Storm Forecast
Valid: Sat 03 Aug 2024 06:00 to Sun 04 Aug 2024 06:00 UTC
Issued: Fri 02 Aug 2024 21:01
Forecaster: TUSCHY

A level 2 was issued for SE Serbia into far W Bulgaria mainly for severe gusts.

A level 1 surrounds the level 2 mainly for hail, strong to severe gusts and heavy rain.

A level 1 was issued for parts of the Ukraine mainly for hail/heavy rain (southern part) and elsewhere for strong gusts and an isolated tornado.

A level 1 was issued for numerous parts of Italy mainly for large hail, heavy rain and strong to severe gusts.

SYNOPSIS

We start to switch back from a temporal more high index (zonal) pattern into another more blocked one as positive geopotential height anomalies over the Bay of Biscay/Iberian Peninsula start to build N into England. In general however the more zonal "touch" of this pattern persists until the end of this forcast.

This change of mid-level height fields occurs downstream of an extensive and rather strong (for this time of year) cyclonic vortex, which is placed just S of Iceland or atop the Iceland Basin. The switch of the vortex' geometry from a zonal one to a more meridionally aligned one backs the building ridge downstream over W Europe.
Elsewehere, weak pressure anomalies/gradients persist over the rest of Europe with a relaxed westerly flow regime. Numerous embedded troughs affect most of S/E Europe and bring rather unsettled conditions - a welcome change to the long lasting blocking subtropical ridge from the past few weeks.

An Atlantic cold front drops S/SE over NW into CNTRL Europe with otherwise no major synoptic-scale fronts forecast (maybe a weak cold front surge into the Balkans). DMC activity is once again driven by the orography, low-amplitude waves, laid out outflow boundaries etc.


DISCUSSION

... CNTRL Mediterranean ...

A sharp low-amplitude wave crosses Sardinia until noon, the Tyrrhenian Sea until midnight before entering the Gulf of Taranto thereafter.
During its passage, temperature values at 550/500 hPa drop by 10K with a clear phase shift of the thermal and geopotential height trough. This results in an overlap of the coldest mid-level airmass with strongest LL WAA with 850 hPa temperature values in the low to mid twenties. Hence we see well capped loaded-gun soundings with offshore MUCAPE values of 3000-4000 J/kg atop the Tyrrhenian Sea. Probably some elevated convection with lots of virga will cross this area but no CI is anticipated.

Hence the main focus for CI remains onshore, where the orography and diurnal driven sea breeze fronts should induce CI around noon onwards, as convection clusters along the Apennines before spreading towards the coasts during the afternoon.

Onshore flow advects an unusual moist marine airmass towards the mountains as SSTs around Italy hover in the 27 to 29 C range, which is 3 to 4 K warmer than the climatology with yesterday's Td readings along the coasts in the low to mid twenties! However yesterday's AMDARs/soundings also revealed that the depth of this marine layer is only uo to 600m AGL with a pronounced inversion aloft (where Tds failed to climb past 7C). Prog soundings support this idea also for today's setup.

Initiating convection will see 1000-2000 J/kg MUCAPE at higher elevation and well mixed BL conditions, so next to hail, strong to severe gusts area forecast. Heavy to locally excessive rain is also forecast with TPWs in the 35-45mm range as pulsating convection (sub 10 m/s DLS) grows upscale into numerous clusters. As thunderstorms build into the more CAPE-rich airmass, they also enounter rapidly increasing CIN values, which should cause a gradual weakening trend until sunset. We broadened the lightning and level areas and went more offshore to account for some outflow driven CI along the coasts.

N-Italy will be placed just along the inflection point to the passing wave-train and is affected by large-scale subsidence during the daytime hours before the next wave and forcing maxima approaches during the night.
Diurnal driven (and also driven by the passing trough) onshore moisture advection along the Po valley brings rich Adriatic moisture far W with dewpoint readings in the lower twenties up to the S Alps (BL mixing ratios in the mid tens in g/kg). This advected moisture could support a few storms along the orography with MUCAPE values in the 1000-2000 J/kg range and DLS in the 10-15 m/s range. Most models keep CI on the isolated side with the local WRF a bit more optimistic, which evolves scattered DMC activity. Any initiating storm will be able to produce hail, strong to isolated severe gusts and heavy rain. This acitivty is forecast to weaken as it leaves the orography to the S despite moving into an high CAPE environment.

The next round during the night initiates in the N Lombardia area and this activity will have a better chance to grow into an eastward shifting cluster with some activity building S / away from the orography. Similar hazards with similar CAPE/shear setup.

...Balkans into Ukraine ...

Another short-wave trough is forecast to enter W Ukraine into Romania during the afternoon while become rather well defined with increase curvature vorticity. Along the surface a weak cold front surges SE and interacts with the complex orography.

CI is forecast over Bosnia and Herzegovina already before noon with thunderstorms moving SE into Serbia, N-Montenegro and Kosovo. Good BL moisture with dewpoints in the lower twenties but weak mid-level lapse rates offer roughly 1000 J/kg MLCAPE with 10-15 m/s DLS. Of interest is the enhanced 0-3 km shear driven by the orography with pockets of 15 m/s 0-3 km seen in some NWP data. Hence there is a risk of organized multicells / an isolated supercell, which grow upscale into a forward propagating MCS as the activity enters drier BL conditions over S Serbia and into W Bulgaria. We added a small level 2 for the more widespread severe wind gust risk but kept most of the activity in a level 1 due to the modest nature of CAPE/shear.

Further NE over Romania into W/CNTRL Ukraine, modest CAPE/shear should keep the activity mostly sub-severe. We added a small level 1 for the Odesa area N with more robust CAPE (hail/heavy rain) into the Cherkas'ka Oblast, where LL shear is stronger and could support a few stronger gusts/an isolated tornado.

... Germany ...

A passing low-amplitude wave during the day and an approaching cold front from the NW result in a prolonged period with convective activity. Unobtrusive CAPE/shear space supports pulsating convection with heavy rain the main risk. The focus for a few excessive rain events should be along colliding convection or peripheral of a transient and convection induced mid-level vortex (MCV-like), which increases rainfall-rates/duration in a 35 mm TPW background. No real focus is forecast (also in EPS data), so we keep this event without a level 1. Local level-1 conditions are certainly possible, which is highlighted by a few ensemble members in ID2, which show a few high QPF spots but with variable placement. During the day, modest LLCAPE and uncapped conditions could support a few funnel/short-lived tornado events.

... N/NE Europe ...

Another day with scattered CI in an airmass with modest BL moisture and weak lapse rates. Slow storm motion could result in a few heavy rain events but missing foci keeps this below a level 1 threshold.