Storm Forecast
Valid: Wed 17 Sep 2014 06:00 to Thu 18 Sep 2014 06:00 UTC
Issued: Tue 16 Sep 2014 18:25
Forecaster: BEYER
A level 2 was issued for Southern France mainly for excessive precipitation.
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A level 1 was issued for Northeastern Spain mainly for large hail and for severe wind gusts.
A level 1 was issued for Western France for a few excessive precipitation events.
A level 1 was issued for Portugal and Western Spain mainly for large hail and severe wind gusts.
A level 1 was issued for parts of Greece, Turkey mainly for excessive precipitation and to a lesser extent for tornados.
SYNOPSIS
The concentrated frontal zone over the North Atlantic splits into two branches over Europe. The northern branch influences Northern Scandinavia, while the southern one is responsible for the development in the Mediterranean.
A more or less persistent upper level low is situated to the east of France and the Iberian Peninsula. On its forward flank warm and humid airmasses are advected into Eastern Spain and Western France as well as the western Mediterranean region. An occlusion front marks the region where the highest humidity values can be found. It is situated over Western France and is only slowly travelling eastward. As second trough is situated over Southeastern Europe leading to unsettled conditions in that region.
A ridge can be found between the upper level low and the trough. It is connected to an upper level high over Scandinavia. At the surface a corresponding area of high pressure can be found that is influencing large parts of Western Russia, Eastern Europe, Southern Scandinavia and also parts of Central Europe.
DISCUSSION
Eastern Spain and Southern France...
...low level streamlines indicate the advection of humid airmasses into the coastal regions of Eastern Spain and Southern France. Mixing ratios are forecasted to lay between 12 and 14 g/kg.
At the same time steep lapse rates are advected from the Spanish mainland to the coastal regions of Spain. The steep lapse rates for Southern France are coming from the Pyrenees where a southerly mid-level flow leads to a steepening of the lapse rates on the lee side of the mountains.
While over the inland of Spain and near the Pyrenees airmasses are too dry for convection, the instability can overlap further to the east/northeast with the already mentioned humid airmasses. As a consequence CAPE between 1500 and 2000 J/kg can develop with the highest values near the coast.
Deep layer shear is quite high (0- 6km: 15 to 20 m/s, 0 3km: 10 to 15 m/s). Locally this shear values are enhanced by the development of a heat low over Central Spain and by the sea breeze that develops during the day in the coastal areas. Therefore organized and long living convection is possible.
Concerning Spain the development of a few supercells is possible. They can bring severe wind gusts as well as large to locally very large hail. Due to the lack a synoptic scale lift mechanism activity should be bounded to the diurnal cycle and thus decrease in the evening hours. However, initiation is little bit uncertain since the difference between LCL and LFC is quite high. Having in mind that some models show convection at least a LVL1 was drawn. If the development shows that convection maybe more widespread a LVL 2 may be needed.
Concerning southern France a large hail event and strong wind gusts are also possible. However the main threat should be excessive precipitation. LL streamlines give a hint for a convergent flow to the coastline of Languedoc-Roussillon. This development is supported by the local topography. The humid airmasses are constantly advected into the valley between the Pyrenees and the Massif Central. Having a southerly flow in mid-levels, those airmasses are blocked at the southern flank of the Massif Central. This may result in repeating and enduring thunderstorm activity in this region. Ppws are forecasted around 35 mm. Therefore excessive precipitations are possible. LMAs give quite large amounts for the 24 h that are covered by this outlook. For instance: WRF shows 150 to 200 mm in 24 h.
A short wave feature that enters the region of interest in the late afternoon also supports the persistency of the convection and may even strengthen it. The provided lift should also lead to some convection to the northeast of the discussed area, later on.
Western France...
...in the surrounding of the occlusion front mixing ratios of 10 to 12 g/kg are forecasted with the highest values in the southern portion of the threat area. This BL humidity shows a quite good overlap with instability. Lapse rates of 7 to even 7.5 °C per 1 km can be found from the Auvergne up to the Normandy, whereas they are lower to the south. The overlap of both ingredients lead to CAPE values between 800 and 1200 J/kg. Locally even higher values are possible due to topographic effects.
Low level convergence can be found due to the presence of the frontal boundary. It is strongest in the northern part of Western France as streamlines indicate. This convergence can help for initiation in the noon and afternoon hours.
PPW values of 34 to even 38 mm are forecasted. Therefore the main threat should be the occurrence of excessive precipitation, especially in the evening hours when storm motion is forecasted to slow down. Due to the CAPE values also a few large hail events are possible. However this threat is limited by the lack of DLS (0- 6 km: around 10 m/s). Strong wind gusts are possible with the strongest storms. Only weak wind velocities in mid layers (850hPa: 15 kn) also limit this threat.
Storm activity should be strongest in the afternoon hours but some storms can still occur in the first half of the night. However, activity should weaken step by step.
Portugal and Western Spain...
...with a southwesterly to southerly flow BL humidity is advected into the western parts of the Iberian Peninsula. It is highest near the coast with mixing ratios that increase until around 12 g/kg throughout the day. The values are smaller going more inland. Rather cool mid-level temperatures in combination with diurnal heating lead to a steepening of the lapse rates. Thus CAPE values of 500 to 1000 J/kg should develop in the afternoon hours with the highest values near the coastline.
Deep layer shear is about 15 m/s, in the late afternoon also values of up to 20 m/s can be found. Therefore better organized convection is possible that can bring strong to severe wind gusts as well as large hail. Since ppws are only between 25 to locally 30 mm and storm motion is forecasted to be quite high (W500: 30 to 40 kn) the excessive rain threat is limited. Only along the coast with several storms moving over the same area a local event cannot fully be ruled out.
First storms should occur even in the forenoon hours. Highest activity is forecasted in the noon and afternoon hours when an IPV maximum enters the region of interest and provides the needed lift ingredient. In the afternoon this maximum travels northward. In combination with the diurnal cooling of the BL, storm activity is forecasted to decrease.
Parts of Greece and Turkey...
...due to cool mid-levels under the influence of the trough and the warm water of the Mediterranean, steep lapse rates can develop. The same is true for Western Turkey due to diurnal heating. At the same time mixing ratios of about 10-12 g/kg (highest values to the south) are present. In effect CAPE values of 600 to 1000 J/kg should be available in this region. While no diurnal difference is present over the ocean, the highest CAPE values over the Turkish mainland are available in the afternoon hours.
Only modest to weak deep layer shear is present (0 -6 km: around or even lower than 10 m/s). Therefore pulsating storm activity should be the most prominent mode of convection. As a consequence large hail should be the exception and coupled to the strongest storms. The same is true for severe wind gusts since winds in 850 hPa are only between 15 to 20 kn. Ppw values are between 32 and 36 mm. Since Corfidi vectors point to the possibility of backward building and training storms, excessive precipitation is possible. Having rather weak 500 hPa winds to the north of the area, waterspouts that can develop at local LL convergence zones are possible. This convergence can develop as a combination of topographic effects and the land-sea breeze system.
Whereas activity over the Turkish mainland weakens in the evening hours, convection can further occur over the ocean.
