Archives For September 2013

Chart on left shows rain forecast for tonight (Fri-Sat).  Chart on right shows some interesting synoptic situation in Atlantic early next week.

Reigate and SE could see some heavy and even torrential rain from late afternoon / evening today and overnight clearing Saturday am. Totals could exceed 30mm in places, but more widely totals between 10-20mm. This LOW is moving from SW in the Channel and proceeding overhead across the SE with a complex of associated fronts. This system heralds the mother of all LOWS which is moving in on Sunday… sweeping an unusually cold Arctic blast across the whole country by early next week. Autumn is here and the Atlantic shows signs of greater activity with a more active jetstream and the appearance of the first ex-hurricane / extra-tropical storm Humberto on the Atlantic Charts next week too.  Humberto is delivering Tropical air to the Pole, while our cold Arctic blast does the opposite.

The big news this weekend is the start of the temperature drop-off which looks to last most of next week.  Saturday will start this process off as cooler air is set to sweep down across the country, mainly from Sunday onwards.  Tmax might reach 12c but in the wind it’ll feel 10c or so and with rain as well it’ll all look thoroughly Autumnal!  Later next week there are signs of high pressure building back in with better weather and a recovery of temps, but let’s wait and see on that one!

850hPa drop this weekend

Weather Watch: update: heavy rain overnight Friday, moving away am; even HEAVIER rain overnight Fri – Sat, moving away am. Some possibly torrential and totals poss exceeding 30mm in next 48 hrs. 

The second significant Autumnal gear change is looking more certain for early next week as a big LOW sweeps between Iceland and Norway across the North of Scotland and a brisk active northerly jet combines to drag in a breezy and cool Arctic airmass from the Poles all the way across the UK.  The isobars on UKMO charts can be traced all the way back to the Poles, so expect chilly weather… possibly creeping into the low teens and feeling like 10c in the wind.

fri fronts

The process starts on Friday as fronts bring potentially heavy rain across the South, including Reigate, especially Friday pm and overnight into Saturday morning. The situation is complicated with fronts lingering over or near the south throughout the weekend so expect rain, some heavy, almost anytime during Friday through to Saturday. Saturday will be cooler than Friday.

The real news, though is the temperature plunge next week. Keep posted for more on this Autumnal development.

There might be a glimmer of some warmer temperatures returning by 21 Sept as a high builds from the SW … temps of low 20c’s might be expected to make a return but certainly not a heat wave!

On 5th Sept 2013 Reigate was, unofficially at least, the hottest place in the UK! (source weather online)top 10 hottest 5 sept

Polar air quickly swept aside the heat of the continent in less than 24 hours. Whilst by no means cold, the 13c reduction in temperature in 24hrs combined with the first rain for 11 days in Reigate made it feel quite Autumnal.  As models suggested the 5mm rain was less dramatic than originally thought… which is surprising considering the sharp change in air mass.

The cool whirlpool of polar air brought down by the jetstream from Iceland will be held in place by a meridional loop in the jetstream for several days.  It’ll gradually warm through but this could spark off a few showers.  So no true Autumnal stormy drama yet, just a gradual edging out of summer heat.

August had 37.6mm of rainfall (July 23.6mm), with more than 20mm falling on just three days: 5th, 22nd and 24 August.

The maximum temperature of 33.5C (July 33C) was recorded on 1st August, hottest in the UK equivalent to Heathrow, the official warmest!

The minimum temperature of 9.7C (July 9.6C) was recorded on the 24 August.

The average for August was 17.4C
157 sunshine hours were recorded by our new solar sensor, way short of the 213 hours recorded in July.
12 rainy days were recorded in August with 10 rainy days in July.
The pictures show that August continued the excellent summer we have had in Reigate.

Reigate has some of the warmest, driest and most pleasant weather of any part of the UK.

This weekend a big LOOP in the jetstream is set to rip up the summer heat still lingering over the southern part of the UK. In a dramatic weather battle in the upper atmosphere above our heads the southerly blowing limb of the jetstream will bring POLAR air to west of the UK to fight it out with the continental tropical heat lingering over the south and east. Heavy rain and strong winds will be the result of this battle but the exact location of the heaviest downpours is tricky to be precise about. Broadly speaking, the heaviest rain will start in the South of the UK on Friday and move North, rotating over to the NW and falling heaviest over the northern hills as the LOW pressure drifts NW during Saturday and Sunday. The South could have comparatively drier days after any heavy rain on Friday, but stay tuned for details on that.  Friday rainfall for Reigate could exceed 10mm starting in the afternoon – so possibly very heavy rain for a few hours Friday pm, with lighter rain through Saturday and possibly none at all on Sunday as the LOW moves away to the NW.

Very warm air drifting up from the continent over England by a northward blowing jetstream will be forced to rise over the markedly colder polar air invading from the NW through Friday and Saturday. The difference between these air masses is very marked: the warm 28ºC surface air currently over Reigate equates to -12ºC at 5000m while the invading cold Polar air mass is -28ºC at 5000m, which will push down surface temperatures on Friday to a Tmax of only 16ºC! It is this contrast between the air masses which is a hallmark of autumnal weather and the key to creating lots of rain: polar air meets tropical air, forcing it skyward, forming rain with gusty winds spiraling round LOW pressure: typical autumnal scenario (except this weekend’s scenario is actually quite unusual: called a trough disruption with the surface low drifting off in an unusual direction: from SE to NW, unlike usual LOWS which track west to east across the UK along a zonal jetstream).  Wind speeds for Reigate this weekend could gust at 25mph at times on Saturday. No thunder is now forecast this weekend as the polar air is relatively stable, being on a return leg to the Poles.  

Thereafter, as you can see from the ensemble below, next week looks to remain cool and unsettled but with some improvement in the south possible later in the week as pressure could rise by next weekend, but no return to hot conditions is expected.

ensemble sept 4

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!