Storm Forecast
Valid: Sun 02 Oct 2016 06:00 to Mon 03 Oct 2016 06:00 UTC
Issued: Sat 01 Oct 2016 17:34
Forecaster: BEYER

A level 1 was issued for parts of Eastern Spain mainly for excessive precipitation and to a lesser extent for large hail.

A level 1 was issued for parts of Western Italy (Campania) mainly for excessive precipitation and to a lesser extent for large hail.

A level 1 was issued for the Adriatic coastline of Croatia, Bosnia-Herzegovina, Montenegro and Albania mainly for excessive precipitation and to a lesser extent for large hail.

A level 1 was issued for parts of the North Sea and the near coastal areas of the Netherlands as well as the East and West Frisian Islands, mainly for tornadoes.

SYNOPSIS

A trough is active over most parts of Central and North Europe. Its southernmost part detaches during the forecast period and forms an upper level low centered over Northern Germany and Denmark. Cold upper level airmasses lead to an unstable environment.

Western and Northwestern Europe are mainly under the influence of a well amplified ridge on the forward flank of a new long wave trough over the Eastern North Atlantic. Stable and warm airmasses are increasingly advected with a southerly component.

Apart from the Aegean Sea most regions of the Mediterranean still have to struggle with an unstable environment. The trough on the southern flank of the addressed upper level low over Northern Germany influences Southern Europa. Its main axis travels from Sardinia at the beginning of the forecast period to the Eastern Balkan Peninsula at its end. However, it is not easy to anticipate a strict axis since the general flow is rather zonal showing several short wave features.

In the second half of the forecast time Southwestern Europa and the Western Mediterranean are getting under the influence of a shallow ridge and thus under stable conditions.

DISCUSSION

...Eastern Spain....

In the highlighted area a good overlap of moist LL airmasses coming from the Mediterranean and steep mid-level lapse rates coming from inland can be found. In addition LL streamlines do show convergence near the coast. Together with ppw's around 30 mm there is a certain risk for excessive precipitation when several storms influence the same area.

DLS is moderate with values around 15 m/s (0-6 km). It cannot be ruled out that one or two storms can get better organized with the help of the vertical wind shear. Thus, also a possibility of large hail exists.

All together coverage is a little bit questionable which makes the possibility of severe storms a little but uncertain. However, if storms occur, some LAM do show this, the named accompaniments are possible.

The severe weather risk diminishes throughout the afternoon, when a ridge approaches from the west.

...Western Italy (Campania)...

Moist airmasses with LL mixing ratios of about 12 g/kg advected from the sea can overlap with steep mid-level airmasses mainly available inland. CAPE values of about 1500 J/kg, locally up to 2000 J/K are the result. With the help of the trough the needed third ingredient lift is available. With the help of the IPV map several short wave features can be anticipated.

The area that was highlighted shows best conditions for excessive precipitation having rather high ppw's (> 40 mm) and convergent LL stream lines. Longer continuing storms can therefore locally bring large amounts of rain.

Shear values are mainly moderate with 0-6km around 15 m/s. However, this does not completely exclude locally large hail or large amounts of small hail.

To the south the dynamic environment is much better (higher DLS) but capping is als rather stronger. Thus no threat level was issued more to south. If however a storm occurs it can also become severe.

This area of interest is mainly affected in the first half of the forecast period. The severe weather risk diminishes slowly during the afternoon, when the main trough axis travels further east.


....Adriatic coastline of Croatia, Bosnia-Herzegovina, Montenegro and Albania...

Conditions are quite similar to Italy. High LL moisture, CAPE values up to 1500 J/kg, locally convergence effects along the Adriatic coast. This altogether can lead to locally excessive precipitation. Shear values of up to 15 m/s between 0 to 6 km offers the possibility for locally large hail or large amounts of small hail.

This area of interest is mainly affect during the second half of the forecast period starting in the afternoon hours and also continuing throughout the night.

...North Sea and the near coastal areas of the Netherlands and the Frisian islands...

Under the center of the upper level low quite weak mid and upper level winds are present. With the available cold airmasses quite steep mid and low level lapse rates are present whereas the latter is mainly due to the warm ocean and daytime heating over land, respectively.

This finally leads to robust LL CAPE values of more than 100 J/kg that enhance significantly the threat for waterspouts. The risk area is mainly coupled to the time of the day and the sea breeze front. During night the waterspouts should mainly be active over sea, however during the day also the coastal areas of the Netherlands and the Frisian islands are under the threat level, when the sea breeze front moves inland.

The overall threat travels with the upper level low slowly eastward during the forecast period.