AND NOW FOR A LOOK AT THE WEATHER: DECIPHERING MOUNTAIN MYSTERIES
Words & photos by Michael DeYoung
Former Air Force meteorologist and professional photographer Michael DeYoung continues his series about the weather with a look at the complexity of forecasting what’s going on in the mountains. Like all his posts, this one’s highly detailed and busting with information you can use to grow your backcountry skillsets. Grab a favorite bevvie or two and buckle up for a real education!
Mountain weather is much too complex to summarize, even in a long blog like this. I've been adventuring in mountains for over 45 years, and even as a trained meteorologist for 30+ of them, I am still trying to further my understanding of mountain climate, weather, and model forecasting skill and accuracy. Let's dismiss the extremes. "Weather forecasts are always wrong, and mountain weather is too unpredictable." Generally speaking, those with that mentality are not going to be here trying to further their understanding of the weather on their backcountry adventures. Global weather models, especially with evolving artificial intelligence, are amazingly more accurate than even ten years ago, but a long way from perfect.
Most adventurers know the basics about mountain weather. We know that mountains and higher altitudes are colder and wetter than surrounding lowlands. This is certainly the case in virtually all mountain ranges in North America. In the Pacific ranges, we know there is a pronounced windward side (Seattle: green, lush, rain) and a lee side (Yakima: brown, dry, windy.) Most of us are taught that mountain weather is indeed "unpredictable." However, unpredictable is a rather subjective term. All weather has some degree of it no matter where you are. Certain large-scale (synoptic) weather patterns result in highly variable weather conditions that major mountains can change drastically. Other patterns have a high degree of predictability. With more understanding of mountain meteorology and more knowledge about forecast models, their capabilities, and limitations, the less "unpredictable" mountain weather will be to you.
I would characterize mountain weather not as "unpredictable" but rather highly variable, capable of rapid changes from one extreme to another or abrupt changes from one side of a range to the other. Sometimes we simply hike into a different weather regime, be it clouds, precipitation, wind, or all three, that was present long before we arrived. Being a keen observer of mountain clouds and winds goes a long way in making mountain weather less surprising. The key to getting weather forecasts for a mountain adventure is to get it from the right sources–including directly from weather models themselves. This rules out getting mountain weather from traditional media like radio and T.V. Fortunately, we have apps today, like windy.com, that allow non-meteorologists with a little training to get location-specific forecasts.
Model forecast accuracy depends on accurate observational data in addition to the accurate mapping of the terrain. Unfortunately, the truth is most mountain ranges are sparsely populated or uninhabited and thus have a profound lack of hard observational data. But thanks to advances in satellite analysis, modern technology, and artificial intelligence, forecast models can measure the properties of the atmosphere in remote areas–including mountains–more accurately.
To make this blog post more palatable and easier to read, I will begin with a set of general mountain weather descriptions to help you get started. If you really want to get into the weeds and detail, some of the things described in this first section have expanded narratives below. Since most folks venture out into the mountains during the summer months, the bulk of our discussion will be about summer weather patterns.
Mountains can create their own cloud cover or suppress clouds that move over them.
Clouds don't always form somewhere else and "roll in" over you. Low clouds, especially cumulus, often develop in place right over you, rain out, and dissipate over you as well, only to repeat the same process the next day.
Clouds are enhanced by low-level winds forced upward by a major mountain range. This is called an orographic lift. Low-level winds that flow down slope do the opposite by suppressing clouds and precipitation. Canadian high pressure to the east that plows westward into the Front Ranges of the Rockies from the Yukon to New Mexico forces air to rise, condense, and cool and can cause significant cloud cover and precipitation on the eastern slopes of the Rockies. High-pressure west of the Front Ranges of the Rockies can cause downsloping winds that suppress cloud formation and precipitation even with cold fronts.
If you look closely you will notice thin clouds hugging the peaks above Spectacle Lakes in the Alpine Lakes Wilderness in the Washington Cascades.
Walking along a high ridge around 6500’ on the Pacific Crest Trail in the Alpine Lakes Wilderness, this shot is 45 minutes later than Image 1.
In coastal mountains from Baja to the Alaska Peninsula, clouds and precipitation can be a result of nothing but onshore flow in the low levels, even in the absence of a front or upper-level trough. Long fetches of flow from the ocean bring low clouds and precipitation (if cloud depth is greater than 2000') to the windward side of mountains, even under high-pressure patterns. High pressure does not always mean clear skies and nice weather. Onshore flow can be deep enough in northern mountains to cause clouds and precipitation even on the leeward side of ranges.
