Summer or Fall?

The presence of cool weather this past week had most of us thinking about fall, with varying degrees of emotion.  Summer, obviously, is not over.  That being said, the average daily high temperature in August falls from around 83 degrees on August 1 to around 77 degrees August 31.  However, daily record high temperatures remain in the upper 90s and lower 100s.

What happens in these final weeks of summer is not that it gradually gets cooler.  Rather, the probability of there being a few days of fall-like weather increases through the month.  The hottest day in August might well be August 31 in any given year, but the probability of a day in the 90s is significantly higher in early August than in late August.

Likewise, a crisp 40 degree temperature is possible any August morning, but becomes increasingly likely later in the month.  So although summer weather is far from over, we should expect less and less of it.

Meteorologist John Wheeler

 

El Nino Primer

The recent surge of press on the impending El Nino has created as much confusion as awareness.  To begin with, the terrible winter of 1996-97 with all the snow followed by the big flood was not an El Nino winter.  The following winter of 1997-98, there was a strong El Nino and our region experienced warmer than average temperatures.

The winter of 1965-66 was also a strong El Nino winter, but the El Nino effect was overblown by other influences, leading to a very cold Northern Plains winter ending with the famous Blizzard of ’66 in March.

El Nino is probably the best indicator we have for predicting the winter season, but it is far from perfect.  There are many other large-scale weather systems which can factor in, many of which are unpredictable very far in advance.  So while it is fair to say the odds favor warmer than average temperatures this winter, perhaps even by 2-1, this bet is not a lock

Meteorologist John Wheeler

El Nino Uncertainty

There are a few things about El Nino that are worth mentioning, given all the attention it has gotten lately.  It is only one of many influences on our weather.  Although the classic El Nino signal is statistically tied to a likelihood of a warmer than average winter, it is not a guarantee.

It is now looking likely that this winter’s El Nino will be a very strong one.  This adds an element of unpredictability.  There have been very few documented “very strong” El Ninos.  Therefore, this El Nino does signal a higher probability of milder winter temperatures, but the unpredictability factor tempers those odds.

Also, there is not a good statistical signal between El Nino and winter precipitation.  Again, the expected strong El Nino coming up adds even more unpredictability to the winter snow forecast.

This El Nino is generating a great deal of press, much of which attempts to simplify the outlook.  But weather is rarely so simple.

Meteorologist John Wheeler

 

 

 

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More the Hail

The four and a half inch diameter stone that fell near Roseau, MN, last week has prompted several questions about how large hail stones can get.  The largest hail stone officially accepted by the National Weather Service is the eight inch stone that fell near Vivian, South Dakota, in 2010.  There are reports of larger stones, but they remain unconfirmed.

There are only three accounts in the history if the United States of people killed by hail.  In 1930, a farmer near Lubbock, Texas was struck and killed.  In 1979, hail killed a baby in Ft. Collins, Colorado.  And a boater was killed by a large hail stone near Fort Worth, Texas in 2000.

Perhaps the costliest hail storm hit Sydney, Australia, in 1999 when hail the size of tennis balls fell for almost an hour and caused more than $3 billion (U.S. adjusted for inflation) in damage to structures.  There is a report of a hail storm in Bangladesh in 1986 that killed 92 people.

Meteorologist John Wheeler

Why the Hail?

Just like some winters there are a lot of blizzards and other winters the snow falls with less wind, summer weather patterns can produce similar storm situations over time.  This summer, there have been several cases of thunderstorms producing large hail and damaging wind.

During the summers of 2013 and 2014, there was actually not a lot of severe weather at all.  The summer of 2012 was very dry and so there were very few thunderstorms at all.  The summer of 2011 was notorious for storms with powerful straight line winds all across the Northern Plains.  Back in 2010, thunderstorms tended to produce tornadoes and there were several occasions with a number of powerful tornadoes.

People tend to think of thunderstorms as individual entities but they are actually a product of their environment.  It is not unusual for a particular environment to repeat itself (approximately) for as long as the weather pattern remains essentially unchanged.

