Storm Forecast
Valid: Sun 11 Sep 2016 06:00 to Mon 12 Sep 2016 06:00 UTC
Issued: Sat 10 Sep 2016 22:26
Forecaster: BEYER
A level 1 was issued for the Alpine region mainly due to excessive precipitation and to a lower extent for large hail.
A level 1 was issued for southern Italy and Sicily mainly due to excessive precipitation and to a lesser extent for tornados
SYNOPSIS
High pressure is still present over most parts of central and eastern Europe having an upper level high situated over Poland.
Western parts of Europe, especially Great Britain are under the influence of an amplifying trough that supports the persistence of the high geopotential downstream.
Northern Europe is influenced by a short wave trough on the forward flank of the main trough. It is traveling from south to north througout the forecast period bringing cool and unstable airmasses to parts of Skandinavia.
Finally, weak geopotential is present over the central and eastern Mediterranean whereas it is quite challenging to anticipate a distinct center. The same is true for the surface pressure. No real low pressure center can be found in the anaylsis. Rather, activity is driven by mid and upper troposphere. On the southern flank of the low geopotential area, a short wave trough can be anticipated that is moving eastward during the forecast time. It can be easily followed by a well defined IPV maximum.
DISCUSSION
... Mediterranean ...
Day time driven thunderstorm activity is forecasted. Inland, the strongest activity is expected during the day with a maximum over the mountains due to the overall lack of synoptic scale lift . Over the seas normally capping is too strong for initation. However, during the night with the help of the sea breeze a few storm are also possible over the ocean.
Apart from that, the approaching short wave trough will bring large scale lift for the southern parts of central and (during the night to Monday ) also eastern Mediterranean. This is why the 50% lightning area is also extented to part of the Tyrrhenian Sea and the Ioanian Sea.
A single excessive precipitation event cannot be ruled out in any case, especially when orography is in the game. However, the overall risk is to weak to issue a large LVL1. Instead we decided to anticipate certain areas where the risk is enhanced.
... Alpine region ...
Having only weak vertical wind shear storms are expected to stay rather unorganized and wih a pulsating character.However, CAPE values well above 1000 J/kg are predicted so that a few large hail events in the beginning stadium of a storm cannot be ruled out.
What is more problematic and wherefore the LVL1 was mainly issued is excessive rain. Having ppw values of up to 30 mm and 500 hPa wind only about 5 kn, slow moving storms can be expected that may bring large amounts rain over the same area within a short time. This especially true having in mind the complex orography that together with the self dynamics of the storm finally result in thunderstorm that stays at the same place for a longer time.
... Southern Italy and Sicily ...
Again excessive precipitation is the main threat. Storms will be initiated during the day with the help of orographical induced lift. CAPE values are forecasted to be highest near the coastlines since humid airmasses are advected by the sea breeze from the Mediterranean. Having the main center of the upper level low situated over southern Italy winds in the upper troposphere are forecasted to stay weak (5-10 kn). This again means slow moving storms in a low shear environment.
In addition surface streamlines show a convergent flow over the continental area. This is mainly triggered by either orography and local effects ike the sea breeze front that can develop due to the overall weak flow regime.
In any case ingredients for excessive precipitation events are present with slow moving storms that have available ppw values well above 30 mm.
A slow upper level flow and convergence effects especially along the coastlines may allow for waterspouts that can also occur outside of the LVL1 area.
... Croatia, Serbia, Bosnia and Herzegovina ...
Rather high CAPE values (about 1500 J/kg) and DLS slightly above 10 m/s may result in on or two large hail events. This should especially be the case in the beginning state of a storm. However, keeping in mind that DLS ist not that pronounced, this risk is to weak to issue a LVL1.
... Southern and Eastern Spain ...
Along the coast line of southern and eastern Spain there is good overlap of high BL humidity coming from the Mediterranean with an easterly to southeasterly surface flow and lapse rates that are getting steeper more inland. This should result in CAPE values of 1000 to 1500 J/kg. However, capping may be quite strong and initiation is a bit questionable.
Shear is about 10 m/s between 0-6km. Thus, where a storm can initiat a large hail event cannot be ruled out. Rather slow moving storms can also lead to an excessive precipitation event. Again, overall threat is too weak for a leveling.
... Parts of Norway ...
Upper level cold air associated with the trough should initiate several showers and a few short-living storms. They may be followed by a few strong wind gust given the high wind velocities in 850 hPa.
