How did the 2013 hurricane season end?

Posted on December 2, 2013 by Weather Guys Editor

The 2013 Atlantic hurricane season officially ended on Saturday. It was the least active year in 30 years. There were only 13 tropical storms, two hurricanes and no major hurricanes. The two hurricanes that formed were very weak. In an average year, there are 12 tropical storms; six or seven go on to become hurricanes and two of those reach major hurricane intensity.

A tropical storm is a rotating weather system with low pressure, strong winds and a storm center with heavy rain producing thunderstorms arranged in a spiral pattern. When the highest sustained winds in a tropical storm reach 74 mph, the storm is reclassified as a hurricane if it is over the Atlantic Ocean or a typhoon if over the Pacific Ocean. When the sustained winds reach 97 mph the storm is considered a major hurricane. Typhoons with winds above 150 mph are called super typhoons.

In late May and early June of 2013, the seasonal forecasts were predicting seven to nine hurricanes. Why was the forecast so far off? For one reason, seasonal forecasts are difficult to make. While the spring and early summer surface pressure and sea surface temperature suggested an above-average season, there were conditions that were not forecast and likely helped this hurricane season to be below average.

During the hurricane season, drier-than-normal air blew off the deserts of Africa into the tropical waters where hurricanes form. This dry air can weaken or inhibit the formation of hurricanes. In addition to this dry air, the average temperature structure was less conducive to thunderstorm development. These are likely two conditions that made this a below normal season. The coming year will bring forth research studies seeking to provide a complete explanation of this unexpected below-average season.

Category: Tropical

Comments Off on How did the 2013 hurricane season end?

Are tornadoes typical in the Midwest during this time of year?

Posted on November 25, 2013 by Weather Guys Editor

No, although in the U.S. tornadoes have occurred in nearly every state and in every month of the year. Wisconsin has reported a tornado in every month except February. There have been nine confirmed tornadoes in Wisconsin during November between the years 1844 to 2012.

Tornadoes form in regions of the atmosphere that have abundant warm and moist air near the surface with drier air above, a change in wind speed and direction with height. It is generally accepted that tornado season begins in the springtime. Because vertical wind shear is closely related to the presence of a jet stream, tornado season in the U.S. moves north and south during the year.

The National Weather Service issued a Particularly Dangerous Situation (or PDS) tornado watch for Janesville and southern Wisconsin on Nov. 17. A weather watch indicates that possible hazardous weather may occur. A watch is intended to provide people with enough time to set safety plans in motion for possible hazardous weather. A PDS is issued to indicate that there is an enhanced risk of very severe and life-threatening weather.

The intense thunderstorms that moved across the Midwest on Nov. 17 hit Illinois the hardest. The severe weather that day killed at least eight people and injured dozens. The weather included tornadoes, large hail and strong winds. The threat of severe weather from these storms caused a delay in the Chicago Bears-Baltimore Ravens football game at Soldier Field.

Category: Severe Weather

Comments Off on Are tornadoes typical in the Midwest during this time of year?

Why does the wind diminish after sunset?

Posted on November 18, 2013 by Weather Guys Editor

A reader wrote us last week with two questions prompted by careful observation: 1. Why does the wind speed tend to decrease after sunset? 2. Why does this effect seem less pronounced in late autumn and winter?

As it turns out, the answers are somewhat related to one another.

The wind speed tends to decrease after sunset because at night the surface of the Earth cools much more rapidly than does the air above the surface.

As a result of this difference in cooling ability, it doesn’t take long for the ground to become colder than the air above it.

The air in close contact with the ground — say in the lowest 300 feet of the atmosphere — then becomes colder than the air above it.

This circumstance leads to the development of what is known as a temperature inversion. Inversions dramatically reduce the amount of mixing that occurs between different vertical layers of the atmosphere. As a consequence, once the inversion sets up (after sunset), it is much harder for fast-moving air above the ground to mix down to the surface, where it could appear as a gust of wind.

During the day it is very easy for the air to mix and cause surface gusts.

If there is a low pressure area or storm in the region the winds will blow day or night. Late autumn and winter bring the strongest storms of the year to our area. These storms have cloud and temperature structures that can often overrule the tendency for inversions to set up at night.

The much stronger winds near the strong storms, coupled with a tendency to avoid inversions near storms, mean that windy nights are more common in the cold season.

Category: Meteorology

Comments Off on Why does the wind diminish after sunset?

Does a cold end to October signal an early winter?

Posted on November 12, 2013 by Weather Guys Editor

October ended up to be about normal in terms of temperature despite the chilly last couple of weeks. In fact, 14 of the last 17 days of the month were at or below normal. A natural question arises in the face of this cool spell — namely, does this portend a cold start to the winter?

The official November-January forecast from the Climate Prediction Center of the National Centers for Environmental Prediction (NCEP) gives us a slight probability of above-average temperatures for the next three months and even chances for normal amounts of precipitation over that interval. This forecast may arise from the fact that sea-surface temperatures in the central tropical Pacific Ocean are nearly normal heading into the cold season.

Our own harbinger of the early winter focuses on the presence or absence of snow cover in northwestern Canada. The idea is that if there is snow cover there by mid-late October, the chances of producing cold air masses there increase as the daylight hours decrease in late fall. Cold air produced in northwest Canada is often involved in our early season cold air outbreaks.

We are testing this idea in current research. So far this year, there has been unusual warmth (and consequently little snow) in northwest Canada. Thus, our prediction is in line with that of NCEP — the early part of the winter will not be memorably cold.

Category: Seasons

Comments Off on Does a cold end to October signal an early winter?

What was that dashed line in the sky on Nov. 3?

Posted on November 12, 2013 by Weather Guys Editor

If you watch jets fly high above, you may notice that sometimes white clouds trail them. These condensation trails are called contrails. Most clouds form as air rises. Contrails form by a mixing process that is similar to the cloud you see when you exhale and “see your breath”.

Contrails form when hot humid air from the jet exhaust mixes with environmental air that has a low vapor pressure and is at a cold temperature. One of the byproducts of the combustion in jet engines is a substantial amount of water vapor. We cannot see the water vapor, but when it mixes with the surrounding air it can condense to form tiny water droplets that soon freeze into ice crystals. These particles we see as the cloud lines behind the jet. Whether a contrail will form depends on the temperature and humidity of the air the jet flies through. If the upper atmosphere is very dry, a contrail may not form.

If you pay attention to contrail formation and duration, you will notice that they sometimes rapidly dissipate, but other times they will spread horizontally into an extensive thin cloud layer. How long a contrail remains intact depends on the humidity structure and winds of the upper atmosphere the jet is flying through. If that atmosphere is relatively humid, the contrail may exist for several hours. However, if the atmosphere has a low relative humidity, the contrail will dissipate as soon as it mixes with the environment. Sinking motions cause the air to warm and dry out, which causes clouds to evaporate away. If the air is moving up and down like a roller coaster ride, the descending air will warm and cause the cloud to dissipate. So, the region in the photograph where there is no contrail is likely air that is descending, and air is rising where the contrail is still visible.

Category: Phenomena

Comments Off on What was that dashed line in the sky on Nov. 3?