Storm Forecast
Valid: Mon 02 Jul 2018 06:00 to Tue 03 Jul 2018 06:00 UTC
Issued: Sun 01 Jul 2018 09:34
Forecaster: TUSCHY

A level 1 was issued across NW France mainly for excessive rain.

A level 1 was issued for far N Spain, SW/CNTRL France and parts of Switzerland mainly for large hail, severe wind gusts and excessive rain.

A level 1 was issued for the Dinaric Alps mainly for large hail and severe wind gusts.

A level 1 was issued for the Caucasus mountains mainly for large hail and severe wind gusts.

A level 1 was issued for parts of W-Russia mainly for large hail (an isolated very large hail event is possible), severe wind gusts and an isolated tornado.

SYNOPSIS

Embedded between two extensive cyclonic vortices west of the Bay of Biscay and Belarus extends a ridge from the CNTRL Mediterranean towards Benelux and UK. The pressure geometry results in an elongated confluence zone, which runs from NW-France to N-Italy towards the S-Balkan States. To its north, a dry and stable air mass precludes deep moist convection, whereas to its south, a hot African air mass caps the available energy especially over the CNTRL Mediterranean. The main thunderstorm chances arise next to this confluence zone.

In a warm sector, stretched by the eastern vortex, exists another focus for DMC activity over parts of W-Russia. Despite the synoptic fronts, which frame that warm sector, no other frontal impacts on today's outlook are anticipated.

DISCUSSION

... France, Switzerland and N-Spain ...

Elevated convection from the previous night continues over NW and parts of N France during the morning. Forecast soundings show not much BL modification is needed for near surface based inflow. Rather modest low-tropospheric moisture and meager mid-level lapse rates keep CAPE and the severe risk on the low-end side. One or two hail events are still possible. The evolving rainfall threat over NW France will be discussed below.

Thereafter, air mass recovery occurs to the south with diurnal heating and improving BL moisture. This offsets meager lapse rates at mid-levels and results in 1000 J/kg nearly uncapped MLCAPE. Beyond noon CI is forecast from the Pyrenees to the Massif Central due to diurnal heating but also in consequence of a crossing trough. Rapid northward shift of that trough places SW France already in weak subsidence during the afternoon hours.
DLS increases from 10 m/s over CNTRL France to 20 m/s next to the Pyrenees so magnitude of CI over SW France remains a bit uncertain. Nevertheless expect any developing thunderstorm to acquire organization with an attendant large hail and severe wind gust threat. This risk expands towards far N-Spain.
Towards CNTRL France, slow moving and clustering convection poses an heavy to locally excessive rainfall risk, although isolated large hail and cold pool driven gusts are also possible. In case of clustering convection over the Massif Central, probabilities for an eastward progressing MCS towards Switzerland increase during the evening hours with an attendant isolated large hail, wind gust and heavy rainfall threat. A few CAMs support that idea and hence the level 1 was expanded east to cover that probability.

Another focus for severe convection exists over NW France during the day, where clustering and slow moving convection poses an heavy to isolated excessive rainfall threat with mainly elevated convection.


... Dinaric Alps ...

Diurnal driven east/northeasterly upslope flow beneath brisk northwesterlies creates robust DLS in the range of 20 m/s with 15 m/s in the lowest 3000 m. LL mixing ratios in excess of 10 g/kg beneath weak alpine lapse rates create a narrow belt with 500-1000 J/kg weakly capped MLCAPE along the mountains. Organized multicells and isolated supercells with large hail and severe wind gusts will be the main hazard. Moving off the mountains into drier air should result in weakening of individual updrafts and a general weakening trend is forecast during the night.

...W-Russia...

A northward surging warm sector - framed by an eastward progressing cold and northward moving warm front - is characterized by rich BL moisture beneath steep mid-level lapse rates. Resulting 1000 J/kg MLCAPE and 15 m/s DLS favor organized multicells with large hail (isolated very large hail is possible) and severe wind gusts in the open warm sector. Strongest forcing remains to the west and hence CI will be more isolated which supports discrete nature of thunderstorms. No upgrade was performed as much stronger shear remains displaced from the CAPE plume.
LL shear increases along the warm front and during the evening also ahead of the cold front and an isolated tornado event is possible. LCLs in excess of 1500 m should keep this risk on the low-end side.

... Caucasus mountains ...

Onshore flow from the Black Sea advects a marine air mass ashore beneath steep mid-level lapse rates. Overlap of resulting 1000 J/kg and 20-25 m/s DLS beneath the subtropical jet create favorable conditions for organized multicells and a few supercells. Large hail is likely with isolated very large hail and severe wind gusts. Weak forcing and isolated to scattered CI precluded an upgrade to a level 2.