The Appalachian Mountains are open to both Gulf of Mexico moisture and Atlantic Ocean moisture and thus have very little windward and leeward rainshadow effect. They are wet and forested on both sides. Western Mountains, especially west of the Continental Divide, are cut off from Gulf of Mexico moisture and are only prone to Pacific Ocean moisture and thus have much more pronounced windward and leeward sides of the ranges.
Temperature changes with altitude are NOT always at the same rate of five degrees per 1,000'. Climbing 2,000' into and under an active thunderstorm can result in as much as 30 degrees cooler–not just 10 degrees. All high mountain valleys can have persistent temperature inversions during night and morning hours unless strong winds are present on the valley floor. Valley floors are not always the warmest places to camp in the mountains. Slopes, ridges, or low gaps 500-1,000 feet above a pronounced basin or valley are usually warmer at night and near sunrise.
A thin layer of stratus covers a remote valley in Alaska’s Talkeetna Mountains at sunrise.
Ridges generally have smaller daily (diurnal) temperature fluctuations than surrounding valleys and lowlands, especially in the western mountains. For example, high mountain valleys in the dry western states may see 30 to 40-degree daily temperature spreads where ridge tops see half of that.
Different cloud types can indicate different temperature lapse rates. Vigorous cumulus clouds signify an unstable atmosphere and possible steeper lapse rate than five degrees per 1,000'. Stratus clouds formed below mountain/ridge/plateau level indicate temperature inversions and warmer air aloft or slower lapse rates.
WIND: (Not associated with thunderstorms.)
Stand on a bank above a moving stream or river, and you can immediately determine which way the current is going. If you are not a whitewater boater and skilled at reading water, you might not notice that behind rocks in the middle of a raging current, the water is calm and or even flowing upstream. The same applies to eddies at the side of the river near shore, especially on inside bends. Stay there long enough, and you will notice that the current pulses and that water can rush into an eddy from either side or over the top of a rock for brief moments.
Similar to the way rocks in a river deflect, divert, and accelerate water flow, mountains do the same thing and can create their own wind currents that are different from the prevailing flow. When a long-sustained current of air, the "atmospheric river" encounters rugged terrain, eddies, wave trains, and chutes of accelerated air form like what you see in a river. The difference is that gases are expandable, and water is not. So, unlike water, gases that are forced into expansion or contraction can affect the temperature. Chinook winds or down sloping winds are warmed more than surrounding air simply by compression.
Downsloping winds race across Convict Lake in the Eastern Sierra Nevada at sunrise.
To get a feel for which direction weather might be coming from, one has to pay more attention to mountain top winds and above. Like river eddies behind rocks can have flows opposite of the current, the same thing can happen in a valley. If you are camped below a mountain with prominent gaps on each side in a strong wind pattern with an approaching front, winds can be highly erratic below ridge tops. They can be calm, gust from either direction and even blow straight down on you.
A key takeaway is to recognize that mountains create upslope and downslope wind patterns and frequently block or accelerate winds. The winds you feel below ridgetops in the short term might not reflect the prevailing current. As described above, if you cross a divide, go up to or down from a ridge, and there is an abrupt change in the wind, chances are you simply walked into a different wind pattern that was present before you arrived.
In the absence of an organized or strong pressure pattern, mountains create diurnal wind patterns frequently referred to as mountain and valley breezes, flowing upslope in the day and flowing down ridges at night. Winds that are forced through narrow passages accelerate. Driving east on I-80 from Ogden, Utah, into the Wasatch toward ski areas, one can often see strong winds at the entrance of the canyon that exist nowhere else. Colder high pressure to the east over Wyoming was forcing air westward through Ogden Canyon (and others around it) much like water accelerating through a culvert. This is a local gap wind often missed by global models.
This is a mature cumulonimbus cloud, the only cloud that produces lightning, thunder and associated severe weather like hail, and damaging winds, at sunset over New Mexico’s Wheeler Peak and the Sangre de Cristo Mountains.
Hiking the Continental Divide Trail in Colorado’s South San Juan Wilderness near 12000 feet during an afternoon thunderstorm and rain shower during summer monsoon.
