Ex-Hurricane Berthas Visit To The UK On Sunday 2 1 The T+00 Met Office fax chart for 6am Thursday 7th August shows Bertha has now become associated with a frontal system south of New Foundland and has acquired ex-tropical characteristics. The centre of the low appears to be on the cold side of the frontal zone now and on the forward side of an upper trough extending south from Quebec down towards the Carolinas. The ex-tropical low is also under an upper-level westerly jet stream. The low pressure system will continue its track NE at 20knts, before tracking more eastward later today, as the steering flow and jet stream above it becomes more westerly along the base of a large upper (long-wave) trough extending south from Greenland across the Atlantic. Now Bertha doesnt deepen a great deal on the model guidance over the next 48 hrs as it crosses east over the Atlantic under an unusually low latitude zonal jet stream, but a shortwave trough in the strong upper westerlies coming out of NE Canada over the weekend looks to sharpen as catches up with the low by then in the southwest approaches later on Saturday. However, the models have been struggling with how the shortwave trough engages with the low pressure system as it approaches UK waters and whether the low falls under the cold side of the jet and in developmental left exit region of the jet where maximum upper divergence takes place. These factors will have an impact on whether the low deepens quickly to our SW and moves NE across Ireland then northern UK on Sunday, as per 00z GFS model output, which would bring a spell of very wet and windy weather to Ireland, England, Wales and S Scotland - with gales around coasts and hills. Or, the low doesnt interact so favourably with the shortwave and jet streak in the southwest approaches as per 00z ECMWF model, thus not deepening and rather continuing on a more southerly track northeast as a shallower open wave through the English Channel and across far north of France, before deepening later on. The updated t+84 fax this morning from the Met Office, drawn by the Duty Forecaster using a combination of model raw output and their ensembles, agrees more with the track of 00z ECM, though the low a tad further north across SE England, with centre of 995mb at noon Sunday in London area. 00z GFS, on the other hand, has low centre at noon Sunday over southern Scotland around 985mb! So you can see the uncertainties for now. Whatever the track, large scale ascent of very warm and moist sub-tropical air getting drawn into the lows circulation from the southwest and strong baroclinic zone created between this air and cold air sinking south from Greenland area - is indicated to produce large rainfall totals from some of the models, perhaps 20-30mm widely according to GFS and NMM over northern and western areas, 40mm+ according to NMM over S/E EIRE and Irish Sea, similarly large totals further south if the low tracks further south. All in all, a risk of some unseasonably wet and windy weather during the second half of the weekend. I will endeavor to bring updates on these thoughts over the next few days. Published on 7th August 2014 10:31 Updated: 7th August 2014 10:42
Posted on: Thu, 07 Aug 2014 12:56:46 +0000
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