DataStreme Activity 11B:

TORNADOES

Do Now:

  1. Print this file.
  2. Print the Wednesday Image 1, Image 2, and Image 3 Files. Images 1 and 2 are best viewed on screen or in color copies when doing this activity.
  3. Print (when available) the Thursday, 29 November 2001, Daily Summary File.

Notice: Please download and print the Fall 2001 DataStreme Participant Evaluation form in the "Image 3" position with today's Activity. Complete the questionnaire and give it to your LIT leader at the final course meeting.

To Do Activity:

  1. Read Chapter 11 in STUDY GUIDE, Part A: Narrative.
  2. Go to STUDY GUIDE, Part B: Applications. Start Activity 11B.
  3. Return here (Wednesday Activity B File) when told to do so.

Go To STUDY GUIDE - Activity 11B Now


WELCOME BACK: Procedure continued from STUDY GUIDE.

Although tornadoes can occur in any month, historically the greatest annual activity occurs from April to June, when weather systems are particularly vigorous. For details on this season's tornado activity, see the Wednesday, 28 November 2001, Supplemental Information file. This season has been near average in total tornado numbers although the spring months were somewhat below normal while this fall has been unusually active. September was slightly above normal, October about twice the average, and November to date almost three times the average for the month. Fortunately, tornado deaths have been fewer than normal this year. Although again, November has recorded 13 deaths when none usually occur during this month.

One earlier, deadly tornado occurred on June 18, 2001, near Siren, in northwestern Wisconsin. At 8:06 PM CDT an F3 (on the Fujita tornado damage scale) tornado touched down 2 miles east-northeast of Grantsburg and continued to the east through the towns of Alpha and Falun before hitting Siren at 8:20. The tornado continued eastward for another 14 miles before dissipating. All totalled, there were two deaths immediately with an additional one later, 6 blocks of the town were devastated, and 16 other people were injured. The tornado travelled 27 miles with a damage path from one-eighth to one-half mile wide. Alerted by radar observations, NWS warnings were issued 35 minutes prior to the tornado's touchdown.

  1. Image 1 includes portions of the Duluth, Minnesota, NWS office Doppler radar display at 0130Z (8:30 PM CDT) with storm reflectivity (precipitation) on the upper left and velocity (radial motion) on the lower right. North is toward the top in each view. The location of the town of Siren is shown by a bold arrow. The dotted area in the upper left image shows the area covered in the lower right image. (A link to the complete report of damage pictures, radar images and discussion is given below.)

    The upper left reflectivity image shows the large "blob" in the lower center of a supercell thunderstorm with a "hook" echo, often associated with tornadic activity, as a lighter curl (oranges and reds on-screen) to the south side of the cell. The location of the hook echo is to the [(southwest) (northeast)] side of the thunderstorm cell "blob". Another cell to the north (middle of view) is also developing this circulation feature.

  2. The most intense precipitation (as denoted by dark red on-screen) within the cell inner area is located just to the [(south) (north)] of the tornado position within the hook curvature. This strong radar echo may also include returns from hail.

  3. The velocity image to the lower right shows two circles (white on-screen) just north and northeast of the town of Siren. These circles are a computer evaluation of the Doppler wind speeds toward and away from the radar site that are judged to be from a mesocyclone, the precursor rotation within a supercell often leading to a tornado. The radar is located to the north of the image. Reds indicate motion away from the radar and blue towards.

    On the west side of the circle nearest Siren (dark reds on-screen), draw an arrow from north to south for wind speeds away from the radar. On the east side (blue on-screen) draw an arrow from south to north for wind speeds towards the radar. Recall from Activity 7B, this radial motion pattern [(would) (would not)] be consistent with a mesocyclone that can produce a tornado. (You might check the other circles for additional rotational signatures.)

  4. The "F" designations for tornadoes refer to levels of destruction based on the Fujita Tornado Intensity Scale, (see Table 11.3 on page 216 of the DataStreme Study Guide, Part A: Narrative). According to the scale, an F3 category tornado creates damage produced by wind speeds estimated to be in the range of _________ - _________ kilometers per hour or _________ - _________ miles per hour.

  5. Image 2 is a Landsat 7 satellite image of the region near Siren the day following the tornado, June 19, 2001. The image is about 50 miles east-west by 20 miles north-south centered on Siren (labelled in the image). The visible reflectivity of the land has been altered from the surroundings by the tornado's passage. The path of the tornado as shown on the satellite view is generally [(lighter) (darker)] than the surroundings. This may be because tree damage along the path removed the vertical orientation that keeps light from being reflected from the surface.

    The image shows that the tornado traveled generally from [(west to east) (north to south)]. Most commonly (but not always!), tornadoes move from southwest to northeast. This is the general direction of midtropospheric winds that steer thunderstorms associated with cold fronts and squall lines.

Meteorologists rely extensively on radar information such as the supercell thunderstorm structure and wind patterns to provide tornado warnings for the public to help save lives and minimize property damage.


The Siren, WI, tornado information can be found at: http://www.crh.noaa.gov/dlh/siren/siren.htm. The most recent listing of deadly tornado occurrences compiled by the NWS Storm Prediction Center can be found at: http://www.spc.noaa.gov/climo/torn/2001deadlytorn.html. Preliminary reports including damage pictures from this past week's tornadoes can be found at:
http://www.srh.noaa.gov/meg/11242001/NorthwestMississippiTornado.htm and
http://www.srh.noaa.gov/lzk/html/svr1101.htm.

Additional reports of other tornado episodes containing NWS radar imagery can be found at the following links:

Your local NWS office homepages may have links to notable severe weather episodes in your area. Finally, for an account of the historic Super Tornado Outbreak of 1974, see: http://www.publicaffairs.noaa.gov/storms/.


Faxing Instructions:

After completing this week's applications, fax the following pages to your LIT mentor by Monday, 3 December 2001:


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