The latest USA Global Forecast model shows a landfalling tropical cyclone near the Tampa are next week. The forecast position is shown above. If this verifies (as either a tropical storm or a hurricane), it will add another layer to what already will be an interesting potlical event!
Category Archives: Extreme Weather
There have been a number of excellent posts on the family outbreak of tornadoes in the South last week. These include, for example,
I want to add a perspective to the discussion. First, as presented in my post
“Little difference was found in total tornado numbers between El Niño and La Niña events. However, significant differences were found in the number of violent tornadoes, and in large number tornado outbreaks. La Niña event years were found to have longer than average track lengths, more violent tornadoes, and a good probability of having an outbreak of 40 or more tornadoes.”
What occurs in La Niña springs in the United States is
- above average directional and wind speed vertical differences (helicity)
- above average convective available potential energy (CAPE).
Both helicity and CAPE are higher in La Niña springs because of the location of the polar jet stream further south of its normal position this time of the year. As defined by the National Weather Service the term helicity
“[a] property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity). Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m2/s2. “
With a more southerly polar jet stream, helicity values are larger in La Niña springs such as we are having this year.
The National Weather Service defines CAPE as
Convective Available Potential Energy [CAPE]. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg.
However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent. CAPE is represented on an upper air sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is warmer than the former. (This area often is called positive area.)”
With a more southerly polar jet stream (and resulting colder air aloft), CAPE values are larger in La Niña springs such as we are having this year.
As we wrote in
Knowles, J.B., and R.A. Pielke Sr., 2005: The Southern Oscillation and its effect on tornadic activity in the United States. Atmospheric Science Paper No. 755, Colorado State University, Fort Collins, CO 80523, 15 pp. (Originally prepared in 1993, published as a Atmospheric Science Paper in March 2005).
“Colder than normal temperatures in the western US/Canada along with warmer than normal temperatures in the southern United States during La Niña events would act to strengthen the interactions between warm and cold air in the midwest (Barnston et al., 1991). There would be an increase in the number of days favorable for tornadic development. This would act to increase the number of violent tornadoes that occur during the late spring-early summer. Large multiple tornado outbreaks are more likely for the same reason.”
We have modeled the formation of tornadoes, as reported in several of our papers which documents the important role of helicity and CAPE in their formation and intensity; e.g.
Finley, C.A., W.R. Cotton, and R.A. Pielke, 2001: Numerical simulation of tornadogenesis in a high-precipitation supercell: Part I: Storm evolution and transition into a bow echo. J. Atmos. Sci., 58, 1597-1629.
Pielke, R.A., J. Eastman, L.D. Grasso, J. Knowles, M. Nicholls, R.L. Walko, and X. Zeng, 1995: Atmospheric vortices. In: Fluid Vortices, S. Green, Editor, Kluwer Academic Publishers, The Netherlands, 617-650.
There have been a number of news articles that claim that a global average surface temperature trend (i.e. “global warming) explains the extreme cold weather and snow that has occurred recently; e.g. see
In this post I want to illustrate why it is the location of the westerlies that determine areas that have extreme cold weather and snowstorms.
The first image below presents the heights of the 500mb pressure surface and the temperatures at 850mb from the ECMWF analysis for January 28 2011 at noon GMT.
The 500mb level is used as it is about halfway through the depth of the atmosphere. The distances between the lines of equal height are proportional to the speed of the winds at that level. Since, in the Northern Hemisphere, winds blow counterclockwise around regions of lower heights, the wind field (not shown) is predominately westerly. This is why the middle and higher latitudes are often referred to as the “westerlies”. Winds at this spatial scale blow almost parallel to lines of constant height. When the height contours are close together, we refer to the higher winds that result as the “polar jet stream”.
Clearly evident in the example below is the progressively cooler 850mb temperatures and lower 500mb heights as one progresses to higher latitudes. Also, clearly seen are the regions of colder air (and corresponding lower heights) that extend towards lower latitudes. When these large equatorward excursions of the westerlies occur, extreme cold weather often happens. On the east side of these cold pockets, where there is a strong contrast with warmer air to the east, winter storms occur. If the temperatures are cold enough, precipitation can fall as heavy snow. These large excursions of the westerlies explains why there have been several extreme snowstorms in the eastern USA and western Europe in recent months.
To illustrate the dynamic character of the westerlies, I have presented below the ECMWF 500mb height and 850mb temperature forecast for next Friday [February 7 2011]. Compare the above figure with the one below. Note, for example, the large excursion of cold air and, therefore, westerlies southward to over the central USA. If this forecast verifies, it will be an extreme cold outbreak with considerable snow (and ice storms) on the southeast flank of this cold region.
It is not scientifically accurate to attribute “global warming” of a few tenths of a degree to explain these extreme weather events.
Moreover, in the latest measurements, the lower tropospheric temperatures are actually cooler than the long-term average! [e.g. see
For other excellent discussions of the recent extreme winter weather, see the posts by Joe Daleo; e.g.
As presented numerous times on my weblog, it is regional atmospheric circulations that matter in terms of the weather we experience. I have an excellent example of this provided by Glenn James Sr. Weather Specialist at the The Pacific Disaster Center in Hawaii.
With his permission, I have reproduced his e-mail and photos below.
A very cold pool of air moved over the eastern side of the Hawaiian Islands overnight Tuesday…into early Wednesday morning.
It brought snow to the tall summits on the Big Island <http://cfht.hawaii.edu/webcams/gemdome/gemdome.jpg>, both of which are a bit under 14,000 feet…which isn’t unusual during the winter months.
The interest that I want to share with you however…is about the snow that fell here on Maui overnight. The Haleakala Crater is near 10,000 feet, and rarely has any of this white stuff falling.
Here’s a picture of this unusual occurrence <http://lh4.ggpht.com/_DDQJHcAYzgA/TTcgguyig_I/AAAAAAAADwc/jqyqwqe5NMg/s640/P1010164.JPG>, shot early Wednesday morning.
Meanwhile, a deep 945 millibar storm low generated a large west-northwest swell recently, which is about to begin pounding Hawaii’s north and west facing shores. We have a touch of winter here in the islands…at least once in a while.
Aloha, Glenn James
There is an interesting possibility of snow on Friday in Captown on Friday; see
This article by Angelique Serrao starts with
“For those who thought they were cold at the Kick-Off Celebration Concert last week, you haven’t felt anything yet.
While tourists from those colder climates up north might laugh at squeamish Joburgers, even they are advised to pack a warm jacket in the coming week.
Temperatures are set to plummet from a high of 19degC yesterday to a maximum of 10C today.
The SA Weather Service has forecast that the rest of the week won’t get much warmer and those heading to Pretoria to watch the South Africa versus Uruguay match tomorrow might be faced with a minimum of -3degC.
The SA Weather Service has warned of very cold conditions, although no snow is forecast in Joburg. Snow could fall in the Free State and Northern Cape.”
This cold and perhaps wet (snowy?) weather is a good mix of weather (climate) and soccer!
Readers who want to view the official forecasts for South Africa can obtain from http://www.weathersa.co.za/web/.