THE MOST UNPREDICTABLE WEATHER
Thunderstorms (convective weather from large cumulus clouds) are the most problematic weather feature in mountains. There are three basic ingredients for thunderstorms: one, there is a sufficient supply of low-level moisture (almost always present in the eastern half of the U.S.), two, an unstable atmosphere-the rate at which the atmosphere cools with height supporting buoyancy throughout a large vertical column of the atmosphere, and three, a trigger mechanism-something that gets the convective process going and clouds building. (Remember, cumulus clouds often form or dissipate "in place" sometimes rapidly and don't always "roll in" from some other location.) Thunderstorms that are triggered by a front or upper-level feature and linked to the Jetstream are much more predictable in their formation and movement. The trigger with fronts or upper-level lows and waves is considered mechanical lift and can be enhanced by orographic lift, which is air forced aloft by a mountain range.
The most unpredictable thunderstorms are those that are only triggered by daily heating. These are airmass thunderstorms–cells that form from daily heating in an unstable atmosphere in the absence of any well-defined weather feature aloft almost always when the polar Jetstream is well to the north. This type of pattern is what triggers the North American Monsoon that affects much of the Southwest U.S. in summer.
Unpredictable is not the same as severity. Thunderstorm activity that is generated by frontal systems and or upper-level waves linked to the Jetstream (which can affect much of the northern tier mountains in summer) are quite capable of producing more severe weather than airmass thunderstorms, but they are more predictable in their development and movements. Placement and movement of individual cells in airmass storms under light flow patterns aloft is what is most unpredictable. However, even airmass thunderstorms, especially at high altitudes in the western mountains, can produce dangerous weather in the height of summer, including exposure to lightning, hail, damaging winds, snow, rain, and creating a sudden danger of hypothermia. There is an expanded narrative on airmass thunderstorms below.
This is a detailed forecast for Baxter Peak, Maine, elevation 5208’, derived strait from the European Center for Medium Range Weather Forecasting current model run from August 6, 2021, presented on windy.com and the windy app.
TIPS FOR GETTING MOUNTAIN FORECASTS
FOCUS ON THE BIG PICTURE: MAJOR PATTERN CHANGES THAT MAY AFFECT YOUR TRIP.
Numerical models define exact start/stop times to precipitation and wind. Don't put a whole lot of stock in that, and don't dismiss forecast accuracy if they are off by a few hours, especially several days out. If I'm looking at the forecast for a week in the mountains, say in the Weminuche Wilderness and CDT in Colorado, I focus on major pattern changes if any are forecast.
Models rarely get major pattern changes wrong. The further away the major forecast changes are, the more the timing and strength of weather systems get adjusted as the change gets closer.
Weather models can accurately predict an area of unstable air that will generate thunderstorms and rain/snow showers over a mountain area. They can't accurately forecast exact cell placement over a particular ridge or valley in an airmass thunderstorm pattern with light and disorganized flow aloft. For example, a line of storms triggered by a cold front that will sweep across the northern Appalachians is much more predictable.
Global and regional models accurately forecast the Jetstream and large-scale wave patterns (troughs, lows, ridges, highs). If I see the Jetstream forecast to drop south of my area, I expect colder weather than seasonal norms and increased chances of precipitation. If there is an approaching cold front, I expect to see accelerated ridgetop winds a day or two ahead. I know not to camp on ridge tops as winds aloft often increase at night with an approaching front.
If I see the polar Jetstream forecast to be well north of my area, then I anticipate quiet and dry weather outside of airmass thunderstorms in the summer season. The southern half of the U.S., when the polar jet is well north, can become vulnerable to tropical weather from a southerly or easterly direction which is a different ball of wax.
If you like looking at graphical weather maps, use windy.com as a source. They are easy to understand. Focus on a few things:
THE POSITION OF THE JETSTREAM AND THE SURFACE PRESSURE PATTERN
If the Jetstream is near you or over, you expect windy weather at high altitudes, especially with possible temporary relief from strong winds in protected valleys. Vertically deep layers of upper-level winds frequently mix down to the surface and valleys during the day. If you are in a valley on a windy afternoon and the winds calm down at night, chances are they didn't just dissipate but retreated aloft.
