El Nino

Friday, July 31, 2015

El Nino continues to make weather news.  This change in atmospheric and sea-surface conditions in tropical regions of the Pacific happens every few years or so, but the one building now has the look of being a strong one.  El Nino’s impacts in the middle latitudes are more pronounced during the colder months.

What will the impacts be here?  It depends on other factors, of course.  This is weather which is highly dynamic.  But a strong El Nino can be statistically correlated to a stormier fall season, a much warmer than average winter with below average snowfall, and a colder but drier than average spring.

But it is important to understand that other factors can overwhelm the El Nino signal.  Of particular interest this winter is a large region of unusually warm water in the northeastern part of the Pacific Ocean.  What role this will play is interesting but unknown.

Meteorologist John Wheeler

Highest Humidity

Dew point temperatures in the 60s and 70s since last weekend have certainly caused a midsummer feel to the air.  Here in the Northern Plains, we do not get as much humidity as they do in the southern and eastern United States, where dew points in the 70s are common all summer long.  (And where summers are much longer.)  Once in a great while, under just the right conditions, dew points in the Northern Plains and Upper Midwest will reach into the 80s.

The highest humidity in the world is along the Persian Gulf.  On July 8, 2003, in Dhahran, Eastern Province, Saudi Arabia, the dew point reached 95 degrees. The temperature at the time was 107 degrees.  The Heat Index calculates to 176 degrees under these conditions.

The next time you find yourself complaining about our humidity, imagine being in the incessant sunshine along the Persian Gulf under these conditions.  And with ocean water temperatures in the 90s, there would be no way to cool off.

 

Meteorologist John Wheeler

2015 Atlantic Hurricane Season Forecast

The Climate Prediction Center has issued its forecast for the Atlantic hurricane season, which officially starts tomorrow.  The developing El Nino conditions in the Pacific Ocean will likely reduce, somewhat, the threat of organized tropical storms across the Atlantic, the Caribbean, the Gulf of Mexico, and along the Southeast and East Coasts of the U.S.  The forecast calls for a 70 per cent chance of a below average number of storms, a 20 per cent chance of near average, and a 10 per cent chance of above-average activity.  Of course, the number of hurricanes is not nearly as important as their locations.  If one bad hurricane strikes a metropolitan area, it is effectively a bad season.  El Nino causes the Atlantic Basin upper level winds to strengthen.  Hurricanes are much more likely to grow strong in an environment of weak upper level winds.

Meteorologist John Wheeler

Indian Monsoon

The monsoon of the Indian subcontinent is one of the most observable and predictable weather reversals on Earth.  During spring and early summer, warm and humid air is drawn northward over the Indian region by semi-permanent low pressure.  The shape of the Indian subcontinent and the peculiar geography of the region, including the tropical Indian Ocean and the cold, dry conditions on the Tibetan Plateau, create ideal heat wave conditions.  Usually by sometime in May, the bubble bursts and widespread heavy rains ensue during the summer.  However, this year, the monsoon has been delayed.  For the last two weeks, weather systems have been diverted eastward or westward away from India.  The result has been two weeks of 115 degree temperatures with sweltering humidity.  Hundreds and hundreds of people have died.  Fortunately, rainy and cooler weather is expected to begin any day now.    Meteorologist John Wheeler

California Drought

Tree ring analysis suggests the present drought in the Southwestern U.S. may be the fourth most severe in approximately the past 1000 years.  The nation’s two largest reservoirs, Lake Powell and Lake Mead, both on the Colorado River, are at their lowest levels since construction.  These two impoundments provide much of the water needs for desert metropolises such as Las Vegas and Phoenix.  This melted snow also provides considerable water for the Los Angeles Basin and the California agriculture industry, which is where a large portion of America’s grocery store produce is grown.  The lakes are at approximately 40 per cent of capacity this spring, and will drop more over the summer.  Hopefully, next winter’s El Nino will help generate an above-average snow pack in the Rockies next winter or the situation will continue to worsen.     Meteorologist John Wheeler

