Weather Topics

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Windstorms

Windstorms are some of the most damaging weather scenarios we receive in the Pacific Northwest. Winds we typically receive in the Pacific Northwest are only high enough to be classified as “blustery”, but with the right conditions aloft, areas of the PNW can be hit by highly damaging wind speeds.

Wind is caused by a gradient in pressure. By “gradient in pressure” I mean a change from low to high pressure over an area. The tighter the gradient, the higher the wind speeds. PNW wind is caused by a low setting up off the coastline, and then moving over the area. The favorable trajectory for high winds is when the low sets up slightly to the north, because the pressure gradient is typically denser on the south side of the low. The SE corner of the low, (where it begins to curve north), has the highest winds, and these winds are known as Bent Back Occlusion winds. 

Wind can also be impacted by local terrain features. Some features can amplify the wind. One of these is the Columbia river gorge. As the low moves closer to land, air streams out of gaps in the mountains, like the gorge, because it is being pulled toward the low pressure center. This creates strong east winds in the foothills. Hills can also block many areas 

from receiving high winds if they are just upstream of where the area is.

Mountainous terrain and its impact on Precipitation totals

Impact #1

Yearly, the Western slopes of the Cascade mountain range receives up to 50 inches more rainfall than the Puget sound region. Why is this? This is due to its terrain features, and proximity to large bodies of water. As warm, moist air at the surface moves over the Puget sound region, it is forced to rise upwards as it smacks into the western slopes of the Cascades. This causes the relatively moist air to condense and change from a vapor to a liquid/solid during its path as an upslope wind climbing in order to get over the cascades. It condenses because as it is forced upwards, it enters cooler air, and freezes. This extra liquid falls in very heavy downpours, and has mostly all fallen by the time it reaches the Cascade crest. This phenomenon is known as Orographic lifting. 

Impact #2

The second impact of mountainous terrain is an effect of orographic lifting, and it is the rain shadow effect. The rain shadow effect is the reason why the Columbia basin scarcely receives rainfall. The rain shadow effect is caused by all the moisture in the air being released on the windward side of the cascades, so by the time the air flows down the leeward side, it is too dry for any precipitation to fall. 

Impact #3

A final way that mountains impact precipitation totals is through convection. Mountains breed convection, which is essentially thunderstorm fuel. You can read my entries about CAPE for more information regarding this. Mountains breed convection because their large terrain features force air parcels to displace rapidly. This elevated convection can lead to heavier showers/downpours, and higher rainfall totals. 

 Snow in the Puget sound lowlands

Section 1: Why it is hard to get Puget sound snow

Sometimes in the Puget sound lowlands, we can go very long periods of time without snowfall, even during winter. This is because of multiple terrain features that are nearby, and these features make it very difficult to get cold air down to surface level at lower elevations. The first of these features is the Puget Sound itself. Water is a temperature moderator, because it stays at a relatively stable temperature. Due to this, typically the further you get from a large body of water, the more variability there is in temperature. A personal experience of this phenomena taking place was when I drove from the Tacoma area to Portland. It was 36 degrees and raining by the Puget sound in Tacoma, but during the portion of the drive just south of Chehalis, the temperature dropped to 29 degrees, and heavy snow was falling. This was because Chehalis was further from a large body of water to act as a moderator. The Pacific Ocean can also amplify this principle. 

A second reason that it is hard to get sea level snow around the Puget Sound is that we have a multitude of mountain ranges that bottle up cold air off to the east, and don’t allow for it to seep out into the lowland areas. The first of these is the Rockies, because when the polar lobes come, the arctic air comes from the northeast, and moves toward the Pacific Ocean. Below the highest peaks, the cold air is blocked, and cannot get down to the surface level. The second operates at a smaller degree, and it is the cascades. The cascades generally block less cold air, but they are still a key factor in keeping western Washington too warm for snowfall. 

Section 2: Cold Air conduits

I previously mentioned that much of the cold air that tries to get into Washington gets trapped by the various mountain ranges. However, there are some paths that cold air can take to seep its way into Washington. The first way, and main way, is the Fraser river outflow.When this occurs, strong winds start at the surface in the Fraser river valley, which empties out north of Bellingham, WA. This brings cold air from B.C and Alberta into the northern Puget sound, and this cold air can rip down the sound via northerlies. 

