AOPA Turbine Pilot Magazine - August 2022 - 89

RUDDER & WRENCH
UTC = The hour at which the forecast is valid,
or if the forecast is valid for a period, the end
of the forecast period.
TMP = surface temperature
DPT = surface dewpoint
WDR = forecasts of the 10-meter wind
direction, given in tens of degrees.
WSP = forecasts of the 10-meter wind speed,
given in knots.
WGS = forecasts of the 10-meter wind gust,
given in knots. " NG " means that no gust is
forecast.
PPO = probability of precipitation occurring.
The precipitation need not be measurable.
P06 = probability of measurable
precipitation (PoP) during a six-hour period
ending at that time.
PCO = Indicates if precipitation, not
necessarily measurable, will occur on the hour.
LP1 = probability of lightning (at least one
total lightning flash) occurring during the onehour
period ending at the indicated time.
LC1 = categorical forecast of no (N), low
(L), medium (M), or high (H) potential for
lightning occurring.
CP1 probability of convection (at least one
total lightning flash and/or radar reflectivity
of 40dBZ or higher) occurring during the onehour
period ending at the indicated time.
CC1 = categorical forecast of no, low, medium,
or high potential for convection (at least one
total lightning flash and/or radar reflectivity
of 40dBZ or higher) occurring during the onehour
period ending at the indicated time.
POZ* = conditional probability of freezing pcp
occurring at the hour.
POS* = conditional probability of snow
occurring at the hour.
TYP* = conditional precipitation type at the hour.
CLD = forecast categories of total sky cover valid.
CIG = ceiling height categorical forecasts.
CCG = conditional ceiling height categorical
forecasts
VIS = visibility categorical forecasts.
CVS = conditional visibility categorical forecasts.
OBV = obstruction to vision categorical
forecasts at the hour.
*In the contiguous United States, these
products are available September 1 through
May 31.
verification-and the agency's performance
is pretty good. But an Embry-Riddle
Aeronautical University study revealed
that TAFs missed low IFR (LIFR-ceilings
below 500 feet and/or visibilities
lower than one statute mile) predictions
44 percent of the time-when conditions
actually turned out to be IFR or higher
ceilings (ceilings at or above 1,000 feet
and/or visibilities at or above three statute
miles). Hmmm. An error rate like that
can put a big crimp in your flight planning.
TAFs come out four times a day at sixhour
intervals, and they are generated by
forecasters at the nation's 122 local NWS
Weather Forecast Offices. These forecasters
are very familiar with the weather
patterns and quirks in their assigned
regions, which is the main reason for our
reliance on them for complying with the
" 1-2-3 " rule set out in FAR Parts 91.167
and 91.169. These have to do with fuel
requirements and alternate airport designations.
If the forecast from one hour
before to one hour after your estimated
time of arrival at your destination airport
is below a 2,000-foot ceiling or visibility
below three statute miles, then you need
to name an alternate. To name a qualified
alternate, that airport's forecast must be
at least a ceiling of 600 feet and a visibility
of two statute miles if it has a precision
approach procedure; if it has a nonprecision
approach procedure, forecast
conditions must be for at least an 800-foot
ceiling and two-mile visibility.
What if your destination and alternate
don't provide TAFs? That's where
the relevant FARs give you an out. Both
FAR 91.167 and 91.169 give credit to
" appropriate weather reports and forecasts,
or a combination of them " for
naming destinations and alternates. TAFs
aren't specifically mentioned, nor is the
Graphical Forecast for Aviation (GFA) tool
on the Aviation Weather Center website
(aviationweather.gov), nor are the Model
Output Statistics (MOS) forecasts found
on ForeFlight and Garmin Pilot.
Nor are the Localized Aviation MOS
Program's (LAMP) forecast products. You
can find these forecasts under the MOS
products tabs on ForeFlight and Garmin
Pilot. The LAMP program crunches meteorological
data using supercomputers
programmed for this, and it spits out forecasts
for many, many airports (1,850 or so)
not served by TAFs, which are published
for 750 airports. LAMP updates its forecasts
every hour, with most of its emphasis
dwelling on recent ASOS reports, out to 25
hours, after which time another forecast
model, the Global Forecast System MOS
(GFS MOS), takes over.
ForeFlight and Garmin Pilot make it
easy to read their MOS products; they're
translated into plain English using abbreviations
we all understand. But you'll have
to dig deeper and work a bit harder to
AOPA PILOT / August 2022
89
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AOPA Turbine Pilot Magazine - August 2022

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