Was February’s SSWE Responsible For UK’s Longest Rain-less Spell Since 76, Strongest + June NAO Since 99?

Written by on July 13, 2018 in Rest of Europe, United Kingdom & Ireland with 0 Comments

After a month of continental style weather, change is on it’s way! Our 3, 4 even nearly 5 weeks of rain free days are well and truly numbered. For some, downpours and minor flooding has already occurred but for many the grass continues to brown and crops, plants wilt having not seen rain in over a month.

Worst ‘drought’ since 1976?

Is it the worst since 1976?, one could argue that one. Is it as bad as 76? Definitely not and the reason is it was much drier in the run up to that summer with a dry 75-76 winter and dry spring of 76 but length of time without rain, well this year could in fact be beating 76 in places but the level of heat this year certainly does not compete with 76 given there was 15 straight days in which somewhere exceeded 32C.

Have you noticed the sea breeze showers and storms becoming more frequent and widespread lately? That’s because our high is gradually weakening overhead and this will weaken further over the weekend. As the high weakens, we begin to loose the cap and that essentially means the mid levels are cooling allowing more lift through the atmosphere.

Scenes arguably not observed since 1995 or 1976

I heard there was a hurricane on the way?

No we don’t have a hurricane coming to UK shores like some think. I stepped into a shop in Penzance on Tuesday and the lady at the till said apparently there’s a hurricane coming next week. Nope, the remains of a former hurricane shall cross the Atlantic.

This feature will have no direct influence on us but indirectly it’s impacts will be great. How so? It’s injection of tropical heat energy into the atmosphere over the North Atlantic will give our atmosphere a kick just like a boulder shifting in a river.

Up until now, there have been no outside forces strong enough to shift that boulder and so day after day of warm to hot sunshine has been the theme.

North Atlantic Oscillation flips from extreme negative to positive

At the end of summer we experienced an extreme spell of snow and cold while at the start of summer we’ve enjoyed the polar opposite. In order for the ‘beast from the east’ event, we firstly observed the SSW or sudden stratospheric warming which in turn flipped the North Atlantic Oscillation to a record negative at end of February. In the starkest of contrasts and quite possibly in a long term ripple effect of that very sudden stratospheric warming we have went to the other extreme with weeks of hot, dry weather and the strongest positive June NAO since 1999.

Deep Greenland, North Atlantic troughs typically correspond to strong ridges downstream over Europe and that’s exactly what we’ve had.

A pattern near identical to 2013 when we last observed an SSWE

It’s been a textbook positive NAO over the last 60 days with this spring and first half to summer is near identical to that of 2013 which is the last year we observed an SSW until February 2018. 2013 was my main analog for this summer and I can’t believe how much it’s mirrored 2013 with the exceptions of a drier and warmer June than 2013.

Note the core of positive heights over Scotland just like 2013, this fits beautifully with my summer forecast issued back on May 1. A great forecast thus far even if I say so myself.

The core of positive back in February is positioned exactly where the core of negative was positioned during the last 60 days.

February

June

As for the unfolding shift now underway over the Atlantic? It’s very common for the tropics to provide a helping hand at shifting upper blocking highs. Think of our high pressure as a boulder in a stream, light movement around it does nothing to shift it but increase the pressure of water movement and the boulder may shift. The northward push of heat from Chris shall increase the jet stream strength (movement of water) and will finally help shift our blocking high allowing the Atlantic to return from holiday.

Heating up this weekend before cooling down

Prior to the Atlantic’s return next week, heights briefly rise over the UK and with the aid of stronger SSW winds, temperatures sharply rise this weekend with 32C quite plausible on Sunday for London.

The wind, rain and cool arrives in Northern Ireland and Scotland this weekend while England and Wales heats up and remains sunny.

Credit: Tropical Tidbits

Credit: Met Office

Credit: Met Office

The front doesn’t get into England and Wales till Monday but even at that, it’s weakening on it’s journey SE so showers are most likely the further south you are.

Credit: Tropical Tidbits

Next week will be more changeable for sure but it’s no washout as there’s sunshine to be had and temperatures remain near normal. There’s no distinct low but heights are low enough to support shower activity triggered by daytime heating.

Credit: Tropical Tidbits

Despite the change, our high never completely bows to low pressure…

Note the CFSv2 weekly 500mb height anomaly for the next 2 weeks.

Credit: Tropical Tidbits

Credit: Tropical Tidbits

Interestingly our positive height field doesn’t appear to collapse. A bone dry UK may have more pull on our atmosphere above than you’d think as dry ground encourages high pressure aloft. While perhaps not as strong it’s been, I wouldn’t be surprised if our high simply bounces back from this break.

The CFSv2 continues to keep 7-day mean precip below normal for the next 2, even 3 weeks and doesn’t show above normal rain into the SW until week 4.

Credit: Tropical Tidbits

Credit: Tropical Tidbits

Credit: Tropical Tidbits

Credit: Tropical Tidbits

 

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