Meteorologists struggle to forecast derecho storms

pgeweather@pge.comJuly 14, 2012 

John Lindsey took this photo of lightning that struck the USS Gray (FF-1054) while sailing between Key West, Fla., and Cuba on the USS Trippe (FF-1075) during a storm in 1983.

JOHN LINDSEY

When I first heard reports of the devastating derecho (pronounced de-ray-cho) storms that battered the Midwest and mid-Atlantic a couple of weeks ago, I thought the severe weather term was relatively new, but it’s been around for over a century.

According to the National Weather Service, it refers to a long duration, widespread, straight-line damaging wind event associated with ranks of rapidly moving thunderstorms.

To be classified as a derecho, a storm must extend for more than 240 miles with wind gusts of at least 58 mph or greater along most of its length.

In stronger derechos, wind gusts may exceed 100 mph. Derechos in the United States occur east of the Rocky Mountains and are most common in the late spring and summer. Because these types of storms are so rare and fast-moving, meteorologists struggle to recognize and forecast them, making them even more dangerous.

According to retired NWS weather forecaster Robert Johns, the term was coined in the late 1800s by Dr. Gustavus Hinrichs, a physics professor at the University of Iowa.

In the 1870s, the state of Iowa hired Dr. Hinrichs to record and study weather phenomena. He set up a network of volunteer weather watchers across the state and began the task of analyzing meteorological data and storm reports from these civic-minded souls. He gradually realized that the nature of damaging winds associated with severe thunderstorms varied considerably. In fact, it appeared as though a few of these destructive wind events were not tornadoes at all, but a straight line of frightfully violent winds.

He decided to use the Spanish language term derecho meaning direct or straight-line to classify these wind events. This term is an analog to tornado, which is also of Spanish origin.

In 1888, he published a paper in the American Meteorological Journal defining this term.

Shortly afterward, the forerunner of the NWS, the U.S. Army Signal Corps, was transferred to the Department of Agriculture. The secretary of this department decided that the Spanish terms tornado or derecho could not be used in any of the U.S. government weather forecasts. Nevertheless, scientists and journalists still continued to use the term tornado. However, because of the rarity of these straight-line wind events, the term derecho would be soon forgotten.

In 1987, nearly 100 years later, Robert Johns and his colleague Bill Hirt published a paper concerning derechos. Since that publication, the term has become progressively more commonly used in describing these long-lived, straight-line wind events.

On the afternoon of June 29, a rare and terrifying derecho developed in Iowa and marched toward Maryland and Massachusetts, reaching those states by nightfall. What began like a standard-issue thunderstorm soon turned violent, generating fierce winds, fearsome lightning and massive power outages.

Central Coast resident Micah Van Bogelen told me he lived in the Midwest for the first part of his life before moving to California. He still remembers the tornado warnings, hailstorms and, of course, the powerful thunderstorms that would wake him up on hot summer nights, but he had never seen anything like what he encountered in Boston last month while on vacation with his family.

Van Bogelen said, “Sheets of water bent and whipped to the ground, forced by powerful gusts of wind. The sky lit up from end to end. Then immense bolts of lightning began to strike the ground, as often as five times a minute.”

He went on to tell me, “Then I saw the most spectacular lightning strike I have ever seen. It was thick and mean as it quickly made its way to the lightning rod atop Fenway Park. The sky cracked as it mended the cylinder-like hole left by the bolt, but it was somehow still on fire in several pockets where the lightning had been. It looked like a plasma fire from a sci-fi movie.”

Kory Raftery, former KSBY reporter and PG&E colleague, now works as the communications director at Calvert Cliffs Nuclear Power Plant in Maryland. On that night, he told me that he had never seen lightning with such frequency and intensity. His entire neighborhood lost electric service, prompting his 3-year-old son, Mickey, to ask Raftery and his wife, Lauren, why was it dark inside their home but bright as day outside.

