Nick F

Another good day for convective cloudscape and rainbows in the clean polar flow, my convective / storm forecast: http://www.netweather.tv/index.cgi?action=convective;sess=

Storm & Convective Forecast Issued 2017-09-13 09:13:43 Valid: Weds 13 Sept 6am to Thurs 14 Sept 6am Day 1 Convective / Storm Forecast Synopsis In wake of Storm Aileen - which will move across Denmark early afternoon, a potent upper shortwave trough in strong belt of upper westerlies will swing in across western Britain today, enhancing the instability of cool and unstable westerly flow across the UK. Much of Scotland will see cloud and general rainfall, limiting convective potential. ... IRELAND, N. IRELAND, ENGLAND and WALES ... Increasingly cold mid-levels will spread in from the west with arrival of mid-level cold pool/shortwave trough ... steepening lapse rates and supporting heavy showers and increasingly scattered thunderstorms, as surface heating in sunny spells allows convective cloud to attain more height for charge separation. Heavy showers intially across western areas this morning, fuelled by seasonally warm SSTs and steeper lapse rates before becoming more widespread further east as surface heating and increasing lapse rates destabilise atmosphere. Thunderstorms most likely across Ireland, Wales, N and W England ... though anywhere across England may see storms this afternoon. Vertical deep-layer shear will be generally weak away from S England (underneath strong jet stream) and CAPE modest in cool Pm flow (a few 100 j/kg), so generally showers/storms will tend not to organise or be prolonged to bring a severe threat. Heavier showers and t-storms may bring hail and enhanced strong wind gusts. Across S England/S Wales lines of showers/storms may organise and align into linear clusters with westerly streamlines to bring a risk of localised flooding and perhaps isolated strong, maybe damaging, convective wind gusts - particularly across SW England and S Wales - where have delineated a MARGINAL risk.

Suddenly getting rather blowy in south London now, replacing old fence panels in the garden that have historically come loose in high winds was on my list to do this month, didn't expect gales to arrive so early in autumn, so been caught out. Can here the fences creaking ready to blow out already, hammer out in the morning I think ... gust of 69mph at the Needles tops so far, though a very exposed site in the English Channel.

Should be some nice convective cloudscapes today in the cool and clear Pm flow, cameras at the ready. Netweather convective / storm forecast: Storm & Convective Forecast Issued 2017-09-11 08:11:51 Valid: Mon 11th Sept 6am to Tues 12th Sept 6am Day 1 Convective / Storm Forecast Synopsis Upper low and collocated surface low close to NE Scotland will move out into the N Sea toward Norway today. An unstable, cool and showery Polar maritime NWly flow covers the UK. ... ENGLAND and WALES ... Pool of cold mid-level temperatures associated with upper low will cover the UK, creating steep low to mid-level lapse rates and supporting numerous scattered showers today across the UK. Surface heating will allow convective clouds to attain sufficient height for thunderstorms to develop later this morning and through the afternoon ... these mostly across England and Wales - where temperatures, moisture and forcing will be greatest. Thunderstorms likely most numerous across central, northern England and eastern England. Vertical shear will be weak, so showers/storms will be disorganised short-lived pulse-type convection. Any showers/storms may produce hail, localised minor surface water flooding and strong wind gusts. Issued by: Nick Finnis

UK Severe Convective & Storm Forecast - Issued 2017-09-10 17:54:35 Valid: Sun 10th Sept 12pm to Mon 11th Sept 6am Day 1 Convective / Storm Forecast Click here for the full forecastCheck the full storm forecast here

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Heavy downpour rolling through Croydon right now and the wind's suddenly gusting despite slow-moving cell. T&L infrequent though.

