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Good evening Yes, it’s the bigy Next Winter 2013/2014 Forecast & Seasonal Outlook. In tonight’s blog post we’re going to focus on May Sea Surface temperatures and how they may impact the winter of 2013/2014 across the United Kingdom. Winter forecasting, is very unreliable. It’s not to be relied on, in fact, usually in the United Kingdom a forecast of more than 5-7 days is very tricky, and dealing with short term impacts of less than 2-4 days can be even trickier. That just gives you a good idea of just how uncertain a winter forecast can be. May sea surface temperatures are usually looked at as a preliminary guide on the “lead up” to a Winter forecast. But, just like most things to do with weather: You can debate it. We focus on looking at the Atlantic, the Sea surface temperatures there. What we look for is the tripole. Bands of warm, cold & warm surface sea temperatures across the North Atlantic. What sort of “May SSTs” are we looking for to get that shovel out?: The Tripole, those bands of warm, cold, warm air in the North Atlantic, Relating to an Increase of Atlantic Blocking (Large area of High Pressure), with Low pressure as a Result to the South of the High, producing Easterly winds across the British Isles, proving cooler & at times snowy conditions. That would be a negative NAO. And no tripole, or no evidence of bands of warm, cold, warm sea surface temperature during May, then you’d look at a positive NAO or “Milder” conditions, with a Tendency to build Low pressure to the North, instead, resulting in a Westerly airflow across the British Isles.
How reliable is this “Theory”?
Now, the Met Office “claims” to put a 70% confidence on this theory. It doesn’t always work. It did last winter, but it might not “work” this time round, for Winter Forecast. Also want to add, this is preliminary. It’s not a forecast, it’s the first of many guidance updates: So treat it for what it is. Oh, and, even at the end of this update if I say current guidance is for cold, a “cold” winter usually has mild periods to. Just like last Winter in the UK. Christmas mas was mild, but the new year turned cold. Here’s the chart from the Wetterzentrale.de reanalysis data. Notice the mild Westerlies at Christmas:
NAO: In Layman’s Terms.
NAO. What is the NAO? The NAO is short for what’s known as the “North Atlantic Oscillation”. It is important to understand that the NAO is not a driver of our winter weather; More a measured index. It tells us what’s going on in the North Atlantic relative to pressure. High pressure blocking, Easterly winds, negative NAO: And a cooler European winter. Low pressure trough replacing the blocking, positive NAO, Westerly winds: And a milder European winter. Here’s an example of a Negative NAO. Notice the Easterly winds across the country with High pressure to the North.
May SSTs & NAO: The Deadline.
The May SST have got that key “Tripole” over the North Atlantic, so that would lend towards a cooler European winter, as shown below. This, in layman’s terms, is low confidence and anything can change. I hope you managed to understand all the technical Jargon used in this blog. I suggest you stay tuned, the next Winter guidance will be on the 2nd of August, a US Winter 2013/2014 Forecast will also be Included.