Storm Forecast
Valid: Mon 26 Sep 2016 06:00 to Tue 27 Sep 2016 06:00 UTC
Issued: Sun 25 Sep 2016 20:36
Forecaster: BEYER

A LVL1 was issued for the eastern coast of Spain as well as the Balearic Islands mainly for large hail, excessive precipitation and to a lesser extent for severe wind gusts

A LVL1 was issued for Northern Tunisia and parts of North Algeria as well as Sicily and Malta mainly for excessive precipitation and to lesser extent for larger amounts of small hail.

SYNOPSIS

An active westerly flow with humid and cool airmasses influences most parts of Northwestern Europe (British Isles to Norway).

At the same time most parts of Northeastern and Eastern Europe are under the influence of a strong ridge with a discrete upper level high in its center.

The Eastern Balkans are still below a slightly weakening upper level low with cold upper level airmasses leading to instable conditions.

The Mediterranean are again influenced by humid and warm airmasses, especially the southern portions of the Western and Central Mediterranean Sea. Several short wave features are travelling eastward throughout the forecast period. At the same time, the upper level flow is rather weak

DISCUSSION

...Eastern Spain and Balearic islands...

Favorable conditions are present just at the beginning of the day. Mixing ratio is highest near the coastline with maximum values of 12 g/kg. At the same time steep lapse rates are advected with a westerly to northwesterly flow from the Spanish mainland into the Western Mediterranean.

As a lifting mechanism works a well pronounced short wave trough that is moving east-southeastward. Its axis reaches the east-coast of Spain at around 12 UTC.

Convective initiation will be quite early in the day thanks to the help of the short wave trough and will continue eastward also capturing the Balearic Islands.

DLS is forecasted to be around 15 to 20 m/s that can overlap with CAPE values of up to 1000 J/kg, depending on the region and the time of the day. Taking all this together a lot of convection can be expected that with the help of the DLS may also be better organized bringing locally large hail. Having rather high ppw's (well above 30 mm) excessive precipitation is another threat. However, there are no signals for stronger convergence zones and backbuilding storms and 500 hPa winds are about 15 to 30 kn.
If a better organized storm can develop a severe wind gust cannot be ruled out. However, apart from that, mid-level winds do not support this threat

...Northern Tunisia, parts of Northern Algeria as well as Sicily and Malta...

Quite humid airmasses are present with mixing ratios between 11 and 13 g/kg. Although lapse rates do not look extraordinary steep, the low level moisture is enough to get about 800 to 1000 J/kg of MLCAPE.

In mid and upper levels another short wave trough is present. In 500 hPa one can even see a closed isohype around Sicily in some time steps. Lift can also be anticipated by the IPV map showing several maxima.

One can conclude that all three ingredients should come together to initiate several storms. Rather slow storm movement and the possibility of backbuilding storms pose an enhanced threat for excessive precipitation. Concerning Northern Tunisia also convergent streamlines can be found near ground. This additionally enhances the named precipitation threat.

Beside that DLS is mostly moderate with 10 to 15 m/s. Thus the possibility of an accumulation of larger amounts of small hail is only a minor threat. The shear enhances more to the south but this is outside of the forecast area

...Eastern Balkans...

An upper level low with cold upper level airmasses leads to steepening lapse rates during the day when lower levels warm up. This will follow into several thunderstorms developing throughout the day. Having mostly around 10 m/s of DLS (at least were instability is present) pulsating storms a forecasted. Although storms movement under the center of the upper level low is forecasted to be rather weak, ppw's are too low to pose an enhanced threat for excessive precipitation.

...Eastern and Central Mediterranean...

Weak upper level flow, low LCLs and local convergence lines are a good combination for an enhanced probability for waterspouts that may occur anywhere where showers or storms form along a pronounced surface convergence.