Today’s Weather Report:

A persistent temperature-inversion layer will continue to produce marine low clouds, fog and areas of drizzle during the night and morning hours along the beaches and into the coastal valleys.

Gentle to moderate (8- to 18-mph) southerly winds along the coastline will keep most of the beaches of San Luis Obispo County overcast through the day with only partial afternoon clearing. The exception could be Los Osos and Morro Bay, where the southerly winds coming off the Irish Hills may produce clear and sunny skies and warmer temperatures during the afternoon hours, a reversal of the normal coastal weather pattern.

Cool marine air will also filter far into the North County as a result of an upper low-pressure trough along the California coastline. Today’s maximum temperatures in the North County will reach the high 80s, while temperatures in the coastal valleys will only reach the high 60s.

Temperatures along the beaches will be in the low 60s, except in Los Osos and Morro Bay, where temperatures could reach the mid-60s.

The first half of next week is looking rather cool across the Central Coast as the low-pressure trough along the coast dominates through at least Wednesday. Cooler temperatures will occur Monday across all locations with Tuesday the coolest day of the week and temperatures at all locations well below normal, generally in the 60s near the coast to the 80s inland.

A massive 1,037-millibar Eastern Pacific High will remain nearly stationary about 1,500 miles to the west-northwest of San Luis Obispo through Thursday. At the same time, the trough of low pressure will gradually move down the California coast through Thursday.

Historically, especially during the summer months, this condition can often bring subtropical moisture from the south toward San Luis Obispo County. At this time, it appears the remnants of Hurricane Fabio will be entrained in this northerly flow and should reach the Central Coast later Wednesday into Thursday with increasing mid- to high-level clouds, scattered showers and a chance of thunderstorms.

The last time San Luis Obispo experienced rain was April 23. Some of the coastal areas, such as Baywood Park and Cambria, have recorded about 0.04 inches precipitation since July 1 because of periods of heavy drizzle during the late night and morning hours.

A warming trend will begin Friday through the following week, with the potential for well-above-normal temperatures for the rest of the Mid-State Fair.

Inland Temperatures, Paso Robles Sun Mon Tue Wed Thu Fri Sat 49 89 51 81 54 77 54 80 56 88 58 95 57 94

Coastal Valleys Temperatures, San Luis Obispo Sun Mon Tue Wed Thu Fri Sat 54 73 54 67 55 68 54 68 55 74 56 77 55 78

Sunday’s Surf Report:

Today’s 2- to 4-foot northwesterly (290-degree deep-water) swell (with an 8- to 11-second period) will continue at this height and period along the Pecho Coast through Thursday.

Increasing northwesterly winds will generate a 3- to 4-foot northwesterly (320-degree deep-water) sea and swell (with a 5- to 8-second period) along the coast Friday through next Sunday.

Arriving from the Southern Hemisphere:

Swell from former Hurricane Emila will arrive along the Pecho Coast on Tuesday into Wednesday. However, it’s coming from a south-southeasterly (165-degree deep-water) direction and will have little effect along our coastline.

Another Eastern Pacific Hurricane Fabio will also be too far east. Consequently, it will have little effect along the Pecho Coast on Friday.

An intense storm is expected to develop off the southeastern coast of New Zealand on Monday. A 1-foot Southern Hemisphere (200-degree deep-water) swell (with a 25-second period) from this storm is expected to arrive along our coastline Saturday. This swell will gradually build to 1 to 3 feet next Sunday into Monday (with a 17- to 19-second period). Another very long interval Southern Hemisphere swell is expected to arrive along our coastline July 25.

Seawater temperatures:

Seawater temperature will range between 53 and 55 degrees through Monday, increasing to 53 to 57 degrees Tuesday through Wednesday. Seawater temperatures will further warm to 55 and 58 degrees Thursday into Friday.

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John Lindsey, media relations representative for PG&E and local weather expert, has lived along the Central Coast for more than 25 years. To subscribe to his daily weather forecast or ask him a question, email him at pgeweather@pge.com.

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