Storm Forecast
Valid: Mon 03 Aug 2009 06:00 to Tue 04 Aug 2009 06:00 UTC
Issued: Sun 02 Aug 2009 21:42
Forecaster: DAHL

A level 1 was issued across NE Italy into Austria mainly for large hail, damaging wind gusts, and excessive rainfall.

A level 1 was issued across Poland mainly for marginally severe hail andd excessive rainfall.

SYNOPSIS

The upper trough centered over France at the beginning of the period will make some eastward progress while developing into a cut-off cyclone over NW Germany, with the southern extension digging into the N Adriatic, likewise developing into a cut-off low laate in the period. Another upper cut-off low will travel from the Batlic states into E Russia. At low levels, rather quiescent SFC conditions prevail over the continental parts of Europe. A rather vigorous Atlantic system will reach the British Isles late in the period.

DISCUSSION

... NE Italy into Austria ...

According to Sunday's 12Z ascents it appears that most parts of Italy and the E Alpine regions exhibit rather low mixed-layer mixing ratios and that the observed SFC moisture is only quite shallow. However, as LIML and Vienna 12Z soundings from Sunday suggest, locally the depth of the low-level moisture may be substantially enhanced. This suggests that at least on a local scale, ample instability could be available on Monday. Rather early development of precipitation/convection could limit allover extent of large instability, however.

Deep shear is simulated to be in the 15 to 25 m/s range, which will be adequate for storm organization, including supercells. Expect large hail and damaging wind gusts to be the main threats. Late in the evening, GFS increases LLS, but mostly outside of the regions where deep instability is expected. So the allover tornado threat seems to be limited, though an isolated event or two may occur. In addition to that, large amounts of convective precip, exceeding severe limits, may occur. Uncertainties on the development/degree of instability currently preclude a LVL2 threat at the moment, but the situation is considered to be on the high end of a LVL1 threat.

... Pre-Alpine regions ...

Farther N, mostly elevated/imbedded thunderstorms are anticipated in a regime of strong low-mid-level frontogenesis. The severe threat should be rather low. However, large amounts of rain may occur amidst the aarea of strong mesoscale/frontogenetic ascent.

... Poland ...

Rather strong instability may develop over Poland, and scattered thunderstorms should initiate during the day. A lack of shear will likely hamper storm organization, but isolated large hail and severe wind gusts could well occur if storms favorably interact with one another. Also conglomeration of cells and slow storm motion may result in excessive rainfall.

... E Spain ...

Models advertise a pocket of high low-level theta-e to persist across E Spain through the period. Early in the day, some DCVA-forced ascent will overspread the region, as well as low-level warm advection. Current thinking is that convective cells will be elevated and imbedded in stratiform precip. The entire activity should decay as large-scale subsidence increases into the day. DLS is simulated to be on the order of 20 m/s, however, so that an isolated large-hail and/or severe wind event cannot be ruled out, should the cells become SFC-based. However, this seems rather unlikely at the moment, so the confidence in organized severe weather is rather low, and a LVL 1 threat area does not seem to be necessary.