Storm Forecast
Valid: Fri 02 Aug 2019 06:00 to Sat 03 Aug 2019 06:00 UTC
Issued: Thu 01 Aug 2019 20:05
Forecaster: PISTOTNIK

Level 3 areas are issued for small parts of NE Italy, Croatia and Bosnia-Herzegovina mainly for large hail, severe convective wind gusts and to a lesser extent for tornadoes.

A level 1 and level 2 is issued for N Italy, S Austria, SW Hungary, Slovenia, Croatia and Bosnia-Herzegovina, Montenegro, Serbia and W Romania for large hail and severe convective wind gusts (mainly in its S half) and for excessive convective precipitation (mainly in its N half).

A level 1 and level 2 are issued for E Romania mainly for large hail, excessive convective precipitation and to a lesser degree severe convective wind gusts.

A level 1 is issued for the Netherlands and NW Germany for excessive convective precipitation.

SYNOPSIS

The main frontal zone runs on a quite southerly position between 45N and 50N from the Ukraine across the Alpine region to the Bay of Biscay. To its north, low-pressure systems are centered over NW Russia and over the N Atlantic west of Ireland. In-between, a formerly blocking Arctic high slips south towards the British Isles, but gradually flattens without interrupting the main frontal zone too much.
Steep lapse rates and rich low-level moisture overlap to the south of the frontal zone, in particular beneath an elevated mixed layer (EML) that is spreading from NW Africa to Italy and towards the Balkans. Cooler maritime air with adequate moisture but weaker lapse rates is present to its north. Finally, cold and dry air spreads into Scandinavia and NE Europe in the wake of the Russian cyclone.

DISCUSSION

... N Italy, S Austria, Hungary, Slovenia, Croatia, Bosnia-Herzegovina, Serbia, Montenegro ...

Thursday's 12 UTC soundings illustrate the ongoing airmass recovery across the area of interest with a quick increase of CAPE / 0-6 km shear from Zagreb (100 J/kg / 10 m/s) to Rivolto (500 J/kg / 15 m/s), Milano (1500 J/kg / 20 m/s) and Cuneo (2500 J/kg / 20 m/s). Surface data showed 2m temperatures in the lower 30s and 2m dewpoints between 18 and 25C across the N Italian plains and along the Adriatic coast on Thursday afternoon, hinting at CAPE mostly between 1500 and 3000 J/kg and a probable underestimation by the Rivolto sounding, which was based on a particularly low 2m dewpoint of only 16C. More airmass recovery is still needed over Slovenia, Croatia and Bosnia-Herzegovina, where the latest cool change left somewhat cooler and drier air with 2m temperatures (dewpoints) around 28C (18C) and even some unseasonable Stratocumulus decks on Thursday.
Aided by synoptic lift from a first vorticity maximum and the frontal boundary slowly passing the main Alpine crest, scattered storms are currently (Thu 19 UTC) initiating over the Austrian and Italian Alps. They will organize well under 15-20 m/s deep-layer shear and will grow upscale into one or two MCSs that persist through the night and move eastward. The leading storms will most likely be located over SE Austria and Slovenia at Friday 06 UTC and continue to translate into Hungary and maybe parts of Croatia afterwards. Additional convection in their wake could become more frequent over the Italian Alps late at night and on Friday morning. These morning storms pose a risk of high rainfall accumulations, some severe wind gusts along their leading edges and large hail in case of tail-end supercells to the south. Considering the strong cap, it is unclear how far south into the increasing CAPE they will propagate.

In N Italy, outflows from these grazing storm clusters to the north and dry nocturnal downslope circulations from the Appennines may deplete the abundant CAPE reservoir to some degree overnight, but much of it will likely remain in place, as the strong cap beneath the EML keeps the thick moisture trapped. By Friday noon, the low-level wind field will likely be dominated by sea breezes again, and moisture across the lowlands should consolidate to a degree similar to the day before.
Forecast uncertainties grow towards the east. Depending on how far east the overnight warm air advection spreads and how far south the morning storm cluster and its outflow push, daytime heating and CAPE buildup in Slovenia and perhaps even parts of Croatia may be limited. Confidence in strong enough insolation increases only towards the border to Bosnia-Herzegovina and beyond.

