How unusual is our roller coaster winter?

Posted on January 20, 2015 by WeatherGuys Editor

It doesn’t take an exceptional attention span to realize that this year’s cold season (starting in November) has been very changeable.

November was 6.1 degrees colder than normal, then December was surprisingly mild (5.8 degrees above normal). As of Thursday — mid-month — January has been 8.5 degrees below normal.

Thus far we have had only 14.9 inches of snow and every month has been substantially drier than normal as well.

This is shaping up to be a very unusual winter in many respects. After a relative warm up, forecast models are suggesting a return to very cold temperatures as we approach the end of January. We suspect this will be enough to keep the monthly average temperature below the normal for January.

Though there is really no way to accurately forecast the monthly average for February, experimental forecasts made by the Climate Prediction Center, issued Thursday, predict a 50-50 chance for a normal February in Wisconsin.

Though detailed records are not immediately available, anecdotally it has been at least 20 years since a cold season has flipped from below normal to above normal in consecutive months from November to February. There is some chance that this will be such a year.

Category: Climate, Meteorology, Seasons

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Are the Great Lakes’ water levels normal?

Posted on January 12, 2015 by WeatherGuys Editor

For the first time in about 25 years, the water level of the all the Great Lakes is above normal. Lakes Superior, Michigan and Huron are about 5 inches above the long term average.

This ends a 15-year period where lake levels have been below historic averages.

Lakes Huron and Michigan were at record low levels in January 2013; that is a rapid rise in water level to be above normal two years later. Such a rapid increase has not been measured since observations began in the mid-1800s.

The water levels of the Great Lakes are determined by the amount of water flowing in and out of the lakes.

Precipitation, runoff, and water from streams and groundwater supply water to the lakes, while evaporation and water flowing out of the Great Lakes system are water losses.

When the input exceeds the output, the levels rise.

The water cycle of the lakes is complex, and weather has played a role in this turnaround in lake levels.

Above-average precipitation and above average runoff in the Great Lakes watershed, particularly in the springs of 2013 and 2014, helped to restore lake levels.

The frigid winter of 2013-2014 also helped by reducing evaporation.

Ice on the lake and cold waters reduce evaporation, which also reduces snowfall in the snow-belt regions of the lakes.

Information on and forecasts of Great Lakes water levels is available from several agencies in the United States and Canada.

The forecast for the water levels is to continue to be above average, though levels could change relatively quickly.

Category: Climate, Meteorology, Seasons

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What rare weather event did we experience last month?

Posted on January 6, 2015 by WeatherGuys Editor

Madison’s overnight low temperature of minus 3 on Dec. 28 was a relatively rare event, all things considered.

As we all know, the temperature dipping below zero in winter is not unusual. But it doesn’t usually happen without snow on the ground, and there are good reasons for that.

Snow on the ground works double duty to keep the air near the ground very cold. First, since the ground is a vast storehouse of heat, the presence of snow insulates the air above from that heat.

The fact that snow is an excellent absorber and emitter of infrared energy — the kind of radiation that nearly everything on Earth emits, including the ground — gives this insulation effect an added radiative dimension. With a couple of inches or more of snow cover, the infrared energy emitted by the ground is absorbed by the overlying snow.

The snow then emits that energy in all directions, some of which are back toward the underlying ground. Thus, much less than half of the energy that would otherwise be emitted from the ground to the overlying atmosphere never gets there.

Secondly, since the top surface of the snow is emitting infrared exceptionally well, the air in contact with that snow surface can get really, really cold. The result is extremely cold surface air.

In our last cold spell, the coldness was imported from locations where this radiative cooling had been in operation — places to our north. This coming week, our low temperatures will be partly “home-brewed” by the snow we just received.

Category: Meteorology, Seasons

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What are geomagnetic storms?

Posted on December 29, 2014 by WeatherGuys Editor

Our sun is an active star that has storms. Sometimes the sun ejects a cloud of gas, called a coronal mass ejection or CME. CMEs are often associated with solar flares, and it takes about two to three days for the charged particles in this gas to reach Earth. Earth’s magnetic field deflects these particles toward the North and South Poles.

Unfortunately, solar electrons and protons from CMEs collide with the Earth’s magnetic field and atmosphere and can stir up electromagnetic storms in the Earth’s magnetosphere. These geomagnetic storms can result in the Northern Lights but also disrupt satellite-based navigation, communications, air travel, power grids and even pipelines. A geomagnetic storm in March 1989 shut down the Hydro-Quebec electric grid in Canada, leaving people without electricity.

In a worst-case scenario, the costs of an extreme geomagnetic storm in the United States could be in the trillions of dollars and would require a recovery period of up to a decade. As a result, the U.S. is developing plans to reliably address impacts of geomagnetic storms on our bulk power system. Networks are being set up across the globe to monitor geomagnetic conditions.

An emerging discipline known as space weather attempts to forecast the effects of solar activity on the upper atmosphere so that impacts to society can be minimized. Instruments on satellites can observe ejections of high-energy plasma from the sun. These can result in NOAA’s Space Weather Prediction Center issuing a warning of a coming magnetic storm, estimating time of arrival and intensity.

Category: Meteorology, Phenomena

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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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