Storm Forecast
Valid: Sun 26 Jun 2016 06:00 to Mon 27 Jun 2016 06:00 UTC
Issued: Sun 26 Jun 2016 06:13
Forecaster: TASZAREK

A level 2 was issued for a corridor stretching from Baltic Countries trough N Croatia, up to N Italy mainly for excessive precipitation, severe wind gusts and large hail.

A level 1 was issued for a broad area covering NE, E, S and SE Europe mainly for excessive precipitation, severe wind gusts and large hail.

A level 1 was issued for S Italy mainly for excessive precipitation, large hail and severe wind gusts.

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


SYNOPSIS

Long-wave trough with a tropical highly unstable and moist airmass on its right flank continues to move eastwardly into E Europe. Within weakening low centred over S Norway and overbuilding high over E Ukraine, an axis of southerly advected high theta-e values will stretch from CNTRL Mediterranean up to Baltic Countries. In this zone a high thunderstorm activity in a warm section and along active cold front in a weakly sheared environment will fall on Sunday. Small portions of jet streaks will be located over N Germany, Denmark, British Isles and along quasi-stationary frontal boundary stretching from CNTRL Finland up to W Russia, mostly outside areas of thunderstorm activity. Thunderstorms are also possible in S Italy thanks to orographic effects and SE Spain where moderate instability may develop thank to steep lapse rates and breeze effect.

DISCUSSION

...corridor stretching from Baltic Sea to W Balkans, including N Italy...

A northwardly moving warm (850 hPa temperature ~ 16-18C) tropical air mass ahead of the westwardly approaching cold front brings an impressive amounts of moisture, that in addition to evapotranspiration resulting from strong diurnal heating, reaches values of 14-15 g/kg (mixing ratio). Weakly mixed air provides relative humidity up to 70-90% even to 3-4 km AGL, thus exceeding 40-45mm PW. Within diurnal heating and steep lapse rates (~ 7 K/km), a CAPE is developing mostly to 2000-2500 J/kg with local peaks of 3000-3500 J/kg over Baltic Countries and N Italy. Due to weak airflow, environment is weakly to moderately sheared with DLS values extending from 5 m/s along eastern flank of level 2 up to 10-15 or even 20 m/s on the western flank. Thanks to deep parcel layer depths, high dew points, few convergence zones and PVA lobes, CI should take place around noon over most of the areas and thunderstorms should remain until nighttime hours. Given moderate shear, convection in the most common way should cluster into multicells, while in few cases thanks to local mesoscale interactions will form supercells. In the areas where shear will be the lowest, pulse thunderstorms will be likely.

Given very moist vertical profile, slow motion of the cells and local training modes, the main threat should fall on excessive precipitation capable of producing local flash flooding. Dry air in mid-levels and thus high delta theta-e values indicate a good potential for downburst and microburst where wind gust may exceed 100-120 km/h and become damaging. The combination of 10-15 m/s DLS, 7 K/km lapse rates and ~ 2500 J/kg CAPE create a good potential for hail up to 3-4 cm. However, thanks to local mesoscale shear enhancement a few cases with hailstones exceeding 5-6cm within supercells are not ruled out.

In current understanding, the highest threat for severe thunderstorms with elevated excessive precipitation and damaging wind gust potential falls on NE Poland where a strong convergence zone thanks to trough axis (or even meso-low) will develop in the afternoon hours. Significant severe phenomena are also possible in the belt from N Italy to Croatia, where thermodynamic instability will be the highest (2500-3500 J/kg). Few MCSs in the evening hours in level 2 area are likely.

...SE Europe...

A weakly pronounced trough over Greece with weakly sheared CAPE up to 1500 J/kg and strong support from orographic and QG-lift will provide an excessive precipitation as a main threat. These will result mainly from a rich in moisture vertical profile (PW ~ 30 - 40mm) and almost stationary thunderstorms near mountain ranges. Similar as in level 2 area, a high delta theta-e values and weak shear will support a downburst and microburst phenomena with wind gust potential exceeding 100 km/h. Given enhanced lapse rates (7.5 K/km), a risk for local large hail up to 3-4cm is also involved. Thunderstorm will develop during the afternoon hours thanks to diurnal heating and should fade in the nighttime hours when instability will drop.

...S Italy...

Thunderstorms in this area will develop thanks to the airflow perpendicular to Apennines, and thus orographic lift in the CAPE up to 1500-2000 J/kg. Similar as in SE European region, a weakly sheared environment and moist atmosphere will create a threat of excessive precipitation and local flash flooding. A threat for severe wind gusts within downburst and microburst phenomena and large hail up to 3-4cm is also involved. Thunderstorms should fade during the nighttime hours.

...Benelux and NW Germany...

Behind the cold front, in the area of steepening lapse rates and increasing boundary layer's moisture, a weak CAPE up to 300-600 J/kg will develop during the daytime over NE France, Benelux and NW Germany. Thanks to converging winds in the axis of the trought's located over North Sea and strong PVA, thunderstorms will develop in the afternoon hours. Although instability will be low, a local overlap with DLS up to 20 m/s will create a marginal risk of local severe wind gusts.

...SE Spain...

An impressive lapse rates exceeding 8 K/km will locally overlap with a breeze effect and 12-13 g/kg mixing ratios. A resulting combination will develop CAPE up to 1000-1500 J/kg. An approaching shortwave axis with increased mid-level airflow on its eastern flank will develop DLS up to 20-25 m/s, and provide a source of QG-lift that in addition to breeze convergence will act as a triggering factor. However, due to considerable values of CIN (~ -100 J/kg), it is uncertain whether updrafts will reach DMC stage. If thunderstorms form, given 20 m/s DLS they may become supercells with a main threat confined to large hail and severe wind gusts.