Severe Weather Reports FAQ
What
are the Severe Weather Reports?
How timely are they?
What's the difference between AWS and synoptic
reports?
Why are the AWS reports listed both by location
and time?
What do the figures in the synoptic report columns
mean?
What do the figures in the AWS report columns
mean?
The report says it's snowing in Cairns. Can this
be true?
OK, what errors do I look for in the AWS report?
And in the manual synoptic reports?
What are
the Severe Weather Reports?
How timely
are they?
What's the
difference between AWS and synoptic reports?
-
Automatic Weather Stations
automatically send in a report every hour or, in some cases, every
half hour. In addition, and of particular interest to those watching
for severe or unusual weather, they also generate a report when
certain conditions of wind, rain, temperature or barometer change
occur. Their strength is in providing detailed wind information,
because they report both the current sustained windspeed as well
as the strongest recent wind gusts. They also generate a string
of reports when heavy rain falls, giving details of rain rates over
just a few minutes.
-
Synoptic observations
are not taken as frequently as AWS reports, but give more detail.
They're taken at all AWSs and also by human observers at several
hundred manual stations. The main stations, and all AWS, report
every 3 hours -- midnight, 3am, 6am, 9am, noon, 3pm, 6pm and 9pm
local time. Many stations in smaller country centres report only
twice daily at 9am and 3pm, and there's a third group that report
more than twice but less than 7 times daily. Most manual stations
do not report at midnight (9pm in WA). The synoptic reports carry
much more detail than the AWS reports and are good for spotting
heavy longer-duration rainfall, such as over a 3 or 6 hour period.
The weather columns list codes for present and past
weather that provide the only routine and systematic observation
of thunderstorms, hail, snow and many other weather phenomena. For
this reason, it is worthwhile taking some time to learn the codes.
Why are the
AWS reports listed both by location and time?
-
Use the AWS reports
by location if you are checking the weather in a particular area.
Stations are grouped first by state, then by weather district. This
makes it easy to see the extent of significant weather.
-
Sometimes, though,
you just want to see what's happening right now. Go to the bottom
of today's report and you'll find any reports received in the past
15 minutes. The reports are listed in UTC time so as to keep them
in proper chronological order across state and time zone boundaries,
but the local time is also shown to the right of the date. The chrono
listing is also useful for watching the movement of major weather
events across the ground.
What do the
figures in the synoptic report columns mean?
What do the
figures in the AWS report columns mean?
-
District:
this
is the name of the rainfall district in which the station is located.
-
Station:
The official Bureau name for the station's location.
-
Date/Hour:
The date is in year/month/day format (e.g. 20020906 is 6 Sept. 2002).
Note that the report uses local time, including daylight saving
time when that is in force. The 24-hour clock is used, and midnight
is 00 hours. The chronological listing also gives the UTC time to
the left of the date to help maintain continuity across time zones.
-
Wind:
three bits of information -- wind direction in degrees from north,
average speed over the past 10 minutes, then the maximum gust recorded
recently. Speeds are in kn/h.
-
Temp and Dew
Poiont: The air temperature and dewpoint.
-
Barometer:
The QNH barometer reading. For most purposes, this aviation pressure
calculation is nearly identical to the mean sea level pressure.
-
Rain:
-
Since 9am -- the
cumulative rainfall in mm since 9am
-
Past 10 min --
the rain in the 10 minutes before the observation time
-
Past hour -- the
rain in the hour before observation time calculated from the
cumulative rainfall reports
-
Rain since last
obs -- the rainfall (mm column) and the number of minutes (min
column) since the last observation
-
Vis,
weather and cloud: At some airports, the AWS reports are
supplemented by human observations of visibility, weather
and cloud. There are
also some canny AWSs that can measure visibility and cloudbase
too. The visibility is in kilometers, and in the cloud columns
the lefthand
figure is amount in octas and the righthand figure is base in feet.
Full details are here
The report
says it's snowing in Cairns. Can this be true?
-
Because these reports
deliberately look for extremes, they're also good at finding errors!
If a report looks crazy, it probably is. When something appears
to be amiss, cross-check. Some of the most common errors, and means
of cross-checking are given below.
-
Manual synoptic weather
reports (the ones that give cloud data) are done by real, live humans.
The 50 or so that come from Bureau field stations at main airports
are done by professionals; the rest are done by dedicated folk who
also have other things to do. Errors in visual observation, coding
the reports and keying them into the WOT (Weather Observation Terminal)
or EFB (Electronic Field Book) do happen.
-
AWS reports (the ones
with / for cloud in the synoptic reports, and all reports on the
AWS report) are prone to occasional computer glitches that can cause
exciting, but wrong, reports.
-
Question everything,
and cross-check, cross-check, cross-check.
OK, what
errors do I look for in the AWS report?
-
Isolated or very high
wind gusts. Check the gust speed against the average speed. If the
gust is more than 1.5 times the average, it may be wrong, and if
it's more than twice the average there's every likelihood that it's
wrong. Some AWS's are more prone to reporting erroneous gust speeds
than others -- e.g. those around Sydney Harbour -- and with experience
you will learn to pick them.
-
Heavy rainfall, but
the satellite picture showed a clear sky! Vibration from strong
winds (and small boys throwing buckets of water) can produce some
phenomenal rainfall. AWS's also sometimes develop a repeating error,
with improbable amounts of rain reported every 3 hours or at certain
times of day.
And in the
manual synoptic reports?
-
Beware of temperature
and barometer errors of 5 and 10 degrees -- when the observer reads
the thermometer or barometer, the wrong graduation mark on the instrument
may be read.
-
Rainfall errors can
occur in two common ways. Two sets of rainfall information are reported,
and unfortunately they use different codes which seem designed to
confuse. Coding errors usually produce flood rainfalls -- if the
station reports 989mm in 3 hours, discard the report. Any unusually
high report should be checked against other stations in the area
-- the stations are listed by rainfall district, which helps this
process.
-
The second error that
can occur in rainfall figures happens when the observer records,
say, 9.2mm but enters 92. Again, cross-check against nearby stations.
-
Use the present and
past weather, and the cloud codes to cross-check. Very heavy, isolated
rain can occur from thunderstorms -- has the observer reported a
9 (thunderstorm) in past weather, or 9 (Cumulonimbus) for low cloud.
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