Storm Forecast
Valid: Thu 14 Aug 2008 06:00 to Fri 15 Aug 2008 06:00 UTC
Issued: Wed 13 Aug 2008 22:32
Forecaster: GATZEN

SYNOPSIS

Intense north-west European long-wave trough starts to weaken during the period. However, another strong short-wave trough will move around its flank reaching the Iberian Peninsula in the evening hours. Ahead of this trough, a mid-level jet streak will eject into northern Italy late in the period. It will affect warm and unstable air mass present to the south-east of a frontal boundary from eastern Spain to south-eastern Germany, south-eastern Poland, southern Belarus, and parts of western Russia on THU evening.

DISCUSSION

Northern Italy region

Latest observations indicate that rich low-level moisture is present over northern Italy. Ahead of the approaching trough axis, cyclogenesis is expected over central Mediterranean and northern Balkan region due to warm air advection and DCVA. Models indicate a convergence line from Sardinia to western Italy late in the period, and moist air mass will remain to the east. During the afternoon, convective initiation becomes more likely as mid-level height falls spread eastwards into northern Italy, and moist air mass will likely become unstable. Limiting factor will be a capping inversion, and initiation seems to be most likely over the central Alps where upslope flow due to southerly wind above the boundary-layer is expected. Thunderstorms that form will likely organize given strong DLS, and large hail is forecast.

During the afternoon and evening, increasing QG forcing underneath an approaching strong mid-level jet and associated increase in vertical wind shear is forecast by latest models. Initiation is expected to spread to the south, west, and east, while severe thunderstorms will be quite likely, capable of producing large hail and locally high winds. Tornadoes will be most likely over the north-eastern Italy region, where backing surface winds are forecast during the evening and night hours, and a few events are not ruled out. Convection will likely go on or even intensify during the night given strong QG forcing and advection of rather steep mid-level lapse rates from the south-west. An MCS may evolve near the Alps, where risk of local flash flooding is expected.

South-eastern Germany, Austria, Czech

Ahead of the developing low south of the Alps, backing winds may advect moist low-level air mass into Austria, and south-eastern Germany. Complex situation limits forecast confidence today, but it seems that instability will develop especially in the evening hours ahead of the approaching jet streak due to mid-level height falls. Great uncertainty exists about the low-level buoyancy, and lack of sunshine and low-level moisture may lead to elevated convection especially in the western part of the risk region, limiting severe potential.

Current thinking is that severe thunderstorms will likely be present in the afternoon hours over the southern Austria region capable of producing large hail. As the upper jet streak approaches, convective cells may also evolve from southern Germany to Austria. Convection that roots to the boundary-layer will likely profit from favourable vertical wind shear and veering profiles, and mesocyclones may produce large hail.

During the night hours, increasing low-level southerly jet is forecast from northern Aegean to eastern Austria and further to Czech Republic and Slovakia, and low-level vertical wind shear may strongly increase to 10 m/s in the lowest kilometre. Given QG forcing in the range of the deepening surface cyclone and advection of steep lapse rates from the south-west, thunderstorms will likely go on while threat of tornadoes may increase, and even strong tornadoes are not ruled out over Austria in the range of strongest low-level vertical wind shear. Weak low-level buoyancy seems to remain the limiting factor, and latest GFS model seems to be quite optimistic over Czech Republic and south-eastern Germany, where elevated convection seems to be more likely at this time. Convection is forecast to merge into an MCS in the morning hours that moves northward into Poland and eastern Germany, where severe threat will gradually decrease due to weak instability.

Slovakia to south-eastern Poland and western Ukraine region

A frontal boundary with weak embedded waves will remain quasi-staionara ahead of the approaching trough axis over northern Mediterranean. A weak upper short-wave trough will enter the region ahead of the strong jet streak leading to some QG forcing especially in the afternoon and evening hours. Given nice low-level moisture indicated by latest observations, daytime heating will likely be sufficient for CAPE. Increasing QG forcing and low-level convergence along the frontal boundary are expected to lead to initiation, and thunderstorms are forecast. While vertical wind shear increases near the frontal boundary, severe thunderstorms are forecast to be quite likely, capable of producing large hail. In the evening hours, tornadoes threat is forecast to increase especially in the southern part of the risk area, where strong southerly winds above the boundary-layer and strong (15 m/s) 0-3 km vertical wind shear is forecast by latest models. Further north, from northern Ukraine to Belarus and western Russia, unstable air mass will likely be to the east of strong vertical wind shear, limiting severe potential.

NE Spain and the area N of the Balearic Islands

A cold front is expected over this region during the period. Capping inversion will likely inhibit initiation initially. As the trough axis approaches during the evening hours with intense forcing, thunderstorms are forecast to develop, spreading very fast to the east as DLS increases to 35-40 m/s. Large hail as well as severe wind gusts are possible with those thunderstorms that will likely weaken in the morning hours due to dry Mistral winds from the north.

Finland and north-western Russia

At the eastern flank of the European trough, a very strong upper jet streak will move northward on THU. To the west, maritime air mass is expected that will likely be associated with neutral to slightly unstable lapse rates. Low-level moisture seems to be questionable over most places given strong southerly winds and low-level mixing. However, diurnal heating will likely lead to instability underneath the trough and showers and thunderstorms will likely develop. Strong low-level vertical wind shear is forecast and well-developed multicells may produce isolated tornadoes. Main threat will be severe wind gusts, though.

England region

In the range of the trough axis, moist air mass is expected to destabilize during the day, and thunderstorms will likely form in the range of convergence lines. Given quite strong low-level buoyancy and weak vertical wind shear, a few weak tornadoes are not ruled out.