St Swithun's Day - is rain predicted?

Today 15th July is St Swithun's Day. And the olde saying goes: 

St Swithun’s day if thou dost rain
For forty days it will remain
St Swithun’s day if thou be fair
For forty days ‘twill rain nae mare

What credance can we give this prediction, will it rain for yet another 40 days if we encounter an unwelcome shower this very summer's day? And what is the provenance of this weather folklore? 

First, the history of this weather legend. St Swithun (or Swithin) was an Anglo-Saxon bishop of Winchester who died in around AD862. The clergymen requested that his remains be interred among the common people outside the church, but in 971, after he had been made patron saint of Winchester Cathedral, his body was dug up and moved to a new indoor shrine. According to some writers this caused sufficient displeasure in the heavens for a terrible downpour to strike the church and continue unabated for 40 days, hence the legend. 

Now this is all firmly Medieval (Middle Ages) territory so MedievalMorsels took to wondering about rainy day food. Pottage it must be! No need necessarily for bread to go with it - the grain is in the pot! Choose from red lentil pottage, spring vegetables pottage, beef or pease pottage! 

Red lentil pottage by MedievalMorsels  for a 12th scale dolls house

Miniature food for a period style dollshouse, vegetable pottage

Comfort food for any Middle Ages rainy season - cauldrons of pottage

MedievalMorsels 1:12 scale dollhouse food - pottage, pease pottage

Nourishing medieval pottages were based on grains and pulses available from essential stores when not in season, plus any available root vegetables, salad leaves (including cabbages) and even fruits. Cooked in a large pot directly over the fire it was a classless dish and particularly suitable for fasting days in better off households. Popular in the Medieval Period as well as in Tudor/Stuart times it was eaten even as late as Georgian  times.

Porridge, pottage, stew by MedievalMorsels, one inch scale food

Medieval Tudor dollhouse food, 12th scale pottage bowls

Now for the veracity of this ancient weather lore. According to the Royal Meteorological Society there is the tiniest glimmer of sense to the rhyme. Because "the middle of July tends to be around the time that the jet stream settles into a relatively consistent pattern. If the jet stream lies north of the UK throughout the summer, continental high pressure is able to move in, bringing warmth and sunshine. If it sticks further south, Arctic air and Atlantic weather systems are likely to predominate, bringing colder, wetter weather." 

The rhyme, according to the RoyMetSoc as it is colloquially known, just needs a little re-rendering:

St Swithun's day if thou dost rain
For forty days, relatively unsettled there's a fair chance it will remain
St Swithun's day if thou be fair
For forty days, a northerly jet stream might result in some fairly decent spells
But then again it might not