Nick F

After a cool down over the next few days, parts of the UK look set to see an Indian Summer during the first week of October on the back of one of the warmest Septembers in recent years. View the full blog here

More for northern Britain and EIRE really, but as well as winds gusting to 50-60mph over Scotland, perhaps 60-75mph far NW Thursday morning, some lively and perhaps thundery showers too: Storm & Convective Forecast Issued 2016-09-28 22:48:32 Valid: 29/09/2016 00z to 30/09/2016 00z CONVECTIVE / STORM FORECAST - THURS 29TH-SEPT-2016 Synopsis Long-wave upper trough to NW of UK edges SEward across the UK during Thursday. A deep area of low pressure passing close to northern Scotland overnight runs east and merges with low pressure complex over Norway. Cold front associated with this low will clear SE UK Thursday morning, with a cool Pm flow following which will become increasingly unstable across Scotland and N. Ireland. ... SCOTLAND, N. IRELAND, EIRE and NW ENGLAND ... Cold front sliding SE across England and Wales is unlikely to produce any strong convection, due to poor lapse rates/ warm mid-upper temps above front. Forward side of large-scale upper trough pushing SE across the UK will introduce sufficiently cold and dry mid-level air atop of warm seas and modest land heating to create an increasingly unstable Polar maritme airmass across northern mainland UK and EIRE. So across the above areas expect scattered or numerous heavy showers, some of which will be accompanied by hail and thunder. Vertical deep layer shear will tend to become weak as the greater instability arrives from the NW in the morning and into the afternoon. Though one or two troughs and occluded fronts in the strong westerly flow could organise convection sufficiently to bring a risk of localised flooding. There is also a risk of strong and perhaps isolated damaging convective gusts associated with downdrafts bringing down strong upper winds within heaviest showers or storms across central Scotland in particular. Therefore have issued a MARGINAL risk of severe weather here.

UK Severe Convective & Storm Forecast - Issued 2016-09-28 22:48:32 Valid: 29/09/2016 00z to 30/09/2016 00z CONVECTIVE / STORM FORECAST - THURS 29TH-SEPT-2016 Click here for the full forecastCheck the full storm forecast here

A powerful trans-Atlantic jet stream brings ex-tropical depression Karl sweeping north of UK Tuesday, followed by another deep low on Thursday. So gales for the north at times and some rain or showers too, though south quite warm. View the full blog here

Storm & Convective Forecast Issued 2016-09-25 08:38:33 Valid: 25/09/16 0600 - 26/09/16 0600 CONVECTIVE / STORM FORECAST - SUN 25TH-SEPT-2016 Synopsis Longwave upper trough moves in from the west across the British Isles. At the surface, N-S aligned frontal system bringing overnight rain clears east coast first thing ahead of low pressure system to NW of Scotland. An increasingly unstable Polar maritime (Pm) flow spreads in from the west through Sunday, with blustery showers accompanied by a risk of hail and thunder affecting the W and NW. ... EIRE, N. IRELAND, N WALES, N ENGLAND, SCOTLAND ... Upper trough moving in from the west will increase lift of moist maritime flow and introduce increasingly cold air in the mid-levels which will steepen lapse rates. As a result, moderate instability will develop across the above areas, aided by modest surface heating, with 300-600 j/kg CAPE forecast sufficient for hail and thunder to accompany the heavier showers expected to spread in/ develop from the west today. Deep layer shear will be fairly weak, so showers /storms are unlikely to produce organised severe weather, though there is a risk of minor localised flooding, hail and gusty winds from heavier cells. Issued by: Nick Finnis

UK Severe Convective & Storm Forecast - Issued 2016-09-25 08:38:33 Valid: 25/09/16 0600 - 26/09/16 0600 CONVECTIVE / STORM FORECAST - SUN 25TH-SEPT-2016 Click here for the full forecastCheck the full storm forecast here

06z GFS whips up an intense depression early next week from the remnants of Karl - which gets swallowed up by the upper westerlies then falls under the developmental left exit of a strong jet where it deepens as it heads toward the Faroe Islands. Gusts of 80mph+ for NW Scotland next Tuesday evening Dragging in some quite warm air ahead of the system, so could see some warm temps to lee of Pennines perhaps? 00z ECM had more of an open wave formed from the remnants of Karl moving NE and clipping northern Scotland Tuesday afternoon, with tight isobars on the southern side of the wave likely to bring gales or severe gales for northern parts. Obviously a good deal of uncertainty with the track of ex-Karl this far off, though would favour a track north of Scotland for now.

Once every few years on average deep depressions formed from ex-hurricanes make a direct aim at the UK. Ex-Karl may head towards the UK next week, after is perhaps becomes a hurricane briefly near Bermuda this weekend. Could be re-named Angus if it does. View the full blog here

A few work colleagues that live in Berkshire gloating about all night storms with constant lightning and thunder, and I get bugger all in Croydon last night, not happy. Electrical activity seems to be easing now, just turning into a damp squib for many in the east. Though looks like a now storm developing ahead of thundery rain over E Cambs / W Suffolk now.

