Are ‘atmospheric rivers’ and the ‘Pineapple Express’ related?

Posted on December 15, 2014 by WeatherGuys Editor

Last week, central and southern California got hit with a substantial rain event, welcome news for a severely drought-stricken area that has not seen heavy rains in many months.

A high-profile element of the storm system that affected the region was a so-called “atmospheric river,” or the local variant of this feature sometimes known as the “Pineapple Express.”

These names refer to the fairly narrow (sometimes only a few hundred miles wide) streams of high atmospheric water vapor content that can result from the encroachment of a middle latitude weather system into the northern subtropics — latitudes of about 30 degrees North. The middle latitude weather system has associated with it a robust circulation that drives air northward on its eastern side and toward the equator on its western side.

This same characteristic is observed with the passage of such storms over Wisconsin at any time of year. When such a circulation finds itself as far south as 30 degrees North, the air that it drives northward on its eastern side is pure tropical air, laden with enormous amounts of water vapor. This results in narrow tongues of exceptional water vapor content rushing northward in what resemble “rivers” of water, but in the atmosphere.

On the Pacific coast of North America, the tropical air can sometimes originate near the Hawaiian Islands, hence the name Pineapple Express. When these rivers of high water vapor content are forced up the sides of the Sierra Nevada mountains in California and Nevada, the water vapor is condensed into liquid and solid form, bringing much needed rains to the area.

So, though this recent storm was powerful, it was not unprecedented and, in fact, is a rather common occurrence along the entire Pacific coast in the late autumn and early winter.

Category: Meteorology, Severe Weather

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What is a ‘panhandle hooker,’ and why is it called that?

Posted on December 8, 2014 by WeatherGuys Editor

A panhandle hooker, also called a Texas hooker, is a name for a storm that forms in the panhandles of Oklahoma or Texas.

These storms bring interesting weather to the Midwest and Great Lake regions. The weather system gets its name from where the storm forms and intensifies.

The “hook” describes the curved path that these cyclones take, first bending to the southeast and then curving northeast to Missouri across Iowa and into southern Wisconsin and on to the Great Lakes.

Cold and warm fronts are associated with these mid-latitude cyclones.

These storms occur from late fall through early spring. They are not very common and bring some of the most memorable weather to our region, including heavy snow, strong winds and blizzard conditions.

As they pass over the Great Lakes, the warm waters can intensify the storm, generating dangerous shipping conditions.

The storm that sank the Edmund Fitzgerald ship on Lake Superior on Nov. 10, 1975, was a panhandle hooker.

The weather preceding the arrival of these storms can be very pleasant.

The Armistice Day Storm of Nov. 11, 1940, was another example of a panhandle hooker. Many Midwesterners woke on that morning to surprisingly warm weather and a day off from work in honor of Armistice Day. Thousands of fishermen and duck hunters headed outdoors to take advantage of the near-perfect conditions.

Most people did not realize that the warm air was the harbinger of a powerful panhandle hooker. Before 2 p.m., the cold front chasing the warm air mass arrived in Wisconsin, Minnesota and Michigan. Temperatures dropped rapidly, in some cases more than 50 degrees.

Rain, sleet, and snow began to fall. High winds brought down trees and wires, whipped up snow into blizzard conditions, and made navigating lakes and rivers nearly impossible.

A total of 145 deaths were blamed on that storm.

Category: Meteorology, Seasons, Severe Weather

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What is the windchill temperature?

Posted on December 1, 2014 by WeatherGuys Editor

What is the windchill temperature?

The windchill describes the increased loss of heat by the movement of the air.

The windchill is relevant to humans and other animals that need to maintain a constant temperature that is higher than their surroundings.

The windchill cannot be measured with a thermometer; it must be computed.

The windchill temperature index, expressed in degrees, translates your body’s heat losses under the current temperature and wind conditions into the air temperature with a 3 mph wind that would produce equivalent heat losses.

The original windchill formula, devised by Antarctic explorer Paul Siple in 1945, was based on research involving the time it took water in a plastic container to freeze.

