Weather Hazards in the Colorado Front Range

D'Arcy Straub - May 13, 2001

Below is some weather information which I presented at the Safety and Leadership Seminar for the CMC's Denver Group in September. If you are new to Colorado, this information may be of use to you. And even if you aren't new, it may still be useful. For instance, what's the best time in the summer to climb high peaks based upon the threat of thunderstorms? I would say from late June to mid July. Read on for why this is so.

The information was intended for CMC trip leaders residing in Denver, but it's probably applicable to front range residents in general.

Finally, this information is based upon personal observations and some data which I have collected from various sources. If you see anything in error or can think of something which I overlooked, don't hesitate to holler. Thanks.

As a leader, you should be aware

As a leader, you should be aware of three hazards due to inclement weather: lightning, frost bite and hypothermia.

Lightning kills a significant number of people each year. If you are caught in an electrical storm, the safest place is below tree line. If you are above tree line, the worst place to be is on top of a summit or on a ridge (an electric charge on any body is strongest along its points and edges). Often times when a thunderstorm is building and overhead, you can begin to hear the rocks "hummm" from the build up of a static electricity charge. This is a sign to descend. More rarely, the static electricity charge is so great that "sparks" of electricity literally arc from the surrounding rocks to parts of your body or backpack (usually your metal ice ax). This too may be construed as a sign to descend. For further excellent advice relating to lightning, see John Devitt's article at

If you find yourself on a ridge or summit, spread the group out and descend from the summit or the ridge (even a hundred foot drop from the ridge is significant, but the greater the drop, the better). If you are unable to move because the conditions are so bad, remove all metal objects from yourself and squat down such that only your boots are touching the ground. This will minimize the effects of ground current if lightning should strike in the near vicinity.

The potential for frostbite is greatest when the wind is blowing fiercely. On trips where this is a potential hazard, remind people to look after each other.

Be on the lookout for slight cases of hypothermia whenever it is very cold or it has rained. Generally, on a single day outing, a slightly hypothermic person will merely be uncomfortable. Most people who die from hypothermia have been exposed to the elements for days or because the temperatures are substantially cold.

Planning Your Trip According to the Calendar

As a CMC leader, you will have to plan your trips several months in advance due to the publication schedule of the Activity Schedule. Quite obviously, you won't know the weather conditions of your trip until a few days before your trip. However, some broad generalizations can be made about Colorado's weather on a monthly basis. The monthly descriptions given below start with July - or summertime - since people are most knowledgeable about those conditions and they are the easiest to describe.


The major threats in July are thunderstorms and lightning. While thunderstorms should be expected at any time in July, the probability for a thunderstorm increases substantially with the onset of the monsoon season (the monsoon results from a tropical air mass which comes from directly from the Gulf of Mexico). The monsoon typically starts around the second or third week of July.

Snow on the highest peaks is not unheard of in July, especially in early July.


The monsoon continues into August, but in a good year, will begin to decrease by the end of the month. In a bad year, it goes in to September. Thunderstorms are a way of life in August.

In late August, you can feel autumn in the air near dusk and dawn, but the afternoons are still reasonably warm.


Wintertime conditions with snow are not uncommon! Don't be caught unprepared!

Thunderstorms are not unheard of in September. However, the day time heating which leads to thunderstorms diminishes considerably in September. Thus, thunderstorms are not the threat they were in July and August, but the threat still exists. Typically, a jacket and pants will be needed to keep warm, especially on the high peaks.


The mountains begin to get their first good snows and the snow pack begins to develop. Don't be fooled - people have died in avalanches in October!

The threat for lightning is minimal, but not zero.

Early morning temperatures are cool, but afternoons - especially in early October - can be comfortable. For this reason, a relatively late start can be advantageous.


The cloudiest month of the year (at least for Denver). Leading a trip in November? Expect to watch the news and wonder if it's going to snow on your trip.

Arctic cold fronts capable of dropping temperatures to the negative single digits in Denver begin to be a possibility around the second week of November.


