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Nick F

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Posts posted by Nick F

    Convective/Storm Discussion Thread - 1st August onwards

    in Storms & Severe Weather

    Posted

    Storm & Convective Forecast

    convmap_130917_1.thumb.png.d8a312189d931eee53c9f3a03fbb2e07.png

    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.

    • Like 2

    Storm Aileen - First Autumn Storm Tuesday night into Wednesday morning

    in Storms & Severe Weather

    Posted · Edited by Nick F

    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.

    • Like 2

    Convective/Storm Discussion Thread - 1st August onwards

    in Storms & Severe Weather

    Posted

    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

    convmap_110917.thumb.png.841096a9371184a775bf75004e7b8c46.png

    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

    Convective/Storm Discussion Thread - 1st August onwards

    in Storms & Severe Weather

    Posted

    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

    convmap_090917.thumb.png.d1244bec87ecacb2e3b992cc70b7f5a8.png

    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

    • Like 2

    Hurricane Irma

    in Hurricanes, Cyclones and Extreme weather worldwide

    Posted

    11 minutes ago, FetchCB said:

    I have a question which hopefully someone can answer

    In the UK it is often said that the more severe the depression/Storm then the more it curves poleward. is it the same with Hurricanes? If so then maybe if GFS is overdoing the depth of pressure within Irma then it would want to curve it poleward ?

    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?

    • Like 1
    • Thanks 1

    Hurricane Irma

    in Hurricanes, Cyclones and Extreme weather worldwide

    Posted

    25 minutes ago, Singularity said:

    Typical... the moment Irma is taken a whisker below category 4 strength, deep convection wraps neatly right around the eye... as if in angry retort!

    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:

     

    • Like 2
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