The National Weather Service updated the formula in November 2001.

This modern-day windchill temperature index takes into account a calculated wind speed based at the average height of a human face (not at the winds measured at the national standard height of 33 feet), the exposure of a human face to cold vs. a plastic container, incorporates heat transfer theory, lowers the calm wind threshold from 4 mph to 3 mph, and has a consistent standard for skin tissue resistance.

As an example, a temperature of 0 degrees and a wind of 30 mph has a windchill temperature index of -26.

If left outside, an object will never reach a temperature of -26, it will cool down to 20 at a cooling rate equivalent to an outside temperature of -26 and a wind at 3 mph.

Under these conditions, frostbite would occur within 30 minutes.

Frostbite occurs when your skin and other tissues cool down to the point that ice crystals form in your bodily fluids.

Cold temperatures combined with strong winds can cause frostbite in a matter of minutes, and the windchill temperature index is used to determine cold hazardous conditions.

Category: Meteorology, Seasons

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How unusual was last week’s cold snap?

Posted on November 24, 2014 by WeatherGuys Editor

Through Friday, Madison experienced 10 straight days with temperatures at or below 32 degrees, or freezing.

And though that streak was interrupted (delightfully) over the weekend, it appears we will experience a return to sub-freezing temperatures by Tuesday.

If anyone needed more evidence of the unusual depth and duration of this early season cold snap, consider this: The most recent time Madison endured a longer streak of consecutive freezing-or-below days in November was in 1880 — 134 years ago.

In fact, this November’s 10-day period of freezing-or-below temperatures in Madison ties it with seven other Novembers, all of which are second only to November 1880, during which there were 15 such days.

Although we are not likely to reach that record, we are very likely to end up alone in second place in this category by the time this November is over.

In addition to the length of this cold period, we have also experienced nine straight days of snowfall through Nov. 19.

Although we have only experienced a trace of snow on five of those nine days (a trace is less than 0.1 inch), such a consecutive snowfall streak in November is quite unusual – much more characteristic of mid-winter.

Over the previous five Novembers, no streak of longer than two consecutive snowfall days has occurred. In those same five winters, streaks of 10 days occurred only three times; in December 2009 (10 days), February 2010 (10 days), and January/February 2013 (11 days). By nearly any measure, this has been a remarkable early season cold snap.

Category: Meteorology, Seasons

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What kind of winter are we expecting?

Posted on November 17, 2014 by WeatherGuys Editor

Seasonal climate forecasts rely heavily on established relationships between climate and key climate forcing mechanisms, such as El Niño.

On seasonal time scales, the influence on the atmosphere of ocean temperature anomalies such as El Niño or La Niña is probably the single most crucial forecast component. This is especially true for forecasts of Wisconsin winters.

It appeared that an El Niño was developing last spring, but that has dissipated and is of little help in this year’s winter forecast. Seasonal forecasts also take account of known atmospheric oscillations.

Monthly and seasonal climate forecasts look very different than weather forecast maps.

There are no low- and high-pressure systems, no fronts and no specific numbers for high and low temperatures.

These seasonal forecasts are limited to predicting changes versus normal conditions instead of giving precise numbers like a weather forecast.

Seasonal forecast maps show chances of above- or below-normal temperature and precipitation.

Three classes of conditions are forecast on seasonal time scales: A, for above normal; B, for below normal; and EC, for equal chances of either above or below normal.

In the case of equal chances, the percentages are roughly 33 percent apiece for above normal, below normal, and normal conditions.

The National Oceanic and Atmospheric Administration seasonal forecast has equal chances of our winter being colder, milder or normal.

The northern-most regions of Wisconsin have a chance of having below normal temperatures.

The seasonal forecast for precipitation is below normal for most of the state.

Our winter last year was exceptionally cold. While November has gotten off to a cold start leading into winter, it is unlikely we will have another extreme like last year.

Category: Climate, Meteorology, Seasons

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