Storm Forecast
Valid: Fri 31 May 2013 06:00 to Sat 01 Jun 2013 06:00 UTC
Issued: Thu 30 May 2013 23:01
Forecaster: PISTOTNIK

A level 1 was issued for Eastern Bulgaria and Eastern Romania for large hail and severe wind gusts, and a level 2 for coastal areas for large hail, severe wind gusts and to a lesser extent for tornadoes.

A level 1 was issued for Moldova, parts of the Ukraine and Belarus for large hail, excessive precipitation and tornadoes.

A level 1 was issued for parts of Lithuania and Latvia for excessive precipitation.

A level 1 was issued for parts of Poland and Germany for excessive precipitation, and a level 2 for Central Germany for excessive precipitation and to a lesser extent for tornadoes.

SYNOPSIS

An impressive, stationary upper-level low over Hungary and various short-wave troughs which pivot around it govern the weather over large parts of Europe. The most progressive trough swerves from the Balkan states towards the Black Sea. A second one rounds the back side of the low and stretches out across the Western and Central Mediterranean region - however, its lift impulse will be more than compensated by pronounced cold air advection, awarding this feature a gentler nature. The third noteworthy vorticity maximum moves from the Northeast into Austria and Southern Germany, where it enhances upslope precipitation but interacts mainly with stably stratified and saturated air in close vicinity to the low's center. The most pronounced jet streak (25-35 m/s at 500 hPa) runs from Spain via the Southern Mediterranean region to the Black Sea, whereas geopotential gradients are weaker along the other flanks of the upper-level low.
At the surface, an unusually large area of low pressure around or even below 1000 hPa covers Central and Eastern Europe. While the parent low pressure center is slowly filling up over Austria and Hungary, new shallow depressions form along the curling cold front further to the North and East. Ahead of this cold front, a broad back-door warm air advection regime hauls warm and moist air from Eastern Europe back into Scandinavia, Poland and Germany.
Thunderstorms and even quite manifold severe weather can be expected in many areas around the dominant low, which will be discussed in detail below. It was decided to follow the physics and circle the low in an anti-clockwise direction.

DISCUSSION

... Bulgaria, Romania ...

A plume of steep lapse rates, originating from Northern Africa, gets ingested into the circulation and overspreads a very moist boundary layer. Thursday afternoon's synop data showed dew points between 16 and 20°C with sustained moist onshore flow in Eastern Bulgaria and Romania. On Friday, this combination should yield again at least moderate CAPE (~1000 J/kg, as already shown by the Thu 12z Bucuresti sounding) in a regime of strong deep-layer shear (20-30 m/s). The short-wave trough will provide ample QG lift, leaving not much doubt about convective initiation. This is also confirmed by noteworthy precipitation signals in the bulk of the model forecasts.
At the time of writing (Thu 21 UTC), the cold front which moves eastward over Romania and Northern Bulgaria has already sparked a long-lived convective line. Against the background of persistent synoptic-scale lift and a rich supply of boundary layer moisture, this convection may weaken but will likely not fully die off overnight. A few hours of insolation on Friday morning should be sufficient to initiate new storms in Eastern Romania and Bulgaria, mainly along the sea breeze front or old outflow boundaries. Due to robust instability release and strong deep-layer shear, multicells and supercells are expected to be the dominant mode. Large hail and severe wind gusts may accompany quite a number of these storms. Even one or two extremely large hail events (>5 cm) are not ruled out, especially towards the South, where deep-layer shear is maximized and an increasing cap may keep storms more isolated, reducing their competition.
Forecast models agree on a CAPE maximum in the 1500-2000 J/kg range along the Bulgarian and Romanian coastline. In case of a well-established, moist Easterly onshore flow, low cloud bases and further enhanced vertical wind shear might also be supportive for a tornado, though this is not considered to be the main threat.
In the afternoon hours, the cold front will gradually shift the activity out to the Black Sea, where water temperatures around 19°C may be just enough to keep the convection surface-based and to extend a severe wind threat for some distance offshore.

... Moldova, Ukraine, Belarus, Lithuania, Latvia ...

