Storm Forecast
Valid: Sun 27 Oct 2013 06:00 to Mon 28 Oct 2013 06:00 UTC
Issued: Sun 27 Oct 2013 05:14
Forecaster: VAN DER VELDE
A level 2 was issued for northeastern France, southern Germany, Czechia, northern Switzerland and northern Austria mainly for severe convective wind gusts and tornadoes.
A level 1 was issued for parts of France, Belgium, Netherlands, Germany, Czech Republic, Poland, as well as Ireland, mainly for severe convective wind gusts and tornadoes.
A level 1 was issued for western France and southern UK for severe convective gust and tornado chances after midnight.
SYNOPSIS
A large depression centered west of Scotland has transported a broad plume of relatively warm low level air deep into Europe. A sharp shortwave trough dramatically visible in IPV fields is racing from western France to eastern Poland during the period. Together with a northwesterly jet axis left exit region, it forces rising air with steepening lapse rates between central France and the Netherlands in the morning, which shifts into Germany during the afternoon and Czechia/Poland in the evening. Thedynamic tropopause(PV)/jet axis intersection region moves just north of the Alps. There is a strong wind field with a corridor over southern Germany with >30 m/s winds in the lowest kilometers.
Late in the night, a side disturbance of the main low arrives in the English Channel region with signs of rapid cyclogenesis en-route to Denmark, producing gale conditions on Monday.
DISCUSSION
...Ireland...
In the first 6 hours of the period, convection and conditions of strong low-level winds and shear 15-20 m/s are present. The jetstream passes over the southern half of Ireland. Expect some convective gusts greater than 25 m/s. An isolated tornado cannot be ruled out as well.
...western France, southern UK...
This region will be affected by two disturbances in 24 hours. At 06Z, unstable air under the influence of the shortwave trough is present as well as 25 m/s mean winds in the low levels, and some 20 m/s 0-1 km shear. Mesoscale convective development is not yet expected to begin, but these conditions combined with low LCL heights can lead to tornadoes, aside from severe wind gusts.
After 00/03Z, the cold front of the second low comes in. It seems to become somewhat convectively active, likely as a forced convective line (there is little CAPE and EL at only -15°), this time with 30-40 m/s mean winds in the low levels over NW France capable of producing damaging gusts at the ground. The front bends back over the southern UK with less strong wind field but enhanced shear and instability sufficient for a tornado or two. The 00Z GFS run predicts the low to take a much more northerly and faster course with more stable conditions than the 18Z run.
...northeastern France to southwestern Poland...
Synoptic and mesoscale lifting appears to increase in tyhe GFS model over northern France and enhance CAPE to about 200 J/kg. A linear PV intrusion is likely to shape the convection as a line, although WRF models suggest more cellular/short line segment modes instead. Indeed even in the GFS model there is a large region of deep convergence but no sharp line at the surface. The jet exit region affects most directly the zone immediately north of the Alps. In entire southern Germany more than 20 m/s of 0-1 km shear should exist, which aids tornadogenesis (with >250 mē/sē of 0-3 km SREH also predicted) and bow echoes with strong to very strong gusts during convective storms. The 00Z GFS run predicts the same timing as the 18Z run but with the PV intrusion displaced slightly to the north, as well as more positive rather than negative tilt.
