Valid: Sat 14 Jul 2012 06:00 to Sun 15 Jul 2012 06:00 UTC
Issued: Sat 14 Jul 2012 04:39
Forecaster: VAN DER VELDE
A level 2 was issued for N Poland mainly for severe wind gusts and tornadoes.
A level 1 was issued for extreme N Russia mainly for large hail, severe wind gusts, excessive convective rain.
A level 1 was issued for S UK, Denmark, S Sweden for spout type tornadoes.
A level 2 and level 1 were issued for NE Italy and Slovenia mainly for large hail, isolated tornado and excessive precipitation.
A level 1 was issued for parts of Poland, Slovakia, Romania Ukraine, Belarus mainly for isolated large hail and severe wind gusts.
Under the influence of a low pressure area centered over southern Scandinavia, a cold front is being pushed into central and southern Europe by strong westerly flow. A strong jetstream is present as well. At the warm side of the front a band of warm humid air resides in the lee of the Alps and increases MLCAPE to over 1000 J/kg. Moisture and CAPE are weaker to the norh and west, but lapse rates relatively steep. The cold front moves through this environment during the afternoon, with a significant shortwave trough and a strong kinematic environment for storms.
...Germany, Poland, Lithuania...
GFS and Hirlam depict the course of the trough quite similarly, but the 0-6 km shear as result of the jet stream is more intense (20-30 m/s) and shifted to the north of Poland in GFS18Z at 15Z. Positioning of not that marginal CAPE in the left exit region of the jet and PV advection region should result in triggering of convection. 0-1 km shear is predicted to be 10-15 m/s and helps tornadogenesis, as do the low LCL heights between 600 and 1000m. Winds at 1-3 km altitude are rather strong at 20 m/s and can enhance convective wind gusts to severe strength at the ground, particularly when a linear storm forms along the cold front. Behind the cold front, over Germany, instability, flow and low level shear remain elevated and an isolated tornado is not ruled out.
...N Italy, Slovenia, S Austria, W Hungary...
Significant MLCAPE builds up with high dewpoints, 0-1 km mixing ratio should reach at least 13 /kg. But capping is also moderate and likely prevents widespread developments except for northeastern Italy and Slovenia. Very strong 25-35 m/s deep layer shear and moderate SREH (200 m2/s2) guarantee supercells. Large and isolated extreme hail and severe wind gusts are possible. The latter are further supported by large delta-thetae and the flow in low levels. During evening and night an excessive rain threat is likely as storms train into Slovenia. 0-1 km shear and low LCL heights cannot be neglected mainly in the level 2 area and can support tornadoes.
...Slovakia, NE Hungary, S Poland, Ukraine, Belarus...
There is not a wide zone of large CAPE here, but strong vertical shear overlapping instability can locally produce supercell storms. Around Hungary/Slovakia some chance of excessive rain is possible as the front and any storms drag along.
In the corner of the map interesting things seem to happen as well, with nice SREH, high cloud bases, with potential for supercells carrying large hail and severe gusts. Models actually only forecast an MCS with almost nothing to the south in the drier air. Shear vectors are quite parallel to the front and up north the MCS may backbuild into the CAPE and drag along the front, producing excessive rain.
...UK, Netherlands and Denmark areas...
These level 1 areas mark regions where conditions appear the most favorable for spout-type tornados. Both area see vorticity fringes in good low level instability, very high relative humidity airmass, as 00Z soundings also indicated. Storm motion should be quite slow and shear weak. These factors are often found during spout outbreaks, and indeed the past days the upper cold pool has had a history of producing funnel clouds and spouts.
Possibly also the coast of the Netherlands, Belgium, N Germany may see some, but instability seems better inland for much of the day, where flow however is likely too strong for this phenomenon. The early morning seems best and currently (04Z) there is a convergence zone in place with line of showers on radar (southern IJsselmeer) , so a level 1 was added here too.