Understand the basic principle that wind blows from high pressure to low pressure. Looking at the surface pressure pattern as well as the low to mid-level wind flow will help you see if you will have upslope, downslope, onshore, or offshore winds. If you have a very light pressure pattern, then winds and weather will be more unpredictable if thunderstorms or big cumulus clouds are present. I pay close attention to the large-scale wind flow pattern from sea level to mountain top level in coastal mountains. If there is a large-scale flow of wind into the Cascades in the Pacific ranges, I expect low cloud cover and fog. Easterly flow off the Atlantic will produce the same effect in New England ranges. Persistent low cloud cover and fog are high-pressure clouds and will mostly affect the windward side of ranges. Again, windward vs. leeward is dependent upon a deep vertical layer of wind. Upper-level winds are often different than they are at the surface.
UNDERSTAND THE NATURE MOUNTAIN INDUCED WEATHER
Especially in summertime thunderstorms. In the western mountain ranges, the drier the low levels of the atmosphere are, the higher the bases of storm cells and the stronger evaporative cooling will be, resulting in drastic temperature drops and strong winds that can catch you off guard. This can put you in a hypothermic situation.
RECOGNIZE UNFORECASTABLE ELEMENTS
The Global model from NOAA has a minimum resolution of 22 km, and the European Global model has a 9 km resolution. This means any area of low clouds, fog, or gap winds that cover a smaller area than this won't be recognized. Even if they cover a larger area, fog and low clouds in the wake of a precipitation event that takes place in an isolated valley or two are often beyond the models' forecast ability. On the grand scale, say all of the Sierra, a few valleys fogged in is nothing more than a few puddles in an otherwise dry airmass.
GET CURRENT FORECASTS FROM THE BEST SOURCE
Either the National Weather Service with a specific forecast for your mountain location or look at what the global weather models are forecasting from the current run. This can easily be done using windy.com. Remember, this site is not producing forecasts. They simply present what the current model runs are forecasting, the same models that virtually all meteorologists use for forecast guidance. You can get a site-specific forecast for any remote mountain location globally that gives you basic forecast parameters in three-hour increments that are elevation-specific. Pay particular attention to getting your elevation accurate. Another excellent source of mountain weather data and forecasts is mountainweather.com
LET'S GET MORE INTO THE WEEDS. AN EXPANDED NARRATIVE ON MOUNTAIN WEATHER
Weather starts in the upper atmosphere, and mountains mess it up.
These are altostratus clouds illuminated by sunrise as they approach over the Chugach Mountains and Lost Lake Trail over Alaska’s Kenai Peninsula.
This image at lost lake is 2 hours after sunrise with low clouds (stratus) already forming over the alpine tundra and rain beginning to fall.
The weather we see at the surface is mostly due to what is moving over our area aloft. Global models can accurately forecast major weather features' strength, position, and trajectories aloft over time, such as the jet stream, upper-level lows, troughs, and their associated fronts, and upper-level highs and ridges. Upper-level weather features such as a strong upper low and upper trough of low pressure over the Pacific can cover untold thousands of square miles. When these large (synoptic) scale systems move over major terrain barriers, things change. The same Pacific front and upper-level trough will produce different surface weather over Boise, Idaho, than it did over Portland, Oregon, even though its properties aloft remain the same over both areas. Oftentimes, surface cold fronts are hard to find as they travel across major mountain ranges of the west.
The accuracy of mountain weather forecasts can largely depend upon the accuracy of the model's terrain profile, and these terrain grids from each model are different and not equal. The European Center for Medium-Range Weather Forecasts (ECMWF) has more accurate mountain weather forecasts than the Global Forecast System model (GFS) that our own NOAA produces. However, other models in the short term (18 hours to three days) can be more accurate than even the ECMWF, such as our own North American Mesoscale (NAM) and our own High-Resolution Rapid Recycle (HRRR). Current model runs from the ECMWF, GFS, and NAM, and their forecast products can be seen on windy.com.
Passionate backcountry adventurers and weather nerds don't need to analyze models and determine which one will perform best for their trip. Leave that to the meteorologists at the National Weather Service. That's their job. Just know that you should get your weather for backcountry adventures from the "horse's mouth," which includes the NWS whose meteorologists live in the area they are forecasting for. Want to climb Long's Peak in Rocky Mountain National Park? The NWS forecast you get is coming from meteorologists in Boulder/Denver using the best-performing model guidance. In the last blog, I showed an example of a forecast for the summit of Mt. Whitney obtained right from the NWS site. I was only able to get "close" with the NWS site, only giving me a forecast for an elevation of 12k. At the same time, on the windy.com and its app, I can use the detailed topo maps integrated with the current model runs and get a more exact pinpoint forecast for the summit of Whitney at 14, 490'. Off the windy.com and app, you can get a forecast for any specific site globally that factors in elevation.
