Storm Forecast
Valid: Sun 02 Dec 2018 06:00 to Mon 03 Dec 2018 06:00 UTC
Issued: Sat 01 Dec 2018 15:53
Forecaster: TUSCHY

SYNOPSIS

Latest NAO forecast indicates a clear change back to positive values. This also manifests itself in an evolving high index pattern over the NE Atlantic. 500 hPa standardized geopotential height anomalies show negative values extending from Iceland to Ireland and then back to the SW over the open Atlantic. At the same time downstream ridging covers most of Morocco and extends towards the Iberian Peninsula. The geometry of the pressure field assists in a broad fetch of subtropical air to advect towards the Bay of Biscay and later on also towards France.

A deep and slowly eastward propagating trough gradually affects Greece and the Aegean Sea during the forecast with another ridge affecting Turkey.

Embedded in this progessive flow regime, numerous synoptic-scale fronts impact W/CNTRL Europe.
Very dry and seasonable cold air affects E Europe and prevents any thunderstorm risk.

DISCUSSION

... Ionian Sea and the E Mediterranean ...

Placed beneath the slow moving upper trough, scattered offshore thunderstorms occur with heavy rain and a waterspout risk the main hazards. A gradually dissolving and progressive LL vortex precludes the development of more organized and stationary convergence zones and hence no level area was issued.
A growing cluster of thunderstorms is forecast to evolve just south of SW Turkey during the end of the forecast period, but most models keep this activity offshore until 06Z. A similar risk is forecast with this activity.

... Parts of Ireland, UK, France, Benelux and W-Germany ...

... Until 18Z ...

A leading occlusion/wave crosses Benelux and Germany until 12Z from W to E but tends to weaken. However, a delayed mid-level impulse (the upper part of the strongly tilted wave) affects the backside of the occlusion (Benelux and W-Germany around noon) and adds some synoptic-scale lift and marginal steeper lapse rate atop the arriving modified subtropic air mass. In addition an upper jet streak tavels from N-CNTRL France to far SW Germany and adds some upper divergence. Hence an isolated thunderstorm risk exists from E-CNTRL France to W-CNTRL and SW Germany with areas further north experiencing weak subsidence behind the wave and an overall limited thunderstorm threat. Strong veering in the lowest 2 km AGL causes enhanced SRH values combined with very low LCLs. Despite more favorable LL lapse rates residing to the north (Benelux and W-CNTRL Germany) with increasing 850 hPa capping to the S, a spatiotemporal overlap exists for enhanced downward momentum of 20 m/s winds in the lower troposphere. In case a stronger updraft manages to evolve a low-end tornado risk exists.

Further west, NW France, UK and Ireland, a tranquil day turns more thundery beyond 12Z as another subtle wave races east. Isolated severe wind gusts and graupel occur.

...Beyond 18Z...

Overall conditions for isolated to scattered thunderstorms improve over the whole forecast area as mid-levels cool atop the NE-ward surging subtropical moisture plume. In general GFS features the most cyclonic flow pattern with IFS and ICON showing a more flat flow regime. Interestingly, GFS LL moisture is on the lower end of the model spectrum with the rest of the models remaining a bit more aggressive. Given the upper flow pattern, GFS currently offers the most active thunderstorm scenario with colder mid-level spreading more south and hence more over the low-tropospheric moisture plume. More model data and probably near nowcasting data will be needed to determine the best overlap of lapse rates and BL moisture, but a general uptick in thunderstorm activity in the diffluent and slightly divergent upper streamline pattern ahead of the mid/upper wave is anticipated. Near BL flow in the 20-25 m/s range is enough for an isolated severe wind gust threat with thunderstorms. The activity will be a mixture of more isolated cells and probably small LEWPs with an isolated tornado risk, given enhanced SRH-1 in a low LCL environment.

Meager amount of CAPE and forcing lower confidence in upgrading parts of the area and hence no level 1 was added for now.