Storm Forecast
Valid: Wed 29 Oct 2008 06:00 to Thu 30 Oct 2008 06:00 UTC
Issued: Tue 28 Oct 2008 23:15
Forecaster: PUCIK

SYNOPSIS

A very deep trough has settled over W Europe, stretching from Scandinavia to Iberian penninsula. Its core is filled with cold airmass, especially in the north characterized by steep mid-level lapse rates. Several short waves will rotate around it during the period, promoting deepening of surface lows. Very strong flow will surround the trough, having more than 40 m/s at 500 hPa level. Underneath the trough, broad surface low pressure system exists. Three distinct centers can be identified - one on the SW coast of Norway, which should fill during the day. Another one will travel from Ireland southwards, deepening as short wave provides strong lift over its center. The last one has formed on the significant fronal system, which has now become quasi-stationary over the Middle Europe. Further deepening of the low is expected and its center will shift NNE wards with time. Strong WAA regime will develop ahead of the front. Eastern Europe will encounter stable conditions as ridge has settled over the region.

DISCUSSION

...Bay of Biscay...

As mentioned above, surface low will move southwards from Ireland. CAA behind the front which overlays relatively warm SST leads to the development of steep lapse rates. Moreover, significant lift is expected both from surface low and short wave above, so further destabilization might occur thanks to this factor. MLCAPEs above 500 J/kg should materialize and combined with favourable lift will result in TSTM development. Strong LL buoyancy will exist over the bay and with nice convergence signals shown by several consequent GFS runs, waterspout threat seems to be quite high. Therefore, marginal Lvl 1 is introduced.

...Sardinia, Corsica to Italy coast...

Several rounds of thunderstorms are possible. Nevertheless, severe threat should arrive with the approaching cold front, which shall affect the region from 15 to 21Z. Troposphere should be conditionally unstable during the whole day, but further destabilization will occur ahead of the front with MLCAPEs above 800 J/kg. Furthermore, strong mid and low level flow will establish over the area with DLS values around 20 m/s and shear in the lowest 3 km around 15 m/s should suffice for good storm organisation. From storm spectrum, multicells should prevail but supercell is not ruled out either. Large hail will be possible with stronger cells, especially by the Italian coast. Strong low level wind field, with windspeeds up to 25 m/s at 850hPa suggest that severe wind gusts may accompany the storms. On the coasts, friction will reduce the surface winds and increase LLS, reaching 10 -15 m/s, so one or two tornadoes might occur too.

...Gargano penninsula to Adriatic sea...

A tongue of steep mid-level lapse rates will point to the area with MLCAPE values above 1000 J/kg. DLS will be between 20 - 25 m/s and MLS above 15 m/s. What is more, favourable veering of the wind with height will raise the SREH, which should reach more than 250 J/kg in the 0-3 km layer. This suggests that rotating updrafts will become a distinct possibility. Although storms should stay mostly multicellular as GFS suggests storm clustering, few isolated supercells might occur. In such case, chances for large hail would increase significantly. Steep lapse rates and also quite strong flow at 700 hPa level will combine to promote a downburst risk. Few tornado / waterspout reports are expected as SREH 0-1 km should increase to 150 J/kg and coexist with high low level buoyancy. As very good storm organisation is anticipated, along with high storm coverage, LVL 2 is issued in the belt, where the highest threat will be present. Most of the severe weather should occur between 06 and 15Z, shifting eastwards. After this period, severe weather risk will gradually dimnish.

.... N Adriatics, N Italy...

Although instability should be relatively meager in this area, passage of the cold front and presence of upper level disturbance aloft should aid in TSTM development. Impressive wind profile will dominate the region, especially by evening hours, with 30 m/s of DLS and over 15 m/s of LLS. SREH in the 0-1 km might exceed 150 J/kg with veering wind profiles. As storms should be rooted in the boundary layer, such strong low level wind profiles will result in tornado / severe wind gust threat. After the cold front passage, threat will disappear.

.... E Germany, W Czech republic...

Mostly elevated instability is predicted over the region. Strong isentropic lift across the front will provide the initiation for thunderstorms. Despite the fact that favourable wind profiles for organised storms will be available, no threat is expected, as storms should stay elevated in nature and instability will be only marginal.