John Hammond’s Month Ahead – High impact cold threatens as Spring approaches

John gives us his latest thoughts on likely weather trends on the horizon over the next few weeks. To ensure you are reading the very latest weekly blog, check out Wow and Why

If you think life’s a rollercoaster, you should try being a long-range weather forecaster at the moment!

In recent blogs, I’ve been flagging up the potential for a quite impactful cold spell to arrive from the east as March approaches. Well, over the last few days the world’s best weather models have been doing somersaults! Their changes in direction about this wintry threat have in part been the result of the strong Sudden Stratospheric Warming event which has been unfolding at the top of the atmosphere.

The models often struggle with this phenomenon. How and when does the resultant reversal of winds (from westerly to easterly) work its way down to the surface? As I write, I’m not surprised that there is still a degree of ‘anarchy’ amongst model forecasts. And depending on which model your weather app uses, you may be given the impression that February will end on a springlike, benign or bitterly cold note. Take your pick! The problem is that these apps have little or no human intervention.

However, despite this confusion, my thoughts about the prospects haven’t really changed since last week’s blog. Whether you’re craving spring or are still hoping for some of the ‘white stuff’, my message is clear – winter is far from over (whatever the calendar might say).

There is a non-zero chance of disruptive snowfall by next weekend.


Springlike start – Wintry finish

The week begins in a fairly mundane way. Atlantic weather fronts will be straddling the UK, erratically edging from west to east. Their progress will be slow, but we can expect cloud and at least a little rain at some stage through Monday. With an Atlantic feed of air, it should be a relatively mild start to the week.

It’s by Tuesday that computer forecasts have been diverging so dramatically on how they expect waves of atmospheric energy to interact. These are subtle differences, but will have a profound effect on events later in the week and beyond. And my thoughts must be put in the context of this inherent uncertainty.

I expect high pressure to build quite dramatically from the Azores, across the UK and all the way into Scandinavia. As this happens, weather fronts will be stopped in their tracks. And, as the ‘Scandinavian High’ begins to dominate, they will start to be pushed bodily back westwards through Tuesday. In the wake of this, colder air will arrive,

By midweek, chilly and frosty weather will prevail. It will be dry for many places; and in the sunshine, with light winds, it will feel quite pleasant. However across some southeastern parts of the UK, on the periphery of the high pressure, cold northeasterly winds will have quite an edge. With time, there is an increasing chance of wintry showers arriving from the east. However, initially, these look unlikely to be very impactful.

Later in the week and into the weekend, there is greater chance of a much more intensely cold airmass approaching from the Continent. If this manages to make it to our shores, the threat of impactful snow markedly increases. There is a non-zero chance of disruptive snowfall by next weekend.

Many a spring lamb will have a shivery shock as they emerge from the womb.


High impact cold

I expect spring to approach on a very wintry note. With stratospheric winds still blowing westwards (instead of eastwards) around the northern hemisphere, there seems little reason why, further down through the atmosphere, the Atlantic jet stream will be re-energised enough to dislodge the strong blocking high pressure that keeps us in the fridge.

With such cold air in place, what comes out of the sky is likely to be mostly solid. I expect snow and, more widely, frost to be enduring threats beyond St David’s Day. Given the dominance of continental winds, it’s the more eastern parts of the UK that are most prone to snowfall. However, as fronts attempt to push moister Atlantic air and rain into the cold air, some western fringes are not immune either.

Cold weather in March can have a greater impact, particularly on the natural world, than during the dormant earlier winter months. Many a spring lamb will have a shivery shock as they emerge from the womb.



The battle for Spring to arrive

I foresee much disagreement amongst computer models as to how quickly the cold block is removed by milder Atlantic weather systems. Their signals will wax and wane. But the likelihood is that gradually,  low pressure will bring wetter weather and a modest rise in temperatures.  However, the process may well be slow and erratic. Further significant snowfall is possible within the battleground between cold and mild airmasses.

The lengthening days and stronger sun mean that, with time, the odds are increasingly stacked against severe cold hanging on towards mid-month. It may be a struggle, but by the time the equinox approaches, Spring 2018 should finally be emerging. Those lambs will be hoping so too…

Given the current volatility of the atmosphere, we will, of course, update this blog during the week if our thoughts change significantly in the days ahead.