Storm Forecast
Valid: Sat 18 May 2024 06:00 to Sun 19 May 2024 06:00 UTC
Issued: Fri 17 May 2024 23:46
Forecaster: PISTOTNIK

A level 1 is issued for parts of England mainly for excessive convective rain.

Level 1 areas are issued for W and central France, parts of Germany, and parts of Poland mainly for excessive convective rain and to a lesser degree for large hail.

Level 1 areas are issued for N Spain, S France, parts of Switzerland, Austria, the Czech Republic, and for NW Italy mainly for large hail and excessive convective precipitation.

Level 1 areas are issued for NE Algeria, NW Tunisia, parts of Albania, North Macedonia, and W Bulgaria for large hail, excessive convective precipitation and to a lesser degree for severe convective wind gusts.

SYNOPSIS

A "high-over low" blocking pattern remains in place. High 500 hPa geopotential and surface pressure stretch from the North Atlantic across the British Isles to the Baltic Sea, whereas an extensive, slowly filling low-pressure system covers the area between the Bay of Biscay, central Europe, the Balkans and Italy. The polar jet is deflected to the far North and only influences Iceland and N Scandinavia. The subtropic jet runs from the W Mediterranean region towards Hungary and Romania at the southern flank of the cut-off low. Moderately warm and moist airmasses cover most of continental Europe and prolong the unsettled springtime conditions.

DISCUSSION

... Spain, France, British Isles, central Europe, Balkans, Italy and central Mediterranean region ...

Daytime heating creates low to moderate CAPE up to 1000 J/Kg across wide areas and allows scattered thunderstorm activity. The plethora of lift sources and triggers - upvalley and upslope circulations, sea breeze fronts, residual convergence zones in the filling low-pressure complex, and dynamic lift ahead of numerous travelling vorticity maxima - makes it almost impossible to go into detail when it comes to the timing and placement of deep convection. However, the majority of storms will follow a typical daily cycle.

Kinematics-wise, the large discussed area can be divided into the following sub-regions:
(1) Most of France, the British Isles, Germany and Poland see weak vertical wind shear. Poorly organized and pulsating storms therefore pose main hazards of localized excessive rain and perhaps plentiful small hail.
(2) Deep-layer shear increases into the 10-20 m/s range in a corridor from N Spain to S France, Switzerland, S Germany, Austria, the Czech Republic and Slovakia. This belt will also see noteworthy synoptic lift, first by subtle warm air advection and then by a short-wave trough that overspreads the entire belt from west to east during this forecast period. Convective mode will be mostly multicellular, which turns large hail into the primary hazard and excessive rain into second position. Following the lift of the trough, the peak of scattered convection will also probably be delayed to the evening in S Germany, W Austria and the Czech Republic, and overnight in E Austria and Slovakia, which makes severe weather less likely.
(3) The jet axis from the W Mediterranean Sea to N Italy, the N Balkans, Hungary and Romania sees lower CAPE and stays mostly bereft of any synoptic lift, hence few to no storms will occur. Nonetheless, it is worth pointing out that deep-layer shear in excess of 20 m/s could easily render any surface-based afternoon storm well-organized with large hail and severe wind gusts.
(4) On the anticyclonic side of the jetstream, another shortwave trough crosses a warmer and moderately unstable airmass from the Maghreb states to S Italy, the S Balkans and Greece. Here, forecast models utterly disagree with respect to timing, placement and intensity of (convective) precipitation. It appears likely that convection will mostly or entirely be elevated, and therefore probably non-severe, in marine and coastal areas. The best chances for surface-based storms in the afternoon to evening exist some distance inland in Algeria, Tunisia, as well as Albania, North Macedonia and adjacent (mountainous) parts of Bulgaria. In that case, deep-layer shear between 10 and 20 m/s favors multicells with a possibility of large hail, localized excessive rain and perhaps a few severe downbursts.

Level 1 areas are issued to cover those regions where a high coverage of surface-based afternoon storms can provoke scattered severe weather events. Isolated events are also possible outside the level 1 areas.

... S Turkey ...

Moist sea breezes and upvalley / upslope circulations beneath a strong NW-erly mid-level flow create a classic "loaded gun" situation with CAPE probably in excess of 1000 J/kg and deep-layer shear on the order of 20 m/s. Isolated to scattered afternoon storms are expected to form along a dryline boundary on the southern slopes of the mountains and to organize into strong multi- and supercells when tapping into the rich moisture reservoir of the lowlands. Each of these storms can produce large to isolated very large hail. In addition, severe wind gusts and excessive rain are not ruled out, the latter mostly in case of backbuilding or repeated formation over the same orographic features. A limiting factor that should keep storm coverage on the lower side is the lack of synoptic lift.

... N Finland ...

Similar to the day before, steep lapse rates and sufficient daytime heating allow marginal CAPE in the warm airmass just south of the polar jetstream, which creates impressive vertical wind shear throughout the depth of the tropopshere (e.g., 0-6 km shear up to 30 m/s and 0-1 km shear up to 15 m/s). However, synoptic lift from a strong short-wave trough does not arrive before evening, and current thinking is that convection forming then would be elevated and not benefit from these excellent kinematics. However, severe wind gusts and perhaps one or two tornadoes are not ruled out if convection can yet root down to the surface, or if early surface-based convection forms already in the afternoon despite a background of pretty dry environmental air.