I'm a total weather nerd and junkie who follows models and tracks their performance on virtually all my remote trips from the Brooks Range to the Sierra to the San Juan’s. The overall forecasts are far more accurate even in the five to seven-day range than many want to believe if they focus on major changes and trends.
It is very rare for a major and long-lasting synoptic change to go unforecasted, even in remote mountain areas.
HOW MUCH COLDER WILL IT BE IN THE HIGH COUNTRY?
We all know it's colder in the mountains than surrounding lowland areas, but in all reality, that is NOT always the case. In the winter, especially at higher latitudes, it can be warmer at altitudes two to three thousand feet higher than the surrounding valleys. For example: under high thermal pressure, Park City, Utah, can be colder than the surrounding ski slopes, especially in the morning hours and sometimes all day. Persistent valley inversions trap cold air and any pollutants that get put into it. In winter, the sun's heat output alone is not strong enough to break surface-based inversions resulting in airmass stagnation that can last for weeks. Only strong winds linked to a strong enough cold front can scour out inversion trapped air. In summer, the sun's heat output is strong enough to break valley inversions all over the west and north.
A backpacker in Alaska’s Talkeetna Mountains. The clouds obscuring the peaks are common in coastal mountains with an on-shore flow but a stable atmosphere.
The temperature changes with altitude are not always at the same rate. The rate at which temperature decreases with height is the adiabatic lapse rate. This rate of temperature change reflects free air temperature changes and does not consider the influence of mountain slopes with unequal heating caused by light and dark surfaces (large snowfield vs. dark spruce forest). In dry air, temperature change can be 5F degrees per thousand feet and 2F-3F degrees in a moist atmosphere. This lapse rate determines atmospheric stability. Observing cloud types and trends can reveal how much temperatures may change as you climb or descend.
In an unstable atmosphere, temperatures often decrease with height at a faster rate than 5F degrees per 1000'. The presence or forecast of vigorous cumulus clouds (the type that produces thunderstorms) indicates an unstable atmosphere and temperature changes that might be more than 5F degrees per thousand feet. Factor in the wind and evaporative cooling from any precipitation and climbing three thousand feet into the mountains with vigorous cumulus development can put you in a potentially hypothermic situation–even though it was 80F when you left the valley floor.
In a stable atmosphere, precipitation-producing clouds will have uniform tops, and the change in temperature with height will be much less than the change in an unstable atmosphere. In fact, in a stable atmosphere, temperatures can increase with height in the lower levels. Temperature inversions are quite common, year-round, in the morning hours in all-mountain valleys from L.A. to Anchorage and beyond. If it's important to you to camp where it is warmest while minimizing condensation, then camp 500-1000 feet above a drainage, especially in fair weather and light winds. You will most likely remain in an inversion layer during night and morning unless there is an approaching frontal system.
Thunderstorm activity can be enhanced and made more severe on ridge tops and peaks by converging winds moving up from valleys below.
THE MOST UNPREDICTABLE MOUNTAIN WEATHER: AIRMASS THUNDERSTORMS
Most of us already know the potential dangers of thunder and lightning. My goal here is not to re-hash what's already been well documented but rather to provide more insight and understanding as to what conditions cause rapidly changing "unpredictable" summer mountain storms. Let's start with the airmass thunderstorm.
The most unpredictable mountain weather is often in the form of "airmass" thunderstorms. These convective clouds are triggered by daily heating, where moisture gets lifted high into an unstable atmosphere and forms billowing clouds that may result in thunderstorms with rain/snow showers, hail, and damaging erratic winds. Airmass thunderstorms are common over much of the southern two-thirds of the Lower-48 states during summer as this part of the country sits under an upper ridge of high pressure with light winds aloft. The southern tier states are prone to weak disturbances from the tropics from an easterly or southerly direction. In particular, the southwest states are influenced by the North American Monsoon, which can feed subtropical moisture from the south and east into places like the San Juan Mountains and the Grand Canyon. Occasionally, monsoonal flow pushes far west into the Sierra and far north into the Northern Rockies.
