Reigate and UK weather looks increasingly unsettled through the next week or so as the jetstream goes up a gear and becomes much more active and autumnal, possibly culminating early next week with the delivery of our first extra-tropical storm of the season, Bertha, on a fast jet from across the Atlantic. More on her later.
This week a couple of significant secondary LOW pressure systems are forecast to spin up rapidly out of the SW bringing wet breezy weather in the South. Wednesday early morning looks especially wet but with a possible rapid clearance behind the active front. Rain totals could be significant and warrant warnings nearer the time, watch the UKMO for these.
Friday looks a similar performance, even wetter for some, with another secondary depression developing rapidly from a parent LOW over the mid-Atlantic near to S Iceland. This secondary low emerges quickly from the south bringing potentially wet weather but it is further off so things could change…
Hurricane season in the Tropical Atlantic basin starts in June and ends in November. The National Hurricane center in Miami keeps an expert eye on every development in the tropical Atlantic but forecasters in the UK also take a great interest in them too. This is because hurricanes can impact the UK as extra-tropical storms.
Several tropical disturbances look like they could potentially affect UK weather in the next few weeks, Bertha being the first. Of course, the UK has never seen a “true tropical hurricane” (i.e. with an eye, wind sustained no fronts etc etc) but we certainly get the remains of tropical storms (called extra-tropical storms when they leave the tropics and arrive in the mid-latitudes). Many / most extra-tropical storms never make it to our shores (e.g. Humberto), many also die a death or veer off long before they have any real impact on the UK. Some arrive as late summer or autumn depressions often characterised by wet and breezy weather: this could be Bertha next week.
Whether or not extra-tropical storms arrive on UK shores they can still inject a lot of heat and moisture and energy into the late summer and autumn mid Atlantic at mid-latitudes. This can perk up UK weather significantly and spoil the dog days of August. This can cause trouble for forecasters as models become unreliable. Bertha is significant because models seem to be agreeing, at this stage anyhow, that she will arrive near the UK sometime late next weekend or Monday. Some models show her deepening into a significant summer storm (GEM), others show her moving south into Biscay and producing a Spanish plume of warm/not thundery weather. In any case, expect more rain this week and next and do keep an eye on forecasts as the weather gets interesting, I would not be surprised to see a flurry / slurry of weather warnings for heavy rain at times.
Finally, the overall outlook remains rather unsettled for us as the charts above show: slightly below average temps with no heat wave on the cards yet (although heat cannot be ruled out at times as a plume event is possible) and plenty of rain spikes with over 150% more rain than normal shown on the NCEP charts. As a rough guide, daily rain totals in Reigate exceeding around 10mm would be considered a really wet day… so there are several potential candidates for some wet days in the next week or so.
Some places in Japan have recently seen more than 250mm IN ONE DAY in tropical storm Nakri, so our worst weather is still nothing compared with some global extremes. In addition, think of Japan as Super-Typhoon Halong bares down on the southern islands this week.
Unusually Bertha appears on Atlantic UKMO chart as a tropical storm status and is downgraded about 12 hours later to ex-TS Bertha. The charts for late Sunday / Monday look astonishingly autumnal with a perky jetstream invigorating the storm under a trough. Nothing certain yet, as exact track and intensity is variable still but this is a turn up for the books. Last year no ET-hurricanes made it to the UK, Humberto drifted off harmlessly in mid September and actually built us a nice HIGH pressure as it made “landfall” over Iceland. Perhaps Bertha might do the same thing?