Pressure Gradients, Storm Tracks and Wind Velocity by Wolf Read

During the landfall of a small, but intense, cyclone on February 7, 2002, a gradient of 10 mb between EUG and PDX resulted--this was a half millibar higher than the gradient produced by the Columbus Day Storm, which kicked up gusts as high as 116 mph in Portland. However, winds at Portland only reached 31 mph during the 2002 storm. What happened? The 2002 event demonstrates a resolution limitation with standard gradient measures. As shown in the hypothetical scenarios depicted in Figure 3, below, a low center can pass close to, but still south of, a northern station, and yield a strong positive gradient. The low's exact position is "hidden" by the rough resolution of official stations. The fact that the northern station is actually in the north sector of the low can be revealed by noting winds, which are typically light and northerly, which brings the discussion to storm tracks.

Storm Tracks The path that a cyclone follows strongly determines where the storm's damaging winds will occur. Indeed, a low's track can have a larger effect on wind velocity at a particular location than pressure gradient. Because of this fact, two broad categories of storms result from the paths that they follow: 1) land-falling systems, which tend to have their high winds confined to an area equivalent to part of a state, and 2) recurving systems that skirt the coast, which can strongly affect several states. Pacific cyclones have made direct landfalls on every part of the Pacific Northwest coast. With a land-falling low, the strongest winds are usually more localized, and are confined to the south side of the low's track. Figure 4, below, plots five idealized tracks that northeast-tracking cyclones can take. The Northwest is broken down into 12 wind zones. Zones typically affected by damaging winds when an intense cyclone follows a path similar to the ones illustrated are indicated under the track. When a representative storm from history roughly meets the characteristics of one of the paths, and is detailes on another webpage, then it is listed above the track. Storm names are either pulled from history, such as "Storm King" and "Olympic Blowdown," or from a hat.

The highest winds tend to follow the south side of a landfalling low, and, as discussed above, are typically from a south direction, often from 170° to 210°, that carries the air approximately toward the low's center. The immediate south quadrants are usually the area of steepest gradient. For very intense systems, this often does not extend beyond 50 to 100 miles from the center. Outside that range, winds decrease markedly, though areas favored for high winds due to unique topography can still get a heavy blast. Gradients are often less steep north of the low, and generally do not produce a damaging north gale. This goes beyond a shallower gradient. There's typically a lack of upper-air jet support for north winds. Also, cyclones often have a north component to their motion, and this speed is subtracted from surface airflow. For example, if a low is tracking northeast at 40 mph, its northward vector is about 28 mph (with a west vector also of 28 mph). That 28 mph subtracts from the average windspeed in the north sector. If the gradient, and other conditions, are present for a sustained 40 mph gale, the 28 mph north vector reduces southward-trending airflow (north winds) to a mere 12 mph. However, on the south side of the low, the storm's northward vector of motion is added to the northward-moving airflow (south winds), making 40 mph potentially 68! Generally speaking, the further north the cyclone's path, the more often it is traveled. Landfalls from North Vancouver Island to the Queen Charlottes are winter's normal progeny, and happen every year, often many times. Though these two classes of storms can produce a good winter gale in Western Washington, and sometimes in Oregon, especially along the coastline, the winds don't often reach the extreme range of interest. For these reasons, no such storm is examined in this volume. The reader is encouraged to watch these cold high-latitude lows unfold during the Northwest's winter season. This will reveal plenty of information on these events. However, keep in mind that the south-of-the-track rule for high winds applies even to a "Queen Charlotte special"; for parts of the British Columbia Coast and Vancouver Island, these storms can and do produce very fast winds. Landfalls from central Vancouver Island southward tend to beget windstorms in the region examined on this webstie, and this is where webpages on specific storm types exist. Depending on just how the low tracks, various regions are affected severely, while others are spared. Storms landing in lower Vancouver Island tend to produce blustery winds in all regions, as the entire area falls south of the low's track, and damaging winds in the northern zones. As the track is moved ever further south, the area of damaging south winds goes with it, and the number of map zones that experience strong winds decreases. However, other events happen on the low's north side: often intense rain and/or snow, especially for regions to the northeast of the low, and blustery west to northwest winds, which can yield near-blizzard conditions. This combination of features, and the regions affected by them, can be used to define the specific classes of landfalling windstorms listed on the map: Olympic Blowdown, Storm King, South Valley Surprise. The recurving class of windstorm is not depicted on the map, for reasons of visual clarity. This does not mean they are of lesser importance. The opposite is probably true. When conditions are right, these storms tend to develop between 130-160° west longitude, and 35-40° north latitude. Cyclogenesis can be very rapid. Typically, the bombing disturbance moves east to east-northeast toward the western United States. Far ahead of these vast systems, air starts to jump the gradient among the mountainous terrain of western North America. Airflow down the valleys, which draws from higher pressure fields to the south, is itself a developing tongue of high pressure at the surface. This zone of high pressure often shoots northward at enough of a clip to act as a block to the storm, and force a northward curve to the low's path. To borrow a hurricane term, the cyclone is forced to "recurve." Most deep lows--even land-falling ones--have a tendency to undertake some level of curve northward as they near shore. A select few, however, bend their paths enough to move roughly parallel the coastline-here termed the recurving lows. When a "recurver" happens while the low center is east of about 130° west longitude, a severe blow can be carried across northern California, western Oregon and Washington (sometimes the whole states), and southern British Columbia. This is especially the case when a cyclone races just a short distance offshore, such as with the Columbus Day Storm of 1962. Because recurvers tend to affect a large region, they also tend to cause the most property damage. These major sou'westers have been given a number of names throughout history, including these book and news media titles: West Coast Disaster, Big Blow and Big Blast. The "Big Three" covered extensively on this website, October 1962, November 1981, and December 1995, are of the recurving type, though the latter one made landfall on Washington's Olympic Peninsula. To pin a specific, yet arbitrary, figure on this kind of windstorm, the track must be oriented between 0 and 35 degrees for at least three hours, and the low center must stay offshore at least to the latitude of Seattle, about 47.5° north, with a landfall on Vancouver Island preferred. By these terms, December 1995 is a marginal recurver, and almost classes as an Olympic Blowdown direct landfall-type system. The size of a cyclone also factors into how large an area it will affect. The Big Three were all titanic cyclones, with intense frontal systems that reached far from the parent low centers, and stretched deep into the western United States as the lows raced northward. More compact systems that follow similar tracks wouldn't yield the same kind of results. The Columbus Day "Big Blow", though it is grouped with these recurving events and is the most intense example, is too unique to be considered the archetype of this class. Arguments can be made (and have been) that place the 1962 tempest in a grouping all its own. The November 14, 1981 windstorm is probably a better "typical" example, if one is to be had.

Cyclone Quadrants Because of the way air swings around a cyclone it can be useful to divide a cyclone into four quadrants: NW, NE, SE and SW, as depicted in Figure 5, below. This can help with the understanding of local wind conditions during a storm. The reported wind directions are for land-based locations.

Northwest Quadrant NNW to WNW winds, cool to cold, sometimes with lowland snowfall, sleet, hail, especially along bent-back occlusion. How much frozen precipitation depends, in part, on the time of year the storm strikes. Northeast Quadrant NE to SE winds, usually light, but can be strong at mountain passes. Cool, sometimes cold around river drainages, which allow frigid, dry air from east of the Cascades to trickle westward. Increasing precipitation from "over-the-shoulder" moisture as low approaches, usually light rain, though can be snow in places, and can get heavy near along the frontal boundary. Southeast Quadrant SE to S winds, sometimes very strong and damaging, warm, as warmer air fields are advected northward in this sector of the storm. Precipitation, almost always rain up to fairly high elevations, can be heavy under ideal low-level jet conditions, especially on SW to S facing mountain slopes. Southwest Quadrant Usually W to SW winds, mild, often with gusty post-frontal showers which can be intense. Damaging S-SSW winds can occur immdiately south of the low center for land-based stations, when cyclone is strong. In the right gradient conditions, a W to SW-erly high wind event can also occur in this quadrant, a considerable time after the low has passed--this is not uncommon for the Puget Lowlands region. The "NW Jump" Signature on Meteograms Another way to look at conditions south and north of a low's track is depicted in Figure 6, below. The key signature for a station that ends up north of a low's track is the wind shift from NE to NW, which makes a sharp jump on the wind direction trace in the meteograms. For a station south of the low's center, a wind shift from SE to SW is typical, which makes a more gradual climb on a meteogram's wind direction trace.

Last Modified: December 5, 2008 Page Created: November 28, 2002 You can reach Wolf via e-mail by clicking here. | Back |