Forecast models are accurate as to the area of coverage of thunderstorm weather but not so much with the exact placement of individual cells and, most importantly, how much they interact with each other. They are unpredictable because they lack any organized steering flow aloft, or the flow is very light. In the absence of an organized weather feature like a frontal system or upper-level disturbance, exact cell placement and movement of individual cells more than a day out are next to impossible to predict. Since these airmass or monsoonal storms lack any organized steering flow aloft, they can dump a lot of rain in a short period over a concentrated area. These are the conditions that create flash flood potential, which will be discussed more in the next blog about desert weather. A small cell may be ten miles by ten miles and can drench backpackers underneath it, while other backpackers 15 miles away might not see any rain at all. Oftentimes, these airmass cells just rain themselves out in place with very little movement. Sometimes you're the windshield, and sometimes you're the bug.
BUMPER CARS. In light and unorganized steering flow aloft, cells interact with each other, playing "bumper cars" and pushing each other around. One day of your hike, you may be under one of these slow-moving cells while hiking on the Continental Divide in the San Juan Mountains and get drenched. The next day a big cell of the same character under the same upper air pattern remains two valleys over, and you get lucky and dodge it. Until the upper air pattern changes, moisture from rain showers can recycle on successive days.
When large columns of heated air rise, they leave a void at the surface to be filled in from surrounding areas. This creates sudden and erratic winds from any direction as convective cells are actively forming. These winds are not nearly as strong or violent as downdrafts that take place when cells dissipate and release their energy.
COLLIDING WINDS CAN ONLY GO UP.
New cells and thunderstorms can often form in between two dissipating cells. When cells dissipate, their downbursts produce "outflow boundaries." When outflow boundaries converge, they can only go up, and upward motion then triggers a new cell and storm. Global weather models can't predict this well except in the short term, usually six hours or less.
WHAT GOES UP MUST COME DOWN.
Mature cumulus clouds and thunderstorms are the results of a lot of energy being lifted high into the atmosphere. Eventually, all that energy is going to return to earth. That doesn't always take place in a gentle way. First, when precipitation falls from a cell, not all of that precipitation reaches the ground. Some of it evaporates, and evaporation is a cooling process. This process is more prevalent in the Western U.S., where thunderstorm bases form higher above ground than in the Eastern U.S. This creates a deeper column of cold air cooled by evaporation. This very cold pool of air can come racing to the surface in a violent and sudden way. Imagine taking a bucket of water and pouring on onto a hard floor surface and seeing it splatter all over the floor in several directions. The higher the bucket is from the floor, the more violent the splatter. This is essentially what is happening on a grand scale, say hundreds of square miles, when a high based mountain thunderstorm or cumulonimbus cell dissipates and produces "downbursts."
Remember, you don't need a cold front or to be at high altitudes for a garden variety thunderstorm to put you in a hypothermic state in a very short period. That temperature drop you feel is mainly the result of evaporative cooling and downdraft winds. Throw in a wet unprotected core, and you can get into trouble in a hurry.
Thunderstorms dissipate from the bottom up. When they "rain out," they can leave layers of mid-level and high-level clouds that can explode into beautiful and colorful patterns at sunset. Outflow boundaries from dissipating cells can create strong winds whose onset is very sudden that can last for hours. Whenever we are camping on high exposed ridges with recognizable cumulonimbus cells around us, we always secure our tent for strong winds even if the wind is calm while we are pitching it.
During airmass thunderstorm patterns, mornings can begin clear and calm. In the Southern Rockies, for example, during the monsoon, storms can have the illusion of appearing suddenly out of nowhere. Is this really the case? Does this count as unpredictable? Not really, especially if you understand the basic dynamics of thunderstorm development. In many instances, convective storms are building in place right over your head. Daily heating and rising air columns don't start until surrounding valleys break the morning temperature inversion. Forecasters can easily and accurately determine what temperature needs to be reached to break the inversion and start with air rising. This, of course, is more accurate around urban areas. Forecasters can also calculate a CCL or convective condensation level. This determines the base at which cumulus clouds will form. This can be very useful when climbing high in the mountains. Will my cloud base be below or above the ridges I'm on?
WHAT ABOUT NIGHT-TIME THUNDERSTORMS?
On day four of a six-day Grand Canyon backpacking trip, it was clear and warm for early May. After sunset in the darkening dusk, I saw cumulus clouds building to alarming heights where none existed during the heat of the day. That told me all I needed to know. The only way cumulus could build with approaching night with decreasing temperatures is by a mechanical trigger. In this case, the trigger was a Pacific Cold front forcing air to rise, condense and form ominous clouds. In this case, the forecast was more predictable. Put the rainfly on. By midnight it was raining, and the rain lasted until 9 AM the next morning. This front dropped snow down to 6000', covering both the South and North Rims with new spring snow.
