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May-June 2013

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Weather Queries

Does the “Pineapple Express,” which results in a continuous barrage of storms along the West Coast, usually occur only during an El Niño event?

Steve Hanan
Morton Grove, Illinois

The “Pineapple Express” refers to a concentrated ribbon of very moist air originating in the tropics, sometimes from the vicinity of Hawaii (hence “pineapple”), that strikes the West Coast of the United States or Southern Canada and sometimes causes excessive precipitation. Figure 1 shows most of the North Pacific Ocean from Japan to the West Coast of North America, with atmospheric water vapor content color-coded. The values along the color bar at bottom are in grams per square centimeter. If all the water vapor condenses in an atmospheric column one-centimeter square, this measure indicates how many grams of water would be realized in that column. An equivalent measure, called total precipitable water, indicates how many centimaters of water would fall at the ground if all the vapor in the column condensed and fell out as rain.

Consistent with climatology, copious atmospheric moisture exists in the tropics, near the bottom of Figure 1. But note the ribbon of moist air extending from north of Hawaii (the black dots near 160°W) to the Pacific Northwest Coast. Features like this are called atmospheric rivers. They are responsible not only for most major rainfall events along the Pacific Coast, filling reservoirs and watering crops, but also, at least occasionally, for devastating floods. The storm of November 6–7, 2006, depicted in Figure 1 caused 8–20 inches of rain in the Cascades and Coastal Mountains.

Caption: Figure 1. An image of vertically integrated atmospheric water vapor obtained by the Special Sensor Microwave Imager (SSMI) aboard a Defense Meteorological Satellite. This image appears on the Web page of the Physical Sciences Division of the NOAA Earth System Research Laboratory (http://esrl.noaa.gov/psd/atmrivers/events/). The color code at the bottom indicates how many grams of water would fall to earth if all the vapor in a vertical column one-centimeter square condensed.

Caption: Figure 1. An image of vertically integrated atmospheric water vapor obtained by the Special Sensor Microwave Imager (SSMI) aboard a Defense Meteorological Satellite. This image appears on the Web page of the Physical Sciences Division of the NOAA Earth System Research Laboratory (http://esrl.noaa.gov/psd/atmrivers/events/). The color code at the bottom indicates how many grams of water would fall to earth if all the vapor in a vertical column one-centimeter square condensed.

Caption: Figure 2. Anomalous weather patterns appearing during strong El Niños (top) and strong La Niñas (bottom). From the NOAA Climate Prediction Center's Web page at http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensocycle/nawinter.shtml.


The Pineapple Express or, more generally, atmospheric rivers, are most prominent during winter, when westerly flow at midlatitudes is strongest and atmospheric dynamics are most active, but they occur almost every winter, whether during El Niño, La Niña, or neutral conditions. Figure 2, from the Climate Prediction Center, one of NOAA's National Centers for Environmental Prediction, shows that the likelihood of an atmospheric river striking particular sections of the west coast of North America does change between years with a strong El Niño and years with a strong La Niña.

In winters with a strong El Niño, when sea-surface temperatures in the tropical Pacific are well above normal, wintertime low pressure tends to persist in the North Pacific south of Alaska, helping to drive a jet stream across the Pacific at lower latitudes than usual. In that the jet stream tends to set the storm track, a greater-than-usual number of storms tends to hit the California coast and track across the southern tier of states, bringing wetter and cooler-than-normal conditions. During El Niño winters, the northern tier of states tends to be warmer than normal.

In winters with a strong La Niña, when sea-surface temperatures in the tropical Pacific are well below normal, high pressure often persists in the Gulf of Alaska. Traveling around this ridge, the polar jet stream draws Canadian air into the Central United States, bringing colder than normal weather to the northern states west of the Great Lakes. The southern tier of states is usually drier and warmer than normal. The subtropical jet stream in the Pacific can be highly variable, but it tends to point atmospheric rivers more often toward the Oregon-Washington-British Columbia coasts than toward California.

Strong El Niños or La Niñas tend to make wintertime weather a little more predictable over North America. At the time of this writing (mid-December 2012), NOAA expects neutral conditions (neither El Niño nor La Niña) for January through March 2013, which means weather patterns will be less predicable in the long term. By the time you read this, you will know what kind of winter it was.

What is the difference between a squall line and a derecho?

Bert Ebi
Dearborn, Michigan

I'll start with four definitions from the Glossary of Meteorology, edited by Todd Glickman and published in 2000 by the American Meteorological Society: A squall line is a line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms. A derecho is a widespread, convectively induced, straight-line windstorm. The term is further defined as any family of downburst clusters produced by an extratropical mesoscale convective system. A downburst is an area of strong, often damaging winds produced by a convective downdraft over an area from less than 1 to 10 kilometers in horizontal dimensions. And finally, a mesoscale convective system is a cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area on the order of 100 km or more in horizontal scale in at least one direction.

For those who dislike the technical jargon, I will try to simplify. A mesoscale convective system (MCS) is basically a large cluster of thunderstorms that produces a large, connected area of rain, often heavy. Both squall lines and derechos are types of MCSs. A squall line is a collection of side-by-side thunderstorms, sometimes organized in a straight line and sometimes in an arc. Squall lines often form with the support of energetic upper air disturbances, troughs in the westerly wind flow, that promote lifting of low-level air on a broad scale. MCSs comprising a disorganized cluster of thunderstorms (not a squall line) may form, for example, at the northern end of a moist, low-level jet, but without the aid of an upper-air disturbance.

Most derechos are associated with a squall line. Their distinguishing feature is strong, often damaging, straight-line winds. One of the most destructive derechos in United States history, affecting large areas of the Northeast and causing widespread, multiday power outages, occurred on June 30, 2012. See http://en.wikipedia.org/wiki/June_2012_North_American_derecho for a detailed description. This derecho formed and strengthened after a squall line organized over Indiana and Ohio and plowed into extremely unstable air. It cut a swath of damage all the way to the Atlantic Coast. Radar displays sometimes show bow-shaped bulges along an otherwise smooth leading edge of a squall line. Meteorologists watch for such features as an indication that a derecho may be developing. Tornadoes may or may not be associated with derechos, but the rotary winds of a tornado affect a much smaller area than the straight-line winds of the derecho.

It has been said that one of the safest places to be during a thunderstorm is inside an automobile. Does this still hold true for a hybrid or electric vehicle?

David Bowman
Lapaz, Indiana

The safest place to be during a thunderstorm is indoors, away from windows, plumbing, and wiring. However, if you are caught in an automobile during a thunderstorm, you will usually be safe from harm, provided that the outer shell of the automobile is metal and you refrain from touching anything metal. It is not safe to remain in an open convertible or in any vehicle whose roof is not metal. If your car is struck by lightning, the current passes through the outer metal shell and into the ground through the tires. (A couple of inches of insulation by rubber tires will not stop transmission of the strong current from the lightning bolt, which has traveled many thousands of feet through air, which is a very good insulator.) Modern automobiles, including hybrids and wholly electric cars, have components that may be destroyed by lightning. There may even be burn marks on the metal frame following the strike. But as long as you are inside the metal shell and not touching metal, you should suffer little more than momentary fright, except in rare instances. This is true for all types of cars with fully metal frames. Cars with fiberglass or plastic shells offer no protection.

Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at weatherqueries@gmail.com, or by mail in care of Weatherwise, Taylor & Francis, 325 Chestnut St., Suite 800, Philadelphia, PA 19106.
 

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