Understanding the GFS HGT500 & Sea Level Pressure Charts

The HGT500 & Sea Level Pressure Charts are perhaps the single most useful chart to a weather forecaster.
They can tell us not only what type of weather to expect, but where the winds are coming from (E.G. the North), how strong and winds are and also what type of pattern to expect - this is particularly useful for long range forecasting.

Anyway, to the actual chart.

This is the chart we will be analysing.
Now. We can see from the key on the right hand side of the chart that different colours represent different pressure readings. The reds and oranges indicating high pressure, the greens, blues and purples representing low pressure. The white lines on the charts are called isobars. They indicate the flow of wind around a cell of pressure. The closer the isobars are together, the stronger the winds will be.

 There is a golden rule when analysing a pressure chart that must be followed in order to make an accurate weather prediction - High pressure ALWAYS rotates clockwise, therefore when working out the wind direction based on the isobars, one must follow the white lines (isobars) in a clockwise direction.
The exact opposite for low pressure. The isobars ALWAYS follow an anti-clockwise direction. For example,  it's clear on this chart that the winds flowing through the United Kingdom will be from the South East, and that high pressure is in control of our weather.

High pressure can often bring sunny, warm and dry conditions (settled weather) to our shores, but there is such thing as a cloudy high, this is where the pressure cell will draw moisture up (usually from the South) into the atmosphere, creating clouds.

Low pressure often brings with it stronger winds and more precipitation. Meaning unsettled weather for many, but there are exceptions to this rule, and low pressure can bring days of sunshine, albeit usually cooler than what high pressure can accomplish.

Thank you very much for reading.

We hope to release another guide shortly.
~Jordan.