Had a few flashes and rumbles in the night in south London, but the bulk of the more active storms appear to have been to the west and NW of London over Hants, Berks, Surrey, east Oxon, Buckinghamshire. At work near Heathrow now and there's some occasional flashes and rumbles from the edge of the storms just NW of London, appear to be edging this way. A few flood warnings and watches across London area from the overnight storms

Seems to be forming along the North Downs escarpment from Farnham to Swanley (NW Kent) - orographic lift?

First loud rumble of thunder of the evening just then here near Croydon, building closer!

Line of storms over from Surrey down to Hants beginning to back build to around 10 mile south of here, just south of Caterham, occasional sheet lightning to the south.

Storm & Convective Forecast Issued 2016-09-15 21:21:06 Valid: 16/09/2016 00z to 17/09/2016 00z CONVECTIVE / STORM FORECAST - FRI 16TH-SEPT-2016 Synopsis Slack upper and surface flow pattern across the UK on Thursday, between upper trough to the west and upper low over France will be replaced by a more progressive / strengthening W/NWly flow at all levels on Friday, behind upper trough and attendant surface cold front moving in from the west. So unusually warm and humid plume that’s been across much of England and Wales since the beginning of the week will be kicked out and replaced by cooler and fresher conditions from the west. During this transition, thunderstorms are likely to become increasingly widespread across S and E England during Thursday night and Friday morning, before clearing east later on Friday. … S and E ENGLAND, MIDLANDS … Convergence north of W-E frontal boundary pushing up across the English Channel and a subtle shortwave moving NE appears to be the forcing mechanism for back-building lines of storms across Dorset, Hampshire and Sussex this evening. These storms are forecast by models to expand in coverage across central S and SE England (including London) and perhaps S/E Midlands overnight, as forcing and cooling increases aloft from the west. These storms will continue the tendency to back-build over some spots – leading to an enhanced risk of flash-flooding from rainfall rates as high as 30-40mm per/hr. Frequent cloud-to-ground lightning will pose an additional hazard. Also these storms will be capable of producing hail and gusty winds. There is a SLIGHT risk for central S and SE England during the first half of Friday morning for of flash-flooding. Eventually these storms and heavy showers will merge into a broad band of heavy thundery rain, with isolated embedded storms, that will spread east across much of S and E England … as the cold front moves in from the west. Lightning may become less prolific and more isolated during the morning as CAPE is reduced by cool outflow/cold pooling from overnight storms and also cooler and drier air undercutting from the west. But there will continue to be an isolated risk of flash-flooding from stronger convective cells, as well as a risk of gusty winds. Band of thundery rain with embedded storms eventually clearing east coast Friday evening. There is a MARGINAL risk over S and E England for the continued risk of flash-flooding, though there is a SLIGHT risk over the far SE of England –where a higher risk for stronger storms with flash-flooding will exist into the afternoon. Issued by: Nick Finnis http://www.netweather.tv/index.cgi?action=convective;sess=

UK Severe Convective & Storm Forecast - Issued 2016-09-15 21:21:06 Valid: 16/09/2016 00z to 17/09/2016 00z CONVECTIVE / STORM FORECAST - FRI 16TH-SEPT-2016 Click here for the full forecastCheck the full storm forecast here

Can just make out the anvil of the storm over west London from where I am at work at Gatwick Airport in Sussex.

The EC rainfall charts can be found here from an Iceland Weather Service site: http://brunnur.vedur.is/kort/ecm0125/2016/09/15/00/ecm0125_nat_msl_t850_6urk.html

Interesting chart from Metdesk posted on twitter: ... ties in with the ECMWF rainfall charts:

The GIF above from a French weather site shows nicely how the warm humid plume across southern Britain is squeezed out both from the south and west as cold fronts approach from both directions, quite an unsual evolution, but the pincer movement of fronts into a rather unstable airmass later should hopefully create some lively storms over next 24-36hrs before the cooler and fresher Atlantic air wins out from the west:

Anaprop I'm afraid,often there, think it's ships! Won't be staying up to see if that thundery rain / MCS over France makes it across the Channel tonight.

