WHAT ARE MONTHLY FORECASTS?
Monthly forecasting covers the time range of 10 to 46 days and is really more correctly described as sub-seasonal or extended-range. It is a time scale in between medium-range weather forecasting (10-day) and seasonal forecasting (3 monthly).
Monthly forecasts can at times provide an insight into weather patterns in the month ahead. For example, will the month ahead be colder than average, wetter than average? However, they should not be used for specific planning purposes as they have generally low skill compared with the 10-day forecast. This is because forecasts beyond one week become increasingly uncertain due to the chaotic nature of the atmosphere. Find more details about chaos and computing in weather forecasting at Met Éireann here.
As the accuracy of Monthly forecasting improves there will be a wide range of societal and economic applications. Monthly forecasts will be used to assist in fore-warning the likelihood of severe high impact weather to help protect life and property. There is a high demand for sub-seasonal forecasts for humanitarian planning and response to disasters, agriculture (especially developing countries) and for disease planning and control. Monthly forecasts can also assist in forecasting of river-flow for flood prediction, hydroelectric power generation and reservoir management.
HOW DETAILED ARE MONTHLY FORECASTS AT MET ÉIREANN?
Extended-range forecasts are not very detailed and often quite general. This is because forecasts beyond one week become increasingly uncertain due to the chaotic nature of the atmosphere. Small inaccuracies in the weather forecast today can become very large by next week. To try and combat this, a set (or ensemble) of forecasts is produced. This set of forecasts aims to give an indication of the range of possible future states of the atmosphere. A probabilistic forecast is born. Read more about probabilistic weather forecasting in Met Éireann here.
The extended-range probabilistic forecast can often give a trend in the weather over the next several weeks. Is it more likely to be colder or warmer than average? Drier or wetter?
THE SCIENCE BEHIND MONTHLY FORECASTING
Medium-range weather forecasting is mostly an atmospheric initial value problem. Seasonal forecasting, on the other hand, is justified by the long predictability of the atmospheric boundary conditions such as ocean, land or ice and by their impacts on the atmospheric circulation.
The time range of 10 to 46 days is probably short enough that the atmosphere still has a memory of its initial condition and long enough that the ocean variability could have an impact on the atmospheric circulation. Therefore, sub-seasonal or extended-range forecasts are produced from coupled ocean-atmosphere integrations.
An important source of predictability over Europe in the 2-6-week range originates from the Madden-Julian Oscillation (MJO). The MJO is a 1-2-month tropical oscillation. Several papers (Woolnough et al. 2003) suggest that the ocean-atmosphere coupling has a significant impact on the speed of propagation of an MJO event in the equatorial Indian and western Pacific oceans. Therefore, the use of a coupled ocean-atmosphere system helps capture some aspects of the MJO variability (Source: ECMWF). Other important sources of extended-range predictability include the stratospheric initial conditions (e.g. Baldwin and Dunkerton 2001) and the soil moisture initial conditions (e.g. Koster et al. 2010)
HOW AND WHEN ARE THE MONTHLY FORECASTS PRODUCED?
Met Éireann uses data from our partners at the ECMWF (European Centre for Medium Range Weather Forecasts). The ECMWF monthly forecasting system is a 51-member ensemble of 46-day coupled ocean-atmosphere integrations. The extension of ENS to 46 days is performed every Thursday and Monday with data available the following day. Over each point of the map, atmospheric variables such as 2-metre temperature, total precipitation, mean sea-level pressure or surface temperature, have been averaged over a weekly period.
The weekly anomaly charts that are used when the forecast is written by the Meteorologist display the difference between the ensemble mean of the real-time forecast and the ensemble mean of the back-statistics or climatology. The graphical products therefore display the shift of the forecast ensemble mean from the estimated “climatological” mean.
Meteorologists at Met Éireann analyse the forecast data throughout the week and the extended range forecast is updated on Tuesday and Friday evenings.
