Page 6 - Nov21T

Winter Flying
WReady or Not, It’s Almost Here
Position Report
by Dianne White
ith the onset of winter, as
pilots, we must shift our flight
planning and weather analysis air is less than 0 degrees C, the raindrops will initially be
from assessing convective threats to icing and cold-weather hazards such as icing, frozen precipitation and fast-moving cold fronts. Going into the colder months, we need to have a strategy for gathering and understanding all the weather data to make sound go/no-go decisions during the winter.
Where to start: First take a look at the big picture, what meteorologists call the synoptic situation. The latest surface analysis, prog charts and significant weather outlooks will give you a pretty good idea of what’s happening in terms of major weather systems affecting your route. Airmets are also good products to review as they advise of weather that may be hazardous – other than convective activity – to light aircraft, as well as larger planes. Airmets are typically issued for conditions that are widespread, meaning it covers at least 3,000 square miles, and are issued in six-hour periods.
As a quick review, airmet components consist of instrument meteorological conditions (Airmet Sierra), turbulence (Airmet Tango) and icing (Airmet Zulu). Gone are the days of trying to decipher and visualize long airmet codes. Today, with a few clicks, we can view a graphical representation of airmets.
Cold fronts, especially fast-moving ones, often trigger convective activity as colder air overtakes and plows under warmer air. In the wintertime, our concern with convective activity is its ability to lift substantial amounts of moisture to altitudes where the temperature is below freezing. The cloud droplets are cooled to temperatures below 0 degrees Celsius and yet can remain, for a time at least, in this liquid form.
Warm fronts, defined as warmer air overtaking and being gradually lifted up and over colder air at the surface, often have a more limited vertical extent than those associated with cold fronts. However, the area over which the clouds spread is often greater, which can spread low ceilings and limited visibility over a similarly larger number of potential destinations and alternate airports under consideration for our planned trip.
The most serious possibility that warm fronts present
is freezing rain or freezing drizzle. Rain forming in the clouds in the overrunning warm air falls through colder air below the frontal surface. If the temperature of this
supercooled. Ultimately, given enough time and distance of fall, something will trigger the freezing process in these raindrops. The greatest potential hazard from this warm- front situation arises when the falling supercooled raindrop encounters some object, like our airplane (also cooled by passage through the subfreezing air to a temperature of less than 0 degrees C), which triggers the freezing process. The result is often a rapid, large buildup of clear ice.
Thankfully, we’ve got some excellent tools for pre-flight planning. The Current Icing Potential (CIP) is an online display of high-precision maps and is updated hourly. It identifies areas of potential aircraft icing produced by cloud droplets, freezing rain and drizzle. The Forecast Icing Potential (FIP) tool depicts icing hazards up to 12 hours in advance. It provides color-coded maps of icing potential from altitudes of 1,000 to FL290 MSL.
In ForeFlight, there is a robust suite of icing products (including CIP & FIP) that allow you to view the forecast for the severity probability of ice and the lowest freezing level. You can also find these depictions on the FAA ADDS website.
Once we have the big picture, we will want to start looking at specifics of the reported and forecast conditions at our departure, destination and alternate airports. En route surface reports may also provide insight into whether a front or pressure system is behaving as predicted. Pilot reports near our departure airport can be valuable data for our decision-making process. As long as the PIREPs are current and in a relevant position, they can provide a glimpse of conditions as they existed at the time of the report, especially cloud bases and tops and any ice encountered.
You are probably familiar with a forecasting product called Model Output Statistics (MOS) forecast alongside the traditional TAF. As the name implies, MOS is derived from weather forecasting models, including the NAM (North American Mesoscale) and GFS (Global Forecast System.) MOS takes the long- and short-range model guidance and attempts to produce an objective and more useful site-specific forecast. MOS is used by forecasters at the NWS to help generate TAFs but is never used solely for constructing the TAF.
MOS does have limitations. It cannot “fix” a bad or faulty model forecast. If you use MOS, verify against METARs, satellite and radar data to ensure the MOS guidance appears on track. Also, MOS tends to be less accurate for extended
4 • TWIN & TURBINE / November 2021