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Extratropical Cyclones

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Diagram of Extratropical Cyclone
Extratropical Cyclone heading toward California

Why do Extratropical Cyclones Usually Occur in the Winter in California?

High pressure is the result of (cold) air sinking over a region. High pressure regions center over the world's major ocean basins. In the North Pacific Ocean, the high pressure system is called either the Pacific High or the Hawaiian High.

During the northern hemisphere's summertime, the Pacific High is located further north due to the position of the sun (the sun is over the Tropic of Cancer around June 22). This high pressure system over the Pacific Ocean, to the west of California, results in California having a dry summer because the storms are forced around the high pressure region. However, during the wintertime, the sun is over the Tropic of Capricorn (23½ ° S), hence the Pacific High moves closer to the equator. The movement of the high pressure system southwards results in extratropical cyclones occurring further south in our region during the winter because the Pacific High doesn't block the storms as often. (The reason Washington State receives rain throughout the year is because the Pacific High is usually south of Washington.)

What are Extratropical Cyclones?

Extratropical (outside the tropics) cyclones (storms with a low-pressure center) occur in the mid-latitudes where typically cold, dry air from the poles meet the typically warm, moist air from the tropics. This zone of convergence is usually called a front (the zone of "conflict").

When these two air masses (the cold, dry polar air and the warm, moist tropical air) come together a storm develops. The stages of the development of an extratropical cyclone are discussed below, but it should be realized that this explanation is a generalization.

First of all, a low-pressure zone develops along the front. In the northern hemisphere (where most of these storms occur due to the position of the continents) the air begins to move from the high-pressure regions toward the low in a counter-clockwise fashion. The winds from the tropics (the south in our case) are moving toward the northeast, "pushing" the warm front ahead of it. The winds from the poles are coming from the northwest "pushing" the cold front ahead of it.

As these local winds continue, the low-pressure region becomes more concentrated (in area) and the winds become stronger. At this point, the fronts form an inverted "V". The weather along the two fronts is quite different, but both generally produce rain or snow. The dominant winds, the Westerlies, push the storm to the east.

Eventually, the cold front overtakes the warm front (forming an occluded front) and the storm dissipates. We will now take a look at the weather along these fronts from a different perspective.

It is a fall or winter evening in Siskiyou County and you have just had a pleasant, clear (although rather chilly) day and you are sitting back listening to the evening news when Jon Galfano or Jeff Heaton comes on with the weather. He talks about an approaching extratropical cyclone and points out the clouds over the North Pacific Ocean. Then an inverted V with red and blue lines (rather like the maps above) is overlaid so the storm can more readily be "seen". OK, so you know an extratropical cyclone is on the way. What kind of weather can you expect?

As mentioned earlier, you already experienced a clear but chilly day. As the storm approaches, you will notice some wispy cirrus clouds overhead. If you look at the Cross Section diagram above, these will be the clouds on the right of the diagram just above the highest part of the red line. This red line signifies the warm front (warm air is pushing this front toward you). On the east or your side of the front (the right side of the red line on the diagram), you will remember that it is rather chilly. To the west of the warm front is warm, moist air coming up from the tropics. Since warm air rises, and since this wind is coming from the southwest, the warm air rises up just above this gently sloping warm front. As this moist air rises, it cools and the water vapor condenses. Because the slope is so gentle, it takes awhile for the air to rise and it eventually rises way up in the atmosphere to the east of the approaching storm. Since the clouds that first form are way up there, it is very cold and so the water droplets freeze. These cirrus clouds that form from ice crystals rather than water droplets are very wispy. Sometimes we call them mare's tails. So your first clue that the storm is approaching are these wispy clouds. The next night you might notice a halo around the moon.

As the storm moves closer, the clouds become thicker and grayer and rather sheet-like. These kinds of clouds are called stratus clouds. Just before the warm front arrives, it begins to drizzle. At this point the clouds are nimbostratus. Because the front has a gentle slope, the storm is not violent and lasts quite awhile, usually all day. Of course, the type of weather you get depends where you are located in relation to the low-pressure center.

Once the warm front passes the weather changes dramatically. Wow, it was quite chilly just a couple of days ago and now it's warm! This is because we are now in the region where warm air is moving up from the south. At this point in time, we are located between the warm front to the east and the cold front to the west. It's not very cloudy, either, so now's the time to get outside and enjoy the day.

The cold front is now approaching, and unlike the warm front, it has a very steep slope. We are still in the region with relatively warm air, but to the west of the front the air is very cold because it is coming from the northwest. Just as always, warm air rises. It, too, will rise along the front. However, the cold front is generally so steep that it results in the warm air rising rapidly! Remember what happens when warm air rises? It cools and condenses and rains (or sleets or snows) if there is enough moisture in the air. This process is similar to summer convective thunderstorms but the uplift of the air is due to the meeting of the warm and cold air so this is called a frontal thunderstorm. This is the kind of storm those of us who live in Siskiyou County are familiar with. The warm front can produce snow also, but these cold fronts come on suddenly and often dump a lot of moisture on us. The towering cumulonimbus clouds can develop rapidly and catch one unawares. However, if there is not enough moisture in the air or it doesn't get cold enough we might not get much precipitation at all.

After the cold front passes, be prepared for clear, cold days ahead. And since it is clear (a high-pressure system called an anticyclonic now prevails) the heat will keep on rising during the night because the clouds aren't there to "blanket" us. This is when we can get some real cold weather. The main danger here is that the roads are generally wet in the day and freeze at night so we have black ice on the roads in the morning.

Why are Extratropical Cyclones Important to Siskiyou County?

Most of the moisture we receive during the year comes in the wintertime with these extratropical cyclones. Because we live in the north in a mountainous region, this precipitation is stored at the higher elevations in the snow pack. Thus, during the dry summer we have our numerous creeks and rivers that supply us with water and provide us with recreation. We have also created reservoirs to hold excess water from high precipitation years as insurance for those all-too-common drought years.

 

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