WHAT IS THE ACCURACY AND VALUE OF MONTHLY FORECASTS?
Monthly or sub-seasonal forecasts from the ECMWF tend to have reasonable skill at both week 1 and week 2 and can highlight trends towards certain weather patterns. At week 3 and 4, there is only marginal skill but the forecast can at times indicate trends towards certain types of weather patterns.
Monthly forecast verification statistics and performance (Figure 1) shows the probabilistic performance of the monthly forecast over the extratropical northern hemisphere for summer (JJA, top panels) and winter (DJF, bottom panels) seasons since September 2004 for week 2 (days 12–18, left panels) and week 3+4 (days 19–32 right panels). Forecast skill for week 2 exceeds that of persistence by about 10%, for weeks 3 to 4 (combined) by about 5%. In weeks 3 to 4 (14-day period), summer warm anomalies appear to have slightly higher predictability than winter cold anomalies, although the latter has increased in recent winters (with the exception of 2012). (Source: ECMWF)
In 2018, week 2 forecast skill for summer warm anomalies was unusually high, but persistence was also at its highest level within the period shown. The corresponding week 3+4 forecast skill and persistence were also near the upper end of values seen so far. Skill for winter cold anomalies in 2018 was close to average, however in week 2 there is an increasingly consistent margin relative to persistence in recent years.
VERIFICATION TOOLS AND VERIFICATION SKILL SCORES OF THE ECMWF MONTHLY FORECASTING SYSTEM
Fig 2: A comparison of the RMSE and Anomaly Correlation of the monthly forecasting system (MOFC) over Europe against climatology (CLIM) and persistence (PERS). Scores are computed from forecasts on a standard 2.5° x 2.5° grid limited to standard domains. The bottom panel displays the anomaly correlation score of daily monthly forecasts of 500 hPa geopotential height over several regions. Anomaly correlation scores are spatial correlation between the forecast anomaly and the verifying analysis anomaly. The top panel displays the Root Mean Square Errors (RMSE) of daily monthly forecasts of 500 hPa geopotential height over several regions. Root Mean Square Errors are the geographical average of the squared differences between the forecast and the analysis valid for the same time (Source: ECMWF).
- The ECMWF Monthly Forecast System root mean square error (RMSE) is lower than the persistence RMSE throughout the forecast period whilst is notably lower than the climatology RMSE through until day 16, thereafter it is similar to climatology but with a slightly smaller error.
- The Anomaly Correlation of the ECMWF Monthly Forecast System is higher than both climatology and persistence through to Day 30.
For further details on the verification of monthly forecast data provided by the ECMWF and analysed by Meteorologists at Met Éireann please visit: https://www.ecmwf.int/en/forecasts/quality-our-forecasts
CASE STUDY: EUROPEAN COLD SPELL FEBRUARY 2018
A particular case when the sub-seasonal forecast proved very useful was ahead of the severe cold spell in Europe in late February 2018. Below we can see the evolution of the forecast from as early as 1 February and compare with the analysed temperature and pressure anomalies. We can see cold anomalies over Europe began to be heralded in early February. In this instance the forecaster had growing confidence that there was a risk of exceptional cold weather in the region in late February.
Fig 3-4: These graphics displays the weekly mean anomalies relative to the past 20-year climate. The first panel corresponds to the anomalies computed using ECMWF operational analysis and reanalysis for a given week. The other panels correspond to the eight monthly forecasts starting one week apart and verifying on that week. The model anomalies are relative to the model climate computed from the model back-statistics. The areas where the ensemble forecast is not significantly different from the ensemble climatology according to WMW-test are blanked. The time range of the forecasts is day 1-7, day 5-11, day 8-14, day 12-18, day 15-21, day 19-25, day 22-28 and day 26-32. This figure gives an idea of how well the predicted anomalies verified against the ECMWF analysis and also about the consistency between the monthly forecasts from one week to another (Source: ECMWF).