there is any mention of thunderstorms in the forecast, is to assess the activity you plan on engaging in. Will you be hiking or biking up to a ridgetop, going over a pass or saddle? Will you be travelling across large areas of open terrain? Will you be on a lake or river?  All of the above put you in areas of high exposure to lightning.

Fishing and boating are high exposure activities in a medium that is a really good conductor of electricity, water. Of all the outdoor activities, fishing accounts for the highest percentage of lightning fatalities in the United States.

Are you going to be on a baseball or soccer field? These are wide expanses of open terrain that leave you highly vulnerable to lightning strikes.

Also, assess how much time and what time of day you expect to be in any of these vulnerable positions?

Safety Check

A thunderstorm does NOT have to be directly over you to throw a bolt of lightning your way. It does NOT have to be raining first, before you should seek shelter. Although, most people never think about seeking shelter until either one or the other of these is happening.

Another Trivia bit: The most dangerous time for a fatal lightning strike, is before the thunderstorm is directly overhead.

When you do seek shelter, buildings or vehicles are the safest places to be. A picnic shelter, baseball dugout, or under a rock overhang are NOT safe places. Why? Because:

1) The picnic shelter is an un-grounded structure that will not fully disperse the electrical charge.

2) Same with the dugout, which may also have a chain-link fence that can conduct the electricity, so at least don’t hang onto the fence!

3) Under the rock overhang, your body is filling in the space between the roof of the overhang and the ground, making it easier for electrical currents running through the rock to close that gap, directly through you!

Postpone or Go Home

No matter what the weather forecast, or how you interpreted it, as your mom used to say, “better to be safe than sorry”. In this case, better to turn around, cancel your plans, of just go home early than to be fried by lightning.

In the mountains at least, I have learned to not let my goals interfere with my decision making. I keep re-assessing the weather, but when clouds are building and as soon as I can hear thunder, I don’t want to be in a vulnerable location to wait it out, I am looking to retreat.

Lightning can be marvelous to watch, but it can also be quite frightening, kind of like being too close to the fireworks. I will write more about thunderstorms and lightning safety in future columns this summer. Have a happy and safe Fourth of July.

Posted by meteorologist Jim Woodmencey

Some of the content of this post first appeared in the Mountain Weather column in the Jackson Hole News & Guide on July 4th, 2018

Thunderstorms around the Jackson Hole area the last two days are a result of warm and moist air coming up from the Desert Southwest Monsoon that is now underway.

A “monsoon” is simply a seasonal wind flow pattern, in this case, one that occurs each summer, primarily in July & August, and extends from the interior of Mexico  and up through the Four-Corners Region. Sometimes, that monsoon flow extends as far north as Wyoming & Montana.

Satellite map below shows the direction of flow on Tuesday morning, at about the same time a cluster of thunderstorms (shown on the lightning strike map) was passing over Jackson, WY. Whereas, everywhere else in the path of that moist air, there were no thunderstorms!

That is the nature of monsoon-related thunderstorms, they can be quite random, and can occur any time of the day or night.

The weather pattern changes over the next two days, to more of a “Zonal” flow (from West to East) across the Pacific Northwest & Northern Rockies, which will keep the monsoon suppressed to the south of Wyoming.

More to come on Thunderstorms & Lightning in future posts.

Posted by: Meteorologist Jim Woodmencey

Happy Fourth of July…. get outside and celebrate America’s Birthday!

A weak upper level disturbance moves by to the north of Jackson Hole this afternoon and evening, bringing a little bit of mid-level moisture and unstable air over Northwest Wyoming.

Expect some cumulus cloud build-up and some scattered thunderstorm activity around the Yellowstone & Teton Park regions later today. But that is probably the only weather to worry about for the rest of the Fourth of July week.

Below is the Accumulated Precipitation Forecast for Tuesday morning to Friday morning (July 3 to 6) for the Western U.S.
Click map to view loop:

Cooler temps Tuesday, then warming up Wednesday thru Friday as a broad Ridge of High-pressure builds over the Rockies. Looks like great weather for the Fourth of July and the remainder of the week.

Below is the Precipitation Type and Probability Map for the entire USA on July 4th, 2018:

No signs at this point of any measurable precipitation Wednesday thru Sunday, although over the weekend we may start to see a few more clouds building in the afternoons.

