Jeppesen Pilot Report

Summer Weather Hazards

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Summer may be defined by some as the lazy hazy days of summer, but pilots know that summer can provide some very dangerous weather hazards. Summer is thunderstorm season, and as we all know, thunderstorms pose the greatest weather hazard to aviation, as they can contain nearly all serious weather hazards in one dark cloud. Severe icing and turbulence, microbursts, wind shear, IMC, hail and lightning can all be experienced in and near thunderstorms. Most pilots have learned to simply avoid thunderstorms, but when planning a longer flight, you don’t always know what will form a few hours from your departure.

If you remember only one thing from this article, it is this: Three ingredients are required for thunderstorms to form:

1.)    Lift
2.)    Moisture
3.)    Atmospheric Instability

Thunderstorm development requires that moist air be lifted to saturation, and then continue to rise. This implies we need a lifting mechanism, which can come from surface heating (thermals), orographic flow over mountains, or a convergence line (includes fronts, surface troughs, sea breeze fronts, intersecting outflow boundaries and dry lines). The lifting needs to occur in a moist atmosphere so that the air being lifted condenses into a cloud. Thunderstorms also need atmospheric instability, which means once the air has been lifted and condenses, the air continues to rise. Atmospheric stability is determined by the rate of temperature change with altitude, also known as the lapse rate. The more the temperature decreases with altitude, the more unstable the atmosphere is.

Spring may be the prime time for severe thunderstorms, but summer has more numerous thunderstorms across the entire country. Summer sees the predominance of air mass thunderstorms, which are caused by intense surface heating. Air mass storms develop during maximum heating in the afternoon, and diminish once the sun sets. Air mass thunderstorms tend to move more slowly, generally in the 5-15 mph range while spring thunderstorms tend to develop along fronts and thus move at a much faster pace. Severe thunderstorms can still form during summer months, usually in the more northern latitudes where better vertical wind shear exists.

There are various stages of thunderstorm development. The image below depicts the different stages, from the dry thermal stage to the supercell stage. In the thermal stage there is not enough moisture for cloud development, but pockets of air are still rising. Anyone who has flown over the Desert Southwest in summer months knows that thermals can still provide some very bumpy air. Once there is rising air and sufficient moisture, we get a cumulus cloud. In developing cumulus clouds, all the air in the cloud is rising. Flying in or directly above these clouds can also provide some good bumps. If there is atmospheric instability, the cumulus cloud will continue to develop into a cumulonimbus cloud, which contains many more aviation hazards, such as severe turbulence and icing, lightning, and severe up and down drafts as well as microbursts and wind shear. If a cumulonimbus cloud exists in a very unstable environment and there is sufficient vertical wind shear, the cloud can become a super cell which normally means a severe thunderstorm where hail greater than 1 inch can exist and surface winds above 58 mph as well as funnel clouds and tornadoes.

  

        Dry Thermal Stage

 

            Cumulus Stage

 

                 Cumulonimbus Stage

 

                Super Cell Stage

The biggest threat to aviation from thunderstorms is wind shear and microbursts during take off and landing. We tend to learn from disasters in aviation, and when Pan Am Flight 759 crashed on takeoff from New Orleans on July 9, 1982 while flying through a thunderstorm,  research began on the hazards, detection and avoidance of microbursts. Research was intensified after Delta flight 191 crashed on approach to Dallas – Ft. Worth on August 2, 1985. Research led to the development and deployment of the LLWAS-2 system and the Terminal Doppler Weather Radar system. These systems enhanced the ability to detect and alert on terminal wind shear and microbursts.

There are additional hazards related to thunderstorms as well. Lightning can impact electrical system and damage the aircraft skin. Pilots should be aware that 75% of all lightning occurs above the clouds base, so within the cloud, from cloud to cloud, or from cloud to air. Only 25% of lightning is from the cloud to the ground. Thus, the hazard from lightning is more common when flying above cumulonimbus cloud base versus below cloud base. Avoiding thunderstorms is usually the best course of action.

Some useful information to detect and predict where thunderstorms will be is the NEXRAD (WSR-88D) Doppler radar network, which has recently been upgraded to a Dual Polarization radar network. NEXRAD scans a large volume of air surrounding the radar site every 6-10 minutes and can provide up to date information on position, intensity, height and movement of storms. NEXRAD should however only be used for strategic navigation, even if that information is available in the cockpit. Only onboard radar should be used for tactical navigation around and through thunderstorms. For prediction of thunderstorm areas, some new products can be quite useful, such as the Collaborative Convective Forecast Product (CCFP), the Convective Outlook, and the new Corridor Integrated Weather System (CWIS) that provides a forecast of NEXRAD activity using current NEXRAD and high resolution numerical model forecasts. The trusty Skew-T/Log P diagram is another good way to view the stability of the atmosphere near the radiosonde balloon site. The Skew-T/Log P diagram usually contains computed stability indices such as the Lifted Index, K Index, and CAPE that help determine the probability and severity of  thunderstorms that may develop. A good site to view Skew-T/Log P diagrams is: http://www.rap.ucar.edu/weather/upper/

The Convective Outlook is a good tool to determine where general thunderstorms may develop as well as where severe thunderstorms are more likely. The green shaded areas depict where general thunderstorms are possible, and yellow shaded areas depict where severe thunderstorms are possible.

Convective Outlook product from Storm Prediction Center in Norman, OK

In the Skew-T/Log P diagram below for Dallas, TX  you can see a moist and unstable sounding with a very low lifted index of -10. On this day, there were severe thunderstorms, large hail and tornadoes in the Dallas metroplex area. The Skew-T/Log P diagram depicts a trace of temperature (red) and dew point temperature (green) with altitude, as well as winds (barbs on far right side). Stability indexes are located along the top of the diagram. This diagram is a thermodynamic diagram that can calculate the amount of energy in different layers in the atmosphere. I believe Skew-T/Log P diagrams are a very valuable tool for pilots to use when flying in and near areas where radiosonde balloons are launched.

       Skew-T/Log P diagram for Dallas, TX April 15, 2012

 

In the U.S. we also experience a monsoon. Not always known, but the North American Monsoon (NAM) begins in early July and persists into early to mid September and affects mostly the Inter-Mountain West areas. Moisture from the Gulf of Mexico and the Gulf of California are transported north across Mexico and into the western U.S. The beginning of the NAM is defined when the average daily surface dew point temperature in Tucson, AZ exceeds 54 degrees F for 3 consecutive days. The impact from the NAM is typically increased thunderstorms and precipitation across portions of AZ, NM, NV, UT, ID, CO, WY and MT. Areas of the Desert Southwest receive up to 75% of their annual precipitation during this period.

Illustration of a North American Monsoon upper level pattern (from Mike Baker of NWS Denver WFO)

Summer is the season when we don’t expect much icing as freezing levels are higher, but  icing is still possible in clouds above the freezing level and severe icing is possible in and near thunderstorms. Turbulence outside of thermals and convective activity is limited since upper level winds are generally light. Smoke from wildland fires can cause some restrictions to visibility generally below FL120 and TFR’s become more numerous over fire fighting areas. Coastal stratus along the West Coast is predominant, so IMC exists during much of the night and early morning hours along the immediate coast.

Coastal Stratus along the CA coast penetrating into inland valleys

The best way to prevent hazardous weather encounters is to obtain a detailed weather briefing before each flight, and if you have any questions, get them answered by a certified briefer or professional meteorologist.