Quick update here focusing on SE especially: please note this applies mainly to Reigate in Surrey and is an amateur analysis for educational purposes. For updates through the storm please see @RGSweather on twitter for the Bertha story as it unfolds for us in Reigate and SE. This is called NOWCASTING (as opposed to “forecasting”).
Ex-Bertha is turning out to be rather interesting meteorologically! A convective potential has emerged today, which means there is more possibility of thunderstorms of some significance as the LOW passes across the UK, especially to the south of the system. Convective gusts of 50-60mph could be possible and the odd tornado cannot be ruled out, though no need to panic because these are quite common and not usually powerful or damaging in the UK. So it is still the case that the overall impact of this storm is still not likely to be extra-ordinary or wreak widespread havoc Daily Express style. It is more likely to be underwhelming for most. Nevertheless, rainfall totals in a short space of time for some places might be high and there could be some interesting weather phenomena associated with active fronts.
UPDATE Sunday 7:30am
Estofex and TORRO have issued severe convective weather warnings for the S UK. Estofex Level 2 storm warning is most unusual for the UK and TORRO do not issue tornado watches lightly.
(back to yesterdays update:) The UKMO fax chart below for Sunday midday shows a “triple point” of three fronts meeting near the SE (warm front, cold front and occlusion) Between the warm front and cold front the warmest humid air is wrapping into the centre of the LOW in the warm sector: this contains much of the moisture to fuel the storm as condensation releases latent heat driving up parcels of air. On top of this a conveyor of cooler drier Polar air that flows over the cold front and warm sector and this increases lapse rates further encouraging lift throughout the system. The warm air eventually flows to the core of the storm as it occludes.
All the time the jetstream to the south is lifting air off the ground (by a process called divergence in the upper atmosphere) and lowering the central pressure causing air to converge into the centre of the LOW… this results in the surface wind rushing into the centre. Converging air at the surface has nowhere to go except up. Rising air, especially where tropical air meets polar air at the fronts, creates condensation, thick cloud and potentially plenty of rain. The potential water available in this storm is large. In addition, cloud top temps, with the influx of cold air aloft, are likely to be as low as -50C causing ice to form in turbulent air that can create charge up thunderstorms. Such storms are only a risk and may not happen at all.
For Reigate and the SE it seems we can expect more rain during the morning than was previously the case in earlier models and forecasts. Latest models suggest widepsread rain in the SE of up to 20mm and discrete patches of high totals possibly exceeding 50mm in the SE. This is about a month of rain in one day, so local flooding could be a problem.
Rain will arrive tonight, after midnight, and persist throughout the morning. Wind speeds, probably 30-40mph max gusts inland, possibly more gusty in any thunderstorms, will increase towards the middle of the day and potentially be highest as the cold front moves away which, on current models looks like early afternoon. Strongest gusts will be associated with any thunderstorms. The good news is that by pm the cloud should break rather rapidly, however, scattered showers could follow in the brisk westerly. This regime will continue for much of the early part of the week.
Needless to say, apart from the rain potential, Reigate is less at risk from tstorms than further N during this episode (Reigate storm shield!)
Even now much still remains uncertain about this storm and it is causing lots of interest and headaches for both professional and amateur meteorologists. The nature of the fronts may produce some organised squall like features and some organised thunderstorms for places but predicting these is extremely difficult. Any such storms can have the potential to deposit a lot of rain in a short space of time.
Late this afternoon Saturday Bertha split in two: one rain system moving north, the other pushing more ENE. This was unexpected. Currently the rain moving north across Ireland is the more significant but things can change.
Let’s finish with the UKMO forecast for Reigate. It shows lots of rain, potential for thunderstorms and some unsually strong winds for the time of year. As leaves remain on trees this might cause loose branches to fall and peak rainfall totals, if met, could cause some local flooding. Certainly nothing to panic about but do look out for any interesting weather features and send them in to @RGSweather! Sadly for @ridelondon the prospects are not terriibly nice in the morning.
Photo mosaic of the squall line that passed over Reigate on 10 Aug afternoon: quite a feature!
and tornado reports of damage from various locations including Hull, plus other news here: