Following the UK’s hottest September day in 7 years yesterday, today will be a very different day as low pressure develops at the base of a southbound trough dropping over the UK. The atmosphere has a lot of energy to disperse of as much colder, autumnal air sinks south while the heat of yesterday gets kicked east into the near continent.

While it was a hot day in the Midlands and South yesterday, it was anything but hot in the North and Northwest where it was widely 13-16C. In one chart I saw, it actually showed 3C in the west Highlands at the same time it was 30C in Kent. A bank of cloud with little rain associated with it was the divide between cool and hot and north of the front, a northerly breeze was transporting cooler air down which set the scene overnight for frost to develop under clear skies and light winds. Quite impressive by UK standards to see 30C followed by a frosty night over parts of the North.

Heavy rain has been spreading north through today and while patchy over south-central and southeast England, head further north and it turns heavier and more persistent. There is concern that as the heavy, persistent rain falls throughout the late day and overnight period over northern England up into southern Scotland, flooding will become an issue. Especially given the fact the ground is fairly hard following a warm, dry summer.

It sure was a cool start compared to recent times and the coldest morning since May with parts of Highland Scotland dipping to -2C. Patchy frost was reported in both Scotland and Northern Ireland.

Below is the GFS surface and precipitation chart for this evening and notice the heaviest rains focused over northern England. We could see 2 inches between 6pm tonight and 9am tomorrow morning with localised flooding. Factor in a stiff north wind, air temp near 12C and it will feel rather miserable out there.

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All this weather is of course associated with a low which has developed and an associated front is driving milder air north while cold air is being forced south, the confliction or fight is increasing the rainfall rates as the front pivots north. You’ll notice the rain band moving in an unusual northwest track. Overall it’s not been a bad day over Scotland and Northern Ireland, albeit cloudy. The best of the sunshine is over northern Scotland and Northern Ireland but through this evening and the rain spreads north and west, setting the scene for a poor night and day come tomorrow.

As you can see from the below chart for tomorrow, the focus of rain is over Scotland, eastern Ireland and north Wales but Northern Ireland will get in on the wind and rain too.

The 500mb chart at 24 hrs shows the level of upper energy associated with this system.

As the low itself works north, the entire canopy of rain lifts initially north, then west and then hooks back into Wales. Wales gets in on the heavy rain this afternoon and evening, eases some as it lifts into Scotland and Northern Ireland but through tomorrow into Sunday, the same area of rain drops back south. Eastern areas of the UK get off lightly this weekend. Not so for western areas. The eastern side of Ireland get hammered.

By the second half of the weekend, the heavy rains move out as heights build over the UK providing a decent start to next week. We could see more patchy rural frost potential as skies should clear and winds turn lighter again.

By Tuesday more low pressure and more rain arrives as we find ourselves in the battle ground between high pressure both west and east of us but ECMWF has a fairly west-east flow across the Atlantic into Europe with no major buckling. Messy conditions will also continue over the Low Countries up through Denmark and into Scandinavia.

By Thursday, despite the height field relatively high throughout the continent including the UK and Ireland, there’s no clear sign of any real settled theme to next week with a lot of areas of rain scattered across the map.

Next weekend sees another fairly significant low develop over the UK which may present similar conditions to what we are seeing develop now.

Here’s the upper chart for this Sunday.

Here’s next Sunday.

The large-scale upper flow continues to have a zonal or west to east look in the means through a large part of September with further spells of warmth and ridging followed by troughiness but no real hint of any cold spell through Sept 20, though the longer term NAO/AO still somewhat of a negative look towards October. The trend till then is ‘slight positive’ which supports the predominantly zonal flow with nothing extreme and no wild amplification.

This setup will keep the polar vortex stronger over the arctic and allow both the sea ice and cold air to increase in intensity and coverage. This could lead to stronger cold shots once the pattern shifts around and we plunge into that negative NAO/AO signal.

October continues with a colder than normal look in the CFS as you can see below.

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