Storm Forecast
Valid: Fri 18 Jul 2014 06:00 to Sat 19 Jul 2014 06:00 UTC
Issued: Thu 17 Jul 2014 22:40
Forecaster: GROENEMEIJER

A level 2 was issued for W France mainly for severe wind gusts, but also for large hail, tornadoes and extreme precipitation.

A level 1 were issued for and area surrounding the level 2 for the same risks.

A level 1 was issued for the NW Balkans mainly for large hail, and to a lesser extent for extreme precipitation.

A level 1 was issued for parts of Romania, Moldova and Ukraine mainly for large hail, and to a lesser extent for extreme precipitation.


SYNOPSIS

A trough located west of western Europe is expected to slowly move eastward. An embedded shortwave over the UK moves northward. A southerly flow has become established on its eastern flank. This flow advects very warm air northward over France, which is moist at low levels. Over central Europe, a weak ridge is located. A cold core low is located over Sweden.




DISCUSSION

France, southern England...

As the main trough approaches France, the mid-level flow is expected to back and intensify, yielding a strongly sheared environment with about 20-25 m/s of 0-6 km shear. In the lower troposphere, a convergence zone should establish approximately near a line from Le Havre to La Rochelle. In vicinity of this zone, both GFS and ECMWF predict high, 20-23 C dew point temperatures, that should yield about 2000 J/kg CAPE in many places, and CAPE locally exceeding 4000 J/kg in northern France according to both models. In vicinity of the convergence zone, high 0-3 km storm-relative helicity is also forecast.

The main question will be how many storms will be able to form in this environment, which is in principle capable of sustaining extremely severe storms. Another question is whether the storms will be surface-based.

Current thinking is that the air-mass slightly to the east of the surface convergence line will be strongly capped. Near the convergence line and to its west, however, convection is likely to break out during the evening as lower tropospheric temperatures drop somewhat in response to quasi-geostrophic forcing for upward motion. Model guidance show very high precipitation rates. It is likely that a few supercells will form initially. In response to increasing forcing for upward motion, a bow-echo may well move northward along the convergence zone. Supercells developing in such a volatile environment, will have a risk of producing giant hail (> 5 cm) and severe wind gusts. Given that low-level shear will increase to 10-15 m/s during the evening, tornadoes will also be possible. Any bow-echo system will have the risk of producing extremely severe wind gusts. In addition, very large precipitation amounts may occur. The focal point of the threat should travel northward across the English Channel and may well affect southeastern England during the 00-06 UTC time frame.

An alternative scenario is that not much convection will be surface-based, and the bulk of the activity will be elevated, reducing the overall severe threat. Because of this uncertainty, an level 3 was not issued at this time.

W Balkans, NE Balkans...

Across these two areas, NWP guidance predicts a fair amount of latent instability to develop (~ 1500 J/kg), so that the risk of severe hail and local heavy rain shoudl be high enough to warrant a level 1, even in absence of strong deep-layer shear.