Jet stream shift to bring summer weather

It’s usually true to say that confidence in a weather forecast decreases the further out you look.

However, we have a situation at the moment where we are actually more confident in the detail for this weekend and beyond rather than over the next few days.

This is because we are seeing some very strong signals for high pressure to dominate the weather from around Friday and then persist for several days afterwards.

In summer high pressure means fine weather, and it also looks like it will become very warm as well.

Normally we can’t give much detail beyond the 5-day forecast period, but with the current situation we can give a forecast beyond that with a higher than usual level of confidence.

So for, say, the Men’s Wimbledon Final on Sunday it looks like it will be dry and very warm with a good deal of sunshine. We are less confident in predicting who will be in the final, or who will be the winner, though.

As we go through into next week, it looks highly likely that one or two places will nudge towards the 30 C mark.

With this in mind, it’s likely that by the middle of next week we’ll have topped the current warmest temperature of the year, which was 27.2 C recorded at Heathrow yesterday (Sunday, 30 June).

So why will we be seeing high pressure develop over the UK to give us this spell of fine weather?

Well, to get this type of situation we need the jet stream to be in the ‘right’ place for this time of year – tracking to the north of the UK between Iceland and NW Scotland.

Chart showing expected position of the jet stream on 6 July 2013.

Chart showing expected position of the jet stream (shown in shades of green) on 6 July 2013.

In this position it guides rain-bearing Atlantic low pressure systems off to the north of the country, meaning the far north-west of the UK gets glancing blows from these systems while the rest of the country has more settled conditions.

We expect the jet stream to move into this position over the next few days.

Leading prediction models from forecasting centres around the world all seem to be in agreement about this development – hence the unusually high level of certainty in the forecast. But, of course, forecasts can still change – so do stay in touch with the latest in our 5-day outlook and out to 30-days ahead.

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4 Responses to Jet stream shift to bring summer weather

  1. nuwurld says:

    Although I agree that this forecast will proceed and be correct I am unhappy and uncomfortable with the publicly announced reasoning.

    This is why.

    “A horizontal temperature gradient exists while moving North-South along a meridian because the curvature of the Earth allows for more solar heating at the equator than at the poles. This creates a westerly geostrophic wind pattern to form in the mid-latitudes. Because thermal wind causes an increase in wind velocity with height, the westerly pattern increases in intensity up until the tropopause, creating a strong wind current known as the jet stream. The Northern and Southern Hemispheres exhibit similar jet stream patterns in the mid-latitudes.

    Using the same Thermal Wind argument, the ‘strongest’ part of the jet stream should be in proximity ‘where temperature gradients are the largest’. ”

    Now, using the above argument, it can be seen that the Jet Stream is a ‘product’. A logical necessity of a rotating, spherical inertial frame subject to sunlight. I really don’t understand why you would think that this ‘ribbon’ of low density, albeit high velocity air could position and form the monumentally massive, colliding and energetically competing, convective cells that produce it. Yes, the conditions are ideal for promoting low pressure systems but really this is the stuff of circular argument. Surely the jet stream is not a ‘law unto itself’, when we have a formal argument that says where it will form and why?

    First we had a greenhouse ‘physical junk’ model, then we adopt another model of sophistry whereby a ‘product’ controls the conditions that produce it!

    I know that you know more. You are modelling sensible GFS parameters that can predict where the jet stream will be, a few days in advance at least. Convince me otherwise.

  2. GT says:

    Reblogged this on FishTweed.

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