Questionable Thunderstorms

[MP-R]

Looking back at chart archives I can pretty much get a sense of the different conditions thunderstorms in the UK form under from Spanish plumes to slack Atlantic lows to active northerly cold fronts a la Jan 28th 2004.

However there are two thundery spells that I would like a little help with, and coincidentally they both fall on the same dates. What were the forcing mechanisms for these particular thundery spells?

10th/11th May 2004:

http://www.wetterzentrale.de/archive/ra/2004/Rrea00120040510.gif

http://www.wetterzentrale.de/archive/ra/2004/Rrea00120040511.gif

10th/11th May 2006:

http://www.wetterzentrale.de/archive/ra/2006/Rrea00120060510.gif

http://www.wetterzentrale.de/archive/ra/2006/Rrea00120060511.gif

They were both warm spells but interestingly with winds not from an overly warm source. And in particular, the 2004 spell came from a northerly. Any help or replies would be welcome.

[Lettucing Gutted]

I think those warm "Northerlies/Northeasterlies" were the result of Returning Continental Tropical airmasses. May 2004 had an unusual pattern in which prior to the Northerlies/Northeasterlies establishing over Britain warm humid Continental Tropical air had got pumped up unusually far north into Northern Scandinavia and that meant our Northerlies were giving us warm humid and sunny Returning Continental Tropical air instead of cool dry Continental Polar air.