The Devil is in the Details
Disclaimer: I have no formal education in meteorology. I take no responsibility for you getting skunked, pissed on, or washed off your objective. This is the Pacific Northwet, after all.
Previously, I discussed weather basics (Part 1) and weather forecasting with useful resources (Part 2). While I was writing these, there were many anecdotes and pieces of advice that I wanted to include, but I felt like those tidbits deserved a separate post. So this article is a little scattered, but offers many trends and patterns that I have observed that can make a big difference.
TLDR: The Cascades are defined by the incredible climatological differences from the west to east side of the range. Microclimates play an outsized role here. Pay attention to details in the topography to learn the details that affect the style of trips you like most.
Regional Cascade Weather Trends
I have tried to organize this rather random piece into some distinct sections. The following trends are more general to the Cascades and their specific geography and are not specific to any particular activity.
Weather can change very rapidly over a few miles of west to east gradient.
One of the defining characteristics of the Cascades is the incredible change in climate from a few miles west of the crest to a few miles east. Driving over the pass, you can often go from drizzle to sunshine in just a few miles. The location of this gradient is sometimes at the passes, or sometimes dozens of miles in either direction. Predicting where the change will occur can mean the difference between a sunny warm day or a day in the clouds. It is a really challenging prediction, even for advanced weather models, so when in doubt, I go further east to ensure that I will be in the clear. But expect high winds if you do find sun, driven by the differences in warming and pressure.
The hydrological crest is different than the topographical, and thus meteorological, crest.
In many parts of the Cascades, the topographical crest is not on the hydrological crest. For example, the Chiwaukum Range is the highest ridge on its latitudinal axis, but sits a good 10 miles east of Stevens Pass. Even though it is technically on the “eastside” and generally drier, I have witnessed many days where clouds push over the lowly peaks around Stevens Pass and stack up against this high crest. Further north, the high chain of peaks from Sahale to Eldorado act as the crest, although the actual hydrological crest extends way east to Rainy Pass. Thus, Easy Pass, which is west of the crest technically, has a surprisingly dry feel with abundant larches.
Cascade Passes are the target of marine pushes.
In the summer, we frequently have “marine pushes” where fog develops over the ocean and is blown onto land during the early morning hours by a pressure differential. We often wake up to clouds and even drizzle while the high mountains sit above this cloud layer. Marine layers sometimes only make it to the Cascade foothills and almost never make it over the passes. Although marine layers usually sit relatively low, when the sun begins to warm the air, the clouds rise with vertical mixing in the atmosphere. When the height of the clouds reaches the passes, the moist air and clouds begin to spill over the passes, drawn by the low pressure resulting from warming temperatures on the eastside. Cascade passes are the easiest place for the atmosphere to equalize pressure, so they often get windy or cloudy. Thus, on a day with a strong marine push, you might think a hike to a pass would get you into the clear, but oftentimes it is actually one of the worst places you can be because it is windy, wet, and cloudy. However, get a few thousand feet above the pass, and you may witness something beautiful. I have had many days like this at Cascade Pass, watching the marine layer spill over the pass.
How strong will the marine push be? At what elevation will the cloud top be? Outside of the familiar Windy.com meteogram and qualitative data from the NWS forecast discussion, I don’t have much better answers. This is probably the most challenging part of summer forecasting here.
Find the microclimates.
Complex terrain creates a plethora of microclimates throughout the Cascades. Learning about these microclimates takes time, and shows a mastery of a local region. Which walls at Index dry out the fastest? Which ski zones at Snoqualmie receive the least wind effect? Here are just a few examples:
- Crystal, sitting west of the crest, but in the powerful rainshadow of Rainier, is remarkably dry. I can only remember one time skiing there where it did not “go blue” (the sun came out) at least briefly. The power of the rainshadow is dependent on the orientation of the storm. Rainier sits due southwest, so storms from precisely that direction exert the strongest rainshadow effect at Crystal.
- On days with easterly winds, winds are funneled up the Alpental Valley, roll over Snow Lake Divide and race across Snow Lake. This can make for an unpleasant skin across the lake later in the day. It also can destroy all the snow around Snow Lake Divide.
- The Boston Glacier is the largest non volcanic glacier in the lower 48. The high massif of Forbidden and Boston Peak sucks so much moisture out of the air that larches, which are usually found in the dry Eastern Cascades, are able to live on the flanks of eastern terminus of the glacier! A few miles to the east, on Ruby Mountain, larches dominate the mountain and there is plentiful sun.
- Sequim, on the NE side of the Olympics, receives less than half the rainfall of Seattle. Just a few miles west or south receives dramatically more. Sometimes there is a literal hole in the clouds above Sequim.
- On days with easterly flow, the western Cascade foothill towns can have extremely strong winds. In the summer, the descending air can also cause these spots to be the hottest around. On hot easterly summer days, avoid North Bend, Enumclaw, and especially Newhalem, which is stuck in a deep gorge of heat and suffering on these days.
- When arctic air flows out of the Fraser River Valley, the air is usually too dry to carry much moisture. However, the air can spurt out of the valley and rise against the northern flanks of the Olympics around Port Angeles, creating hyper localized snowfall.
