Storm Forecast
Valid: Mon 23 Jul 2018 06:00 to Tue 24 Jul 2018 06:00 UTC
Issued: Mon 23 Jul 2018 00:24
Forecaster: BEYER

A level 1 was issued across Southern Italy mainly for excessive precipitation. To the south also for large hail and severe wind gusts

A level 1 was issued across the Balkan mainly for excessive precipitation. In the southern regions also for large hail and severe wind gusts

A level 2 was issued across parts of Romania, Serbia and E Bosnia-Herzegovina mainly for excessive precipitation and to a lesser extent for large hail and severe wind gusts.

A level 1 was issued for parts of Eastern Europe going from Belarus and Ukraine into Russia and further south covering Georgia and Armenia mainly for excessive precipitation. In the southern regions also for large hail and severe wind gusts.


SYNOPSIS

A persistent general weather situation is about to develop over large parts of Europe. While a low pressure system can be found from Iceland to the Norwegian Sea, an upper level ridge is strengthening over Southwestern Europe, extending its influence into Central Europe during the forecast period.
This ridge is about to get into contact with an persistent upper level high over far Northeastern Europe, forming a high-pressure bridge at the surface.

At the same time a broad upper level low is situated over Eastern and Southeastern Europe. Two centers of low geopotential can be found. One situated over Eastern Europe and the second over the Balkan. The latter shows a well-defined upper level jet (subtropical jet) at its southern flank. This is the only region where some dynamics come into play while over large parts of Europe no dynamics and synoptic-scale fronts impact the thunderstorm forecast.


DISCUSSION

... Southern Italy ...

This area is influenced by the trough. Rather cool mid-level temperatures lead to steep lapse rates. Humidity is enhanced near the coast but also inland specific humidity values are sufficient to build up several hundred J/kg of CAPE. Starting with ongoing convection from the previous night, insolation will lead to a steepening of the lapse rates and thus an increase of thunderstorm activity throughout the day.

Having an only moderate upper level flow (500 hPa: 15 kn) but high values of precipitable water (35 to locally 40 mm) large amounts of precipitation within a short time frame are the main threat. However, developing storms are mostly unorganized and local orography can influence its development.

This is different in the far southern region of the LVL1 area. The nearby upper level jet results in enhanced values of DLS (0-6 km) up to 20 m/s. As a result better organized storms are possible that can also bring large hail and severe wind gusts.

Overall thunderstorm activity will rapidly decline in the first half of the night due to the diurnal cycle and the movement of the upper level trough to the east.

... Balkan ...

This area is influenced by the southern center of the broad upper level low. Steep lapse rates are present thanks to low mid-level temperatures and diurnal heating of the lower troposphere. At the same time the present airmass is characterized by enhanced values of specific humidity that are especially high to the north of the LVL1 area and next to the coasts. Thus CAPE values of up to 1 kJ/kg can be expected for the afternoon hours.

As a result widespread thunderstorm activity is forecasted. However the lack of DLS results in more or less unorganized storms that start to develop over the mountains or by the influence of the orography and then shifts further to other regions with the help of its outflow boundaries and developing areas of convergence

The main accompaniment is excessive precipitation due to the ppw values and only 10 to 15 kn of 500 hPa wind.

More to the south the risk shifts step by step from excessive precipitation to large hail and severe wind gusts. The forward flank of the upper level trough brings better dynamics into play. DLS of 15 to 20 m/s as well as long, more or less straight line hodographs are forecasted. Thus in the southern part of the LVL1 area the development of organized storms or even supercells is forecasted. Since the airmass is not that rich in humidity than further north, CAPE values are lower and capping is stronger. Thus storm coverage will be not that widespread.

Storm activity will only slowly decrease but moving eastward together with the eastward shifting trough.

... Romania, Serbia and E Bosnia-Herzegovina ...

This region shows mainly the same conditions and the same airmass as the surrounding LVL1. The following arguments, however, lead to an upgrade. First of all the specific humidity values are even higher as further south. The same is true for precipitable water that shows values that locally exceed 40 mm. In addition, situated in the center of the upper level low, the upper level flow is rather weak (500 hPa: 5 to 10 kn).

The main argument is a well-defined convergence line throughout the whole lower troposphere into the mid troposphere that is forecasted to persist over several hours. This then should lead to repeated activity of slow moving storms over the same area. As a result large amounts of precipitation can be expected that locally may lead to flash floods. This is especially true for Southern Romania where streamlines indicate a persistent flow of humidity from the Black Sea. Bordered by the Carpathians really high values of humidity can enrich during the day.

This is also visible in the resulting high values of CAPE. Up to 1750 J/kg are forecasted for the region between the Carpathians and the Black Sea. Thus, locally large hail and the risk for wet downburst is enhanced.

... Eastern Europe ...

The second upper level trough is responsible for the thunderstorm activity over this region reaching from Belarus and Ukraine into Russia and further south covering Georgia and Armenia. The combination of steep lapse rates and high values of specific humidity will develop into several hundred J/kg of CAPE. With the help of the Black Sea even higher values are forecasted (1000 to 1500 J/kg in Western Ukraine and Russia)

Winds in the lower troposphere show a well-defined convergence line that has a meridional extension in the western part of the LVL1 and a zonal one more to the east. Along this convergence line widespread initiation of storms can be expected.

Again slow storm movement and high amounts of precipitable water determine the main risks of developing storms. Thus excessive precipitation should be the main accompaniment. More to the south a little better dynamics (DLS: 10 to 15 m/s) enhance the risk for severe wind gusts and large hail. However, also more to the north, the high amount of liquid water poses a threat for severe wind gusts by wet downbursts.

The diurnal cycle will lead to a decrease in storm activity over large parts during the upcoming night.