From noon onwards, storms from the Italian Alps, Slovenia and SW Hungary will finally start propagating towards the SE. Additional initiation becomes more likely with every hour at outflow boundaries or sharpening drylines in the wake of the Appennines and the Dinaric Mountains, respectively. Under 20-25 m/s deep-layer shear and slightly veering wind profiles, these early, discrete storms will quickly organize into (mostly right-moving) supercells with main risks of large to very large hail, severe downbursts and localized excessive precipitation. The tornado risk is enhanced if a right-moving supercell follows the dryline or passes to its moist side, where 0-1 km shear increases to 10 m/s, though not particurly low cloud bases control this risk to some degree.
The main upper-level trough crosses the area of interest in the afternoon to evening. It arrives at the optimum daytime in N Italy, where strong synoptic lift and a strengthening wind field also at lower tropospheric levels could promote convection to grow upscale into a bowing line with widespread severe and isolated to scattered extreme wind gusts between the lower Po river, the Appennines and the Adriatic coast. Its later arrival over the N Balkans will likely shift the peak of convective activity into the evening there, but the severe weather risk starts to decrease after sunset. Nonetheless, clustering and elevated storms will travel into Serbia and maybe even Romania overnight, and heavy rain and a few severe wind and hail events are sill possible.

Despite rather impressive displays of several strongly-right moving supercells in the forecast maps of convection-resolving models, only small level 3 areas are issued for the expected dryline positions where the risk of "convective failure" is minimized. The majority of the discussed area is covered with a level 2 due to the mentioned uncertainties originating from the foregoing night.

Storms that propagate too far SE will betimes fall victim to an impermeably strong cap beneath the EML. Aided by orography, isolated and short-lived afternoon storm with a large hail risk are possible as far south as central Italy and Montenegro, though.

... Romania, Moldova into SW Ukraine ...

Likewise on the warm side of the frontal zone, moderate CAPE on the order of 1000 J/kg overlaps with moderate deep-layer shear between 10 and 15 m/s. Stronger shear is present towards the Ukraine, but could be displaced from substantial CAPE, while an overlap of higher CAPE and enhanced vertical wind shear is likely in the sea breeze regime in E Romania.
Apart from the lingering diffuse frontal zone, synoptic conditions do not provide much lift. Scattered daytime-driven storms are expected mainly over the mountains, but a few may propagate some distance into their eastern forelands in the later afternoon and evening. Multicells will be the primary storm mode, and moderately large hail, heavy rain and isolated severe downbursts are possible. A higher severe weather risk evolves if storms from the eastern Carpathians tap into the sea breeze regime over E Romania. Some of these storms may organize into supercells. If that happens, isolated very large hail is possible and a tornado is not ruled out if the wind field is favorably contracted at the sea breeze front.

... Netherlands, Germany, Czech Republic, N Austria ...

Some hundred J/kg CAPE form in response to daytime heating on the cool side of the frontal zone. Scattered storms may already be active in the morning over the Netherlands, where a mesoscale cyclone (possibly even with a shallow warm core) moves onshore. Isolated waterspouts are not ruled out near the center of this cyclone or in the German Bight, which may also see morning showers.
In the late morning to afternoon, scattered convection will initiate further inland. Slow storm motion under weak vertical wind shear can promote a few excessive rain events. Towards the south, deep-layer shear increases to 15 m/s and convection over the Bohemian Massif (bordering region of Germany, the Czech Republic and Austria) can organize into multicells and possibly one or two transient supercells. In that case, marginally large hail and isolated severe wind gusts become additional risks.
Convection will mostly decay in the evening.