Netweather's official storm forecast for Thursday: http://www.netweather.tv/index.cgi?action=convective;sess= Storm & Convective Forecast Issued 2016-09-14 22:05:55 Valid: 15/09/16 00z to 16/09/16 00z CONVECTIVE / STORM FORECAST - THURS 15TH-SEPT-2016 Synopsis Quite a complex synoptic pattern evolves across the UK/NW Europe over the next 24 hours, an upper low over Biscay will gradually drift across western France while filling … while Atlantic upper trough approaches from the west. At the surface, a shallow low will drift north across the English Channel and in across southern England during Thursday. A slack warm and humid but unstable airmass across much of the UK will be capped across most areas, however, thunderstorms may develop across central parts of England by the evening ahead of cold front arriving in the west. … S ENGLAND, MIDLANDS, E WALES, N ENGLAND … A MCS moving N/NW over France tonight may eventually affect the Channel Islands, English Channel and low risk of affecting southern coastal counties of England overnight before gradually weakening during the first half of Thursday morning. Any elevated storms that make landfall over Channel Islands or south coast could bring a risk of localised flooding, frequent cloud-to-ground lightning and gusty winds. Then during Thursday, despite GFS building up to 1000+ j/kg CAPE in the afternoon, as warm humid airmass is heated in the sunshine, this potentially unstable airmass will generally be capped. However, shallow surface low drifting north across southern England will likely induce breeze convergence in a N-S line from central S England through the Midlands and up towards N England. So along this zone sufficient lift may break the cap to allow isolated storms initially in the afternoon … before greater forcing for ascent arrives from the west in the evening as upper trough and surface cold front move in from the west … allowing storms to expand in coverage over SW and C S England, E Wales, The Midlands and N England. Given weak winds aloft – storm motions will be fairly slow … combine this with PWAT (Precipitable Water) values of 30mm+, there is a risk that storms could produce large rainfall totals in a short space of time locally (25-30mm per/hr). Also, given large CAPE values, storms may also produce brief bouts of isolated large hail (2-3cm) and frequent cloud-to-ground lightning … before storm updrafts are cut-off by downdrafts in weakly sheared environment. Have issued a MARGINAL risk for severe weather across the SW/CS England, E Wales, MIDS and N England. … E SCOTLAND … There is also a more isolated risk of storms across eastern Scotland … as cold front/upper trough moves in from the west and creates forced ascent of warm and humid airmass. Any storms could produce localised flooding. Issued by: Nick Finnis

UK Severe Convective & Storm Forecast - Issued 2016-09-14 22:05:55 Valid: 15/09/16 00z to 16/09/16 00z CONVECTIVE / STORM FORECAST - THURS 15TH-SEPT-2016 Click here for the full forecastCheck the full storm forecast here

Storm & Convective Forecast Issued 2016-09-13 10:12:30 Valid: 13/09/16 06z to 14/09/16 06z CONVECTIVE/STORM FORECAST - TUES 13TH-SEPT-2016 Synopsis Atlantic upper trough to the west of UK will continue to extend SE towards SW Europe today before disrupting into a cut-off upper low over northern Iberia by the end of the day. At the surface, a waving cold front … lying N-S from western Scotland down through the Irish Sea, Bay of Biscay and Portugal …will eventually pivot around a developing low over Biscay/W France. Heavy rain and thunderstorms will pulse north across western just ahead of the cold front as colder air and divergence aloft across the west destabilise the western edge of the warm and moist plume further east. … SW ENGLAND, WALES, W MIDLANDS, NW ENGLAND and S SCOTLAND … Increasing divergence/lift and cooling aloft on eastern flank of upper trough combined with drier air and stronger positive vorticity pushing north in the mid-levels (500mb) from NW France is destabilising western edge of warm and humid plume, characterised by high (16C+) theta-w values (wet bulb potential temperature) spreading north across England and Wales. Thunderstorms will spread north across SW England and south Wales this morning as result of this destabilisation, before the storms carry on north across north Wales, NW England this afternoon before arriving across SW Scotland this evening. These thunderstorms will be generally elevated and rooted above a stable boundary layer, but will be in environment of deep layer shear values of 40-60knts, therefore will likely organise into multicell clusters and perhaps upscale growth into a MCS (Mesoscale Convective System), mainly bringing a risk of excessive rainfall (up to 30mm per/hr) and frequent cloud-to-ground lightning. With fairly large CAPE values, there is an isolated risk of large hail (2-3cm diameter) too. Therefore have issued a SLIGHT risk for severe weather across these western areas. A few isolated thunderstorms could develop across central southern England and West Midlands too this evening, given unstable warm and humid airmass advecting north today, characterised by CAPE values reaching 1000 j/kg+. But this looks less certain Issued by: Nick Finnis

UK Severe Convective & Storm Forecast - Issued 2016-09-13 10:12:30 Valid: 13/09/16 06z to 14/09/16 06z CONVECTIVE/STORM FORECAST - TUES 13TH-SEPT-2016 Click here for the full forecastCheck the full storm forecast here

Certainly tomorrow's date record of 28.3C set back in 1934 is under threat. Ironically, eastern England is forecast to be hotter than southern Spain tomorrow, which has just come out of a record heatwave when last week temperatures reached 45C (114F) Max temps last Tuesday across Spain: Looking at the medium range of the operational runs today, GFS appears to have been more progressive in keeping a deeply cyclonic flow close or over the UK, whilst the ECM keeps the Atlantic trough further west towards the end of the run, with a ridge being thrown north across the UK, so summer warmth may return again. 12z EC and GFS comaprison reflacts this difference: