Severe storms are expected across southern Alpine region along/ahead of the strong cold front blasting trough in the afternoon/evening hours. High instability and strong shear overlaping together perfectly. Near or above 2000 J/kg MLCAPE will come together with DLS of 50kt+ and LLS near 30kt. Enhanced SREH values up to 300m^2/s^2 will support rotating storms. A few strong supercells with threat for large hail, severe winds, excessive rainfalls and also tornadoes are possible. While the main threat with this setup are excessive rainfalls along the Dinaric mountain range with peak above 200mm/24h locally as strong SSW flow will create intense orographic rainfalls.
Here are more details about this day and some additional maps from different models.
An interesting setup is shaping up for today and surprisedly its showing on the maps since last Sunday already, actually no big differences have been done on the outputs. Anyways, its a classic early summer setup with extreme CAPE and shear overlaping together over southern Alpine region. 50-60kt DLS and easily 1500 J/kg of MLCAPE will be available during the afternoon/evening hours over NE Italy and N Adriatic sea. Additionally, classic Lee cyclogenesis takes place in the early afternoon which will support veering wind profiles and enhance SR helicity up to 200 or even 300 m^2/s^2.
What is interesting to note is that SST of N Adriatic sea are currently near 27°C but they will rise a bit more as strong southerly flow (Scirocco) is bringing warmer SST from the south. Along with it, of course, will also increase the LL moisture and therefore more energy will be available. Surface dewpoints are expected to reach near 23°C locally over NE Italy, especially along the coast. But, looking over those very strong sustained 25-30kt H925 winds, those low 20s dewpoints should be well across whole BL and not just near the ground.
According to GFS model, there is excelent overlaping of at least 50kt DLS with 1500 MLCAPE over NE Italy. LL shear up to 20kt while SREH3 peaks up to 300 m^2/s^2 where SREH1 is pretty impressive as well, reaching 150m^2/s^2! It has to be said, this is global model and local models do have even slightly better conditions, as ALADIN has better shear and instability in the afternoon. LL shear is the most impressive, coming with 25-30kt at H925 from SSW at 15z while H85 is near 20-25kt from SW at the same time. At 18z, winds are even stronger and backed in the lowest layers with 20-30kt at H925 from SSE. At H85, winds will will be backed to straight southerlies as well. So according to these wind profiles, aything that could manage to pop up along any convergences, it could go tornadic quite rapidly. Especially as convection will be explosive with those MLCAPE values.
But on the other hand, ECMWF model has a trough axis and surface low more to the north and then winds more unidirectional, as well as shear. Then, more linear mode of storms can be expected and hard time making right movers. Later on, a large MCS is expected along the southwards blasting front, likely going across the region overnight and morning hours.
As for Slovenia, it seems that at least W SLO will see extensive and excessive rainfalls, most of them being pre-frontal rainfalls. Strong ML flow and moderately strong CAPE should lead into strong orographic lifting along the slopes and several hundreds of mm could become reality in the next 24hrs. The same for NE Italy and N Friuli. The rest of Slovenia is actually quite questionable for the initiation of severe storms given the anvil layer flow being almost parallel to the ML flow and DLS, simply said there might be lack of insulation due to cloud cover if storms go widespread over N Italy too early in the afternoon. However, E SLO will likely have weak easterly LL flow which will enhance helicity and any storms would then be in the game to become severe and rotating. Less CAPE and SREH limits the higher threat, but this setup reminds a bit of last year’s July 13th 2008, when an extremely severe cyclic supercell crossed the whole Slovenia. It started in the northern Adriatic sea, moving NE and caused an extensive wind damage over parts of Slovenia, including a tornado over village Gozd near Kamnik. Similar conditions are seen this time again, but such events are less likely to occur. Especially as cold front will enter NNE SLO by the evening and cells which will cross the boundary, will become elevated and then less severe.
This outlook above was based on the latest global and local models from last days, while today’s model output have a bit more unidirectional shear. Local BOLAM model goes along with ECMWF, being unidirectional as well. Still DLS and MLCAPE are pretty high and nicely overlaping for strong supercells, but slightly less tornadic conditions that it looked before. A view over 0-4km shear vectors gives a good deal that cells, especially those more to the south, will have good options to become right movers. Those cells closer to the mountains will have more linear mode and will serve as an excessive rainfall threat there.
The front will be slightly more horizontally oriented than thought before, but still SW-NE oriented. There shouldn’t be any problems with cloud cover to the southernmost cells, so they should stay isolated while cells more to the north/mountains will be more linear. The storm motion has to be noted, as cell will be blasting like 240/40kt across the region which is pretty fast to chase over such a complex road network we have in NE Italy.
Latest model outputs show storm initiation in the early afternoon over pre-Alpine areas, while conditions are not changed a lot. Still looks supercellular and party even tornadic. For early September, that is more than impressive! Here is a map of SREH3 and supercell composite parameter..
Here is an ESTOFEX forecast discussion for today and overnight storm activity.
Valid: Fri 04 Sep 2009 06:00 to Sat 05 Sep 2009 06:00 UTC
Issued: Thu 03 Sep 2009 16:29
A level 2 was issued for NE-Italy, W-Slovenia and W-Croatia mainly for large hail, severe wind gusts, excessive rain and to a lesser extent for tornadoes.
A strong upper trough over the North Sea moves eastwards and is placed over Sweden and Norway during the end of the forecast. Embedded disturbances cause unsettled conditions for most parts of central and northern Europe. Warm and stable conditions prevail over most parts of the Mediterranean and eastern Europe.
… S-Austria, Hungary, W-Ukraine, Slovenia, parts of Croatia and NE-Italy …
All models show abundant convective signals, but timing of the convective precipitation remains uncertain. ECMWF is a bit reluctant with the main activity during the afternoon hours, GFS has initation all day long and UKMO indicates an heavy rainfall scenario. We stay with GFS and therefore expect a prolonged period with thunderstorms as numerous disturbances cross the area from the west.
The temperatures at mid-levels cool down slightly, whereas the lower levels remain warm and moist due the the blocking effect of the Alps in respect of the cold front passage. This should support a prolonged period of modest CAPE release in a strongly sheared environment. In fact, ECMWF and GFS are in line with up to 2000 J/kg MLCAPE along the NE- Adriatic Sea. Large hail and severe wind gusts are forecast which could reach the criterion “extreme”. In addition, a few tornadoes are forecast with the highest risk over western Croatia, central and western Slovenia and extreme NE-Italy. Due to the repeated thunderstorm development over the same area and persistent influx of moist air from the south, excessive rain is forecast, too. The severe risk gradually moves southwards as cold front approaches from the north.
Further towards the east, including S/E-Austria, rest of Slovenia and western Hungary, CAPE decreases somewhat with values in general less than 1000 J/kg MLCAPE. Shear at all levels remains supportive for large hail and severe wind gusts. During the night hours, the cold front finally pushes southwards and so do the thunderstorms.
See the rest of the forecast here: