Extended Forecast
Valid: Thu 23 Jul 2009 06:00 to Fri 24 Jul 2009 06:00 UTC
Issued: Tue 21 Jul 2009 19:51
Forecaster: TUSCHY

A level 2 was issued for Switzerland, parts of Germany, the Czech Republic and parts of Poland mainly for large hail, severe wind gusts and tornadoes.

A level 1 was issued for surrounding areas of the level 2, France, Lithuania and Belarus mainly for large hail and severe wind gusts

A level 1 was issued for Belgium, Luxembourg and the Netherlands for large hail and tornadoes.


SYNOPSIS

The amplified pattern continues with a far southward building upper-level trough just west of Europe. Embedded in an active southwesterly jet along its eastern fringe, numerous short waves cross western and central Europe. Persistent WAA downstream of this trough assists in the northward development of a strong ridge with a very hot airmass spreading far to the north.

At the surface, a frontal zone serves as focus for initiation next to the short-waves. It separates hot and dry air to its east from cool moist air to its west.

DISCUSSION

... East/southeast Germany, western Czech Republic and parts of Poland ...

An healthy short-wave, crossing France in the morning to afternoon hours and northern Germany during the evening and early night hours, is situated over south Sweden during the morning hours. This feature is present in the NCEP-ensemble but also in all global models with variable strength. Confidence is therefore increasing that forcing will spread northeastwards, affecting the highlighted areas from the southwest.

At the surface, an extensive boundary is present, which runs from N-Spain, SE-France, SE-Germany to NW-Poland with an overall slight eastward motion during the forecast. East of this boundary, very hot air is forecast to spread northeastwards with readings of 20°C and more at 850hPa all the way to Belarus. A robust elevated mixed layer continues to spread northeastwards with mid-level lapse rates of 8-9K/km east of the boundary. Dewpoints in GFS forecasts are in general in the upper tens or lower twenties, which seems reasonable regarding the quality of the airmass further upstream. Locally higher dewpoints are possible due to moisture pooling e.g. along convergence zones or the front itself. Spots with dewpoints in the mid twenties (e.g. SE-Germany or Lithuania) are discounted but those areas will be monitored regarding the magnitude of boundary layer recovery.

Instability along and east of the boundary will be in the 1000-2000 J/kg MLCAPE range with regional oscillations due to differing boundary layer quality or topographic effects (e.g. foehn conditions along and north of the Alps). The airmass is well capped, so initiation will await mid-level forcing.

Shear at all levels is in the strong to extreme range with 0-6km shear of 20-40m/s, 0-3km shear of 20-30m/s and augmented directional and speed shear in the lowest 1000m. Combined with the aforementioned CAPE values, rapid thunderstorm organisation is forecast with favorable conditions for mid-level rotation. Very large hail is probably a significant risk, especially during the more discrete phase of those storms. Betimes, small lines of storms with a damaging wind gust risk will probably develop. The strength of the 0-3 km shear suggests that these may locally be extremely severe. The tornado risk will not yet be evaluated as exact shape of the surface trough is still uncertain, but thunderstorms over parts of E-Germany, W-Czech Republic and NW-Poland could also produce tornadoes, which can be strong.

Still roughly 2 days ahead, only those areas were highlighted with a level 2, where models agreed well in past few runs but also among each other. Further modification will be needed regarding the shape of the level 2 but also the final risk level.

The large hail and severe wind gust risk expands well to the east, including Belarus, which is also highlighted with a level 1.

... N-France, Belgium, Luxembourg and the Netherlands at 18Z onwards ...

Agreement amongst the models (especially GFS/WRF and NOGAPS) exists that another short-wave crosses the highlighted area from the SW, placed beneath the left exit of a strong 60m/s high-level jet. Influx of moisture ahead of the wave will support at least isolated to scattered thunderstorms, which cross the area from the SW during the night hours. The strength of the wind field depends on the path and constitution of the wave but if latest model forecasts verify, at least a low-end tornado risk is forecast. Thunderstorms may also affect extreme NW-Germany in the morning hours but probably with a decrease in severe.