“Partly Sunny” means an entirely different thing on the west side of the crest than the east side.
Partly Sunny and those ambiguous forecasts can be tough to plan around. In my experience, partly sunny on the west side means “mostly cloudy with pockets of sun”. On the east side, this usually indicates predominantly sunny weather just with some mid level clouds floating through. This could be due to the nature of the clouds: the west side is often dealing with thick marine pushes, whereas the clouds on the east side in the summer are usually more convective in nature.
Praise thee easterly flow, our savior.
Snoqualmie Pass should not be a viable ski destination. At 3,000 ft and so close to Puget Sound, it should not be consistently cold enough to get large quantities of snow. If you look at 3,000 ft on Mt. Si, there is rarely enough snow to ski. However, Snoqualmie Pass gets easterly flow. Cold air seeps over the pass from the east side, lowering temperatures just enough to frequently produce snow. Sometimes you may drive up to the pass and watch the temperature drop 4-6 degrees right upon reaching the pass. When easterly flow ends, you can often feel it in the air, with a sudden temperature change and the snow quality turning to poop or even rain.
Except when easterly flow brings fog to the passes.
The downside of easterly flow in the winter is the inevitable low level fog it brings to the passes. The forecast almost never gets this right. If you think you will get a sunny day at Snoqualmie Pass proper in the winter but there is easterly flow, think again. However, you do not usually have to climb much to get above this low level fog, just like with marine pushes.
Incoming storms are predictable. Outgoing storms are not.
I find that the timing of incoming storms is much more reliable than outgoing storms. I have spent many days stuck in a cloud, waiting for the weather to supposedly improve, but it does not. When a storm is impending, it is usually very obvious with high cirrus clouds that begin to spread across the entire sky. Then the highest peaks get lenticular clouds, the cloud deck continues to lower, and eventually it starts to rain. This process usually takes a few hours, giving you time to react.
The Models do not understand volcanoes.
There are many days when the forecast predicts 10 inches at Paradise but something ridiculous like 25 inches up at 12k on Rainier. Does it actually snow that much at elevation? I really doubt it, and the insane winds will blow most of it away anyways. It seems that the volcanoes are so anomalous compared to other peaks in the Cascades that the models get “tricked” into these extreme forecasts. Remember that a model is only good at predicting scenarios where it has similar training data, and it is hard to get data on snowfall totals high on a super prominent volcano.
Do not underestimate the power of a single large peak to influence the local weather.
Volcanoes and our other large peaks can have an outsized influence on the local weather. It is impressive to climb peaks in the Stuart Range, watching sacrificial Stuart catch all the clouds and leave the rest of the range in the clear. Or to ski under blue skies at Crystal, protected by the big hunk to the southwest. Hell, even the “Issaquah Alps” create tons of moisture, as I have learned since moving to Issaquah.
Don’t forget the Olympics.
Last but not least, do not forget our friends in the Olympics! It is easy to become too Cascade focused when sometimes the better weather is to be found over on the Peninsula. The Olympics often escape the worst of the smoke and heat during the summer. With outgoing storms, the eastern side of the Olympics can clear before the Cascades because the better weather is arriving from the west. My Bailey Range Traverse was a last minute audible, and we nailed it with much less smoke and no rain, whereas the Cascades at the same time period had heavy smoke and a giant thunderstorm.
Snow Specific Trends
These trends are specific to snow and ice for winter and spring skiing.
Too much fresh snow is a bad thing.
This can be counterintuitive, but I would argue that too much fresh snow (> 12-16 inches in 24 hours) often makes for, in my opinion, less fun and more challenging skiing, especially if the snow is our typical “Cascade Concrete”. A smaller amount of fresh is definitely less work on the legs and easier for beginners. Less also usually means better avalanche conditions.
Temperature change is equally as important as absolute temperature.
We usually focus on ambient temperature to predict ski quality, but I would argue that the layering in the snowpack is more important. For example, if it is 31 degrees but the temperature was recently 33 degrees and is trending downwards, you will likely get a “right side up” (light over heavy) layering that makes for very enjoyable, easy, fast skiing. 4-6 inches of lighter snow on top of a firm base is amazing! But if it is 31 degrees is snowing on top of some actual low density snow that fell when it was 25 degrees, you have a “upside down” layering that will not be supportive but also heavy. Additionally, avalanche conditions will be more touchy with a “strong over weak” layering. Thus, look for storms that cool as they taper off, creating right side up layering in the snowpack.
Pay attention to the vertical temperature gradient.
With ski areas, we can often see the temperature at the top of the mountain and base. If there is a substantial temperature gradient (> 3 degrees per thousand feet) then the snow that hits the base will likely be surprisingly low density given the ambient temperature at the base. Conversely, when temperatures are the same at top and bottom, this usually indicates an atmospheric inversion or impending warm front. The snow will be much heavier and even could produce freezing rain if it is cooler at the surface than up high.
It can snow with temperatures above freezing.
