Storm Forecast
Valid: Thu 02 Mar 2017 06:00 to Fri 03 Mar 2017 06:00 UTC
Issued: Thu 02 Mar 2017 00:54
Forecaster: GATZEN

A level 2 was issued for western and southern Turkey mainly for tornadoes, severe wind gusts, and to a lesser extent excessive rain and large hail.

A level 1 was issued for the Benelux countries, central Germany, Poland, northern Czech Republic, Belarus mainly for severe wind gusts and to a lesser extent tornadoes.

SYNOPSIS

Intense long-wave trough extends across north-western and northern Europe. A strong jet streak moves at its periphery over northern France, Benelux, and Germany and develops an amplifying short-wave trough across eastern Europe. Downstream, another intense trough cuts off over the Aegean and moves across southern Turkey. Western Europe is affected by a high-amplitude trough late in the period.

DISCUSSION

Western Turkey

A very intense short-wave trough approaches over western Turkey on Thursday. Associated strong mid-level jet streak moves from the south Aegean along the southern Turkish coast. It will provide very strong QG forcing across western and southern Turkey with a tropopause fault extending down to 600 hPa. Strong stretching below this stratospheric intrusion will cause low-level lapse rates increase and steep lapse rates spread from western Turkey along the southern portions during the day. These lapse rates overlap with around 6 g/kg low-level mixing ratio and CAPE in the order of 500 J/kg can be expected, mostly realized at low levels and only weak lifted index at mid-levels, although equilibrium level temperature will be below -20°C. Vertical wind shear will be around 10 m/s in the lowest 3 km.

Current thinking is that a line echo wave pattern forms along the main cold front and moves from the east Aegean towards the south-east. High winds can be expected along this convective line as well as tornadoes. In the wake of the Turkish plateau, initiation of numerous storms is expected during the day. Vertical wind shear will be moderate to strong with favourably veering profiles. Supercells can be expected in this environment capable of producing tornadoes, severe wind gusts, and large hail. An additional threat is excessive rain.

Storms will spread south-east during the day together with the cut off low, so that weakening / decaying storms can be expected from the west in the afternoon and evening hours.

The Channel towards Benelux region, northern / central Germany, Czech Republic and Poland into Ukraine, Belarus, and western Russia

A well-developed tropopause fault rapidly moves east across the area, producing strong stretching of the low-level air masses, and steep lapse rates will form and advected to the east. These lapse rates overlap with 5 g/kg low-level mixing ratio in an occluded warm sector that spreads east. Poor low-level lapse rates within the occlusion will improve in response to the stretching and some CAPE, maybe elevated CAPE, could form. With equilibrium level temperatures below -20°C, thunderstorms are expected within the occlusion. These may be embedded in more stratiform rain, however, the development of a convective line in this high shear, low-CAPE environment is not completely ruled out. In the latter scenario, severe convective wind gusts can occur from eastern central Germany towards Poland and Belarus. However, current thinking is that convective storms will be embedded in stratiform precipitation and will be partly elevated, reducing the threat.

In the wake of the occlusion, strong large scale pressure gradient will result in stormy gusts over western and central Europe. The convective contribution to these gusts is expected to be limited. However, ongoing stretching supports steep lapse rates across central Germany, spreading into the Czech Republic and Poland. Some storms may develop and could evolve bowing lines given the very strong vertical wind shear. In this case, some convectively produced wind gusts will add to the overall wind gust threat.

Portugal

Late in the period, intense sharp trough enters south-western Europe. A cold front will reach Portugal in the morning hours. Latest GFS model indicates rich low-level moisture ahead of this front and steepening lapse rates from the west. Some overlap can be expected over northern Portugal. This could result in some embedded storms along the cold-frontal rain band. Some severe wind gusts cannot be ruled out completely although the degree of convective organization is expected to be low given weak CAPE and 0-3 km vertical wind shear.