Visual clues as to the strength of a thunderstorm's updraft can help the weather spotter be able to identify those storms which are more likely to produce severe weather.
If the storm you are watching has a vigorous updraft, a small portion of the updraft may rise higher than the surrounding anvil, which is the flattened out cloud formation at the top of the storm. This will form a bubble of cloud sticking up above the rest of the anvil. The bubble is called an overshooting top. Most thunderstorms will have small short-Lived overshooting tops. However if you observe a storm with a large dome-Like overshooting top that lasts for a fairly long time, more than 5 or 10 minutes, chances are good that the storm/s updraft is strong enough and persistent enough to produce severe weather.
If the anvil is thick, smooth edged, and puffy then the storm probably has a strong updraft and is a good candidate to produce severe weather. If the anvil is thin, fuzzy, and wispy then the updraft probably is not as strong. If the anvil is large and seems to be streaming away from the storm in one preferred direction, then there are probably strong upper level winds in the storm/s environment, which is a condition that is favorable for the production of large hail and damaging winds.
These are just a few clues to watch for in the upper portion of the thunderstorm cloud.
Did you know that in order for a thunderstorm to produce hail the size of a dime, the approximate upward wind speed needed is 37 mph? golfball size hail requires an updraft of approximately 56 mph. Three inch diameter hail, just bigger than a baseball, has upward wind speeds of 100 mph. If the air is rushing upward at 50 to 100 mph, you had also better be on the lookout for possible tornadoes.
This public information statement will be the last one for this year/s severe weather awareness week, which has been in conjunction with the national emergency preparedness week.