Understanding key weather charts and graphs

Free access to charts generated using data from a range of Numerical Weather Prediction (NWP) computer models is available on TheWeatherOutlook Chart viewer. Numerical weather prediction uses mathematical models of the atmosphere and oceans to predict the weather based on current weather conditions and in the era of digital computers has become the most important tool used in weather forecasting.

Some of the charts and data discussed below is not available on the TWO mobile site. Currently most smartphones and small tablets are redirected to the mobile site by default. Larger tablets such as the iPad use the full desktop site.

NWP models

NWP models can be either global or regional. High resolution regional models usually only forecast 2 or 3 days ahead and are intended to give a detailed picture of the short term prospects. Global medium range models may go up to 16 days. Global seasonal models may go up to 8 months ahead or even longer.

Forecast accuracy of the computer models varies and generally it drops away quite quickly after about 5 to 7 days ahead. Seasonal forecast models are quite new and at the moment they offer a low level of accuracy for the UK and north western Europe because of the close proximity to different air masses.

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High Resolution Limited Area Model (HIRLAM)

A high resolution model updated twice each day providing forecast data up to 48 hours ahead. Plots available are for the UK only. Stepped at 3 hour intervals.

Global Forecast System (GFS)

A global model updated four times each day providing forecast data up to 384 hours ahead. Plots available for the UK and the northern hemisphere. The first 180 hours are stepped at 3 hour intervals with the rest at 12 hour intervals.

Global Ensemble Forecast System (GEFS)

A global forecasting system which updates four times each day and is based on the GFS model providing data up to 384 hours ahead. The GEFS is an ensemble system consisting of 20 runs plus the operational GFS run and the control run. The multiple runs use slightly different initial atmospheric conditions that are all plausible given the past and current set of observations or measurements. They are an attempt to account for uncertainty in the model. The aim is for the actual outcome to fall within the predicted ensemble spread. The amount of spread should be related to the uncertainty of the forecast. A bigger spread means more uncertainty and lower forecast confidence.

ECM (European Centre for Medium Range Weather Forecasts)

A global model updated twice each day providing forecast data up to 240 hours ahead. A limited set of charts is currently available for the UK and the North Atlantic / Europe region.

CFS (Couple Forecast System)

A global model updating four times each day providing forecast data up to 8 months ahead. Charts for the northern hemisphere showing 500hPa and 1000hPa anomalies and 850hPa temperatures are available.

Fax

Hand drawn charts provided by the UK Met Office. On the site they are updated two times each day.

WW3 (Wave Watch 3)

A global model updating four times each day providing forecast data on wave heights and periods. Charts for the North Atlantic region are available.

Beginners guide to the charts

The NWP models output a range of weather forecast variables at different time steps, and these can be plotted as charts or graphs to produce a visual representation. For example, maximum temperature, 850hPa temperature, mean surface level pressure, and wind gust speed, are examples of variables. The time steps for these may by +3, +6, +9...+X hours ahead.

Many of the charts on the site are relatively easy to understand, for example, maximum temperature, cloud cover, and precipitation rates. Others such as 500hPa GPDM and 850hPa temperatures require some explanation. Information on the more commonly used ones is provided below.

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500 hPa (GPDM) and MSLP

Geopotential height which the US National Weather Service defines as: roughly the height above sea level of a pressure level. For example, if a station reports that the 500mb height at its location is 5600m, it means that the level of the atmosphere over that station at which the atmospheric pressure is 500mb is 5600 meters above sea level. This is an estimated height based on temperature and pressure data. It is shown by the coloured shading

Mean Surface Level Pressure (MSLP) is usually show on the 500hPa GPDM plots with contours.

The 500hPa plots are useful for a range of reasons, one being that computer models usually forecast this level better than surface conditions at longer ranges.

In the winter months areas of higher heights (yellow, orange and red shading) to the north and east of the UK are often associated with colder weather. Lower heights to the north and east often mean a typical westerly flow bringing mild or close to average temperatures and unsettled weather.

