April dips into cool spell #Reigate

April 19, 2016 — Leave a comment

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Currently Reigate and the SE is enjoying a settled sunny spell under the continental flow of a HIGH pressure sat directly over the UK.

Light surface easterly/SE winds and unbroken sunny skies under high pressure have brought temperatures up to 17C locally today.

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The 500mb chart above shows the upper southerly / SE flow bringing recently warm conditions in the daytime.  Unfortunately things change by the end of the week as the upper flow swings round to arrive from the north over the weekend and push temperatures well below average for the time of year.

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The jetstream will be arriving direct from Greenland by Sunday, as HIGH pressure builds over the Atlantic and falls to the east of the UK.

Airmass temperatures in this northerly polar flow (measured at 850hPa) could even be low enough for showery wintry precipitation.

Cold late Springs are not unheard of in the SE, 1981 was a cold Spring with snow, for example. Recent Springs over the last 25 years have been milder than the long term, so this event is still “normal” weather.

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The HIGH is set to regress NW into the Atlantic and merge with a high pressure over Greenland.  Pressure over the Poles is rising as well, shown by the swing to negative Arctic Oscillation and related NAO.

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The result of higher pressure over the Polar regions and low pressure in mid latitudes is to push cold Arctic air south.  If this was January it would be a truly cold period coming up.

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With LOW pressure over the North Sea the resulting funnel of cold northerly winds will dig south across the UK and into Europe especially from Sunday into next week.  Sunday is the London marathon and, whilst showers are forecast coming down the UK during the day, it looks a mostly dry but cool start for runners.

Most of the wintry action looks set for Benelux but here the Tmax during the daytime are likely to be around 10C, but fall to near freezing at night, with frosts under any clear skies.  Occasional wintry showers are possible, especially Monday. Note the Tmax temperatures on Monday daytime shown below.

Cool for the time of year for sure, as shown by the contrasting anomaly charts below between this week and next.

During this cold spell next week fresh falls of snow are expected across mountain regions of Europe, especially Norway and the Alps and the highlands of Scotland and Northern England.

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At lower elevations snow might fall at times during this cold spell, however, strong April sunshine (as strong as August) will quickly raise temperatures between showers so that, even with any wintry precipitation, ice and frost will melt rapidly away.  Icy roads overnight are also unlikely at lower elevations because soil and surface temperatures have risen well above freezing.  Nevertheless, gardeners should be aware of the chance of frost and 10cm soil temperatures will fall to near 5C over the course of next week.

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How long will this last?  Charts show the cool spell lasting for much of next week but with some amelioration throughout likely.  Stronger sunny periods will make it feel satisfactory out of any wind but night time on east coasts in heavy wintry showers, for example, will feel distinctly chilly!

Where’s the heat gone?  It’s worth pointing out the very high temperature anomalies over N Africa and Greenland at the moment.  The highest and earliest surface melt across the Greenland ice cap is likely to continue next week in the upstream flow around the Atlantic block. The build up of heat in N Africa (shown below) could, in the right conditions, bring continental heat to the UK later and, perhaps some decent thunderstorms, but this is all just speculation.

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