Archives For warm plume

2015-05-02_06-58-07

analysis chart shows HIGH edging out with moist Atlantic winds ready to pounce

The analysis chart above shows a weakening ridge of HIGH pressure over the UK being edged out north by a slow moving Atlantic LOW to the W/SW.  Reigate is still currently (Saturday am) in cool dull easterly winds generated by the HIGH but a significant switch in wind direction will take place over the next 12 hours into the bank holiday period as a wholly different mild and humid Sub-Tropical Atlantic air mass, with a source region round the Azores, takes hold from the SW.

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weather cross-section

A mild moist S/SW wind drives in from the Atlantic as the LOW edges north east tonight. The first mass of rain is edging onto radar from the SW and is expected to arrive in Reigate by around mid-late pm today.  Most rain is likely for places further north and west but the SE is still likely to pick up plenty of wet weather overnight with low cloud and rain into Sunday morning when it could turn heavy and showery for a while in the early morning as the trough passes directly overhead and pressure continues to fall. Things are expected to clear to brighter conditions later in the afternoon as pressure rises and winds turn more westerly. Cloud cover will hopefully break and cloud height will lift during the afternoon becoming more cumuliform.

2015-05-02_10-49-17

trough and fronts migrate north, showers follow

If the sun comes out then there could be a low risk of an odd heavy shower Sunday afternoon, possibly thundery, but these are more likely further north of our area where more unstable air makes progress across the Midlands and East Anglia.

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During Sunday winds will be occasionally blustery with moderate convective gusts possible, especially on hills and nearer the coast, and make the mild temperatures Tmax 16C feel considerably cooler. Temperatures overnight could hold up to a balmy 12-13C.

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Overnight Saturday-Sunday rain could linger as showers through the morning

Winds turn from SW to more southerly through Monday and pressure should up-tick slightly giving a mostly dry and warm day and less windy as things stand currently.  Troughs could progress east during Monday and build cloud and produce some showers.  More importantly there is a looming threat of something special for later Monday-Tuesday night.

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As the northern block (high over Greenland) holds on, the Atlantic LOW just west of Ireland will usher in a mild and moist S/SW flow of air from the continent.  An unstable LOW brewing in the topical Atlantic today (Saturday) is forecast to sweep up and intensify from Biscay later Monday and into Tuesday and this might bring heavy rain and winds to the south and SE and a possible thundery episode later Monday but more likely overnight into early Tuesday for SE.  The jetstream is dipping well south and is forecast to perk up and approach the UK from an unusually southerly direction by Tuesday.  If this happens the jetstream could deepen this low considerably, as modelled by some charts (latest UKMet shows 980mb).

Depending on the evolution we could find ourselves in the unstable left exit region of a jet where divergence aloft enhances convective action and creates heavy rain.  Warm air from the south will also contain more moisture.  A dry slot at mid-levels might also enhance instability (rising dry air cools more quickly increasing lapse rates and CAPE, enhancing lift).  High dew points near the surface temperature also encourage condensation and indicate extremely moist warm surface flows.

So all these ingredients stirred up could be a good recipe for some briefly moderate-severe weather in our region especially some briefly torrential rain, though totals are unlikely to amount to more than 10mm.  Gusty winds and gales near coasts could also accompany this system.  Latest metoffice chart shows pressure dipping to 980mb in the North Sea which is significantly LOW pressure for the time of year.

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coastal gales and convective gusts inland

However these episodes have a habit of tracking across Holland and merely clip Kent with thundery showers and miss us entirely.  Models also generally exaggerate these early on and then things flatten out nearer the time considerably.  Nevertheless, it is worth watching this develop as our first potential “warm plume” of the year.  If we take a direct hit the SE could have some heavy rain.

The GEFS summary below clearly shows the two main rainfall spikes tonight and Monday night.

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GEFS 850hPa temperatures and rainfall London

Later mid-week the LOW is expected to drift east across the UK bringing in a more westerly pattern so unsettled showery weather is likely for a while. Thereafter, a rise in pressure from a developing Euro high pressure may then take place from the south and settle things down for us in the SE, though this might only make faltering progress.

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2015-05-02_07-05-01

faltering pressure rise later next week

This sat pic from 9am over Biscay shows the weather action delivering our wet and windy weather. The northward limb of a meridional jetstream curving up from a lengthy warm journey over a warmer than usual Tropical Atlantic is pushing a lot of unstable moist air our way. Note the wave clouds in the cloud cover over France caused by a fast northerly jet. Note also the whopping cumulonimbus clouds NW of Spain and Portugal, these thunderstorms might reach UK later today or tomorrow bringing the odd crack of thundery showers to the South of the UK.  Very mobile weather controlled by the jetstream will continue through this week.  Some models are showing a deep LOW pressure autumn storm developing over the weekend and crossing the UK early next week.  One to watch as this could deliver gales and rain… possibly more to the south of the country if models prove correct.  

Quick update Tuesday: Friday sees upper trough sinking over Ireland with dramatic loop in jetstream rapidly developing a surface low East England tracking NW taking rain with it. Worst of rain set for NW. Looks wetter for Reigate on Friday but potentially reasonable for rest of weekend, even temps recover somewhat during day. Changeable and interesting, check back.   

The late summer mini-heat wave currently building nicely mid-week with temperatures up to 28ºC possible for Reigate looks likely to end with rain on Friday and then some heavy thundery showers developing through the weekend and possibly into early next week.  A cold front is moving south bringing cooler surface air on Friday and this will interact with a warm plume of upper air coming the opposite direction from Spain at the weekend.  

The result could be heavy thunderstorms. Whilst convective showers are typically hit and miss, it is probably safe to assume it will be a wet weekend at times in some places in the south.  Total rainfall for Friday through to Monday for Reigate could potentially exceed 20mm.

unstable sunday 8 Sept

So, periods of heavy rain and possible thunderstorms are looking more likely for Reigate and parts of Southern England from Friday through the weekend (especially Sunday / Monday) as the cut-off LOW over the Bay of Biscay winds up and pushes a warm upper air flow across the south which will interact with a cold front moving across the region on Friday. Indicators like CAPE and Lifted Index which measure instability (the propensity for buoyant air to LIFT / convect upwards, condensing and forming cumulonimbus clouds) are all set for some potentially perky convective thundery activity at points anytime from Friday through to Monday. Heavy thundery showery rain is possible during this period.  Nevertheless, such activity is prone to miss places entirely and drift off at the last minute so… check back for updates!