Storm Forecast
Valid: Sun 30 Jun 2019 06:00 to Mon 01 Jul 2019 06:00 UTC
Issued: Sat 29 Jun 2019 20:24
Forecaster: TUSCHY

A level 1 was issued across NW-Spain mainly for large hail and severe wind gusts.

A level 1 was issued across parts of France, Switzerland, Germany and Austria/N-Italy mainly for large hail, severe wind gusts and heavy rain.

A level 1 was issued across parts of Norway/Sweden mainly for isolated large hail, severe wind gusts and a low-end tornado threat.


SYNOPSIS

The long-lasting regime of the subtropical ridge continues during this forecast period with its center still offering +2 sigma standardized geopotential height anomalies. The highest thickness values however start to shift from the Alps towards Romania with an overall thickness decrease along its western fringe over W-/CNTRL-Europe.
An upper trough is analyzed over NW-Europe with a lead short-wave crossing the North Sea until noon from SW to NE, pushing a cold front to the E/SE towards CNTRL France/Germany and NW-Poland until 00Z.
Beyond 00Z a second wave (broader and less distinct) takes shape over Ireland/Scotland/England and enters the North Sea until the end of the forecast period.

A compact short-wave just west of Portugal lifts N over the Bay of Biscay but phasing with the second wave over NW-Europe remains uncertain. A long-wave trough over E-Europe drifts east and starts to lose contact with another trough over the E-Mediterranean Sea.


DISCUSSION

... Parts of France, parts of Germany, parts of Switzerland, extreme N-Italy and S-Austria ...

The mobile cold front over N-Germany into NW-Poland becomes quasi-stationary further south over CNTRL-France into CNTRL-Germany with a reduction of the angle between the background wind vector to the front. A plume of high moisture (latest MIMIC-TPW readings in the 30-40 mm range over far W-France to Scotland) along this cold front advects into our area of interst which occurs simultaneously with moisture recovery from the NW-Mediterranean Sea to the N. Hence rich BL moisture beneath an NE-ward fanning EML provides plenty of MLCAPE from the Massif Central to the Vosges (1.5 - 2.5 kJ/kg) and somewhat lower values over SW-Germany/Switzerland during the night. Another CAPE max. probably evolves along the S-Alps.

Despite capping issues during the daytime hours, lowering thickness and 850 hPa temperatures next to plenty of sunshine should support a few thunderstorms along the mountains. Latest data keeps this activity attached to the orography probably due to the ongoing NWP CIN forecasts, but would not be surprised about a few large and mobile thunderstorm clusters in this favorable thermodynamic setup. Despite weak DLS in the 5-10 m/s range, initiating storms pose a large hail and severe wind gust threat (due to a well mixed subcloud layer) before excessive rain becomes more dominant in case of upscale growing and slow moving convection. Overall still hostile synoptic-scale environmental conditions should keep the thunderstorm activity in a level 1 regime.

During the night, we probably deal with ongoing clusters between the Massif Central, the Vosges and Switzerland with similar hazards. Beside that, LL convergence signals become more focused/augmented from E-France into Germany along the wavy/quasi-stationary frontal boundary. This occurs simultaneously with subsiding LL CAA and ahead of the approaching second short-wave. CIN will be an issue along/south of the front, but LAMs indicate variable CIN strength with regionally lowered values. Despite mixed signals in past few QPFs of global models/CAMs, we expect at least a few elevated thunderstorms to develop along the N fringe of the front (e.g. E-CNTRL France into CNTRL-Germany). It remains questionable how far they can tap into the BL which is important regarding the severe risk as they would then move into 1500-2000 J/kg CAPE and take profit of effective 15-20 m/s DLS / 20 m/s 0-3 km shear. However we currently don't expect storms to take full profit of those ingredients and hence a few eastbound shifting progressive clusters with isolated large hail, strong to severe wind gusts and bursts of heavy rain are forecast. However this part of the level 1 remains still uncertain and needs to be monitored closely during the night.

... N-Pyrenees ...

The NE-ward lifting wave interacts with a plume of 1000-1500 J/kg MLCAPE. Attached to that wave is a strong mid-level speed max, which also affects the area between 9-15Z and helps to push DLS in the 15-20 m/s range. Finally this wave also crosses a dissolving synoptic-scale front over far NW-Spain. Hence, orographic initiation is likely with developing strong to severe thunderstorms. The main hazard will be large hail and severe wind gusts. Betimes, a cluster is forecast to move offshore over the Bay of Biscay and into a more hostile environment with a gradual weakening trend anticipated during the night.

... S-Norway and Sweden ...

A broad warm sector is forecast over this area with a cold front moving in from the W during the afternoon and overnight hours. Despite 10 g/kg mixing ratios in the BL, weak mid-level lapse rates keep CAPE values on the lower side. 200-400 J/kg MLCAPE along the NE-ward surging warm front and roughly 500 J/kg in the open warm sector/ahead of the cold front are forecast. Elongated/straight hodographs
occur in the warm sector with 20-25 m/s DLS so any deeper/more persistent updraft will be able to produce large hail and severe wind gusts.
CI along the warm front remains questionable due to marginal CAPE, but any thunderstorm, which evolves along the S fringe of the warm front experiences 0-3 km SRH in excess of 300 m^2/s^2 and DLS aoa 30 m/s. LCLs between 500-1000 m AGL overlap with those ingredients, so an isolated tornado event next to a few severe wind gusts is possible.