Automatic Regional Weather Forecast in Hong Kong & Pearl River Delta Region

Notes

The Automatic Regional Weather Forecast (ARWF) website provides the following products in the form of time series and animations:
1. hourly forecasts of air temperature, relative humidity and wind direction and speed for the next nine days at several stations in Hong Kong and a total of 240 grid boxes over the Pearl River Delta Region;
2. 3-hourly forecasts of the weather icons for the stations in Hong Kong, as well as an overview of the state-of-sky for each day over the next nine days;
3. daily forecasts of the probability of precipitation at the stations in Hong Kong for the next nine days;
4. rainfall nowcast for the next two hours over Hong Kong and the Pearl River Delta Region;
5. lightning nowcast for the next hour over Hong Kong and the Pearl River Delta Region (beta); and
6. hourly forecasts of air temperature and relative humidity for the next three days at several urban meteorological monitoring stations in Hong Kong. (beta)

9-day Forecasts

The products are updated twice a day at around noon and midnight. The former provides hourly forecasts from noon the same day to midnight on the ninth day. The latter provides hourly forecasts from midnight the same day to midnight on the ninth day.
The products are generated using a multi-model consensus forecast system, which integrates the outputs of a number of global numerical weather prediction (NWP) models. The model outputs are first corrected based on the observation data collected at various weather stations over Hong Kong and the Pearl River Delta. A consensus forecast is then derived by combining the corrected model forecasts weighted according to their past performance. Technical details of the forecast system are available from the Hong Kong Observatory Article No. 1064.
Users can view the forecast time series of a particular station or grid box by clicking on the map directly. Users can also toggle between station view and grid view of the forecast map by clicking on the bottom left corner of the map.
The data in the grid view of the map shows the general weather situation over the indicated region as forecast by the multi-model consensus forecast system. As each grid box represents an area of about 10 km x 10 km, it may comprise a wide range of geographic characteristics such as land mass, sea surface, hills and valleys. The weather conditions described by the products may be different from those recorded at individual weather stations within the grid boxes.
For the station wind forecasts, the number inside each of the compasses indicates the forecast wind speed in kilometres per hour, while the red arrow head points in the direction to which the forecast wind will blow. In the above example, the wind is forecast to be 23 kilometres per hour from the southeast (i.e. from the southeast towards the northwest).
In the time series chart for wind speed, the arrow represents the hourly forecast wind direction. "←" and "↓" indicate easterly and northerly winds respectively.
The forecast for certain stations or grid boxes may sometimes be not available due to missing observation or forecast data. In this case, an "M" will be displayed.

The weather icon describes both the cloud cover and the rainfall expected each day or over the corresponding 3-hour period. Details are listed in the table below:

Weather Icons for Automatic Regional Weather Forecast

WEATHER ICON
CAPTION	Sunny	Sunny Periods	Sunny Intervals	Sunny Periods with A Few Showers	Sunny Intervals with Showers
WEATHER ICON
CAPTION	Cloudy	Light Rain (0.5 mm or above)	Moderate rain or heavier (10 mm or more)
WEATHER ICON
CAPTION	Fine (use only in night-time on 1st of the Lunar Month)	Fine (use only in night-time on 2nd to 6th of the Lunar Month)	Fine (use only in night-time during 7th to 13th of Lunar Month)	Fine (use only in night-time during 14th to 17th of Lunar Month)	Fine (use only in night-time during 18th to 24th of Lunar Month)	Fine (use only in night-time during 25th to 30th of Lunar Month)
WEATHER ICON
CAPTION	Mainly Fine (use only in night-time on 1st of the Lunar Month)	Mainly Fine (use only in night-time on 2nd to 6th of the Lunar Month)	Mainly Fine (use only in night-time during 7th to 13th of Lunar Month)	Mainly Fine (use only in night-time during 14th to 17th of Lunar Month)	Mainly Fine (use only in night-time during 18th to 24th of Lunar Month)	Mainly Fine (use only in night-time during 25th to 30th of Lunar Month)
WEATHER ICON
CAPTION	Mainly Cloudy (use only in night-time on 1st of the Lunar Month)	Mainly Cloudy (use only in night-time on 2nd to 6th of the Lunar Month)	Mainly Cloudy (use only in night-time during 7th to 13th of Lunar Month)	Mainly Cloudy (use only in night-time during 14th to 17th of Lunar Month)	Mainly Cloudy (use only in night-time during 18th to 24th of Lunar Month)	Mainly Cloudy (use only in night-time during 25th to 30th of Lunar Month)

