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viking_smb

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    Storms and Convective discussion - September 2023

    in Storms & Severe Weather

    Posted

    Just now, Azazel said:

    Unsurprising. Something has definitely changed meteorologically to make this area a complete dead-zone for storms during summer.

    We still usually get the odd thing outside of the summer months but that’s normally during a showery regime 

    having said that we had a good storm on a thursday 2 weeks ago in weymouth, forked lightning and had to get the cadets off the water sharpish as we had sailing etc on. but the storm brewed up very quickly without warning though

    Storms and Convective discussion - September 2023

    in Storms & Severe Weather

    Posted

    27 minutes ago, Eagle Eye said:

    Could do. High LCL's weakens entrainment CAPE and with it being so warm sub cloud level, hail won't be as strong as analogs and they might not be accounting for the height. Those 850's aren't really typical for larger hail events. Given the 2 inch analogs, some people might say that my 1.5 inch forecast was very conservative but I think it was actually pretty high end. 

    the current synoptic chart (18z) and 12 hrs confirms my theory 

    • Like 1

    Storms and Convective discussion - September 2023

    in Storms & Severe Weather

    Posted · Edited by viking_smb

    6 minutes ago, Eagle Eye said:

    Sorry for the late forecast. Made this earlier today but was waiting for Jay to do the map. Was playing cricket so couldn't help him when making it either. 

    Convective Outlook ⚡️ 

    A large Theta-E airmass advection will allow for near tropical moisture rates along the Irish Sea and the coasts of Eastern Ireland and western England up to western Scotland. 1000+ J/KG of MLCAPE looks to advect and create showers and storms Wednesday night/Thursday morning, with potentially frequent lightning in some of them. They will move roughly north as it goes into Thursday morning.

     

    This kind of saturation and moisture levels tends to allow for very frequent lightning once the cap is broken. If the capping isn't broken then it's a bust but it looks fairly likely to break and so some storms around the previously mentioned coasts could have quite frequent lightning rates.

     

    Further into Ireland may see some scattered storms at random points throughout the night. These may have fairly frequent lightning, having good lapse-rates along with near tropical like moisture amounts and fairly good MLCAPE.

     

    850's Theta-W at 16°C+ is enough for the elevated storms to form. Lots of elevated CAPE and good hydrolapse rates continue to suggest frequent lightning rates at times possible.

     

    The inflow layer is fairly large with good below zero CAPE. Along with good hydrolapse rates and a supportive hodograph, severe hail is quite possible.  Hail could potentially be up to 2 inches but it's probably unlikely to reach it's potential but I suspect some 1.5 inch hail is possible. Some surface flooding is also possible.

    Timing wise and it's on Wednesday night, storms should form in the southern part of the Irish Sea and into the SW of England, moving north towards SW Scotland potentially by around mid morning. These storms will be mostly longer-lasting pulse-type reaching 9-10 km before new storms should form after one falls. They might form into a weak but wide MCS with some near severe wind gusts depending on DCAPE strength.

    20230906_200002.thumb.jpg.216804991e2b6ca8ad32fa0553f6a637.jpg

    Going off your forecast and other info, I think wales west of swansea will be most at risk 

    • Like 4

    Storms and Convective discussion - September 2023

    in Storms & Severe Weather

    Posted · Edited by viking_smb

    10 minutes ago, Ben Sainsbury said:

    Signs of mid-level instability here too. Satellite is promising and as @ChannelThunder said, AROME 12z is promising too. UKV remains decent for far SW. Fingers crossed.

    Looking further ahead, Saturday night and Sunday looking more of a widespread risk.

    with the widespread risk, I think it will be more northern areas that will cop it, as UKV showing some activity in the south on sat night, but that will change closer to the time

    Storms and Convective discussion - September 2023

    in Storms & Severe Weather

    Posted · Edited by viking_smb

    34 minutes ago, ChannelThunder said:

    On mobile at the mo but hopefully this works

    https://www.wxcharts.com/?panel=default&model=arome,arome,arome,arome&region=eng&chart=convective_overview,accprecip,wind10mkph,snowdepth&run=12&step=016&plottype=10&lat=51.500&lon=-0.250&skewtstep=0

     

    according to the met office inshore forecast, the showers are going to be west of lands end and in the irish sea as I use the inshore forecast frequently in work 

    Storms and Convective discussion - September 2023

    in Storms & Severe Weather

    Posted

    16 minutes ago, WeatherArc said:

    A thundery breakdown is looking possible at the end of next week, at the moment i would think Fri to Sun are the main days of focus.

    We still need some things to go right for us in the models though.

    Looking at Sunday we can see that cape is pretty marginal, i would say the max is about 1000 j/kg in Cornwall, near Bristol and in South Wales. 

    Screenshot2023-09-03170312.thumb.png.4b8af87d31859ccf1c56014e070cadf8.png

    The problem is that the GFS is thinking that the heat being pumped up from the south will be relatively dry. Relative Humidity values in some parts of the country on Saturday will be below 35%. 

    Screenshot2023-09-03170427.thumb.png.4cee7e5e116409d095e6ff9890c6c9ca.pngScreenshot2023-09-03170448.thumb.png.f21561d4ff95d829ae77aeebce402441.png

    We also have to keep an eye on where that high pressure sets up. I don't think it will be a problem by the end of the week though. 

    In my opinion and experiences with forecasting American storms the GFS really underdoes cape values, it's only when more short range high-res models get into range that we get a better idea of the thunderstorm potential. In terms of location right now i would favour the west for surface based homegrown storms with the south getting imports from France. 

    Screenshot2023-09-03170448.thumb.png.f21561d4ff95d829ae77aeebce402441.pngScreenshot2023-09-03170512.thumb.png.8a8c37485ca7ff7475169f526d9c1894.png

    Massive difference on Saturday between the GFS and euro with the dew point values. The only problem here is that the Euro has much lower temperatures, limiting cape. 

    Screenshot2023-09-03171115.thumb.png.866d98bcbd79f8fd63637974ab6d364e.pngScreenshot2023-09-03171018.thumb.png.ab5897f437a41756438ac63ee4261dd9.png 

    Comparison of GFS and Euro for Sundays cape. 

    Screenshot2023-09-03171841.thumb.png.eb5b1604e5bfa239969d104944292183.pngScreenshot2023-09-03172325.thumb.png.57d7cc882c0265cac49ae7977fe787fc.png

    Interesting sounding from around Gloucester on Sunday, the wind profile really stands out, plenty of directional shear with winds at the surface being south easterly, 850s being direct southerly and 500s being from the south west. Nice looking hodograph. Obviously the speed shear, cape, low level lapse rates and cin leave a lot to be desired. 

    So overall, I genuinely think the potential is there for some very good late summer thunderstorms however we need quite a few things to go our way. We are still 150+ hours out so expect we will see a few different solutions on how this heat will breakdown, hopefully not with frontal rainfall from the Atlantic.  

    Edit-The 12z Gfs has come out and looks much better for t-storm potential on Sunday, could be the start of cape values being upgraded over next few runs. 

     

    its too far ahead yet, like you said, I would rather wait till weds at the earliest to see what happens 

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