Professional Pilot, October 2018

which develop from time to time The strength of these lows and the corresponding cold and warm fronts tend to be strongest in the fall because its the time of year with the greatest difference in temperature between the 2 opposing air masses Thus in the middle latitudes it is the conditions associated with these cyclonic storm systems that are usually the major weather factors affecting aviation Fronts are a part of the middle latitude weather picture in every season but a fall frontal passage can include elements of both summer and wintertime storm systems including strong squall lines wind shear and icing The temperature differential especially at top of the troposphere is also much greater as fall matures This means that the discontinuity that fuels the jet stream tends to be at its strongest during fall As a result jet speeds are fastest during the season The implication is that when the low sits beneath a region of strong jet winds it can deepen far more quickly and reach lower pressure than when it is coupled to a weaker jet flow as might be experienced during other seasons These fall cyclone bombs can double normal cold front speed which in turn are far more effective at lofting the still hot and humid air ahead 102 PROFESSIONAL PILOT October 2018 of them This speed can also create waves that might generate thunderstorms several hundred miles ahead of the front And while the warm sector of these systems remains hot the cold sectors are quite cold Rain falling through these colder regions can quickly become supercooled freezing rain Pilots should pay close attention to the outside air temperature when transiting beneath a fall cold front where rain or clouds are present Winds and fog Winds and fog can also become dangerous in these systems In the closely spaced isobars behind the central low wind speeds can easily reach hurricane strength making crosswind landings difficult if not impossible Low level windshear is also not uncommon in these conditions as winds become turbulent as they flow around terrain buildings and other obstacles Beyond the midlatitude cyclones that can bring dangerous flying conditions to half a continent the extra few hours of night as fall progresses are frequently enough to initiate a radiation fog In many places fall is when airports see the greatest frequency of foggy mornings If you are operating in and out of an area near a river or in a deeper valley you should consider the likelihood that any morning flights will be delayed especially if the overnight sky is clear winds are light and the air is relatively humid In addition the later sunrise and lower sun angle are more likely to delay evaporation of the fog until later in the morning Fall weather forecasts As with any season anticipating the weather is paramount to a safe and uneventful flight There is no real magic to this National hydrometeorological agencies such as NOAA often provide seasonal forecasts that provide regional guidance regarding the likelihood of things like above or below normal precipitation These agencies as well as a host of private weather companies to which many pilots subscribe also produce shorter range forecasts from a few hours to as much as 15 days ahead Large scale features such as the development and movement of lows and fronts can be predicted with reasonable accuracy but the details of conditions they may bring may only be forecast with any accuracy a day or 2 beforehand Pilots can make their own forecasts ahead of a flight by examining the weather maps from the past several hours and extrapolating where the fronts and major fall weather makers may move in the hours leading up to the flight Of course speaking with a trained briefer will help greatly in understanding some of the meteorological nuances regarding the current and forecast weather conditions Naturally when planning a flight it is best to draw the route of flight across a weather map to see where your path might intercept adverse conditions and determine if minor deviations might help to keep the flight smooth As always please report any conditions you experience that are not Karsten Shein is cofounder and science director at ExplorEiS He formerly was an assistant professor at Shippensburg University and a climatologist with NOAA Shein holds a commercial as forecast license with instrument rating Storm system disrupts aviation across much of North America in October 2010 Conditions associated with these systems include thunderstorms icing wind shear fog and blizzards Caution should be exercised whenever operating around a midlatitude cyclone