Storm Forecast
Valid: Thu 27 May 2010 06:00 to Fri 28 May 2010 06:00 UTC
Issued: Wed 26 May 2010 22:37
Forecaster: GATZEN

A level 1 was issued for central Italy mainly for large hail and tornadoes.

A level 1 was issued for portions of central and southern Poland mainly for tornadoes.

A level 1 was issued for Austria, Czech Republic, Slovakia, northern Croatia and Serbia, Hungary, Romania, south-western Ukraine mainly for large hail.

A level 1 was issued for eastern Iberia mainly for large hail and excessive rain.

A level 1 was issued for northern Algeria mainly for large hail and tornadoes.

SYNOPSIS

A westerly mid-level flow affects most portions of Europe. The intense centre of the weak geopotential is situated across the North Sea. With the westerly flow to the south, a short-wave trough travels from southern France across the Adriatic and the Balkans into Romania during the period. Another short-wave trough moves into the south Mediterranean Sea ahead of weak geopotential over Iberia. At lower levels, cool and dry low-level air is present from the British Isles to Scandinavia and Belarus, while rich boundary-layer moisture extends along of a frontal boundary from central France to southern Germany, Poland, and southern Ukraine on Thursday. Rather rich low-level moisture is also forecast to evolve over portions of southern and south-eastern Europe underneath a tongue to warm air spreading north-eastwards on Wednesday and Thursday. Together with the warm air, rather steep lapse rates do spread north-eastward and may reach Romania late in the period. A mid-level low will affect Turkey and the Aegean Sea.

DISCUSSION

Central Italy and Adriatic

The boundary-layer moisture across the west Mediterranean Sea has started to recover underneath a warm elevated mixed layer originating from the Atlas mountains as indicated by latest Palma de Mallorca sounding. Latest models indicate that a tongue of rich low-level moisture will spread north-eastward ahead of a frontal boundary that crosses the west Mediterranean during the day.

Given rather steep lapse rates across the region, CAPE is likely due to diurnal heating especially over northern and central Italy. As the mid-level short-wave trough approaches during the noon/afternoon hours, DCVA will likely yield increasing lapse rates and latent heat release becomes quite likely especially along the frontal convergence. Severe convection is forecast to be most likely across central eastern Italy, where vertical wind shear and CAPE will overlap with about 25 m/s DLS and 1000 J/kg MLCAPE in the early afternoon hours. Low-level veering will be not too strong, but about 100 m²/s² 0-3km storm-relative helicity is forecast by latest GFS and supercells are expected to develop. These storms will be capable of producing large hail. Additionally, tornadoes are forecast given the high low-level moisture and low LCL heights as well as rather high low-level buoyancy overlapping with south-easterly winds near the Adriatic Sea in the wake of weak sea-breeze fronts. The 0-3km vertical wind shear may excess 15 m/s in these regions, and tornadoes are forecast especially where the storms will interact with sea-breeze fronts.

The convective activity is forecast to spread eastward with the low-level convergence and will likely decay over the Adriatic Sea due to weak low-level buoyancy.

Poland, Austria, Czech Republic, Slovakia, northern Croatia and Serbia, Hungary, Romania, south-western Ukraine

Rich low-level moisture is present to the south of the frontal boundary and has lead to thunderstorms across France and southern Germany on Wednesday. This rich low-level moisture will advect further eastward as a surface low will move from Germany to northern Poland. Additionally, a rather warm air mass with steep mid-level lapse rates is present further south over Hungary und Romania ahead of the Italian trough axis. Diurnal heating is expected to lead to instability that exceeds 1000 J/kg in the southern portions.

Convective activity becomes most likely along the cold front that moves across Poland in the late morning hours. Thunderstorms will likely merge along the cold front given moderate deep layer vertical wind shear and rather poor low-level profiles across central Poland. A gusty front may result with a weak chance of tornadoes. With decreasing moisture further east, this linear convective cluster will likely weaken late in the day over eastern Poland, spreading into Belarus in the evening and night hours.

Further south, about 1000 J/kg CAPE is expected across Hungary. Warm air advection will likely go on during the day as low-level winds are forecast to be slightly backed in the wake of the Alps. Initiation is forecast along of a warm front from northern Austria to Slovakia and northern Hungary/north-western Romania at noon. While the deep layer vertical wind shear will be weak, isolated cells may evolve rotating updrafts given enhanced low-level helicity due to easterly low-level winds in the range of the warm front. Large hail seems to be the main threat with these storms.

Later in the period, deep layer vertical wind shear is forecast to increase ahead of the approaching Italian trough, and lift will likely help to break to cap further south over northern Croatia/Serbia and southern Hungary in the evening hours. Supercells may develop capable of producing large hail. With increasing QG forcing and due to rather substantial instability, steep mid-level lapse rates, and cold pool potential, storms may merge into an MCS that moves into the western Ukraine later on. Isolated severe wind gusts are not ruled out as well as locally excessive precipitation during the night hours.

Iberia

In the range of the axis of a mid-level trough and rather moist low-level air, diurnal heating will create instability and showers and thunderstorms are forecast to develop that will move eastward with the westerly flow. While the severe potential of these storms is forecast to be rather low, it is increased in the eastern portions where low-level winds will be backed during the daytime due to the sea-breeze. Some storms may evolve rotating updrafts with a chance of producing locally large hail. Locally excessive rain is also forecast due to the slow storm propagation.

Northern Algeria

Substantial low-level moisture and rather steep lapse rates will create a broad region with CAPE during the day. At the northern flank of a jet streak moving north-eastwards across central Algeria, strong DLS and CAPE will overlap over central and northern Algeria, where supercells will likely develop. Large hail will be the main threat, but tornadoes are also forecast given the strong 0-3km vertical wind shear of about 15 m/s.