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