Storm Forecast
Valid: Wed 20 Jun 2007 06:00 to Thu 21 Jun 2007 06:00 UTC
Issued: Tue 19 Jun 2007 22:29
Forecaster: GATZEN

SYNOPSIS

Intense mid-level trough centered W of British Isles is forecast to remain. Ahead of this feature, latest model guidance shows that steep lapse rates will be replaced over most of west Europe before WED, 6Z. Additionally, westerly low-level winds will likely advect cool maritime air mass into British Isles, Benelux, and most of France during the night hours. Even though a strong jet streak will be present during the day from Bay of Biscay to southern British Isles, QG forcing is limited due to a short-wave ridge moving northwards during the day.

The following short-wave trough is forecast over Iberian Peninsula in the evening hours, and will propagate eastward reaching from western France to northern Italy at the end of the period. Associated strong upper jet streak will move into western Mediterranean during the night hours, providing QG forcing over southern France, and western Alpine region. Height falls/steepening lapse rates and remaining low-level moisture in the range of stalling frontal boundary will likely result in increasing CAPE.

Another region with strong instability is further northeast from southeastern Germany to Balkans in the range of 850hPa thermal ridge. Weak DLS is expected, while forcing along the frontal boundary will likely be sufficient for initiation over Germany during the day. Further south, strong capping inversion is expected as latest soundings indicate warm EML over Mediterranean that will likely spread into Balkans.

Over east Europe, long wave trough axis is expected from Finland to Black Sea. In the range of this trough, relatively cool air mass will likely be well mixed above the surface. Although low-level moisture is quite poor, latest GFS model run shows that instability will develop from eastern Ukraine to western Russia and Finland.

DISCUSSION

Southern France, western Alpine region, northern Italy

Expected convective set-up strongly depends on Tuesday's convective evolution. Current thinking is that frontal boundary will be in place over southeastern France/western Alps during the day. Low-level moisture flux convergence and diabatic heating will likely occur. Initiation is likely along the frontal boundary/outflow boundaries around or even before noon. Later in the day, approaching jet streak/upper vort-max will lead to QG forcing and increasing DLS, while thunderstorms spread into central Alps and northern Italy. Although low-level winds will likely remain weak, 100 J/kg 0-3 km SRH is forecast that is sufficient for at least a few supercells. Additionally, strong multicells are forecast to develop. Large hail may be the main threat during the day, but severe wind gusts are also expected. Even very large hail is not excluded, and a level 2 may is warranted over northern Italy, where 20 m/s DLS will likely overlap with CAPE. Weak low-level shear is forecast, but an isolated tornado is not ruled out especially in the evening hours when low-level vertical wind shear will slightly increase. Given persistent QG forcing north of approaching jet streak, thunderstorms will likely go on/rebuild during the night, posing a threat of large hail, severe wind gusts, and maybe a few tornadoes. Local flash flooding is also quite likely, especially over the southern/western Alps.

Northern Alpine region, southeastern Germany, western Austria, Czech Rep, and western Poland

In the range of the thermal ridge, strong instability is expected to develop by latest GFS model run. While steep lapse rates now present over France/Alpine region will likely spread northeastward, low-level moisture will be a problem over most of the region. However, at least weak instability is forecast during the day. Thunderstorms will most likely initiate along the cold front over central Germany, moving eastwards. Given rather poor moisture expected over southeastern Germany, thunderstorms may weaken initially. However, given steep low-level lapse rates/inverted-v-profiles and quite some instability, strong wind gusts and large hail are not completely ruled out. Later in the period, approaching vort-max is expected to lead to QG forcing ahead of the frontal boundary, and latest model guidance shows that thunderstorms will move northeastwards over southern Germany during the night. Severe threat seems to be quite weak given cool low-level air mass, and relatively weak vertical wind shear. Intense precipitation and local flash flooding is possible, though.