Post by meteorologist Jim Woodmencey
July 3rd, 2018

The Summer Season officially began at 4:07 am MDT Thursday June 21^st.

Up to this point, June of 2018 in Jackson Hole has had its ups and downs. The first week or two of the month it was warm and dry, reaching our first 80-degree day on June 9th. Then, the last several days, going back to the start of the Father’s Day Weekend, we got a deluge of rain that put down 1.61 inches of rain in the Town of Jackson is less than 4 days. Sunday & Sunday night, June 17th, accounted for 1.05 inches of that total. On Monday, June 18th, our high temperature was only 56-degrees.

Now that summer has “officially” arrived, hopefully we can get on with some more summer-like weather conditions. Normal High temperatures would be in the mid 70’s in Jackson this last week or so of June. We may get back to that again next week, but this weekend looks like it will be a little cooler than that, as a couple of upper level low-pressure Trofs roll across the Pacific Northwest & Northern Rockies.  Those disturbances will cause  some showers, but not the deluges like we saw this past weekend.

Some Solstice Facts

The time of the Summer Solstice (4:07 am MDT Thursday June 21^st) is precisely when the sun will be directly over the Tropic of Cancer, at 23.5 degrees North latitude.

The Solstice also marks the longest day of the year in the Northern Hemisphere.  Actually, sunrise/sunset times do not change much around the Solstice, as the last few days, since June 19th, have all had about the same length of daylight. The days don’t really start getting shorter for another week or two, by a minute or two.

We will have almost 16 hours of daylight in Jackson the rest of June  — not including twilight — compared to less than 9 hours around the Winter Solstice in December.

The first graphic below shows the tilt of the earth and its orientation to the sun’s ray on June 21st.

This second graphic shows the change in the tilt of the earth and its orientation to the sun.  This is what’s responsible for the seasonal changes we see.

Around the Summer Solstice the sun will also appear as high in the sky as it gets around here, about 70 degrees above the horizon at noontime. It will also be rise a little more north of due east and setting a little more north of dues west, due to the shape and curvature of the earth.

That’s your solstice science lesson for today. Happy Summer!

Posted June 21, 2018 by meteorologist Jim Woodmencey

Low-pressure moves inland from the Pacific Northwest Coast and settles in over the Great Basin for the Father’s Day Weekend. More clouds and scattered showers & Thunderstorms will be generated as moist and relatively unstable air is circulated over the Rockies, Friday thru Monday (June 15 to 18).

The flow aloft over western Wyoming and Jackson Hole will be coming out of the South (Southwest to Southeast) this weekend, so be on the lookout for moisture & T-storms generally tracking from those directions, especially over the surrounding mountains.

Forecast of this weekend’s precipitation is shown in the map below. Click the image to run a loop of the accumulating precip amount, every 3-hours, from Friday Noon to Monday Noon, June 15 to 18. (This is just one model’s interpretation, from CAIC).

Central & Eastern Wyoming in this model’s prediction seemed to get more precip Saturday night/ Sunday morning.  Southwestern Wyoming & the Jackson Hole area seem to get the most precip later Sunday & early Monday.

Here are the Convective Outlook Maps for Saturday & Sunday.  These maps display areas with a risk of Thunderstorms. Thunderstorm coverage over the Rockies is widespread, but risk of strong thunderstorms is limited to areas east of the Rockies.

Posted: June 13th, 2018
by meteorologist Jim Woodmencey

Looks like we will be saying goodbye to the totally sunny days we have had lately. After  a week or more of nearly perfect weather to begin the month of June 2018, we can expect some changes at the end of this week and through the Father’s Day Weekend.

Here is Wednesday morning’s weather map:

The big blue “H” (High pressure) that has been parked over the Western U.S. & Rockies and kept the atmosphere dry and stable, is beginning to break down. That will be replaced by a Low-pressure system that will move inland the next few days & hang out over the Great Basin into early next week.

That will lead to some showers & thunderstorms around many areas of the West, including Western Wyoming and the Jackson Hole area beginning as early as Thursday afternoon/evening and then each day thru Sunday, Father’s Day.

Here are the general Forecast Maps for the USA for Thursday & Friday: