Professional Pilot, September 2016

the cresting wave can destabilize the air causing it to begin its ascent In a typical summer environment this uplift can push the air upward to a point where it becomes unstable and is able to rise in free convection The creation of these convective updrafts is the first step in the formation of thunderstorms The motion of the air mass boundary serves as a fueling mechanism ensuring a steady supply of displaced warm and humid surface air all along the front This displacement is further enhanced by the outflow from the subsequent convective downdrafts as the cells mature Squall lines may also form along the outflow boundary of a decaying thunderstorm or even from the collective outflow of an MCC or other MCS These squall lines are often less extensive but can be just as severe as frontal squall lines They also often take on a characteristic bow echo on radar scopes Squall lines may be active for many hours and may even continue through the night restrengthening with the next days heating They may also move at alarming speeds some clocked at nearly 70 kts and the largest of the cells they produce may create a wall of convection extending well above FL400 Because of the continual displacement mechanism they can easily kick off convection in places where otherwise the ambient dynamics of the atmosphere might not support the formation of a storm meaning a mature cell can crop up nearly anywhere along the line Additionally even though most squall lines 46 PROFESSIONAL PILOT September 2016 Squall line of mature thunderstorms near the Cayman Islands Squall lines are a type of MCS that may form along a boundary between 2 air masses along an outflow boundary from a decaying storm or where an atmospheric wave has initiated uplift of conditionally unstable surface air are only a few kilometers deep they may extend for hundreds of kilometers And as the storm cells are firing next to each other their outflow can serve to generate enhanced uplift in the gap between them Furthermore the atmospheric flow ahead and above a front tends to be parallel to the front and toward the central low pressure As a result the storms of a squall line also have some lateral movement in addition to their forward motion These are big reasons why flying though radar gaps in a squall line is unadvisable even if those gaps appear generous In the highly dynamic environment that supports an organized MCS new storms can form in minutes and even where there is no storm towering cumuli are often present obscuring what may lay behind Flying in the vicinity of MCS The best advice for navigating a MCS is not to If your aircraft has a sufficient ceiling you may be able to safely overfly a weak MCS but with many tops stretching into the high flight levels that may not always be an option If the cloud tops are above your flightpath the 2 safest options to you are either to deviate around the MCS or land and let it pass Most MCS are fast moving and a short rest stop will often allow the system to pass so you can continue your journey in more favorable weather A good rule of thumb for flying in the vicinity of any thunderstorm is to stay clear by 1 mile for every 1000 ft of cloud height with 20 miles of clearance minimum So a cumulonimbus with tops at 45000 ft should require a 45 mile exclusion zone around it This safety margin goes for takeoffs and landings as well as punching through gaps between cells A gap of 10 miles between a pair of 30000 ft cumulonimbi is just not going to provide adequate safety In that scenario you are only 5 miles from either storm and you can count on both storms working to create an offspring in that gap Even if the gap doesnt close while you are traversing it either or both storms can toss hail your way and the turbulence in the gap may be strong to severe Furthermore though the gap may show up as a rain free zone between 2 precipitation shafts if the gap is already clouded over it could actually be a developing storm with a wicked updraft that simply has not yet begun to drop precipitation Pilots could find themselves trapped if the storm matures when theyre flying halfway through the gap Often pilots shave those safety margins much closer especially during arrivals and departures Many commercial airports will not stop operations until lightning actually strikes inside the airport boundary And with approach radars displaying near real time echoes a pilot may believe that controllers will keep them out of harms way Likewise for a myriad of reasons pilots in aircraft equipped with onboard weather radar or those who are below the cloud deck and can see the surrounding rain shafts may feel just keeping clear of the storms by a few miles is sufficient But as with a gap Photo by Karsten Shein Cluster of thunderstorms over central Florida in July 2016 Not all widespread thunderstorm activity is associated with a mesoscale system