A few hours later than Bulgaria and Romania, QG lift and steepening lapse rates will also overspread the regions further North, while a Southeasterly low-level jet advects rich moisture from the Black Sea deeply into the Ukraine. CAPE in the range of 500 to 1200 J/kg is expected to build ahead of the cold front, which slowly shifts to the Northeast.
The main uncertainty lies in a possible wave development: While ECMWF and nested limited area models show only weak signals for such a feature, GFS and WRF would foresee a stronger cyclogenesis, starting in Moldova in the late morning and then moving Northwestward into Northeastern Poland by the end of the day. This development is forecast to boost 0-3 km storm-relative helicity into the 150-200 m^2/s^2 range and 0-1 km vertical wind shear into the 10-15 m/s range in the warm sector along its track. Together with deep-layer shear around 20 m/s and substantial CAPE, these values could easily support tornadogenesis.
All in all, scattered to widespread thunderstorms are expected to form in the afternoon along and ahead of the cold front and to organize into multi- or even supercells. A few large hail events pose the main risk, while the tornado and excessive rain threats do more critically depend on the track and intensity of the wave development. Nonetheless, an enhanced tornado risk cannot be discounted, as well as a possibility of excessive precipitation in case of upscale growth into an MCS.
According to the more aggressive GFS and WRF forecasts, strongly veering wind profiles will spread as far North as Northeastern Poland, Lithuania and Latvia towards evening, even though the overall shear magnitude would gradually decrease. However, due to the model uncertainties and the late arrival it is doubtful if any leftover convection will still be able to benefit from these better kinematics. Earlier in the day, these regions will see a round of air mass thunderstorms under moderate instability and weak vertical wind shear. With precipitable water in excess of 30 mm, localized excessive precipitation is possible, which is why a low-end level 1 was extended into Lithuania and parts of Latvia.

... Poland, Germany ...

A formidable tongue of warm and moist air wraps around the low and enters Germany from the Northeast. Even though the axis of maximum shear is displaced further towards the Alpine region, the nose of this warm air advection regime will feature impressively helical wind profiles across the entire Southern half of Germany. Current thinking is that cloudiness, stratiform rain and too cold low levels will preclude surface-based convection in most of the area with the best wind profiles. However, towards the end of the warm air advection regime, breaking clouds and some hours of insolation can suffice to make storms root down into the boundary layer with rich moisture and a still helical flow - this transformation can most notably be seen by comparing the Lindenberg soundings from Thu 12z and Thu 18z, or the Wroclaw and Legionovo soundings from Thu 12z, respectively.
Current thinking is that scattered to widespread thunderstorms will form from Northern Poland into Central Germany in the afternoon hours. Even though the forecast models still disagree by /- 100 km with respect to the position of the belt of strongest rainfalls, all models agree on the existence of such a belt, and the combination of very high atmospheric moisture, a plausible scenario of training storms and a history of more than plentiful rainfall in the past few weeks indeed emphasizes a robust flooding threat. Storms will move to the Southwest and will sooner or later become elevated and get incorporated into a large-scale rain field. However, just a few hours of insolation may suffice to sustain surface-based storms well into the region of the best hodographs. In that case, strong convergence, enhanced low-level shear and very low cloud bases may be sufficient for one or two tornadoes as well. All in all, a small level 2 area was drawn for Central Germany, primarily for excessive rain and secondarily for tornadoes, where model agreement on plentiful convective precipitation and on an overlap of instability with strongly veering wind profiles is best. A larger level 1 for excessive rain surrounds the level 2 and covers further parts of Germany and Poland.
In the evening hours and overnight, remnants of this convection will continue to move southwestward until they hit the Alps in the bordering region of Germany, Austria and Switzerland. Due to the late start and uncertainties about a further involvement of embedded (and elevated) convection, it was decided to stick with a level 1 here despite an agreement of the model pool on an onset of abundant upslope precipitation.

... Central Mediterranean, Italy, the Balkans, Hungary ...

Moderately steep lapse rates, insolation and some residual moisture will again create patchy CAPE in the wake of the cold front. Scattered thunderstorms are forecast to form over land, while they will be more isolated over the sea, but still possible thanks to the cool upper-levels.
Westerly background flow is brisk but stronger deep-layer shear is confined to the regions further South, where an increasing cap and a lack of forcing make convective initiation highly doubtful. One or two hail or wind events cannot be ruled out, but overall threat is too low for a categorical risk.
Closer to the base of the cut-off low, a cyclonic wind field, cold upper-levels and weak winds throughout the troposphere point to a slightly enhanced risk of funnel clouds and a brief landspout in Hungary.

... Scandinavia ...

While most parts of Scandinavia still see dry and unseasonably warm conditions, scattered afternoon thunderstorms can form over the Southern parts. Convective initiation is facilitated by a vorticity lobe, as the remnants of an old upper-level low slowly move northwestward. Severe weather is not expected due to weak vertical wind shear and limited instability.