Major mountain ranges can enhance or suppress cloud cover. This is the Alaska Range which is along an east-west axis here in Denali National Park.
Changes in valley winds are often connected to temperature inversions. Imagine taking a tray of ice cubes and placing them under running tap water. Initially, there is nothing felt at the bottom of the ice cube, and there is no motion. Eventually, the ice will melt, and flowing water will reach the bottom of the ice tray. Similar things happen in the atmosphere with mountain and valley winds. Especially in western mountains in dry patterns, it is common to have a strong flow of wind. Temperature inversions over valleys essentially place the valley floor in an ice cube keeping it protected from winds aloft. When the sun's heat breaks the inversion, winds aloft then mix down to the surface (remember the atmosphere is 3D) and blow until the evening when the temperature inversion sets up again. This sets up a cycle of increased winds during the day and calms at night in valley locations. This often gives the impression that the winds died down. Technically they just decoupled from the surface and are most likely blowing at night at ridgetops above.
This is something to think about when choosing a high exposed camp. With approaching fronts, winds often increase at night, first on ridgetops before reaching the valley floor. Fronts are strong enough to break temperature inversions from above (like melting an ice cube above with warm water) at night. Valley winds at night can often keep or delay the formation of an inversion resulting in warmer night-time and sunrise temperatures. If it is significantly warmer in a valley location in the morning than it was at night, that is usually a good indicator of an approaching storm and weather. Anytime I camp on a ridge, even on a clear and calm afternoon, I always prepare for increasing nocturnal winds.
In the absence of a strong pressure gradient associated with an active large-scale weather system, mountains develop their own mountain and valley breezes. Due to unequal heating, especially on south-facing slopes, valley breezes develop in the day, blowing up ridge and mountain breezes from cold air flowing downhill develop at night. In glaciated mountains, strong katabatic winds can develop under light large-scale pressure patterns. Cold air is denser than warm air, and unequal heating in low forested areas can cause a rush of downward flowing air from a glacier or icefield in the afternoon.
THAT PESKY JET STREAM
When a stream bed changes from a smooth sandy or gravel bottom to huge boulders standing waves form and are easy to see.
In the summer, the "atmospheric river," aka the polar Jetstream, is significantly weaker and more fragmented (like a braided river). It hangs around the northern tier of the U.S. and into southern Canada. The polar jet serves as a steering flow for large-scale fronts and upper-level disturbances that can affect the northern ranges even in the height of the summer months. Storm systems linked to the jet are more predictable in their motion and weather produced. If the Jetstream is flowing over your area, you will be in wetter, more unsettled weather patterns, especially on the windward side of mountain ranges. If the jet is south of you, you will be in colder than normal conditions.
If the Jetstream is north of your area, you are influenced by the subtropical ridge of high pressure (warm air aloft). With maximum summer sun intensity, daily rounds of thunderstorms and rain showers develop under and west of the ridge in light winds if there is a sufficient source of low-level moisture. Conversely, the very same high pressure in the same position in October when daily heating is much less can bring delightful warm, clear weather.
When the subtropical ridge aloft builds into the northern tier, and the polar jet goes even further north into central Canada, the southern tier mountains become open and vulnerable to tropical weather from the east and south. Tropical weather, from easterly waves to hurricanes, can bring catastrophic weather from flash floods to mudslides. Warmer air simply holds more moisture than colder air. When moisture-laden warm air collides with colder mountain air, more moisture is drawn out than would be over flat terrain. Take all tropical weather warnings seriously. Late summer and fall are when southern mountains are most susceptible to tropical weather systems.
Michael DeYoung is a photographer, a wilderness traveler, and a weather guru who, together with his wife and adventure partner Lauri, has bucked corporate tradition and forged a life as a freelance adventure and travel photographers in the ’90s. They have shot campaigns for a wide variety of tourism clients and outdoor publications, including many assignments promoting Alaska as a visitor destination. When not out in the wilderness, they spend their time near Taos, New Mexico, in a 100% solar powered, sustainable straw bale home and office. Michael has turned more toward leading and guiding photography workshops and uses his meteorological expertise to teach others forecasting for wilderness travel and finding the best photographic light.
Photo Tours: Active Photo Tours
Sign up for our Newsletter