Storm Forecast
Valid: Sun 11 Jul 2010 06:00 to Mon 12 Jul 2010 06:00 UTC
Issued: Sun 11 Jul 2010 05:45
Forecaster: SCHLENCZEK
A level 1 was issued for N Spain and W France mainly for severe wind gusts and excessive rainfall.
A level 1 was issued for E France, Benelux, W-central Germany, W Austria, N Italy and Switzerland mainly for excessive precipitation and to a lesser extent for large hail and severe wind gusts.
A level 1 was issued for Denmark, S Norway and S-central Sweden mainly for severe wind gusts and excessive precipitation and to a lesser extent for tornadoes.
A level 1 was issued for NE Romania mainly for excessive precipitation and to a lesser extent for large hail and severe wind gusts.
A level 1 was issued for N Ukraine mainly for excessive precipitation and large hail and to a lesser extent for severe wind gusts.
SYNOPSIS
An unstable airmass with 1000 - 2000 J/kg MLCAPE covers large portions of western / central Europe. Some mid-level vorticity advection is forecast for western / central Germany during the afternoon hours, leading to a convergent flow at lower levels as shown by 10m / 925 hPa wind maps. Later in the evening, the vort-max of an upper shortwave trough will reach western France.
The other zone of moderate instability but very weak vertical shear is expected over SE Europe from W Russia, Ukraine and Belarus towards Romania.
DISCUSSION
...N Iberia, W-central France...
A local maximum in instability is expected in the vicinity of the exit region of a 30 m/s upper jet streak. In the late evening / night hours, the associated upper vort-max will cross N Iberia and W France from SW to the NE. QG forcing should be sufficient for convective initiation and the development of a linear MCS should be likely as 0-6 km shear increases to values around 20 - 25 m/s. The main threat will be severe wind gusts as Delta Theta-E in order of 20K are expected and 700 hPa winds reach 15 - 20 m/s. Current thinking is that the severe wind gust threat will be more locally as the front-normal component of the storm motion vector is very short and storms will tend to move along the frontal boundary. In this scenario, excessive rainfall should become the main threat as the training effect will become important. An isolated large hail event is possible as well.
...N Italy, Switzerland, W Austria...
Rich BL moisture, weak vertical shear and locally more than 2 kJ/kg CAPE are expected over parts of N Italy and surroundings. In the late morning / early afternoon hours, some multicell storms will develop, posing a threat of large hail, isolated severe wind gusts and excessive rainfall.
...E France, W Germany, Benelux...
A zone of deep convergence should be present over eastern France and central Germany in the late morning / early afternoon hours. The cap should be relatively weak in both regions with strong mid level convergence as shown by a very small LCL - LFC difference. Recent soundings show 10 - 15 m/s winds around 6 km AGL which is slightly more than predicted by GFS. The directional shear is rather disorganised in most soundings but nevertheless there are some small spots of slightly enhanced SRH3 forecast for N- central Germany in the afternoon hours. According to the thermodynamic and kinematic environment, the development of multicell storms should be most likely. These storms may produce isolated large hail and isolated severe wind gusts but the main threat will be excessive rainfall due to slow storm motion and clustering. Later in the afternoon, storms may tend to merge into a large MCS which will move to the north / northeast.
...Denmark, S Sweden...
A MCS over the North Sea is forecast to move to the NNE in the next hours. Given some hundred J/kg MLCAPE and 15 - 20 m/s deep layer shear ahead of the MCS, this system should likely produce heavy rainfall and severe wind gusts. SRH and LL shear are expected to increase over S-central Sweden and storms in this environment will likely develop rotating updrafts which may allow an isolated tornado. Thunderstorm development should start in the early morning and will likely continue into the late evening.
...Romania and Ukraine...
Continuous advection of very humid and unstable air towards E-central Ukraine and NE Romania will lead to approx. 1500 J/kg MLCAPE and storms that develop in this environment are likely to move very slowly. The heavy rain threat of the previous forecast period will continue but weaker signals in effective precipitable water and LL convergence suggest that the heavy rain threat will be more locally. Apart from excessive rainfall, large hail and severe wind gusts are possible.