It's winter, okay? This means cold temperatures and arctic cold fronts capable of dropping temperatures in Denver to 20 below zero. However, often times the cold dense air from an arctic front can't make it into the mountains. Thus, it may be warmer in the mountains than in Denver.

Snow in the winter months is dry. Hence, above tree line the wind usually blows the snow off the exposed high summits and ridges.


The severest arctic blasts - or those capable of dropping temperatures from 20 to 25 degrees below zero in Denver - are most likely in January and through the second week of February.

Chinook winds can lead to some darn nice winter days in Denver with temperatures in the 60's. However, these temperatures are created by air moving from the west that is compressed as it descends mountain valleys (the compression leads to the warming of the air). Yes, the weather might be grand in Denver, but it may be rather windy in the mountains and especially in the foothills. Strong westerly winds at this time of the year may also create orographic - or mountain created - snow storms.


By the middle of February, the threat for the coldest temperatures has moderated.

One can notice the higher sun by the end of February. This is both good and bad. On one hand, the "warm" sun feels good. On the other hand, it melts the snow and can create icy conditions on cross-country ski trails.


The cold mornings still remind one of winter. However, the higher spring sun is very noticeable, both in terms of comfort and skiing conditions.

March marks the onset of the wet spring snows. The snow that falls is not as likely to be wind blown above timberline. Consequently, a spring ascent of a peak can be much more challenging than a winter ascent.


Just a bad month to do anything in the mountains. The snow is usually lousy for skiing - except off of the highest peaks - and the snow pack is at its highest at the beginning of the month. In addition, the spring snowpack can be frustratingly unstable, even with snowshoes and skis.

Look at past CMC Activity Schedules - most CMC trips stick to the foothills. However, April is one of the snowiest months for Denver and the nearby foothills. Snowfalls of two to three feet are not uncommon in the foothills at this time of year. In a "bad" year, the mountains can get hit with a lot of snow.

The threat of thunderstorms, along with lightning, has returned to the high country.


The snowpack has usually stabilized, although one should still be cognizant of avalanches. Great glissading conditions usually exist.

Whether you lead a hike with or without snowshoes depends on the elevation of a hike. The snowpack and its depth can be followed at In fact, the snowpack can be followed year around at this web site.

Both snow and thunderstorms can be expected. More than one Memorial Day Weekend has seen significant snow fall.


The snowpack may affect trips through mid June, especially in the range of 10,000 to 12,000 feet.

Spring snow storms are not rare in the beginning of June, but considerably less likely by the end of the month.

Thunderstorms are a threat, but by the end of the month, the spring storms have diminished and the air is typically drier than in the spring or during the monsoon in July and August.

General Considerations

The weather in Denver doesn't always represent the weather in the mountains. Typically, Denver gets snow when a condition known as an upslope develops (i.e., these are winds from the east that push and raise the air up against the Front Range). Further, merely cloudy conditions in the summer and fall can be due to upslope. The result? Often times it may be snowing or cloudy in Denver, but it may be relatively fine in the mountains (many times you can recognize the classic upslope conditions by the emergence and disappearance of snow or clouds at Floyd Hill on I-70).

Arctic cold blasts don't always make it into the mountains. The cold air of a arctic front is relatively dense and can't make it into the mountains. Thus, it may be warmer in the mountains. However, it will still be cold, but perhaps not as cold.

Thunderstorms, with lightning, are most likely from late April to late September. To predict whether thunderstorms are imminent later in the day, look at how the clouds are building. If the clouds have no vertical formation, then you're in for a good day. Quite obviously, if the clouds are building rapidly and have dark blue under bottoms, watch out.

Snow isn't bad except if it's accompanied by high winds. And how do you predict when it will begin to get windy? Because wind is caused by pressure gradients in the atmosphere and there's no way for you to perceive this gradient in the field, you have no way of predicting when winds will decrease or increase. Thus, if snow is falling or expected, you need to be comfortable with your route and/or how to use your map and compass or GPS in case the conditions and visibility deteriorate rapidly.

