Storm Forecast
Valid: Sun 07 Nov 2010 06:00 to Mon 08 Nov 2010 06:00 UTC
Issued: Sat 06 Nov 2010 17:44
Forecaster: SCHLENCZEK
A level 2 was issued for parts of Greece and Albania mainly for severe wind gusts and tornadoes and to a lesser extent for large hail and excessive rainfall.
A level 1 was issued for the central Mediterranean including Sicily and S Italy mainly for severe wind gusts and to a lesser extent for large hail and tornadoes.
A level 1 was issued for parts of Ireland, western UK, NW France and NW Iberia mainly for severe wind gusts and to a lesser extent for tornadoes.
SYNOPSIS
On Sunday there are two weather systems affecting Europe which may pose a risk of severe convective and non-convective weather:
(i) a strong extratropical cyclone initially located northwest of the British Isles will race across the NE Atlantic and the Irish Sea towards the Bay of Biscay while occluding. This low pressure system should gain its maximum intensity on Sunday around 18 UTC between Iceland and Ireland. The origin of this low is a baroclinic wave east of southern Greenland next to an impressive 100 m/s upper jet streak.
(ii) a large upper trough over the Mediterranean with strong upper level winds ahead of it, pointing towards S Italy and Greece / Albania. Moderately warm air on the right side of the jet streak should lead to at least some hundred J/kg MLCAPE, probably with maximum values close to 2 kJ/kg. Increasing 0-6 km shear and 0-3 km shear in the late evening / night hours and sufficient QG forcing from the upper vort-max may lead to widespread severe weather, especially wind gusts and tornadoes.
DISCUSSION
...Sicily, S Italy...
A plume of highly unstable air which originates from Tunisia / Libya is advected northeastwards and will reach Sicily and S Italy on Sunday afternoon. An increase in deep layer shear to values around 20 - 25 m/s in an environment with 1000 - 2000 J/kg MLCAPE will create almost ideal conditions for severe multicellular systems (probably a squall line or a LEWP structure) and supercells. Strong winds in the 700 hPa layer (15 - 20 m/s) may lead to severe wind gusts due to downward transfer of momentum. Isolated large hail should be possible as well. A tornado is not completely ruled out but the probability is rather low as LL shear will stay below 10 m/s and SRH1 is low. The predicted rain sums are below 20mm / 24h and therefore excessive rainfall and flooding are not very likely.
...Greece, S Adriatic, Ionian Sea, Albania...
In the late evening / night hours, the unstable air should have reached the Greek and Albanian coast. Current thinking is that a severe MCS will develop over the Mediterranean, racing northeastwards towards Greece / Albania. Large hail is expected with the strongest storms. Increasing LL winds and increasing directional shear combined with > 200 mē/sē SRH1 suggest that a few tornadoes may be possible, even a strong one (F2 - F3) not ruled out. The main threat should be widespread severe wind gusts as some 25 - 30 m/s at 700 hPa can easily be mixed down. There are some signals that accumulated precipitation may reach values around 50mm / 12h especially in S Greece. A level 2 is issued for the region where widespread severe / isolated significant severe events are most likely.
...Aegean Sea, W Turkey...
Similar kinematic conditions as mentioned above are also expected over the Aegean Sea and over W Turkey but the thermodynamics are somewhat different: CAPE is in order of some hundred to about 1000 J/kg and the airmass over the Aegean and Turkey is strongly capped with CIN in order of -100 to -200 J/kg. As no real source of upper level forcing is in place and the LL convergence is probably too weak to initiate convection, no thunderstorm area was issued. This region has to be updated to at least level 1 if convection starts to develop.
...W and C Mediterranean, Tyrrhenian Sea...
Some low-end CAPE should be created during the day and scattered showers and thunderstorms are expected. Overall shear is quite low (< 15 m/s) and therefore severe weather seems to be unlikely. Near the upper cold core of the low, calm winds at all levels and rich BL moisture suggest that an isolated waterspout cannot be ruled out.
...W Ireland, Scotland and extreme SW England...
After the warm sector has passed the western half of the British Isles in the late evening / night hours, the cold front will approach Ireland and western UK. A few hundred J/kg CAPE / ICAPE are forecast which should allow a strongly forced but rather shallow convective line that may produce severe / damaging wind gusts. As the non-convective gusts will likely reach values around 25 m/s and the CAPE signal is rather weak, a level 1 should cover the risk of convective events properly. As seen in other similar cases, SRH1 and SRH3 tend to reach extremely high values near the cold front and isolated tornadoes may occur within the line.
...Bay of Biscay, NW Iberia, NW France, British Isles...
A broad number of global models (GFS, ECMWF, GME and others) agree that convection will become rather unlikely in these places (except for NW France in the afternoon hours before the cyclone will reach western UK). The warm front / warm sector region of the cyclone may lead to widespread severe wind gusts over Ireland and UK which are mostly non-convective. In the early morning hours on Monday, the cold front of the low pressure system will arrive at NW France but instability should be too low for deep convection. Some convective enhancement of the wind, especially in the southern Bay of Biscay, cannot be ruled out and therefore a level 1 was issued for those regions which are most likely affected.
Please note that we do not issue threat levels for non-convective events!
