Storm Forecast
Valid: Mon 04 Jul 2022 06:00 to Tue 05 Jul 2022 06:00 UTC
Issued: Sun 03 Jul 2022 22:54
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for south-central Europe for excessive convective precipitation, large hail and severe convective wind gusts.

A level 1 is issued for NW Russia, E Latvia, E Lithuania and N Belarus mainly for excessive convective precipitation.

A level 1 is issued for parts of the Balkans for excessive convective precipitation, large hail and severe convective wind gusts.

SYNOPSIS

The dominant feature on the weather maps is a mid-level low over the Norwegian Sea. It is circled by a mid-level jet from Iceland to the British Isles, the Baltic Sea and Finland. Maritime, moist and rather cool air makes a slow but steady progress into the NW half of Europe. The cold front crosses Poland, the Czech Republic, the Alpine region and S France on Monday and is the main focus for thunderstorm activity.
In contrast, high geopotential and extremely hot air still stretch from the W and central Mediterranean region all the way to Hungary and Romania. A ridge filled with warm air also extends northward into NW Russia, but maintains its slow weakening trend.

DISCUSSION

... south-central Europe ...

Surface dewpoints in the mid- to upper tens in conjunction with daytime heating support moderate CAPE, mostly in the 300 too 1000 J/kg range, in a broad band ahead of the cold front. The richest moisture (and therefore the highest CAPE) is in place close to the cold front on the one hand and in the upvalley flow regimes of the Alpine forelands on the other hand, though capping is stronger in the latter regime. Sunday's 12 UTC soundings confirm this assumption by showing the most robust CAPE (900-1800 J/kg), but also a fairly strong cap around the 850 hPa level across N Italy, which is grazed by an elevated mixed layer that has detached from Spain and NW Africa.
Numerous small vorticity maxima in the SW-erly to W-erly mid-level flow provide large-scale lift, whereas the contribution of temperature advection is largely neutral ahead of the front and turns strongly negative in its wake. On smaller scales, the cold front itself plus various outflow boundaries (which in turn may become dominant enough to modulate the front line in the course of the day) provide a fairly convincing lift mechanism. Vertical wind shear is moderate with 0-6 km values varying between 10 and 20 m/s.
To sum it up, all conditions for organized convection are in place, though none of them stands out. Agreement of the model pool on mesoscale details is poorer than usual, hence this forecast leans towards a mean of the provided spectrum plus a pinch of climatology.

Scattered elevated convection is expected already in the morning near the cold front (i.e., along the NW rim of the highlighted area), most notably in S Germany and possibly adjacent parts of the Czech Republic, Austria and Switzerland. Either these storms root down to the surface or new surface-based convection forms from noon onwards. Scattered to widespread activity is then expected in the entire region from the Czech Republic across the Alps and their forelands to S France. The expected primary storm mode is multicellular, while the odd temporary supercell is possible if storms propagate towards the capped but plentiful CAPE reservoir in the S Alpine forelands. Towards evening, several large storm clusters may emerge.
The expected hazards are fairly evenly distributed between excessive rain, large hail and severe convective wind gusts. Regionally speaking, the excessive rain hazard is maximized over the Alps, where deep convection can start quite early and several rounds are possible before the cold front finally replaces the unstable air. The hail and wind hazard are somehwat higher towards the south, where lapse rates are steeper and CAPE is higher, possibly reaching up to 2000 J/kg in N Italy. Other regions of a conditionally enhanced wind hazard include the Czech Republic, where 700 hPa winds are stronger (but convection in the afternoon "prime time" is also more questionable), and the SE Alpine forelands, where an organized regime of strong to severe outflow winds in the evening can run into Hungary and Slovenia.

The strongest convection will gradually weaken overnight, while a few additional, but mostly non-severe storms may form all night long further upstream ahead of various vorticity maxima.
Towards the end of the forecast period, cooler mid-level air in conjunction with weakening winds and a lake-parallel flow suggest prime conditions for a few waterspouts over the 23-25C warm water body of Lake Constance.

... Scandinavia and north-central Europe ...

Weaker lapse rates and limited moisture keep CAPE mostly confined to a few hundred J/kg. Scattered convection is expected with a peak in the afternoon both at the cold front (most notably in Poland) and ahead of various post-frontal short-wave troughs (most notably in S Sweden and Denmark). Under moderate deep-layer shear between 10 and 20 m/s, multicells are likely. A few marginally severe hail and wind events are not ruled out with the strongest ones. Convergence by the sudden increase of friction might be able to spin up one or two (probably non-mesocyclonic) tornadoes at windward coastlines.

... from NW Russia to the Balkans ...

Scattered, disorganized and slow-moving afternoon storms are expected in an environment of moderate CAPE and weak vertical wind shear. The main risk is localized excessive rain. In the increasingly hot air towards the south (Romania and southward), evaporation-driven downbursts and marginally large hail are possible as well, and lightning strikes may also ignite wildfires.

... Portugal, Spain, Morocco, Algeria ...

Robust CAPE develops in the sea breeze regimes propagating inland in the course of the day, but capping beneath the elevated mixed layer spreading from the hot and dry interior is very strong. Subtle synoptic lift might erode the cap just enough to allow isolated convective initiation over mountains. If persistent updrafts form, the environment would allow excellent storm organization especially in E Spain and NW Algeria (up to 2000 J/kg CAPE and up to 25 m/s deep-layer shear) with an associated hail and wind hazard. However, confidence in persistent and surface-based updrafts is too low to add more than some low probability lightning areas. Other than that, scattered elevated convection from Altocumulus clouds could develop limited lightning activity but does not pose a severe weather hazard.