Hurricane Forecast

There are three major, large-scale factors which can have large scale effects on the Atlantic hurricane season.  One is the general sea-surface temperature.  If the tropical part of the Atlantic Basin is warmer than average, there is more thermodynamic energy available for hurricanes.  Another is the amount of dust from the Sahara.  Actually, it isn’t the dust, but the deep layer of dry air which accompanies these dust clouds that rob tropical storms of moisture.  The other key is the overall strength of upper level winds.  When the winds aloft are strong, tropical storms tend to shear apart before they can grow large and powerful.  The presence of a building El Nino such as there is at present often produces stronger winds above the Atlantic, usually signaling a quieter Atlantic hurricane season.   Meteorologist John Wheeler

Big Hurricane Absence

TIn the nine years since Category 5 Hurricane Wilma struck South Florida in October of 2005, there has not been a single major hurricane (Category 3 hurricane or stronger) to strike the U.S.  This is the longest absence of landfalling major hurricanes since records began in 1851.  Although Atlantic hurricane activity has been, overall, below average, since 2005, there have been many major hurricanes.  It is just that they have missed the U.S. mainland.  There were two very strong Category 2 hurricanes in 2008 that missed being Category 3 by just a few miles per hour.  And, of course, there was the Hurricane Sandy disaster in the New Jersey/New York area in 2012.  But Sandy was not a typical Category 1 storm.  It had a major hurricane caliber storm surge even though its winds had weakened just prior to landfall.  So this absence of big hurricanes is more of a statistical oddity than a change in hurricane patterns.   John Wheeler

Continental vs Maritime

The temperature this month in Fargo Moorhead has ranged from eleven below zero to 75 degrees for a range of 86 degrees in a month.  It’s hard to dress for that.  Fargo Moorhead is a perfect example of a continental climate in which the temperature can vary as air is blown around the region by various random weather patterns.  The greatest one-day range was 44 degrees on March 14 when it was 24 in the morning and 68 in the afternoon.  In contrast to this, the temperature this month in Key West, Florida, has ranged from 69 degrees to 84 degrees for a range of 15 degrees.  The greatest one-day range was 13 degrees from a low of 69 to a high of 82.  Key West weather is maritime.  The temperature of the surrounding ocean dominates the weather, keeping daily as well as day-to-day changes minimal.  Meteorologist John Wheeler

First Day of Nothing

Friday, March 20, is the Vernal Equinox.  Many people call this the first day of spring without thinking much about it.  Can spring start at the same moment everywhere on Earth?  Think about the equatorial regions.  At the equator, March and September are when the sun is directly overhead at noon.  It is hot all year at the equator.  But it is often hottest in March and September.  The weather is also rainiest and most humid along the equator around the time of the equinoxes because the direct sunlight causes peaks in evaporation rates to go along with the peaks in temperature.  Around the time of the Solstices in winter and summer, the middle of the world has its two slightly less hot seasons.  So there are relationships between the equinoxes and weather, but they are general and varied around the world.  The Vernal Equinox is really an astronomical moment with no immediate impacts on the weather anywhere.  This makes the “first day of spring” a fairly useless concept.    Meteorologist John Wheeler

Quiet Atlantic Hurricane Season

The Atlantic hurricane season officially comes to an end this weekend.  It was a good year for people and a bad year for hurricanes.  There were just eight named tropical systems (sustained winds of at least 39 mph) in the Atlantic Basin.  Six of them became hurricanes (sustained winds of at least 74 mph).  Only two became major hurricanes (Category 3,4,5 with winds of at least 111 mph).  The relative absence of hurricanes is due to the absence of good hurricane conditions.  Upper level winds over the tropical Atlantic were strong and kept clusters of thunderstorms from becoming organized.  Also, a lot of dry air was in place over the Atlantic. On the other side of the Americas, the eastern Pacific was the most active since 1992.  It is not uncommon for the two oceans to be out of sync like this.  The record year for Atlantic hurricanes, 2006, was a very quiet year across the Pacific.  Meteorologist John Wheeler