The daily forecast of the probability of precipitation (PoP) indicates the chance of rain (with rainfall of 0.5 mm or more) at the station on a certain day and it has six categories: <10%, 20%, 40%, 60%, 80% and >90%.
For example, if PoP is 60%, it means that the probability of rain occurrence at the station is around 60% on that day. In other words, rain occurs about 60 times on average in every 100 forecasts, with the error of around 10 times on average from the actual number of occurrence according to the verification statistics. It should be noted that PoP only indicates how likely there will be rain, and it is not directly related to the size of area affected by rain, the time of rain occurrence, its duration or intensity.
As the products integrate the outputs of a number of NWP models, errors in individual NWP model forecasts will limit the capability of the products in predicting extreme weather events.
The above products are directly generated from a computer forecast system without manual adjustments, and may be different from the official weather forecasts issued by the Observatory or the measurements recorded at individual weather stations. Please refer to the official forecasts issued by the Observatory.
For more information on numerical weather prediction, probabilistic forecasting and other meteorological topics, please visit:

2-hour Rainfall Nowcast

The rainfall forecasts are generated by the Hong Kong Observatory's nowcasting system SWIRLS (Short-range Warning of Intense Rainstorms in Localized Systems). The main inputs of SWIRLS include the radar echoes (the reflected radar signals from raindrops) collected by the Observatory's weather radars, as well as the rainfall records registered at local raingauges. Simply stated, four main steps are involved in rainfall nowcasting, namely
1. tracking of the radar echoes
2. the real-time calibration and conversion of radar echoes into rainfall rate
3. the extrapolation in time of the radar-derived rainfall (assuming that both the rainfall rate and movement of echoes remain unchanged); and
4. computation of the accumulated rainfall amount at the surface grid
The technical details of SWIRLS can be found in the Hong Kong Observatory Technical Note No. 100, Article No. 561, Article No. 637, Article No. 1024, Article No. 1056 and Article No. 1258(some in Chinese only).
The horizontal resolution of the grid used to generate the rainfall forecast product is 2 kilometres. The areal coverage of the product is from 21.328 to 23.487 degrees North and 112.956 to 115.291 degrees East (i.e. within about 120 kilometres of Hong Kong). Under normal operating conditions, the forecast rainfall maps are updated automatically once every 6 minutes, following the completion of the radar scans starting at the 00th, 06th, 12th, 18th, 24th, 30th, 36th, 42nd, 48th and 54th minute of every hour. Normally, it takes 20 minutes or so to finish the processes of radar scanning, computation in SWIRLS, graphics generation and web page upload to the Internet. A total of 16 forecast distribution maps of 30-minute rainfall accumulation will be disseminated in every update to show the corresponding evolution in 6-minute steps.
In reality, rainstorms can develop very rapidly. Significant strengthening or weakening, or even changes in the direction and speed of storm movement may occur between successive updates of the product. Should such developments happen, the assumptions of the invariability of rainfall rate and storm movement will be violated, resulting in large forecast errors.
The colour of each pixel in the rainfall forecast maps merely represents the average rainfall amount expected to fall on the respective 2 by 2 square kilometers grid box. In other words, the sub-grid rainfall distribution is not resolved. The time series chart of a selected grid grossly expresses the change of forecast rainfall every 6 minutes.

Half-hourly accumulated rainfall less than 2.5 mm

Half-hourly accumulated rainfall between 2.5 and 5 mm

Half-hourly accumulated rainfall more than 5 mm

Users can choose switching to a 3-dimensional display of rainfall forecast. The half-hourly accumulated rainfall is represented schematically in vertical bars of different height and colours. Users can also adjust the position and angle of rainfall map according to the following method:

Movement	Mouse	Keyboard
Left or right	Hold left key and drag left or	Press left or right button
Up or down	Hold left key and drag forward or backward	Press up or down button
Rotate horizontally	Hold right key and drag left or right	Press “SHIFT” and left / right key together
Rotate vertically	Hold right button and drag forward or backward	Press “SHIFT” and up / down key together
Zoom-in / Zoom-out	Move mouse wheel (if any) forward or backward, or click “+”or “-“ icon on top right hand corner of website	Press “=” / “-” key

