Storm Forecast
Valid: Mon 25 May 2009 06:00 to Tue 26 May 2009 06:00 UTC
Issued: Sun 24 May 2009 22:55
Forecaster: PUCIK/GROENEMEIJER/TUSCHY

A level 2 was issued for N Spain, S and Central France mainly for large hail and severe wind gusts.

A level 1 was issued for Spain mainly for large hail and severe wind gusts.

A level 2 was issued for N France and Benelux mainly for severe wind gusts, large hail, tornadoes and excessive precipitation.

A level 1 was issued for much of England for excessive precipitation and large hail.

A level 1 was issued for Germany, Switzerland, Austria, Slovenia for large hail and severe wind gusts and excessive precipitation.


SYNOPSIS

A mid-level cut-off low with its center over western Spain will slowly connect with a short wave trough, which is travelling in a strong westerly flow above the Atlantic. This trough will deepen during the day and a blocking situation will be established with a significant ridge stretching from the central Mediterranean to southern Scandinavia. Strong southerly flow is simulated between these two features. To the east, yet another trough will stagnate over Eastern Europe. Strong WAA is already underway on the lee side of the ridge and a tropical airmass is expected to reach Northern Germany and Southern Britian. Wavy cold front will slowly propagate eastwards and a surface low has developed under the left exit region. This low will rapidly propagate northeastwards and deepen and its center is forecast to be over the Northern Sea by Tuesday morning.

DISCUSSION

...Spain....

Under a strong mid-level jet, a belt of high DLS with values over 25 m/s is anticipated and over 15 m/s in 0-3 km layer. Sufficent forcing will be provided by the presence of a cut-off low and thunderstorms will initiate especially in the region of cold front. Instability should be only modest, with MLCAPE values between 500 and 1000 J/kg, with decreasing to the south. However in combination with very good wind shear profiles, well organised storms, including supercells are anticipated and a large hail can occur with the stronger cells. Moreover an isolated severe wind gust is not ruled out because of relatively dry airmass at lower levels and a level 1 seems to be warranted.


... Belt from N Spain to Central France...

Moderate to strong instability release is anticipated in this region with MLCAPE values locally well over 1000 J/kg thanks to the presence of humid airmass at lower levels and steep mid level lapse rates. Initiation of storms seems to be most likely between 12 and 15Z but it is quite probable, that some convection will be ongoing from the previous day in the form of decaying MCS with its outflow forming a boundary, on which a new activity might start. A developing trough should provide QG forcing especially for the southern part of the region and a high storm coverage is expected along the cold front. Impressive kinematic profiles, with 20 - 25 m/s of DLS and locally up to 20 m/s of shear in the lowest 3 km. Moreover, enhanced SREH is anticipated due to the backing low level flow on the forward side of the developing surface low and within such environment, well organised storms will easily form.

At first, isolated development might occur, with some supercells very probable. Due to the large CAPE release, large to very large hail will be possible with any stronger and well-organised storm ( especially in case of supercells). A rapid storm clustering is expected later on, with one or two MCS developing along the front and moving rapidly northwards. Bowing segments might develop within MCS, mainly towards the northern parts of region and severe wind gusts could become a serious hazard in such structures.

....N France, Benelux....

Two waves of thunderstorms are simulated by models in this region.

The first one is expected with an isolated storm development in the afternoon hours. Impressive instability release is predicted with MLCAPEs probably reaching locally more than 3000 J/kg and in combination with moderate wind shear ( 15 m/s in 0-6 km layer), any developing thunderstorm will have a chance to produce large, or even giant hail. Delta Theta E values should reach more than 16 K, implying a threat of downbursts in the storms. A development of MCS is not ruled out as storms will progress to the north. The initiation of storms seems to be most likely over NW France along the warm front that is moving northward.

The second wave of storms will reach the region in night hours, probably in form of one or more well developed MCS. At the same time a surface low will undergo a deepening process and strenghtening wind speeds at lower levels. At 850 level, strong southerly flow is anticipated wind windspeeds over 20 m/s. Increasing low and mid level shear suggest a serious wind gust threat with those MCS, especially in case bow echoes form. Furthermore, tornadoes are possible given the strong low level wind shear, low LCLs and a possibility that some embedded rotating structures will develop within MCS ( this is supported by the strenghtening SREH in the vicinity of the surface low). Very strong low level convergence, humid airmass and high rainfall amounts simulated by both GFS and ECMWF models point to the threat of heavy convective rainfall with MCS.

....England...

A threat of large hail is anticipated over the SE England in the evening hours, as more than 1000 J/kg of MUCAPE are quite probable as well as moderate to strong DLS ( 15 - 20 m/s). Storms will likely cross the region in form of MCS and during the night hours the system should reach the Northern Sea. Also here a threat of excessive precipitation is present especially over the NW part of Level 1.

...S Germany, Switzerland, Austria, Slovenia...

Diurnally driven thunderstorms might form over the region in spite of the presence of strong ridge aloft. Due to the strong instability release with MLCAPEs likely above 1500 J/kg, large hail will be likely with any stronger multicell thunderstorm. Also, with steep lapse rates at mid-levels, chance for severe wind gusts will be also present. Due to the slow movement of the storms and a mountaineous terrain, over which thunderstorms are likely to stagnate or redevelop, local heavy precipitation event is not ruled out.