How to understand T850's on the GFS

Following in from the previous topic talking about the HGT500 and SLP charts on the GFS, we now have a T850 chart to analyse. T850's are instrumental to the temperatures we receive on the ground and give a good indication on how warm, or cold a chart is. The HGT500's are geopotential height, and so give low pressure cooler, lower amounts, and high pressure, higher amounts, however, in reality, the source of the air may go unregarded on HGT500 charts, and so T850's (or upper air temperatures) will give a more realistic viewpoint.

Anyway, here is the chart I am analysing:

As you can see, very cold upper air (-10c or below) is stored in Russia, Canada and Greenland. This is usually known as cold pooling, and generally a surge of energy moving out of these cold pools will give Britain it's famed 'freezes', if directed in the correct position. Britain is under 0c to 5c uppers, and this is generally above average for the time of year (temperatures around 0c under average pressure will deliver average temperatures in late autumn and in early spring. Summer uppers will usually peak at 8-10c in July, but in 'heatwaves' they can reach past 15c, and in Winter, uppers usually may be a degree or so under freezing, and can get down to -15c to -18c in the coldest of blasts, but usually get down to -8c/-10c at least once in the average British winter.

Usually, a heatwave in mid-summer would require uppers of over 10c or more, but under high pressure, temperatures usually can rise past 25c with average uppers. The warmest heatwaves can bring 20c uppers to the south coast, like August 2003, where Britain sizzled and Gravesend managed 38c!

For snow at sea level, you usually need about -5c uppers from a dry continental flow (from the north east, east or south east), and -7c/-8c for wetter maritime flows (the north, north west or west), this is due to higher instability in the atmosphere during maritime flows than continental flows (a larger temperature difference from say, 5000m in the air and sea level).

The rule usually is you take 1c off the required uppers every 150m you go north, but in continental flow, this often rises to 300m, here is an example-

In a maritime flow:

Sea level: -7c uppers required for snow
150m: -6c

300m: -5c

450m: -4c

600m: -3c

and so on, this often means that uppers of 0c or so can deliver snowfall to the mountains of Scotland, that can reach 1300m or more, this is why snow can fall almost all year round on some mountains.

I hope you now know how to understand T850's and how to forecast warm or cold weather with them, and happy forecasting!

Thanks,

~Arjan

Understanding the GFS 'Height of 0 degree isotherm' chart

Hi guys. Tonight I decided to write about a chart that is crucial to forecast where precipitation will fall as snow. And since we're closing in rather quickly to winter, it will help substantially with your forecasts.

The above is the 0 degree isotherm chart produced by today's 12z GFS run.
Basically, the chart shows the height (in meters) in which the temperature is at 0C. The chart is fairly straight forward, and is especially useful for predicting the snow level on mountains.
The key on the side of the chart is colour coded. It measures the height in meters. The chart goes up to 6000m, and down to -500m. As a general rule you will find that the isotherm will need to be at 200-300m for snow to fall at sea level.

So as an example: This current chart tells us that snow will not be falling in the UK, even over the highest peaks of Mt. Everest. It does however tell us that the 0 degree isotherm is very low over Siberia, it's at 0m and therefore any precipitation is likely to be snow rather than rain.

That's all there is to it. One of the more simple charts produced by the GFS.

Thanks,
~Jordan.

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.

Welcome to Convergence Zone

Hello and welcome to Convergence Zone.

This is a blog created by two members of the popular weather forum; Netweather.tv
It is basically a blog for those who need help understanding the basic charts of the GFS, ECMWF and UKMO.

Over the coming weeks we will be releasing new content, and we would appreciate some feedback. We have just managed to get up and running, so content at this stage is obviously going to be lacking somewhat.

The first articles will be published tonight, and will be looking at how to read some of the basic GFS charts, hopefully this will allow you to produce your own weather forecast to some extent, without the need to look at what the Met Office think the weather will do.

It's a very fascinating subject, and we will be learning just as much as you will be.

Thanks for reading.
~Jordan.