Storm Forecast
Valid: Tue 29 Jul 2014 06:00 to Wed 30 Jul 2014 06:00 UTC
Issued: Tue 29 Jul 2014 00:45
Forecaster: BEYER
A level 2 was issued for Eastern France and Central Germany mainly for excessive precipitation
A level 1 was issued for Northern Italy, Austria, Western Poland, Czech Republic, Slovakia, Hungary, Western Romania and Bulgaria, as well as part of Greece, Serbia, Slovenia and Macedonia (FYROM) mainly for excessive precipitation
A level 1 was issued for Tunisia, the Tyrrhenian Sea as well as Central and Southern Italy mainly for large hail and severe wind gusts
A level 1 was issued for Northwestern Russia and the easternmost parts of Finland mainly for excessive precipitation
SYNOPSIS
Most parts of Europa are under the influence of a trough that shows several regions of interest. The frontal zone is situated quite far to the north. Scandinavia is lying on the forward flank of a trough situated a little bit to the east of Iceland. Rather humid and warm airmasses are advected especially into the southern parts of Scandinavia. An occlusion front coming from the corresponding surface low is influencing Great Britain which is also affected by the trough axis and cooler airmasses.
A pronounced upper level low is slowly moving to the south during the forecast period. It travels from southern France to the western Mediterranean. On the southern flank of this upper level low, that is hardly visible in the surface pressure distribution, a pronounced short wave trough is travelling from the west to the east. It will reach the Tyrrhenian Sea at the end of the forecast period and is accompanied by a strong mid- and upper level jet.
Another short wave trough is situated in a zonal orientation over Eastern Europe. At the surface a corresponding weak low pressure system can be found. Embedded in this low is a tongue of humid air that extents all the way from eastern France to Germany. Also most parts of eastern and southeastern Europe are under the influence of warm and humid airmasses. A frontal boundary can be drawn in the forecast maps to separate those airmasses from drier and slightly cooler airmasses to the west.
DISCUSSION
Eastern France and Central Germany...
...during morning hours there will be some elevated activity still going on from the night. During the day several hundred up to 1500 J/kg of ML-CAPE should develop in an airmass that is rich of BL moisture showing mixing ratios of 11 to 13 g/kg and dewpoints between 17 and 20 degree. Although no steep lapse rates are visible between 2 and 4 km this will be enough to develop narrow but high reaching CAPE profiles (until 11 to 12 km) as visible in the forecast soundings.
Following the streamlines at the surface as well as winds in lower levels (as for instance 925 hPa), one can find a pronounced convergence zone that develops throughout the day over most of the LVL2 area. This can thus act as an initiator for strong lift processes.
After a minimum in the forenoon hours and with the help of some diurnal heating, activity will increase around noon and in the afternoon hours. Diurnal heating is the main question since many clouds will be left over from the night time hours. However, with the help of the aforementioned lift processes atmosphere should overcome this lack.
Those thunderstorms that develop will have primarily the potential to bring excessive precipitation. This is mostly due to high values of ppw (35 to 40 mm with locally even higher values) as well as weak winds in 500 and 700 hPa (5 to 10 kn). This will lead to slow moving and training stoms which is also indicated by Corfidi vectors. The slow storm motion can also be inferred from the fact that storm motion vectors (pointing almost to the west) align with the zonal orientated confluence zone. Those storms may locally bring really high amounts of rain that can locally accumulate to 50, or even 100 mm, especially when several storms move over the same area.
Apart from the preferential rain threat a severe wind gust cannot be ruled out where a cold pool is able to organize or where a local microburst event coupled to the high precipitation load, takes place. In addition a marginal hail event (2-3 cm) is possible at the beginning of the thunderstorm development. Since the lifting condensation level is rather low especially in the western part of the LVL2 area a tornado cannot be ruled out although low level shear is quite weak.
Thunderstorms activity can last throughout all the night while the convergence and thus main activity line is shifted to the south / southeast. However, overall activity should weaken step by step.
Northern Italy, Austria, Western Poland, Czech Republic, Slovakia, Hungary, Western Romania and Bulgaria, as well as part of Greece, Serbia, Slovenia and Macedonia (FYROM)...
...in all the LVL1 area (that surrounds the LVL2) a similar airmass is present showing mixing ratios of 10 to 12 g/kg that finally lead to values of 500 to 1200 J/kg of ML-CAPE. The difference of the LVL2 area is the lack of a pronounced confluence area. Nevertheless numerous storms should form especially in the afternoon hours. Due to the lack of a pronounced synoptical lift mechanism mostly mesoscale lift should trigger those storms. These are topography, old outflow boundaries as well as local convergence effects.
Those storms are able to bring excessive precipitation having a rather slow storm motions and ppws between 30 to 40 mm in most regions. Besides the precipitation also marginal hail and locally severe wind gusts cannot be ruled out.
While shear in most parts of the LVL1 area is below 10 m/s higher values can be found in the southernmost parts (Balkan area, northern Italy) where DLS (0-6km) has values between 10 to even 15 m/s. In those areas storms should be able to better organize. Thus large hail and severe wind gusts become a more pronounced threat.
Tunisia, the Tyrrhenian Sea as well as Central and Southern Italy...
...with the help of the upper level low and its cool mid-level airmasses, CAPE values of 1000 to 2000 J/kg can develop in the afternoon and evening hours. Main problem is a rather high capping indicated by rather high CIN values and large LCL-LFC differences. Since a strong short wave trough is travelling over the western Mediterranean all the way to the Tyrrhenian Sea during the forecast period (accompanied by a strong mid- and upper-level jet) this lift mechanism may overcome the strong capping and initiate severe thunderstorms.
Having strong shear (0-6 km: 20 25 m/s, 0-3km: 15 20 m/s) supercells may organize that can bring large hail and severe wind gusts. The latter is mostly due to 850 hPa winds of 20 to locally 30 kn. In addition wind vectors have a large perpendicular component to the thermal frontal boundary that is also moving to the east.
The main activity will reach central and southern parts of Italy during the night hours until the end of the forecast period. If an organized line of thunderstorms or and mesoscale convective system should form, since then, this will lead to severe wind gusts as well as large hail in parts of central and southern Italy. Since initiation during the day is questionable and further (elevated) organization is not definite, only a LVL1 area was drawn.
Northwestern Russia and the easternmost parts of Finland...
...mixing ratios are between 10 and 12 g/kg that can overlap with some steeper lapse rates that develop throughout the day. This leads to CAPE values of 800 to 1500 J/kg, locally also 2000 J/kg are possible. Streamlines show confluence in some regions that may hold as the needed lift mechanism. Shear is mostly weak with values around or even below 10 m/s between 0 and 6 km. Therefore storms will mostly have a pulsating character.
Storm motion is between 25 and 30 kn but Corfidi vectors, nevertheless, show the chance for training storms especially in the later afternoon and evening hours. Ppws are forecasted to be mostly between 30 and 35 mm. To cover the precipitation threat a LVL 1 area was drawn for the region with best confluence. Besides that marginal hail and a local severe wind gust are possible.
Southern Scandinavia...
...given around 10 g/kg of mixing ratio diurnal heating should lead to the development of steep lapse rates and thus 500 to 1000 J/kg of CAPE. Shear values are around 10 m/s. Some storms are forecasted to develop during the day. Although marginal hail, strong wind gusts and a few stronger precipitation events cannot be ruled out, overall condition do not allow a leveling of this area.