A second conduit of cold air that brings Arctic air into places west of the Cascades is the Columbia river gorge. This is the event of colder air from eastern Washington and Oregon spilling into the Portland metro area from the Columbia River gorge via strong Easterlies. This cold air is then pushed by Northerlies south into the Willamette Valley.

A final cold air conduit flows into eastern Washington, and it is called the Okanogan River outflow. This is cold air that pushes southward out of the Okanogan river valley from B.C into eastern Washington. This outflow has many similarities to the Fraser river outflow, but it is a much smaller scale conduit of cold air. 

Section 3: Pacific Ocean snow Machine

One way that snowfall amounts can be enhanced is by the Pacific Ocean snow effect. This occurs when a low from B.C swings out over the Pacific Ocean, and brings moisture with it. As long as the low is substantially cold, the moisture that the Pacific Ocean contains can evaporate up into the levels of the atmosphere, and condenses, causing it to all fall back the the surface after it crashes inland, leading to heavier snowfall because of the excess of moisture in the atmosphere. This can lead to drastic snow totals.

 However, if the low is not cold enough, the relatively warm Pacific Ocean can warm the air too much and 

Put too much moisture into the air, causing it to fall as rain.

Update/continuation on CAPE

I previously posted that…

In essence, CAPE is thunderstorm fuel. CAPE (convective available potential energy) is a measure of instability in the levels of the atmosphere. Typically, CAPE is measured in J/kg, which is joules per kilogram. Now, what is instability? Instability is as it sounds, an unstable airmass with large pockets of air interacting with other pockets of air and moving vertically very frequently. This creates energy, which fuels thunderstorms. These air pockets move vertically when there is a large difference in temperature as you go up in the atmosphere. That is why large supercellular thunderstorms are associated with very tall clouds, because the temperature difference is creating violent updrafts, that pulls the air upwards. Here in Washington state, thunderstorms often occur in the spring, when cold air aloft, moves over relatively warm air at the surface. 

Updated portion…

In hindsight, I would like to talk more about updrafts. The reason that the temperature gradient pulls the air upwards is due to the laws of energy transfer. Energy transfers from high to low. So due to this, when there is a rapid decrease in temperature with height, the warmer air wants to move up into the cooler air, to reach equilibrium. This air moves rapidly upward to attempt to achieve this, and as a result, many parcels of air are displaced causing an increase of instability, or J/kg. Note, J stands for joule, which is a measurement for energy. This is why larger thunderheads have taller clouds, because more instability, means more updrafts, and more updrafts means more instability. Here in the Pacific Northwest, high levels of instability are much rarer, so our thunder clouds tend to be much shorter. Thanks for reading.

 Freezing rain

We have all woken up to those days where the “world” is a sheet of ice. This is due to freezing rain. Freezing rain is rain the falls when the temperature is below freezing, and freezes upon contact with the ground, or whatever else it falls on. Why, you may ask, does the rain not fall as snow? This is due to cold air (below freezing) being trapped at the surface, that has a layer of warm air above it, so that the rain does not have time to turn to snow before it hits the ground. This is why freezing rain most commonly occurs when there is cold air in place, and then a warmer system rides into the area, up over the top of the cold air. It rides up over the top because due to the terrain, very cold air can be very hard to scour out at the surface. A place that gets frequent freezing rain is the Colombia river gorge, because very cold winds blow  in at the surface from the east, so when a warmer system comes in from the west, the cold is trapped at the surface. Thanks for reading.

Topic 1:evaporative cooling.

Evaporative cooling can occur year round, but is most common in the winter months. Evaporative cooling is when an airmass is so dry, and typically somewhat cool, that any moisture moving into that airmass typically evaporates before it hits the ground. When it evaporates, it takes heat with it, and cools the atmosphere further. For example, if the temperature is 35 and the dew point (measure of moisture in the atmosphere) is low enough, let’s say 25, the airmass may cool to around freezing (32 degrees Fahrenheit). During the temperature fall, the dew point will rise. Once they are close enough, precipitation may begin to fall, and if enough evaporative cooling takes place, it could even fall as snow. However, there is a catalyst for this reaction. The precipitation must be rather heavy and frequent, otherwise not enough can evaporate, and cool the atmosphere.