Wind is dangerous in the wintertime as it can lead to very dangerous wind chills. If it is windy, be on the look out for frost bite. Also, the wind tends to be the worst at the crest of saddles and ridges. Avoiding the leeside part of a ridge, saddle or summit can improve conditions considerably.

Rain is just the pits. Let it snow or let it blow, but please keep the rain away. When it rains, be on the look out for people who aren't properly prepared and might suffer from slight hypothermia.

Regardless of the weather forecast, some areas are just more susceptible to rain and thunderstorms. For instance, it seems that people climbing Fourteeners are usually contending with thunderstorms on the Maroon Bells, El Diente and Mt. Wilson, and Redcloud and Sunshine Peaks. Much like water will flow around a rock in a river in the same regular pattern, air currents are influenced by the mountains and this results in some areas being more susceptible to thunderstorms than other areas. And on a related note, expect more snow in those areas that are close to ski resorts (they're there for a reason, you know). For example, during the winter you can often see a cloud bank over the James Peak Group, while Mt. Evans is cloud free. This explains the location of the Winter Park ski area.

If you're leading a trip, you'll probably begin to watch the weather forecasts about three to four days in advance. In the winter time, you should be concerned about Pacific fronts approaching from the west or Arctic fronts approaching from the north. In the summertime, you need to be concerned about the monsoon flow. If a high pressure system is over the state, that means good weather.

Good and bad days seem to come in bunches because the jet stream affects our weather and it changes its path gradually over time. Keep in mind, however, that transition days do exist. Most often, a good winter storm is preceded by nice, warm conditions.

The weathermen aren't always right! Furthermore, as implied above, the weather in the mountains can be very different from that in Denver. If you know prior to your trip that inclement conditions will exist, you may be faced with canceling or postponing your trip. Factors to consider are the level or classification of the trip, the experience of the participants, and your experience in the mountains. Peak baggers signing up for a December ascent may not mind a little snow and cold, whereas a social hike may not have much point if it's snowing and cold. And of course, you should never lead a trip if you think the weather would make it unsafe to do so.


Paul Wilson adds:

what's the best time in the summer to climb high peaks based upon the 
threat of thunderstorms? I would say from late June to mid July. 

This is true, although mid July is getting pretty close to boomer time. The summer climb is not without risk of thunderboomers in the high country. Study the weather and how to bail when they get close. Try to always have an escape route (the bail) to get out of the hazard zones when the threat is within a mile or so.

My take on the subject of how to avoid the boomers is pretty simple. Spring and fall

In the spring after the snow consolidates the hiking and climbing is just great. We are getting close to this season now. The only thing one needs to remember is that afternoon will bring soft sun soaked snow and snowshoes are necessary. I usually carry them all day and use them for the descent.

In the fall I have had great climbs through thanksgiving most years. The biggest issue is routefinding in the dark. A good headlamp is required and good map/guidebook study is required. Scouting the day before to get you from your camp for several hours up the route is my best suggestion. The return is pretty easy since you would have, hopefully, done a good study on the way up. That is if you return the same route. The thunderstorms in Oct and Nov are quite infrequent.

In winter boomers dont exist but the avalanche hazard raises its head. Probably a more hazardous issue.

All these climbing hazards are recognized by all Colorado climbers and the CMC works diligently to train their members via the various schools. Denver, Boulder, Fort Collins, and the Pikes peak (Colo Springs) groups all offer these schools. When participating in these schools one would meet the old hands and new climbing partners and one can take advantage of this to gain more experience.

Steve Bonowski adds:

Paul and others: for what it's worth, I've found that there is a "weather window" at the end of July when the mid-summer boomers die down, and before the start of the August monsoon. I did the "train 14ers" in '93; N. Maroon & Pyramid in '96; and Crestone Peak last year; all in the "window." This "window" doesn't appear every year, and some years, there is still some very late day shower activity, like after 6:00 PM on a couple days last year in the Crestones. Still, it's something to shoot for. I plan to go "Bear hunting" (Little Bear) this year in the "window." Hopefully it will be there again.