And onto tomorrow, another chance for some rumbles and flashes, for those like me, who didn't get any today ... though risk for SE probably not until late afternoon/early evening. Storm & Convective Forecast Issued 2017-09-08 21:42:58 Valid: Sat 9th Sept 6am to Sun 10th Sept 6am Day 1 Convective / Storm Forecast Synopsis Upper low and surface low pressure system will move east across the North Sea on Saturday, while a ridge builds in across the far west, with a cool and unstable showery northwesterly flow covering the UK. ... ERN REP. OF IRELAND, N. IRELAND, SW SCOTLAND, ENGLAND and WALES ... An upper trough will shift SEwards across the UK on Saturday as upper low moves east over N Sea, pool of coldest mid-level temps in axis of upper trough will create steep lapse rates which combined wth large scale ascent on forward side of the trough will support bands of heavy showers and thunderstorms which will move SE. Intially heaviest showers and thunderstorm risk will be across Ireland, Irish Sea, Wales and western parts of England during the morning, before heavy showers/storms tend to shift further SE across northern England, Midlands, C-S England in the afternoon then reaching SE England and E Anglia in the evening. Vertical shear will weak away from Ireland, so generally no organised severe threat seems likely, though bands of showers/storms forming under trough axis may bring a risk of localised flooding. Showers/storms may also be accompanied by hail and gusty winds. Wind stream convergence towards western and eastern coasts and light winds aloft may allow a few funnel clouds to develop with stronger updrafts. Issued by: Nick Finnis

Nice shelf cloud over Southend, Essex this evening

Waters of 30C+ west of Bahamas may strengthen Irma again after its slight weakening during eye wall replacement - which has now completed.

This may partly answer your question http://www.aoml.noaa.gov/hrd/tcfaq/G5.html In a nutshell, the semi-permanent sub-tropical ridge steers the hurricanes, easterlies on the equator side steer storms that form mid-Atlantic westwards towards Caribbean, as we've seen with Irma and Jose. But when they reach the western edge of the sub-trop ridge the flow swings southeasterly and southerly, the western periphery of the ridge currently over SE U.S. and Bahamas, so when a hurricane reaches here - they tend to head NW then N. Now, differences between model tracks may lie over how much influence or weakness there is in the sub-top ridge on its western edge, as a slacker flow or trough digging SE over eastern U.S. will tend to pull a hurricane poleward, GFS perhaps doing this too soon with slight miscalculations in the steering flow on western edge of sub-trop ridge?

NHC forecast discussion mentioning that Irma is undergoing an eyewall replacement. With it continuing to move over warm waters of 30C and weak shear, likely it only slightly weaken and will maintain as cat 4 before Florida landfall, risk it could strengthen again, hopefully not for the sake of northern Cuba, SE Bahamas and ultimately Florida in its path. Some interesting facts about Irma tweeted yesterday:

Jose now a major hurricane, upgraded to cat. 3 with 120mph winds, unfortunately it looks like it could affect Antigua and Barbuda on Saturday, before swinging away NW then N towards Bermuda, by which time it may have weakened a fair bit.

12z ECMWF ensembles suggest there's no dodging a bullet for Florida

Storm & Convective Forecast Issued 2017-09-07 22:09:53 Valid: Fri 8th Sept 6am to Sat 9th Sept 6am Day 1 Convective / Storm Forecast Synopsis An upper low and collocated surface low will drop southeastward across northern Britain on Friday. A strong SWly jet aloft will parallel a slow-moving waving cold front across southern England, to the north of the front, a cool and unstable showerly NWly flow will cover much of the UK. ... S ENGLAND ... Frontal wave moving east and formation of triple point, as wave occludes, will likely combine with increasingly divergent cyclonic vorticity advection aloft ahead of trough moving SE to force ascent of warm moist air to south of frontal boundary. This may support, with ample vertical shear of 30-40 knots and strong SWly flow aloft, organised convection, perhaps some bowing line segments, capable of producing localised flooding, isolated strong wind gusts, hail, thunder and lighhtning. Have issued a MARGINAL risk for potential for isolated flash-flooding, given signal from GFS and ECM for 300-600 j/g CAPE and fairly high preciptable water (PWAT) content of warm sector airmass across S England. ... ELSEWHERE ... Steep lapse rates generated by cool mid-level temps of upper low drifting SEwards will, with surface heating and warm SSTs, support scattered showers and weakly-electrified thunderstorms. Small hail, isolated localised flooding and gusty winds are possible with stronger showers or thunderstorms. Issued by: Nick Finnis

Eye heading NW just north of Puerto Rico

Still strengthening as it heads toward Puerto Rico. 914/915mb, just wow, what a monster

Florida landfall looking more certain, just a question of where and how strong, two major hurricanes hitting mainland U.S. within a few weeks pretty jaw dropping ... oh then Jose could take a direct aim at Barbuda, St Martin and Puerto Rica ... so cruel

