Monday, May 16, 2011

Six For Safety: Thunderstorms

Try your knowledge of aviation safety. This month: Thunderstorms.

As noted in the "Illusions of Flying" (March), the accident rate for inadvertent VFR flights into IMC is very high. Equally concerning, especially in terms of survivability, are encounters with thunderstorms. There are several factors that need to be considered when planning a flight in and around convective activity. For this month's safety quiz we look at some of those factors in the questions below. Answers and discussions can be found at the end of the quiz.

1) It's the weekend and you are looking forward to getting away with friends and family. While reviewing “all available information” you take notice of the Day 1 Convective Outlook chart. Based on the image below how might the Categorical Outlook image influence your flight plans?

a) A trip to Hersey, PA might be out of the question given the slight chance for well organized severe storms.
b) A trip to Cape May or maybe Cape Cod might be a reasonable alternative even though general thunderstorms are forecast.
c) This is a no-go scenario.
d) Both a and b.

2) Given the slight chance for well organized severe storms in PA you wonder about the likelihood of severe weather. Using the images below how might the probability of severe weather (by type) influence your flight plans? 


Image Descriptions:

  • The left graphic shows a 5% chance (brown area) of a tornado within 25 miles of any given point. 
  • The middle graphic shows a 15% chance (yellow area) of damaging thunderstorm winds or wind gusts of 50 knots or more within 25 miles of any given point.
  • The right graphic shows a 5% (brown area) of one inch diameter hail or larger within 25 miles of any given point. 
Answers for question 2:
a) A trip to Hersey PA is not advisable but you might consider other factors.
b) A trip to Cape May is not out of the question but does pose a risk.
c) A weekend in Cape Cod is probably the way to go.
d) All of the above are reasonable conclusions.

3) Your passengers are really eager to spend the day at Hersey Park and not exactly thrilled with the idea of going to Cape Cod. Succumbing to their pressure you consider the Area Forecast and TAF.

KMDT 051152Z 20010KT P6SM -SHRA BKN050 OVC100
FM051800 25010KT 2SM RA BR OVC040CB
FM042300 31015KT P6SM OVC050
Given the above, which of the answers below best represents the forecasted conditions?
a) The ceilings are relatively high providing plenty of clearance to avoid any storms.
b) The storms are not expected until suppertime... long after arrival.
c) It's reasonable to expect a severe thunderstorm along the route of flight.

4) Resigning yourself not to disappoint anyone you take off for Hersey, PA. However, once airborne you decide to dial-up SOLBERG HIWAS upon which you hear the following:


(Graphical representation of SIGMET)

True or False: The best course of action is to return to your departing airport and terminate the flight.

5) What's the significance of 20 to 25 miles (or more) in relationship to thunderstorms?

a) Turbulence is known to extend this far laterally from a thunderstorm.
b) It's not unheard of to encounter hail this far from a thunderstorm.
c) Both a and b.
Answers and Discussion:
1) The best answer is d). The image for question 1 was taken from the Storm Prediction Center's Convective Outlook (AC) Day-1 site and represents a categorical depiction of both general thunderstorms (green area) and severe thunderstorms (further broken down by risk category - yellow, red and pink - that define coverage and intensity). As displayed on the image, SPC predicts a 10% or greater chance for general thunderstorm activity along the east coast. Over central PA, we also see (in yellow) a slight risk for severe thunderstorms meaning well organized severe storms but in small numbers within a 25 mile point. In this scenario that point could be stationary, such as Hersey Park, or any point along the intended flight path that penetrates this area. One interesting consideration is that SPC does not provide a probability for the slight risk of severe storms. Instead, you must look at the severe weather potential by threat - wind, tornado, hail (see question 2). The categorical outlook is not necessarily intuitive at first glace. To simplify, the green category represents a general risk of thunderstorms. For example, what you might expect towards the end of hot afternoon or isolated storms associated a warm, slowly moving air mass. The severe weather categories are yellow (slight risk), red (moderate risk) or pink (high risk). For example, severe storms associated with a powerful cold front. There is one caveat... if the green category displays within it's boundaries the words "See Text", this implies a severe storm risk exists; however, the risk is not expected to approach or meet the categorical criteria for "slight" in terms of coverage or intensity. For those keeping count, that makes for five risk categories in all.

2) The best answer is d). Answer d represents the notion of balance between risk and reward. Keep in mind that convective outlooks are forecasts by risk category and threat potential and do not forecast absolute likelihood. For the scenario presented in question 2, a chance encounter of an actual weather threat within a 25 mile radius of the plane's location along its flight path at any given moment is relatively low in terms of probability. However, if severe weather were encountered the results could be devastating. Therefore, other factors must be considered. What if the prevailing forecast is VFR - CAVU? Given that scenario a VFR pilot might use a see and avoid approach and, if necessary, land until the threat passes. What if the ceiling is 5,000 feet and broken? Such conditions might raise the risk levels considerably in terms of safe flight operation given the difficulty of seeing and avoiding severe weather. And what if the visibility is considerably hazy? Five miles visibility is VFR but you may never see that rain shaft ahead until it's too late and fly right underneath a storm cell. These are all factors to consider regardless of whether general or severe storms are forecast.

The SPC Convective Outlook is a very technical but very useful forecast for flight planning purposes. Consider reviewing with an FSS specialist while obtaining your next weather briefing. For more detailed information, click on the following links:

3) The best answer is c). Obviously, ceiling height is not part of the thunderstorm avoidance playbook while conducting VFR flight operations. In terms of timing, storms are forecast for the early afternoon (converting to local time of course). But given the forecast conditions (particularly CB tops) in combination with what we learned from the convective outlook, a discerning pilot would anticipate severe weather and seriously consider other factors such as visibility, ceiling, frontal movements, etc. And of equal importance, what "out" options are available to you? That is, if a storm pops up along your flight path or directly ahead of you: can you navigate safely around the storm; do you have enough fuel to divert; is there a field or airport nearby to land at?" Of course, a good pilot should ask these kinds of questions on the ground before even taking off.

4) The best answer is True. To continue traveling to the destination would be foolish and very likely disastrous, if not fatal. Fortunately, there is an easy out - just return to the departing airport.

5) The answer is c). Depending on the severity of the storm in combination with other severe weather phenomena the best advice is to give storms plenty of lateral clearance due to the potential threat of severe turbulence, wind shear, IFR conditions and hail. The general rule of thumb is to avoid severe storms by at least 20 miles; however, it is impossible to predict the lateral extent a storm might have. At the same time, while landing or departing, keep in mind the potential for wind gusts, wind shear and downdrafts. If you are approaching the field or in the pattern and see threatening cells and/or rain shafts along the flight path or around the field then consider, if possible, a go around or divert to another airport entirely until the cells move away. A couple of other reminders: don't forget about severe icing if you happen to fly into one of these monsters; don't forget to keep an eye out for the anvil and the associated threat of hail.

For further reading on this topic, see AIM Chapter 7 - Safety of Flight, and in particular 7-1-20 Thunderstorm Flying.

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