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Cloud height
TAF
f 9 kmh (5 kt) up to 46 kmlh (25 kt)
+_ 20% above 46 kmth (25 kt)
f 200 m up to 700 m
f 30% between 700 m and 10 krn
Occurrence or non-occurrence
+ 2 oktas
f 30m (looft) upto 120m(400ft) + 30% between 120 m (400 ft) and 3 000 m (10 000 ft)
TREND FORECAST
f 9 km/h (5 kt) up to 46 kmk (25 kt)
f 20% above 46 krnh (25 kt)
f 200 m up to 700 m
f 30% between 700 m and 10 km
Occurrence OT non-occmence
+ 2 oktas
f 30 m (100 ft) up to 120 m (400 ft) + 30% between 120 m (400 ft) and 3 000 m (10 000 ft)
80% of cases
80% of cases
80% of cases
80% of cases
70% of cases
70% of cases
70% of cases
90% of cases
90% of cases
90% of cases
90% of cases
90% of cases
90% of cases
ANNEX 3 ATT B-1
Annex 3 - Meteorological Service for International Air Navigation Attachment B
Element to Operationally desirable Minimum percentage of
be forecast accuracy of forecasts cases within range
FORECAST FOR TAKE-OFF
Wind direction + 30' 90% of cases
Wind speed + 9 km/h (5 kt) up to 46 kmh (25 kt)
f 20% above 46 km/h (25 kt)
90% of cases
Air temperature + 1°C 90% of cases
Pressure value (QNH) + 1 hPa 90% of cases
AREA, FLIGHT AND ROUTE FORECASTS
Upper-air temperature + 3OC (Mean for 900 km1500 NM) 90% of cases
Upper wind f 28 km/h (I5 kt) upto flight level 250 + 37 km/h (20 kt) above flight level 250
(Modulus of vector difference for 900 km1500 NM)
Significant en-route Occurrence or non-occurrence
weather phenomena and
cloud Location: f 100 kmf60 NM
90% of cases
80% of cases
70% of cases
Vertical extent: + 600 m/2 000 ft 70% of cases
ATT B-2
ATTACHMENT C.. BACK-UP PROCEDURES AT
THE WORLD AREA FORECAST CENTRES
(See Chapter 3, 3.2.2 of this Annex.)
1. INTRODUCTION 2.2 WAFC Washington interruptions
A WAFC will provide any or all of the WAFS services as
needed when the interruption of the operation of the other
WAFC occurs. WAFC London and WAFC Washington have
studied a number of potential service intenuption and outage
scenarios, reviewed the current commilnication links between
the two WAFCs, and agreed upon the appropriate responses to
each interruption in service.
2. INTERRUPTIONS IN NUMERICAL
WEATHER PREDICTION (NWP) OR
SIGNIFICANT WEATHER (SIGWX)
FORECAST PRODUCTION
2.1 WAFC London interruptions
2.1.1 A global gridpoint NWP model runs in Exeter. The
output from this NWP model forms the basis for the wind and
temperature forecasts in the Gridded Binary (GRIB) code
form. Two, largely identical, supercomputers are housed in
separate computer halls, allowing the model to run on either
machine. Therefore, routine or non-routine maintenance can
be performed on one of the supercomputers without affecting
the operational capability of WAFC London. In the rare event
that the model cannot be run on either supercomputer, the
following course of action would be taken. When the first
model run is delayed significantly or is not available, a decision
will be made to use the output from the previous model
run, incremented by the appropriate number of hours. In the
event that the output from a second,' consecutive run is
unavailable, WAFC London would process the CRIB data
routinely received from WAFC Washington to produce the
required WAFS forecasts. If the WAFC Washington data cannot
be processed, WAFC Washington may be notified to begin
the production of the back-up SIGWX forecasts. These forecasts
along with other WAFS data are then sent to the WAFC
London message-switching centre for normal distribution.
2.1.2 In the event of a failure at the primary operational
workstation, the forecaster would use one of the back-up
workstations located on the forecast floor, or in the back-up
operations centre. If WAFC London is affected by a massive
failure, paragraph 5 describes the action under "Total outage of
a WAFC".
2.2.1 A global spectral model runs at the National
Centres for Environmental Prediction (NCEP) and produces
the WAFC Washington NWP output. This forms th,e basis for
the wind and temperature forecasts in the CRIB code form.
NCEP maintains the ability to run the global model on redundant
systems in the case of a failure of the primary system. If
the NWP is significantly delayed or missing due to the failure
of the primary or secondary computer systems, a decision is
made to use the NWP data routinely received from WAFC
London at the National Weather Service Telecommunications
Gateway (NWSTG), incremented by the appropriate number
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