Mary Gilbert adds:

You make a very interesting point here. About this time in the summer it seems that the monsoons start a bit earlier further east in Colorado. The end of July through very early August is a notorious "bad weather" window on the northern Front Range, east of the Continental Divide. Doesn't happen every year, but some REALLY nasty stuff has occurred within this time frame.

Consider the following:

There have been a few other severe storms along the Front Range around this time in other communities besides the above.

In 1988 (the tinder-dry year of the Yellowstone fires) I attempted McHenrys Pk from an overnight camp. RMNP was very dry the entire summer, but we pick the ONLY weekend when they had a monsoon! The dates? ~ July 31-Aug 1. After that outing I now never schedule a CMC trip on the Front Range at this time.

So, for anyone, if you want to hike/climb somewhere at this point in the summertime, get off the Front Range and go to places deep in Colorado, where Steve has apparently experienced some fine weather.

D'Arcy Straub replies:

In terms of predicting weather on past experiences, it must be realized that we're dealing with probabilities here. Perhaps you flip a coin and it comes up heads 7 out 8 times. However, the probability of heads or tails is still one half and just because you hit a stretch of heads doesn't mean that will continue. There are such things as statistical variations, which of course should be expected for things involving probability.

If you really, really want to find out about precipitation probabilities during the climbing season (throughout the year for some), here's a real good link to visit:

You can select stations throughout Colorado and it gives climatic averages based upon lots of annual data. To see the dip in precipitation in late June and early July, select the link "Precipitation Probability by Quantity" and then you can create graphs using various criteria. Selecting an amount of "0.01" ", "1 Day" will give you the probability that it will essentially rain for that day (and you can play with that "smooth value" which accounts for statistical variations from day to day). Or, you can just hit "Create Graph" and not worry about all those numbers and assume the computer will give you the most useful graph.

If you do enough stations, you can get a flavor of precipitation patterns. (And you'll be able to tell if Steve's window is real or if he's carrying a lucky four leaf clover in his pack he ain't telling anyone about. :)

Mary Gilbert adds:

I'd like to add to the short-term forecasting discussion. There are four factors that affect weather change--clouds, wind direction, air pressure and humidity. The more you have, the more accurate the s-t forecast. One by itself is not enough; at least two are good. I use mostly clouds and wind.

So, clouds alone don't make a good forecast. Low level cumulus doesn't always guarantee a fair day, and higher level cumulus ('sheep', mackerel sky) doesn't mean a 100% chance of rain.

For example, if your first cloud is cumulus, you can instantly determine wind direction because it's a low cloud. If it's a west wind, chances are, it's going to stay fair. Watch cumulus. If they mostly stay thin, have irregular bumpy bottoms (but not always) and a 'diffused' look to their tops, they'll likely not do anything mean. If they get flat bottoms and start to show well-defined 'cauliflower' heads, they're generating into thunderstorms.

If your first cloud is a herd of 'sheep' or mackerel sky, you'd really want to get a wind direction because that means uncertain weather. If you have 'sheep' and winds anywhere out of NE thru S, chance are, you might be in for a splash. (The nearest low pressure system is ~90 to 120 degrees to your right if you face into the lower wind.) If you have 'sheep' and winds not NE thru S, you'll likely stay dry.

If you want to get a wind direction and don't have the most reliable low clouds present, you can start off with a wet finger. Or look at how treetops move--the higher off the ground, the more accurate the lower level wind direction. Or wait until low clouds show up.

If you have both low and high clouds present, you can already make a seat-of-the-pants forecast.

I've found with experience if just a teeny tiny bit of 'sheep' or mackerel sky show up, it usually means nothing--but I try to get a wind direction anyway. If the 'sheep' or mackerel sky is in fair abundance, the sky's very likely making a serious statement of something yet unseen.

If you wake up to thick fog in the morning of your outing, it can be problematical. It may or may not burn off. In fog you cannot see falling weather or approaching storms; so, it can suddenly rain on you without warning.