Storm Forecast
Valid: Wed 08 Jul 2009 06:00 to Thu 09 Jul 2009 06:00 UTC
Issued: Wed 08 Jul 2009 06:26
Forecaster: PUCIK

A level 1 was issued for northeastern Poland and western part of Baltic states, mainly for excessive precipitation, severe wind gusts and tornadoes.

A level 1 was issued for Belarus, Western Russia and Northern Ukraine mainly for large hail, severe wind gusts and tornadoes.

A level 1 was issued for belt from Northern Italy to Western Romania mainly for large hail, tornadoes and excessive rainfall.

A level 1 was issued for Denmark and surrounding regions mainly for spouts.

A level 1 was issued for Eastern Spain mainly for large hail and severe wind gusts.

SYNOPSIS

A strong impulse will rotate around the trough that has been influencing the weather across Central Europe. This impulse (positioned in the left exit region of strong jet at mid and upper levels of troposphere) has become the main player in the rapid cyclogenesis, that will deepen the surface low developing as a "wave" on the cold front now only slowly progressing to east. A short wave trough is expected to develop in result at mid-levels, with strong mid and upper level flow translating it to the north and therefore, this will also be the expected track of the surface low, which will reach Baltic States by evening hours. Strong cold front will speed up to the east in reaction to the strenghtening of the surface low and should reach Russia/Ukraine borders by Wednesday 06Z. Polar maritime airmass will advect behind the front with postfrontal shower / thunderstorm activity over many regions of Central and Western Europe.

DISCUSSION

... Poland and Baltic states...

As mentioned above, low pressure system will travel across the region and will deepen considerably, potentially having pressure under 1000 hPa in the center. GFS and ECMWF are in good agreement concerning the track and pressure values, though for example UKMO model advertises only shallower low. Some LAMs are even considering lower pressures and stronger windflow around the low than GFS and ECMWF. Nevertheless, we expect that ahead of the cold front, unstable airmass is placed and during the daytime heating, MLCAPEs locally reaching values around 1000 J/kg are possible.

The most prominent threat should be excessive rainfall, due to the advection of unstable and humid airmass towards the surface low. GFS and also other models predict high rainfall amounts, especially on the "warm part" of the frontal wave and northwestern part of the surface low, where the strongest isentropic lift is anticipated. It is a bit questionable whether most of the precipitation will be of convective nature, but at least embedded thunderstorms and generally, deep convection is expected (as suggested by the fact, that unstable airmass will be advected towards the low), so that they will probably significantly enhance the total rainfall amounts. As of 05Z Wednesday, convective system has already formed over Poland and is expected to propagate to the north / northeast.

With strenghtening of the flow especially behind the progressing cold front, reaching up to 20 m/s at both 925 and 850 hPa, strong low level shear is forecast ( over 10 m/s). Moreover, good overlap with moderate MLCAPEs are simulated by GFS, especially over Baltic states. High SREH values ( over 300 J/kg in 0-3 km layer) should be observed in the vicinity of the low due to the strong veering of low level flow with height. Due to the strong forcing and strong flow at lower levels, squall line might form on the leading edge of the cold front with both severe wind gusts and tornadoes possible ( the latter threat in case that LEWP structures form, or isolated rotating cells ahead of the cold front).

... Belarus, Northern Ukraine, extreme Western Russia...

In the warm sector of the surface low, unstable airmass is predicted with rapid destabilization during the noon hours. MLCAPE values might locally exceed 1500 J/kg. DLS should stay around 15 m/s, with models simulating strenghtening LLS especially towards late afternoon and values over 10 m/s will be possible. Well organised multicells might form and in this case, isolated large hail event is forecast. Moreover, strong shear in the lowest km, as well as low LCLs imply that a slight chance of tornadoes also exists. During the passage of cold front, squall line might form at its leading edge and in that case, we can not rule out that severe wind gusts will occur (but most of the gusts should stay sub-severe).

... Belt from Northern Italy to Western Romania...

Strong DLS ( values over 20 m/s) and MLCAPE values in order of hundreds of J/kg should allow for well organised thunderstorms, possibly including a (low-topped) supercell. In case that supercells manage to form ( this supported by fact, that enhanced SREH values should be available in the region), large hail might become a possibility, as well as severe wind gusts. A convective system that has formed over Italy during the previous forecast period will now affect Balkan states and travel slowly into Romania, posing risk of local excessive rainfall. Models indicate that low level shear should strenghten during the day, especially across western Romania and for this region, a slight risk of tornado is included.


... Denmark and Northern Germany...

Good environmental conditions for spouts will exist over this region, including strong CAPE release in lowest 3 km, low LCLs and convergence zones simulated in the area. Some of the spouts might cause slight damage and a low end Level 1 is introduced for this threat.

...Germany...

Strong LLS is simulated with values above 10 m/s during the day time thunderstorm activity. In combination with low LCLs, environment could be supportive of tornadoes. However, DLS around or even below 10 m/s suggests that only weakly organised storms will exist, without any stable cells including rotating updrafts. Therefore, tornado threat should be very limited and Level 1 is not warranted, even though we can not rule out a very isolated event.

...Spain...

Under the belt of strong mid and upper level flow, high DLS values are anticipated, in order of 20 m/s. Moreover, surface low, that has formed over Central Spain is inducing weak easterly winds (with westerly flow aloft) which will carry humid airmass further inland, as well as experiencing orographic uplift on their way. Belt of enhanced SREH (over 200 J/kg in 0-3 km layer) is forecast to exist in the region and environment should be quite supportive of supercells. Large hail can be expected with any of such cells and due to the dry mid-levels, also threat of severe wind gusts will exist.