Storm & Convective Forecast Issued 2017-09-01 05:59:17 Valid: Fri 01 Sept 6am to Sat 02 Sept 6am Day 1 Convective / Storm Forecast Synopsis Upper trough axis will continue to move east of the UK on Friday, slack N to NWly flow will be unstable across E England closer to upper trough while ridge builds in across the W. … E/SE ENGLAND and E MIDLANDS … Steep lapse rates towards E England with proximity to colder mid-levels of upper trough, surface heating and local breeze convergence will support development of scattered heavy showers and isolated thunderstorms today. Weak vertical shear will mean severe weather is unlikely, showers/storms will be slow-moving – so may produce localised flooding, particularly where they cluster along convergence zones. Small hail is possible and conditions along convergence zones will be ideal for a few funnel clouds. Issued by: Nick Finnis

Quite unusual for a major hurricane to form so far east over the tropical Atlantic too.

Storm & Convective Forecast Issued 2017-08-30 21:58:02 Valid: Thurs 31 Aug 2017 - 6am to Fri 01 Sept 2017 - 6am Day 1 Convective / Storm Forecast Synopsis Long wave upper trough axis will slide east across the UK during Thursday. A slack cyclonic and unstable westerly flow, veering NWly across the far west, covers the UK, supporting widespread showers and scattered thunderstorms. ... IRELAND, ENGLAND, WALES and S SCOTLAND ... Upper trough and associated cold mid-levels will create steep lapse rates across much of the UK as it slides eastwards on Thursday ... which will support the development of heavy showers and scattered thunderstorms which, with light winds aloft, will drift slowly eastwards. Troughs in the flow may focus showers/storms into slow-moving clusters capable of producing localised minor flooding, otherwise weak vertical shear / unidirectional wind profiles should discourage an organised severe risk, though there maybe isolated instances of hail to 1 - 1.5cm given steep lapse rates and potential for strong updrafts with surface heating in late August sunshine. Also, a few funnel clouds, which may even reach the ground as a brief/weak tornado, can't be ruled out - given favourable conditions of several wind convergence zones, light winds aloft and fairly low cloud bases. Issued by: Nick Finnis

Storm & Convective Forecast Issued 2017-08-29 09:20:33 Valid: Tues 29th Aug 6am to Weds 30th Aug 6am Day 1 Convective / Storm Forecast Synopsis An upper trough axis lies SW to NE across the far NE Atlantic and a cut-off upper low sits over Iberia to the west of an upper ridge over central, southern and eastern Europe. At the surface, a weak cold front will slide slowly SE across England and Wales, with a warm and huimid airmass across SE UK and an unstable showery flow across NW UK. A shallow area of low pressure covers the near continent which will deepen slightly and bring an area of heavy and perhaps thundery rain across SE UK tonight. ... SE UK ... The NW edge of a plume of very warm and humid airmass residing on the near continent will sit across SE England today and overnight. Isolated elevated heavy showers, perhaps with sporadic lightning, may occur where Ac cas becomes deep enough with subtle forcing along NW edge of plume. Otherwise, overnight there is an increasing risk of embedded elevated thunderstorms within pulses of heavy rain developing and moving NE from N France / East English Channel across SE England ... as baroclinic zone and surface convergence developing over northern France in response to falling heights/temps from the NW create increased ascent of plume over near continent. There is a risk of minor localised flooding from these storms, mainly Sussex and Kent overnight. ... N. IRELAND and W SCOTLAND ... With steepening lapse rates from the NW, as upper trough / cold pool slides SE, convection may become deep enough amongst the showers expected here to produce sporadic lightning and small hail. Issued by: Nick Finnis

Brief heavy shower passing over here near Croydon, blue skies to the south, better shot of the London convective skies (earlier) in the tweet below

A few heavy showers popping up across the SE, though no sferics, tower to my SE near Croydon

06z AROME breaks out heavy showers (maybe thundery?) across SE corner early Tuesday morning 00z ECMWF had some rainfall across SE England between 06z and 12z Tuesday too. Some weak CAPE spreading NE across the SE on Tuesday morning on both AROME and EC, but GFS shows no CAPE or rainfall what so ever. Be interesting who's right in the end, AROME is normally quite good a picking up these elevated destabilisation convective situations.