Message #14209

[martinastro]

19Z Update

Analysis

On the 500mb Chart a broad deep layer trough persists from Northern Scandinavia to the Western Med.  A broad Omega Ridge of High Pressure located to the West of the UK ridging NE.  A weak upper air disturbance is moving South into N Scotland and a strip of Vorticity coming south on the western edge of the trough is enhancing precip across N England and now into the Midlands

OVer the coming 48hrs a cutoff upper high forms a quasi stationary upper block to the north of the UK with High pressure at the surface intensifying over Scandinavia and stretching from South of Iceland to NW Russia. A New low over the Med  is moving northeast.

Discussion

Strip of Vorticity coming SW within the western portion of the upper trough is enhancing precip coming south across Yorks into the Midlands.  Trajectories and 4KM Meso take this SSW , though appears to be somewhat more organised than the T+6 frame from the 12Z run.  Take this SSW towards SE Wales and Bristol area.  SHowers moving into the SE are organising into a shower train across Kent some heavier ones giving 10-15cm overnight  also similar amounts over the Yorks Moors and 1-6cm elsewhere

Temps very low already across most of the UK -5 to -14C already across many parts, cloud coming south across Scotland tonight likely to interfere with overnight lows and make them somewhat erratic, clearest skies across Southern Scotland tonight and Eskdalemuir is expected to be very cold tonight. Along with parts of Wales and NIreland.

Saturday sees snow showers dying back to the SE as the vorticity strip moves away to the west with sunny skies in places but clouding over slowly from the SE with a very strong ENE wind developing across Southern England, F7s off the Kent coast later tomorrow.  Bitterly cold feel to the day.

Tomorrow night the Snow Showers organising again as another weakness develops aloft.  The showers organising into bands to give 10-15cm later tomorrow and overnight across the Downs in Kent and Sussex.

Into Sunday and the development of a complex process of slight warm advection / upgliding aloft combining with a new developing upper low which breaks forward from the main continental vortex and moves west to the south of the UK. The two effects combining to produce extensive precip across the South.   The GFS has finally come on board with developing some more extensive snow - though its a little out of sync in terms of timing when compared with the UKMO GM and ECMWF also somewhat further north.   There is reasonable agreement from the UKMO and consistent guidance from the ECMWF for outbreaks of Snow to develop and extend WNW across England into Wales on Sunday and persisting into Monday across Wales and The SW.  Heavier falls possible in places esp inland from coasts where added Sea temps providing additional moisture input.  ECMWF has up to 10cm across parts of the SW and Wales (Eastern Upslopes) and 5-10cm over other areas.  Northern Limit is less far north in the ECM, UKMO than in the GFS. Snow dies out from the East during sunday but persists into the night across the SW and Wales into Monday.

Monday sees Snow slowly dying out from the ESE but remaining very cold in places esp over newly fallen snow.

Into the Middle of next week there are considerable "detail" problems in the models.  The GFS tries to bring a low into the SW and has an incursion of milder air into the far south before colder air pushes back in post T+144.  The UKMO model brings a low into the Channel before the low complex sinks south and a SE is kept across most parts.  The ECM is rather strange tonight, it brings a low up the channel around T+102/T+108 crosses the far SE and into the Southern North Sea.  This would produce a sig amount of snow across most of England. A further low then comes into the SW around T+144, but this extends NE and introduces less cold air across the South but it remains cold in the North with further bands of rain pushing into the SOuth and turning to Snow across Northern England and Scotland. All the time high pressure persists across Scandinavia.

The variety of solutions for Mid next week onwards make any detail largely impossible to predict, but the general detail i feel remains the same, Low pressure coming increasingly close to the SW and South threatening Snow but also increasing the potential for less cold air to start to move north.  How quickly it does this and in what shape / form .. well the jury is still out on that one. .. 

TOI 19.29

Paul Blight
UKww Manager - Education/Warnings
Associate Fellow Royal Meteorological Society