If the air aloft is cool, snow can continue to fall in above freezing temperatures because it takes time and energy to melt the snow. During periods of intense precipitation, all the melting, which takes considerable heat input (latent heat), can lower the temperature enough to hold snow far lower than expected. It might not be good snow to ski, but any snow is better than rain.
It can also rain with temperatures below freezing.
When there is warmer air aloft and cooler air near the surface, rain can fall all the way to the ground. This also can produce freezing rain when the rain freezes at the ground or near the ground. This sometimes happens when easterly flow is ending and warm air has moved in aloft, but a pocket of cold air remains near the ground. This is terrible for skiing and especially driving.
We can get mid-winter psuedo-corn cycles if a rain event precedes high pressure.
Sunny, warmish days of high pressure in the winter usually ruin chances of any powder on solar aspects, but there can be some psuedo-corn to be harvested if there was the right set up. If it dumped powder then just got progressively warm and sunny each day, then you will mostly have a certain texture of mashed potatoes. But if it was a rain event and then freeze, and then diurnal freeze thaw with plentiful sun, there is usually the potential for psuedo-corn. While it might not be the ripe corn in May on a volcano, it can still be decent.
Powder seems to settle faster later in the winter.
I do not really have any explanation for this, but it seems that storm snow settles, reducing the avalanche danger, faster towards the end of the winter like in March and April. Perhaps it is due to the warmer ambient temperatures, a deeper snowpack, or just smaller snow dumps. But it seems like storms slabs can stay touchy for a few days mid-winter but you can sometimes safely ski powder the day after in late winter. Take this with a grain of salt; I am not an avalanche forecaster.
Powder and ice can survive relatively warm temperatures in the shade, if humidity is low.
If powder falls at a reasonable density, it can settle and hold good skiing for many days in shade and out of the wind, even if temperatures rise much above freezing. However, if it gets cloudy, the moisture in the air, even without rainfall, can create a thin crust and ruin the snow. Similar stories are true with ice, although crust-ification is not really relevant. Clear skies overnight allow radiative cooling and ice growth. This freeze – thaw is actually ideal for ice formation.
After early April or so, fresh snow is a bad thing.
Once we are done powder skiing, we want the snow to transition to a spring snowpack. Intermittent fresh snow dumps disrupt this process, globs on our skins, and grabs our bases when it inevitably warms up post storm. These frustrating “resets” happen periodically during the spring ski season and it is usually best to give them a few days to “burn down”. A two foot dump could take up to a week under the wrong conditions to burn down. Rain is similarly not the best for the spring snowpack, but resets after one clear day or so, much faster than new snow.
The snowpack transitions into spring from SE to NW.
Between winter powder and a true, isothermal spring snowpack is a transitionary period that can be sub optimal in the Cascades. This transition can take a month or two due to our deep snowpack and frequent rainfall and powder “resets” during April and May. The southeastern Cascades, with a thinner snowpack and sunnier weather, melt out and corn up much earlier. For example, the Teanaway is often firing by mid to late April, while do not expect reliable corn up on Shuksan before mid to late May, but good skiing can continue past the summer solstice. Lower elevations corn up earlier, why the high volcanoes often take the longest. Obviously, the specific timing depends on the year, but you get the idea.
Cloud cover is more important than freezing levels for getting a refreeze.
Skiers pay a lot of attention to freezing levels, deservedly. But in the spring time, sometimes cloud cover and sun are more important. Even if ambient temperatures are well above freezing overnight, the snow surface can refreeze due to radiative cooling if the sky is clear. Actually, I find the absence of cloud cover to be the single most important factor in determining if a refreeze will occur. It is nearly impossible for the snow here to refreeze with clouds overnight, and not just due to the clouds trapping warmer air. The sun has a strong effect also and oftentimes I feel a crust forming in the snow just after a slope has reentered the shade, well before sunset, while temperatures are still above freezing. There is actually usually a brief “reverse-corn” window on western aspects as the sun sets and mashed potatoes begin to firm up.
Seeking corn, work the clock.
By April and May, the sun is incredibly powerful. When planning a spring tour, I think about “working the clock”: timing the aspects I travel on as the sun progresses from east-northeast to west-northwest. East and northeast aspects corn up earliest in the day, while west faces can be prime in the afternoon. North aspects, receiving indirect sunlight, can be good later in the day also.
East facing corn descents are the most challenging to time.
Because east faces get blasted by sun immediately, timing a corn descent can be challenging. It usually requires being at the top of the line just an hour or two after sunrise, which means an uncomfortably early start. For this reason, I am wary of trying to do east facing corn descents that require a significant approach.
These are all just observations I have made, and I have spent a relatively small amount of my life in the mountains compared to many other people. But I am always studying the weather and conditions, leveraging our modern tools to get a deeper and more precise understanding of localized weather patterns. The biggest takeaway from this article should be the importance of paying attention to details yourself. Be studious. Take notes. If you screw up and find trap crust when you were hoping for powder, stay positive and ask yourself what you can learn from it. Learning never stops, and the weather never sleeps.
In my final post in this weather series, Part 4 will cover my process of evaluating weather and making trip plans!