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Pilot's Handbook of Aeronautical Knowledge
Aviation Weather Services

Aviation Forecasts

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Pilot's Handbook of Aeronautical Knowledge

Preface

Acknowledgements

Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making

Appendix

Glossary

Index

Example:
SFOR WS 100130
SIGMET ROME02 VALID UNTIL 100530
OR WA
FROM SEA TO PDT TO EUG TO SEA
OCNL MOGR CAT BTN 280 AND 350 EXPCD
DUE TO JTSTR.
CONDS BGNG AFT 0200Z CONTG BYD 0530Z .

Explanation:
This is SIGMET Romeo 2, the second issuance for this
weather phenomenon. It is valid until the 10th day of the
month at 0530Z time. This SIGMET is for Oregon and
Washington, for a defined area from Seattle to Portland to
Eugene to Seattle. It calls for occasional moderate or greater
clear air turbulence between 28,000 and 35,000 feet due to the
location of the jet stream. These conditions will be beginning
after 0200Z and will continue beyond the forecast scope of
this SIGMET of 0530Z.

Convective Significant Meteorological Information (WST)
A convective SIGMET (WST) is an inflight weather advisory
issued for hazardous convective weather that affects the safety
of every flight Convective SIGMETs are issued for severe
thunderstorms with surface winds greater than 50 knots, hail
at the surface greater than or equal to ¾ inch in diameter, or
tornadoes. They are also issued to advise pilots of embedded
thunderstorms, lines of thunderstorms, or thunderstorms with
heavy or greater precipitation that affect 40 percent or more
of a 3,000 square foot or greater region.

Convective SIGMETs are issued for each area of the
contiguous 48 states but not Alaska or Hawaii. Convective
SIGMETs are issued for the eastern (E), western (W), and
central (C) United States. Each report is issued at 55 minutes
past the hour, but special reports can be issued during the
interim for any reason. Each forecast is valid for 2 hours. They
are numbered sequentially each day from 1–99, beginning
at 00Z time. If no hazardous weather exists, the convective
SIGMET is still issued; however, it states "CONVECTIVE
SIGMET…NONE."

Example:
MKCC WST 221855
CONVECTIVE SIGMET 21C
VALID UNTIL 2055
KS OK TX
VCNTY GLD-CDS LINE
NO SGFNT TSTMS RPRTD
LINE TSTMS DVLPG BY 1955Z WILL MOV EWD
30-35 KT THRU 2055Z
HAIL TO 2 IN PSBL

Explanation:
The WST indicates this report is a convective SIGMET. The
current date is the 22nd of the month and it was issued at
1855Z. It is convective SIGMET number 21C, indicating that
it is the 21st consecutive report issued for the central United
States. This report is valid for 2 hours until 2055Z time. The
convective SIGMET is for an area from Kansas to Oklahoma
to Texas, in the vicinity of a line from Goodland, Kansas,
to Childress, Texas. No significant thunderstorms are being
reported, but a line of thunderstorms will develop by 1955
Zulu time and will move eastward at a rate of 30–35 knots
through 2055Z. Hail up to 2 inches in size is possible with
the developing thunderstorms.

Winds and Temperature Aloft Forecast (FD)
Winds and temperatures aloft forecasts (FD) provide wind
and temperature forecasts for specific locations in the
contiguous United States, including network locations in
Hawaii and Alaska. The forecasts are made twice a day
based on the radiosonde upper air observations taken at
0000Z and 1200Z.

Through 12,000 feet are true altitudes and above 18,000 feet
are pressure altitudes. Wind direction is always in reference to
true north and wind speed is given in knots. The temperature
is given in degrees Celsius. No winds are forecast when
a given level is within 1,500 feet of the station elevation.
Similarly, temperatures are not forecast for any station within
2,500 feet of the station elevation.

If the wind speed is forecast to be greater than 100 knots but
less than 199 knots, the computer adds 50 to the direction
and subtracts 100 from the speed. To decode this type of data
group, the reverse must be accomplished. For example, when
the data appears as "731960," subtract 50 from the 73 and
add 100 to the 19, and the wind would be 230° at 119 knots
with a temperature of –60 °C. If the wind speed is forecast to
be 200 knots or greater, the wind group is coded as 99 knots.

For example, when the data appears as "7799," subtract 50
from 77 and add 100 to 99, and the wind is 270° at 199 knots
or greater. When the forecast wind speed is calm or less than
5 knots, the data group is coded "9900," which means light
and variable. [Figure 12-13]

 

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