Archives For high pressure

2016-03-20_09-26-16

High pressure has dominated the last week of our weather but it has turned out disappointing here due to cloud cover lingering under a persistent temperature inversion, not unusual for this time of year.  Lingering decaying fronts have caused drab stratocumulus cloud to spread out beneath an anomalously warm upper air mass causing anticyclonic gloom for much of the SE.

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Any convection has been limited to the lowest 1km and been unable to break the inversion, so cloud, unable to rise into cumuliform tufts associated with the stronger sunshine in April, simply spreads out into a boring grey blanket, especially when the flow arrives from the North Sea bringing additional moisture in the lower layers. An inversion is when temperatures increase with height through a part of the atmosphere, usually referring to a lower layer.

So, whilst upper air temperatures have been anomalously warm, the surface temps have been kept disappointingly low.  Somewhat “upside-down” weather.

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This is because the Spring sunshine has been unable to break through the cloud and warm the surface.  The exception has been the north and west of the country, especially the hilly parts of Wales and NW England, which have enjoyed more sunny days.

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anticyclonic gloom

Today it was the turn of the E/NE coast to get the sunshine as weakening fronts shifted south around the high.

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The coming week sees the HIGH slowly deflating, like a sad party balloon, into the sub-tropical Atlantic.  A couple of powerful late winter storms emerging out of the NE US and Newfoundland start the onslaught to break a westerly unsettled flow back across the Atlantic by Easter.

For us in the SE this change to unsettled conditions happens slowly but ensemble runs are showing around 20mm of rain is possible before the end of March. So we might expect some wet and windy days before the end of March, including a risk of a wet bank holiday period.

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A return to more mobile frontal conditions is not all bad news, especially in the SE.  A westerly flow with weakening Atlantic frontal systems will break the gloomy cloud cover and bring sunny intervals and showery episodes to clear the pollution phase we have experienced lately.  The risk for us is any fronts stalling over the SE in front of a European HIGH – this situation can dump fairly large amounts of rain. Chart below shows wind speed and an active cold front for Sunday, too far off to be reliable but worth watching.

 

The multi-model charts below show this change from HIGH to LOW during the next week.

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Note the model agreement below by Saturday for SW winds, bringing temperatures up possibly into the mid-teens in the SE.

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In short, we can expect the weather to turn the “right way up” again and we should enjoy more mobile, fresher, brighter conditions, albeit with breezy episodes of potentially heavy rain at times. The charts below show temperatures rising in the SE but rain returning by Friday.

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Further into April there are hints that pressure rises across the Pole which will bring the potential for further unsettled cooler conditions during the school holiday period. RGSweather is off to Iceland (East fjords) again so this could mean some nice cold conditions for our trip there as the AO is expected to turn negative and the flow northerly, at least for the N Atlantic.  The UK appears to get stuck in an unsettled trough for early-mid April. Worth watching as JMA and CFSv2 both agree on this blocked pattern.

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2016-01-05_20-38-55

GEFS cool dip mid January

A cold snap looks more likely next week from around 13 January. Nothing extreme, just a long-overdue “normal” wintry feel is on the cards.

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12Z gefs and ecm ensembles show more distinct dip in temps

 

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ECMWF upper air goes cold

Temperatures are due to take a dip below the seasonal norm.  It’s still a way off so details will change but here’s the current idea moving ahead.

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The current run of wet Atlantic westerlies (above) that has brought flooding to many parts of the country, is due to weaken as pressure rises over the Atlantic and further north over the Pole.   As the persistent Atlantic LOW pressure gradually moves East this week it will bring more rain across the UK.  As it moves further east over the weekend it is forecast to draw down cooler northerly winds from the Arctic next week, at least for a while. (see below). (update: “Atlantic block” noted on chart below is probably overstating it a bit … as HIGH is likely to give way fairly promptly)

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2016-01-05_21-04-00

Arctic Oscillation goes negative january 2016

The cause of this Arctic outbreak is indicated by the Arctic Oscillation (AO) going negative.  The AO is a measure of air pressure over the Pole relative to mid-latitudes.  It has been positive for most of the autumn and winter so far and this usually means a strong jetstream and mild wet westerlies for the UK.

When the AO goes negative it indicates building pressure over the Pole and more likelihood of Arctic air “leaking” into mid-latitudes.  The chart below shows the 500mb mean heights for 8-10 days time.  Spot the anomalously high pressure over the Pole relative to the mid-latitudes.

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8-10 day mean 500mb heights (ecm and gfs models)

The negative AO has been predicted by many expert long range forecasters for a long time partly because of a lack of sea ice in the Kara Sea (part of the Arctic Ocean near Siberia).  This is a long-term indicator for potential pressure rises in this region.

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In addition, the recent enormous pump of warm air, courtesy of the Storm Frank, will have encouraged tropospheric height rises over the Pole.  The result is an inflating balloon of relatively cold air waiting to pop into the populated mid-latitudes!

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surface temperature anomalies Jan 14 2016

As well as a push of cooler Arctic air, pressure is also due to rise over the UK.  With increased pressure we can thankfully expect a drier period.

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wintry pressure rise

A classic winter high might be expected to bring dry, bright clear days with cold nights with views of the stars.  On the other hand, frost, fog and icy conditions might also be expected too.  In the SE huge dumps of snow look most unlikely next week from current model runs but lows can move south in the Arctic flow and cause unexpected events. The duration of the cold snap doesn’t look long** as the Atlantic HIGH is swept away by more westerlies.  However, this is uncertain so stay tuned for more regular updates on twitter.  Of course, seek professional forecasts for decision making purposes.

update 06/01 **looking potentially more prolonged now. cold snap turning into a spell.2016-01-05_20-55-31

January 2016 cold snap

2015-16 winter forecasts have long considered the possibility of cold weather in the latter stages from Jan thru to feb. This was explored in a post here

https://rgsweather.com/2015/12/29/winter-is-nigh/

2015-09-18_19-00-43

run reigate weekend wins out for a great day

The second annual Run Reigate half marathon and 10km race on Sunday is a major event for the town.  Happily the weather looks PERFECT for both runners and spectators!

A ridge of HIGH pressure is building in as the trough which brought a few sharp showers this afternoon moves off NE. So we expect no rain at all this weekend.

A beautifully settled early autumn weekend is expected.  Cool nights at Tmin 10C with some autumnal mist early morning are likely.  Days will brighten up in sunshine to reach possibly Tmax 20C in the afternoon.

Light winds and little cloud will be appreciated by runners and spectators. The outlook is for unsettled cooler than average conditions to gradually return on Monday and into next week with rain at times as Atlantic fronts progress across the country with low pressure. Further ahead there is a chance of a warmer and drier end to September as a Scandinavian high builds to the east.

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outlook shows a fortunately settled weekend for Run Reigate