Storm Forecast
Valid: Wed 13 May 2015 06:00 to Thu 14 May 2015 06:00 UTC
Issued: Tue 12 May 2015 22:29
Forecaster: PUCIK / GROENEMEIJER
A level 2 was issued for E France, N Switzerland, SW Germany and extreme NW Austria mainly for (very) large hail, severe wind gusts, tornadoes and to the lesser degree for excessive precipitation.
A level 2 was issued for SE Austria and SW Hungary mainly for large hail, severe wind gusts and excessive precipitation.
A level 1 was issued for SE Germany and W Austria mainly for large hail, severe wind gusts and excessive precipitation.
A level 1 was issued for SW Slovakia and Hungary mainly for severe wind gusts, excessive precipitation and to the lesser degree for marginally large hail.
SYNOPSIS
A complex synoptic-scale scenario is forecast over Europe with a strong zonal flow forecast in between a ridge over the Mediterranean and a trough-ridge-trough complex over the northern half of Europe. Underneath the zonal flow, a diffuse frontal boundary is forecast to stall, stretching from the central France through S Germany eastwards. South of the boundary, tropical airmass will be advected in SW-ly flow from Iberia to S France and the Alpine region with EML from Iberia. With enhanced low-level moisture along the boundary, CAPE is forecast to build up along and south of the boundary underneath the strong westerly flow.
DISCUSSION
... E France through Alpine region into Hungary and SW Slovakia ...
A potentially dangerous but also highly questionable setup will develop over the region. NWP generally agrees on the impressive overlap of CAPE with strong DLS which should reach 25 - 35 m/s over the whole Level 2 area. Furthermore, as wind veers with height, SRH values should also reach above 200 m2/s2, especially near the border of Switzerland, Germany and France. The highest CAPE values are simulated over France and NW Switzerland, where the best overlap of low-level moisture and EML is likely with MLCAPE values between 1000 and 2000 J/kg. Towards the east, CAPE will be generally less.
With diffuse boundary and no strong forcing factor, convective initiation will become a crucial factor in this setup. CIN values should decrease as geopotential heights fall during the day and heating mixes the boundary layer. However, cloud cover and potential rain along the boundary may hinder the destabilisation. Low-level convergence zones along the boundary or induced by orography will likely be the primary initiation mechanism.
Due to the impressive kinematic environment, developing storms will likely become supercells, capable of large to very large hail and severe wind gusts. The highest threat of very large hail will be over E France, NW Switzerland and SW Germany. Towards the evening, as 850 hPa flow will increase and LLS will exceed 10 m/s. This should elevate tornado threat, which will be the highest over NW Switzerland, extreme E France and SW Germany. Storms rapidly tracking east to southeast will likely grow upscale in the evening hours, possibly forming an MCS over S Germany as PV max crosses the region. MCS could easily reach SW Slovakia and central Hungary by Thursday morning. Such MCS would pose threats of severe wind gusts as 0-3 km bulk shear exceeds 25 m/s and of excessive rainfall, especially over and close to the Alpine area.
... Pyrenees and NW Spain ...
NWP generally does not favour widespread initiation over this area but e.g. GFS or ECMWF break out spotty precipitation over the mountains and south of them. With strong DLS and steep lapse rates, cells that manage to persist in the dry environment may eventually become high-based supercells capable of large hail as DLS exceeds 20 m/s. Thus, Lvl 1 is issued to cover this scenario.
