The Jet Stream - Upper air flow and Severe
Understand that without upper air winds, a thunderstorm would grow vertically,
the rain would fall down directly into the updraft, inhibiting the updraft or
destroying it. So upper air winds are vital to long lived severe
thunderstorms and tornadoes. This is why checking upper air maps is
an important factor for chasers to watch carefully when chasing. Most of
this data is gathered by weather balloons, and they are sent up only twice
daily, 12 hours apart, at stations spread all over the US, many with hundreds of
miles between them. This is one of the aspects about upper air that chasers and
forecasters grapple with, the limited data due to times available for upper air
|Continuing since 1904 balloons are still sent up twice daily to
gather needed information. Here are two photos from NOAA and the NWS
showing the simple technology used to send up the data gathering
instruments into air to get this data back to official at local offices.
radiosonde balloon from the Sterling WSFO facility.
Image ID: noaa6129, NOAA's Online World Collection
Weather Service Forecast Office Washington, D.C.
Preparing to launch America's first "ballon-sonde." Since this first
launch, literally millions of weather balloons have been launched by the
National Weather Service and its predecessor organization. In: "The
Principles of Aerography" by Alexander McAdie, 1917. Page 12.
Image ID: wea01136, Historic NWS Collection
Location: St. Louis, Missouri
Photo Date: 1904 September 15
CREDIT: National Oceanic & Atmospheric Adminstration (NOAA)
This infrequent gathering of information is part of the reason why predicting
exactly where the storms will have the greatest potential for severe weather and
tornadoes is so difficult. I have watched jet stream predictions change 100
miles or more in the 12 hours between balloon launches. Just read the jet
stream prediction maps and then when that time actually arrives check to see
where they actually are. You will find sometimes they are off by 100 miles,
while other times they are dead on. Upper air winds can be stronger or
weaker than forecast, move further north or south, drastically changing the
storms life, intensity, and location.
On a good storm day, as storms build you
will see thunderstorms leaning as they build, much like a small tree bending in
a strong wind, being pushed about by the differing wind directions.
this free video Clip) This wind shear can be seen with the eye just by
watching the storms grow sideways. This leaning allows the falling rain to fall
away from the updraft, and not in it, thereby allowing the updraft to continue
and even strengthen. On some days the wind shear is so strong you will see
storms leaning so much that their tops get blown off and then they are seen
racing away from the rest of the cloud. As the storms strengthen they over
come this shear with enough force to keep building and stay together with the
end result of strong shear within the storm, and often spin or storm rotation.
A spinning (rotating) thunderstorm is called a
mesocyclone when it meets certain criteria. Properly used, mesocyclone is a
radar term; it is defined as a rotation signature appearing on Doppler radar
that meets specific criteria for magnitude, vertical depth, and duration. But
with experience you will become able to pick out most mesocyclone storms with
your eye as they are usually very evident to the seasoned chaser. Experience
will help you identify them accurately.
Next, if a jet stream moves over the top of a building storm and pulls the
top of the building storm away from the main updraft, it kicks the storms into
high gear causing some of the most violent storms and tornadoes. Rather than the
top of the storm flattening out you see it being pulled one direction,
stretching the top out and elongating it sometimes for many miles.
(Don't try this at home, it makes a real mess and
can be deadly.)
EXAMPLE: If there was a small fire in your fireplace and then something covers
up the chimney. What happens? The smoke backs down the chimney
into the house, it has no place to go. Same with storms. They build to a point
and then hit a ceiling of air that stops them from growing.
Now if that same cover was removed from your chimney and what happens to the
smoke? It can rise up and out freely again, just as strong upper air winds can
allow storms to keep rising and building.
Now one more example. With nothing covering your fireplace chimney and
the flute is wide open, and it is a windy day with winds of 40 or 50 miles per
hour racing over the top of your chimney. What would happen then? Well if you
have ever been by a fireplace when it is really windy you will see the fire
burns hotter unless you restrict the updraft in the fireplace by closing down
the flute. A powerful wind over your chimney makes the updraft stronger.
The same with a strong jet stream placed over a storm, it speeds the updraft
So, a well positioned jet, will vent the storm, pulling the rising air up
faster and away from the main body of the storm, letting the updraft strengthen.
This kind of venting on storms is called divergent flow aloft, and is a main
contributor to violent storms. I have seen storms like this and they act like
giant vacuum cleaners, sucking all the clouds and moisture up and into them. I
have seen clouds racing into the storm only to seem to disappear into it, but in
reality are racing upward in the strong updraft of the storm.
One more area of added potential for violent storms is an area of maximum
winds within the jet stream, called the jet Streak or jet max. This area in
the jet has a big impact on storm growth and development. The jet streak
moves along the jet stream at a slower speed than the wind in the maximum area.
As jet stream air enters and leaves this area of high winds, the accelerations
act to induced upper level divergence in the forward left quadrant (I), thus
promoting convective activity. Rapid thunderstorm growth can be
anticipated as the jet max approaches the warm sector of the surface low
pressure area. (Barnes and Newton, 1983)
So keep an eye on upper air maps, where is the jet stream forecast to be, and
where is the low pressure in relation to this. Look for an area of warm air that
is being pulled into the low pressure area by the jet, and you may have the
target location of the day.
(Much of this information was learned from
Thomas P. Grazulis, "A chronology and Analysis of Events,
Significant Tornadoes 1680-1991". (Available at most
We learn through reading.