Storm Forecast
Valid: Sun 16 Aug 2009 06:00 to Mon 17 Aug 2009 06:00 UTC
Issued: Sat 15 Aug 2009 15:16
Forecaster: TUSCHY

A level 1 was issued for N-Italy and parts of Austria mainly for excessive rain.

A level 1 was issued for N-Tunisia mainly for large hail.

A level 1 was issued for N-Latvia, Estonia and extreme SE-Finalnd mainly for severe wind gusts and to a lesser extent for tornadoes.

SYNOPSIS

Various depressions rotate around the outer verges of a compact polar vortex. They affect northern Europe and cause unsettled conditions. Rising geopotential heights and WAA result in warm and stable conditions over most parts of S / SE and E-Europe.

DISCUSSION

... E-France, central and western Germany ...

The senario today is marked by a southeastward sliding front, which decays gradually during the forecast period. The uncertainties with the front, including its position and strength, are still present in the models with ECMWF and GFS being the outliers in respect of initiation (none and scattered thunderstorms respectively). The southeastward moving cold front stalls somewhere over central and western Germany on the anticyclonic shear side of a zonal aligned mid-/upper wind field to its north. This causes overall marginal upper-level support with missing forcing and streaks. A weak impulse crosses east France and west Germany during the peak heating hours, but we think that the focus for appreciable forcing is the surface front.

Yesterday's dewpoints were already between 14-17 °C and that's, what WRF forecasts, whereas GFS even has the 20°C isodrosotherm over far eastern France. Moisture pooling along the front could indeed push dewpoints to those values and initiation looks most likely in the highlighted area. Low probabilitiy thunderstorm lines (not yet made public) also include most parts of central and southern Germany due to the discrepancy in model outputs. Shear is stronger to the north with better instability over E-France and a level area may be included for isolated large hail later-on.

... N-Italy and parts of Austria ...

Thunderstorm activity will be partially daytime driven but also influenced by weak short-waves, which affect the Alps during the forecast. Shear at all levels is weak but the main concern is very slow storm motion. Surface dewpoints soar into the upper tens or lower twenties next to 0-1km average mixing ratios of 11-13 g/kg. A level 1 was introduced for excessive rain amounts with those storms. The orientation of this level is bound to the orientation of a SW-NE aligned convergence zone. Thunderstorms decrease after sunset but isolated thunderstorms are possible well into the night hours.


.... N-Algeria and N-Tunisia ...

No change to the past few days as a persistent branch of the subtropical jet stays put with impressive 0-6km bulk-shear values of 20-30 m/s. Aside from that, the interface still persists between the very moist and warm maritime boundary layer along the coast and the well mixed mid-layer, so elevated instability in excess of 1000 J/kg MLCAPE is forecast. The main issue again is the missing forcing. Weak impulses cross those areas, but the past few days already showed that the activity remained short-lived. However, large to very large hail is still possible with each more persistent updraft and hence a level 1 was included, where initiation looks most probable. Thunderstorm coverage however will be very isolated in nature.


... Parts of Latvia, Estonia and extreme southeastern Finland ...

A sub-1000 hPa depression crosses the Baltic Sea druing the forecast from the southwest. Attendant warm front affects those areas during the morning hours but the front departs rapidly to the northeast. Thereafter, a sheared warm sector covers those areas during the noon and afternoon hours. A cold front draws near from the west during the evening hours.

The main issue is, if the boundary layer moisture can recover enough for surface based thunderstorm initiation during the afternoon hours. Instability will be better further south , e.g. Lithuania, but forcing is more questionable. Next to the vorticity lobe, which crosses Estonia in the afternoon hours, a 30-35m/s mid-level streak draws near and places Estonia beneath its left exit region. Strong speed shear, some directional shear coupled to roughly 500 J/kg MLCAPE are enough for organized thunderstorms, probably shallow ones given quite warm EL temperatures. Dependent on the final quality of the boundary layer, a tornado risk exists with those storms with a more widespread severe wind gust risk, especially if thunderstorms can organize along the cold front (evening hours). The level 1 was expanded well to the northeast with this scenario in mind.