Storm Forecast
Valid: Sat 29 Aug 2009 06:00 to Sun 30 Aug 2009 06:00 UTC
Issued: Fri 28 Aug 2009 23:53
Forecaster: KOROSEC

A level 1 was issued for parts of N Italy, N Adriatic sea, Slovenia and the rest of NW Balkan states mainly for excessive rainfall, damaging winds, large hail and lesser extent for tornadoes/waterspouts.

A level 1 was issued for E Slovakia, E Poland and parts of Belarus and Lithuania for damaging winds and excessive rainfall.

A level 1 was issued for coastal Netherlands, Denmark, N Germany, N Poland and S Sweden mainly for weak tornadoes and waterspouts.

SYNOPSIS

A broad upper trough moves across north-central Europe with a strong cold front extending from central Scandinavia southwards across Poland and further SW-wards towards the Alps. Severe thunderstorms are expected along this eastwards moving front across the southern Alpine region and northwestern Balkan peninsula during the forecast period. An upper level low over southern Mediterranean finally fills during the day and finishes an excessive rainfall period over those areas.

DISCUSSION

... parts of N Italy, N Adriatic sea, Slovenia and the rest of NW Balkan states ...

Hot and unstable boundary layer conditions are placed ahead of an approaching trough/front. Weak to moderate instability will be available over the highlighted areas with peaks over northern Italy and Adriatic sea. By Saturday morning, a cold front will be confined to the Austrian/Slovenian border, extending westwards along the southern Alpine flank. Storm initiation is expected along the progressing frontal boundary in the morning hours. Some uncertainities about the morning activity are present, depending on the previous night activity over the Alps. If large convective complex forms and digs more southwards then models predict, then storms could also develop or will be already on going well ahead of the front to the south. GFS/WRF models simulate this possibility while ECMWF has storms initiation mostly along the front across central Alps during the early morning.

Then, when upper trough slowly approaches, the cold front is pushed southwards and storms will continue to develop along it. Over the eastern Alps, a surface cold front will arrive quite early with NE-erly winds behind it spreading southwards towards the central Balkan. Storms there will tend to be more elevated. While instability across western Slovenia and northern Italy will stay mostly untouched. Moderately steep lapse rates and upper 10s to low 20s surface dewpoints suggest that near 1500 J/kg of CAPE should easily release there. Favorable upper level divergence will support large scale ascent on the jet's right entrance placed right over the southern Alpine region. About 15-20m/s of deep-layer shear will be available within the jet streak, overlapping with the instability pretty well.

Only marginal backing winds seem available over the northern Adriatic and N Italy. Storm which will manage to stay more isolated in these areas, will likely become severe given the moderate instability, shear and helicity in place. Threats for large hail exist. Models suggest quite impressive water contents through the troposphere which suggest excessive rainfall likely within the strongest cells. Only weak low-level shear/helicity limits the tornado threat.

A large scale ascent ahead of the trough and QG forcing suggest widespread storm coverage. Large MCS cluster seems likely to form in the afternoon hours over NE Italy and SW Slovenia. The complex/MCS will then be slowly moving SE-wards along the NW Croatia where GFS model simulates enhanced 0- 1km shear/helicity. Therefore higher probabilities for a couple of tornadoes/waterspouts exists with the more isolated strong cells. Especially along the coast, where stronger Scirocco SE-erly winds will be in effect.

An aforementioned faster progress of colder air over the eastern Alps will push the front faster across eastern Croatia and Hungary. At least some hundreds of CAPE will be present along/ahead of the front, but only partly overlapping with the shear/helicity which will stay behind it. The main threat with the storms there will be an excessive rainfall while large hail in unlikely with the limited instability.

... coastal areas of Netherlands, Denmark, N Germany, N Poland and S Sweden ...

The polar airmass will be spreading eastwards within the strong trough located west of Norway. Quite cold mid-levels support steep lapse rates and several hundreds of CAPE will result over the warm waters of North Sea during the day and afternoon hours. Numerous showers and thunderstorms will easily form. Low LCL heights, weak flow/shear and high low-level lapse rates suggest that a few tornadoes/waterspouts are possible over Denmark and coastal areas of Netherlands, N Germany, N Poland and southern Sweden. A level 1 was issued for these areas where higher probabilities of occurence are expected.

Cold mid-levels will be slowly moving eastwards towards Baltic sea until the Sunday morning and storms will continue to develop there. No threat levels were issued in those areas, as severe storms or high coverage of waterspouts is not expected. But an isolated hail event or waterspout may still occur.

... E Slovakia, E Poland and parts of Belarus and Lithuania ...

A deep-layer shear of near 15-20 m/s will overlap with weak to moderate instability there. Storm initiation will be along the frontal convergence, storms will gain organization into multicells and a few discrete supercells. Given the almost parallel flow relative to the shear vector, early storm clustering is expected and a linear MCS cluster will likely form along the eastwards trailing cold front. Clustering storms suggest that locally excessive rainfall with flash floods may occur given the slow moving storms and high PWAT values. Models simulate also some backbuilding of the cells and therefore rainfall may persist longer over the same areas and enhance flooding threat. An enhanced helicity up to 300 m^2/s^2 is expected which could support rotating storms especially at the early stages, if storms tend to be more isolated. Then, a threat for a few events of large hail will exist.

... SE/coastal Spain ...

A couple of storms will likely form there along the Spanish coast during the afternoon hours. Low-level easterly/southeasterly winds over the western Mediterranean will bring moisture onshore and storm will be mostly confined to the wind convergence and along the mountainous terrain there. Around 500-1000 J/kg should become available, as well as around 10m/s of deep layer shear. A few organized storms can form, posing a risk for large hail and severe wind gusts. Given the low coverage of the convective activity, no threat level is needed at the moment. Storms could persist well into the night hours.

... Southern Italy, Sardinia and Tunisia ...

An upper level low which was in effect in the last days finally diminishes, but still some cool mid-levels suggest that mostly diurnal driven showers and thunderstorms will form along the sea breeze convergences. Slightly higher probabilities for a severe storms exist over Tunisia and Sicily where still high amount of water contents within the troposphere are present. Slow moving storms and good low-level CAPE release suggest that local flash flooding risk and a waterspout or two can occur.