The rainfall records given in the ordinary weather reports are based on data collected by surface rain gauges. Such records are relevant to specific locations only and are different from the above-mentioned forecast rainfall over a gird box (which are essential areal averages). Direct comparison between the two cannot be made.
The Observatory will consider a number of factors when issuing weather warnings associated with rainstorms, including the current and forecast intensity of the rainstorms, their coverage and how long they will last, etc. Besides, those factors that are not taken into account in the nowcast system, such as atmospheric stability and influence of the atmospheric circulation would also be considered. In short, the forecast rainfall amount derived from the nowcast system is only one of the many factors that have to be considered.
Due to the above limitations, the rainfall forecast maps and time series may be different from the weather information disseminated by the Hong Kong Observatory and the neighbouring meteorological authorities. Users should not assess the likelihood of a rainstorm warning based on the products directly. For taking precautionary actions against inclement weather, the public is reminded to refer to the latest weather forecasts and warnings issued by the Hong Kong Observatory and the respective meteorological services in Guangdong and Macao as the authoritative sources:

1-hour Lightning Nowcast (beta)
1. The lightning forecasts are generated by the Hong Kong Observatory's nowcasting system SWIRLS (Short-range Warning of Intense Rainstorms in Localized Systems). Reference on the technical details of SWIRLS can be found in section 3(i) above.
2. The horizontal resolution of the grid used to generate the rainfall forecast product is approximately 700 metres. The areal coverage of the product is from 22.0 to 22.7 degrees North and 113.7 to 114.6 degrees East. Under normal operating conditions, the forecast rainfall maps are updated automatically once every 3 minutes. Normally, it takes 20 minutes or so to finish the processes of radar scanning, computation in SWIRLS, graphics generation and web page upload to the Internet. A total of 10 forecast distribution maps of lightning will be disseminated in every update to show the corresponding evolution in 6-minute steps.
3. Thunderstorms can develop very rapidly. Significant intensification or weakening, or even changes in the direction and speed of storm movement may occur within the forecast period resulting in forecast errors. This is a limitation of the current state of science and technology in weather forecasting.
4. Purple boxes on the forecast lightning map indicate forecast lightning locations corresponding to the selected time on the time slider. Users can choose switching to a 3-dimensional display of forecast lightning locations. The position and viewing angle can also be adjusted using the same method in the above 3-dimensional rainfall map.
5. Lightning icons indicate whether there will be lightning within the designated 30-minute period of forecast. A red lightning icon indicates lightning within 10 km, while an amber icon indicates lightning within 15 km. Icons for the second 30-minute forecast period are depicted with hollow shapes to indicate lower certainty of forecast.
  
  Lightning within 10 km in the first 30-minute period
  
  Lightning within 15 km in the first 30-minute period
  
  Lightning within 10 km in the second 30-minute period
  
  Lightning within 15 km in the second 30-minute period
6. Based on verification result, the probability of detection within 15 kilometres in the first 30-minute period is around 70%. The forecast accuracy is lower for the second 30-minute period.
7. The forecast weather condition as presented by this web page may be different from other weather information disseminated by the Observatory for the general public. The user should not assess the likelihood of rainstorm or other warnings based on the lightning forecast information directly. To take precautionary actions against inclement weather, the public is reminded to refer to the weather warnings special weather tips, and the latest local weather forecasts issued by the Hong Kong Observatory.
3-day Forecasts at Urban Meteorological Monitoring Stations (beta)
1. The products are updated twice a day at around noon and midnight. The former provides hourly forecasts from noon the same day to midnight on the third day. The latter provides hourly forecasts from midnight the same day to midnight on the third day.
2. The products are generated using a multi-model consensus forecast system, which integrates the outputs of a number of global NWP models. Reference to the technical details can be found in section 2(ii) above. Please also refer to sections 2(vii), (xi) and (xii) for points to note when using the products.
3. Users can view the forecasts of air temperature and humidity at urban meteorological monitoring stations located in several locations of the urban areas, and Sha Tin by zooming into the map. The forecast time series of a particular station can be viewed by clicking on the map directly.
Points to note
1. The webpage is best viewed by browsers that comply with HTML5 standard and support JavaScript (e.g. Chrome, Firefox, Internet Explorer 11 or above, Safari). If your browser does not support HTML5 and WebGL, you may not be able to use some special features and the response could be relatively slow. Presentation of the webpage may vary according to different browsers, computers and operating systems.
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