In the summer months higher heights over the UK are an indicator of drier and probably warmer weather.

500hPa ridge - trough patterns can be identified on these charts. Rain is more likely under a trough and dry weather under a ridge.

Sample

MSLP and 500hPa charts are available using different map projections and covering different areas. Ths one below covers the North Atlantic region and Europe. The thick black line shows the 552hPa geopotential height and is shown to make it easy to identify the trough - ridge patterns. This chart shows surface level high pressure areas and upper level ridges to the south of the UK. Areas of lower pressure are to the north. That usually indicates unsettled conditions with temperatures close to the average in the winter months.

500hPA

850hPa temperatures and MSLP

850hPa is approximately 1500m above sea level. This means there is no diurnal variation (day to night change due to the surface warming and then cooling) making it very useful to identify air masses.

In winter 850hPa temperatures below -5C can often be cold enough for snow. At times it can be cold enough for snow even when 850hPa temperatures are only just below 0C. This usually only happens in the UK under an easterly or south easterly air stream.

In the summer months temperatures at ground level are less dependent on the 850hPa air mass and more dependent on the stronger sun heating the ground. Thus in summer even if  850hPa temperatures aren't very high it can still be warm..

Sample

MSLP and 850hPa temperature charts are available using different map projections and covering different areas. This one covers the North Atlantic region and Europe. Yellow, orange and red shading is used to show temperatures above 0C. Purple, blue and greed shading is used to show temperatures below 0C. This chart shows the UK under a relatively mild air mass for January with a westerly flow and 850hPa temperatures across the country ranging from just below 0C to about +3C.

Thickness values (dam)

Thickness values give an indication of how warm or cold the air is. Lower thickness values indicate colder air. Thickness values are one of the parameters sometimes used to help determine whether precipitation will fall as snow or rain. If the air is of a continental source snow can fall with thickness values of about 530dam or lower. If the air is from a maritime source values of 522dam or lower may be required for snow.

Sample
Thickness value charts are available using different map projections and covering different areas. This one covers the North Atlantic region and Europe and also shows forecast precipitation. Here the 528dam line is far to the north of the UK, just fringing with Iceland. The 546dam line is near the south coast and the 564dam line near the bottom left of the chart to the west of the Azores.

Jet stream charts

Jet streams are ribbons of strong winds moving around the globe. The polar jet front marks the boundary between tropical air masses and polar air masses. Low pressure areas usually follow a path close to the jet stream. A flat (zonal) jet pattern close to the UK often suggests unsettled conditions with close to average temperatures in the UK. A buckled (meridional) pattern often leads to cold or warm spells as the UK sits to the north or south of the polar jet front.

Sample

Jet charts are available for the North Atlantic and Europe region, and the northern hemisphere. On this chart the jet stream is shown exiting the eastern coast of the USA and heading across the Atlantic towards the UK, but becoming weaker.

GEFS line graphs

As noted above the GEFS consists of 20 individual runs (referred to as perturbations, P1 to P20) plus the control run and the GFS operational run. A convenient way of quickly assessing what all of the perturbations are showing is to plot them together on a line graph. Plots of different variables are available, but the most commonly used one shows 850hPA temperatures and rainfall amounts.

Sample
This shows forecast 850hPa temperatures on the top half of the plot and rainfall amounts on the lower half for up to 16 days ahead. On the 850hPa temperatures plot in particular look for the lines to be clustered close together. Tighter clustering indicates less spread and a higher degree of confidence in the forecast. On this plot there is quite a wide spread by Jan 15with 850hPa temperatures ranging from about +5C to -12C. The thick white line shows the average of all runs. The thick green line is the GFS operational run, and the thick blue line the GEFS control run.

Finally....

This guide is intended to be as simple as possible, giving an overview of some of the key charts available on the site. If as a result of this there is something you either don't understand or think is misleading please Contact us via the link near the top of the page.


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