Storm Forecast
Valid: Tue 14 Jul 2009 06:00 to Wed 15 Jul 2009 06:00 UTC
Issued: Tue 14 Jul 2009 06:17
Forecaster: VAN DER VELDE

A level 2 was issued for N Spain, parts of France and the north side of the Alps mainly for large hail and excessive precipitation.

A level 1 was issued for parts of Russia (and Ukraine) mainly for excessive rainfall and large hail.

A level 1 was issued for Czech republic, southern Poland and Austria for large hail and severe gusts.

A level 1 was issued for Ireland (and the UK) for spouts and isolated excessive rainfall.


SYNOPSIS

A stationary warm front is situated over S/C France and southern Germany. Strong mid level winds blow parallel to the front and create vertical shear favorable for severe storms. A low pressure system moves slowly over the British Isles.
Another stationary frontal system lies over Ukraine and Russia.
Low pressure is also present over the western Mediterranean Sea and eastern Spain, where a shortwave is predicted by GFS to pass and move during the night to the Alps.

DISCUSSION

...N Spain, France, S Germany area...

In the wake of last night's MCS activity over France and SW Germany, the region over central Germany and northeastern France is likely to be affected negatively initially with little available energy, remaining precipitation and even quasi-geostrophic subsidence. To the south of the front, warm unstable air (CAPE around 1000 J/kg) and insolation are available and wind profiles should veer with altitude as a result of warm air advection. Deep layer shear of 15-20 m/s and 0-3 km SREH will turn a number of cells into supercells (strong rotating updrafts) and these can produce large hail.

The storms may cluster and form once again MCSses (in series) which can lead to excessive rainfall, since the flow is parallel to the front that triggers the cells, which can then drift for a prolonged time over the same corridor, motly during the night. This is also possible for the area of E France and SW Germany by advection of convective activity from central France and likely helped by the upper shortwave during the night.

Over NE Spain, good QG forcing should be present and enhance storm activity. For 21Z a ridge should pass and may suppress storm activity, but it can revive later at night.

Over Czech Republic, Austria, and Poland the precipitation threat is lower and the chance of gusts and large hail larger, with a CAPE maximum, higher LCL and delta-theta-e (16 K) and shear under a greater angle to a convergence line bending to the north, as well as better QG support.


...Ireland...

Given slow storm motion in the trough that stretches over the country, good 0-3 km CAPE and weak winds, spouts can form, as well as locally large precipitation sums.
This may also occur over the UK, but flow is stronger there which should diminish the chances somewhat.


...Russia and Ukraine...

Weak flow parallel to the waving frontal zone should help storms with locally excessive precipitation. Over Russia, SREH is also fairly large (200 m²/s²) and large hail can occur from strong multicells or supercells.

...Netherlands, N Germany, Denmark...

While deep layer shear is strong, cloud top heights are forecast to be low, barely reaching the -20°C level. At the same time there appears to be subsidence through the unstable region which may suppress activity. On the other hand, GFS gives the contradictory signal of spots of deep convergence.
No level was issued, but the showers or storms, if they form, may produce some gusts and up to 2 cm hail.