Meteorologist John Wheeler

 

What the Hail?

 

Last Wednesday afternoon and evening, thunderstorms dropped very large hail on several locations across northwestern Minnesota, including one hail stone near Roseau which measured four and a half inches in diameter.  There have been several rounds of storms with large hail and damaging winds but no tornadoes during July and August this year.

During each of these rounds, the atmosphere has been extremely unstable (hot and humid below while cooler aloft) with lots of available moisture.  These conditions have allowed thunderstorms to develop intensely strong, rotating updrafts capable of supporting large hail stones.

Very weak winds in the lowest part of the atmosphere have made it unlikely for tornadoes to develop, however.  But the strong updrafts have led to powerful downdrafts capable of damaging wind gusts.

Meteorologist John Wheeler

Compressional Heating

The hot weather of the past few days has been caused in part by a process in the atmosphere known as compressional heating.  When the high pressure forms in the upper atmosphere, this air aloft warms up and expands.  It expands in all directions including downward and this downward moving air heats the air near the surface by compressing it.

Large high pressure systems in the upper levels of the atmosphere are known for having large areas of generally downward moving air which is further compressed by the increasing air pressure of the lower atmosphere.  In this way, air can be heated in place so it is not necessary for there to be a significant wind to cause significant warming.

Meteorologists refer to this as dynamic warming, as opposed to wind blowing in warmer air, which is known as warm advection.

Meteorologist John Wheeler

Big Storms? Maybe But Not Necessarily

When this heat wave breaks, there are going to be some massive thunderstorms, right?  Not necessarily.  Although there is potential energy present in hot and humid air, these are just two of many elements that are required for severe thunderstorms.

When air is hot and humid, there is certainly more thermodynamic energy available once that air rises enough to cool and condense.  However, there needs to be enough wind aloft and it helps if the wind aloft is blowing across the wind near the ground.  This keeps the downdrafts of rain from snuffing out the air rising into the storm.

Such wind shear also encourages rotation which can add to the wind velocity of the updraft and make the storm more dangerous.  Other upper atmospheric conditions, such as temperature and humidity, can have a lot to do with how strong storms can be.

So the presence of hot and humid weather is not a signal to head for the basement when a storm moves in.

Meteorologist John Wheeler

 

Other Worldly Lightning

Around the world, lightning happens about three million times a year, or about 30 times a second.  Not all lightning strikes the ground.  Lightning can also occur within a cloud or go between clouds.

Most of us associate lightning with rain storms because that is where it is most commonly seen.  However, lightning can also happen in volcano eruptions.  In arid climates, lightning often happens in storms producing little or no rain.  In winter, lightning occasionally happens when it is snowing.

We usually don’t think about weather on other planets, but lightning has been observed in the atmospheres of Mars, Venus, Jupiter, and Saturn.

In all of these cases, lightning is a big spark caused by charge separation due to static electricity.  The spark heats the air it passes through, causing the air to suddenly expand, which causes a shock wave we hear as thunder.

Meteorologist John Wheeler

Hurricane Outlook 2015

NOAA’s Climate Prediction Center has revised its 2015 Atlantic Basin Hurricane Season Outlook downward.  The developing El Nino in the Pacific Ocean is causing strengthening upper level winds, a general basin-wide sinking motion, and cooler than average sea surface temperatures over the tropical Atlantic region which are all unfavorable for hurricane development.

The latest forecast, issued last week, calls for a 90 per cent chance of a below average season.  More specifically, the outlook calls for a 70 per cent chance of 6-11 named storms (tropical storm or hurricane), from 1-4 actual hurricanes and either none or one major (Category 3 or higher) hurricane.  The outlook says nothing about how many of these tropical systems will strike land as opposed to remaining at sea.

When an El Nino is building in the Pacific, it is typical for the Atlantic hurricane season to be relatively calm.

Meteorologist John Wheeler