Storm Forecast
Valid: Sun 15 Sep 2019 06:00 to Mon 16 Sep 2019 06:00 UTC
Issued: Sat 14 Sep 2019 19:32
Forecaster: TUSCHY

A level 1 was issued across parts of Spain mainly for excessive rain, a low-end tornado and hail threat.

A level 1 was issued for N-Algeria and offshore for large hail, isolated severe wind gusts and excessive rain.

A level 1 was issued for parts of far E-Sweden to Estonia/S-Finland into Russia mainly for severe wind gusts and a low-end tornado threat.

SYNOPSIS

A deep cyclone over the Norwegian Sea shifts east towards Norway or rather evolves a new center over the N-Baltic Sea. It pushes an occlusion over the Baltic Sea east.
A quasi-stationary upper low sits over the Iberian Peninsula
with falling surface pressure south of the Balearic Islands. Uncertainty exists to which degree a LL vortex evolves but nevertheless a modest NE-erly LL flow persists along the E-coast of Spain, maintaining an onshore flow of a moist and unstable air mass.


DISCUSSION

... Iberian Peninsula and far N-Africa ...

Beneath the upper low, scattered thunderstorms evolve in a low to moderate CAPE (MLCAPE in the 400-800 J/kg range) and weak DLS (roughly 10 m/s) environment. Therefore slow moving thunderstorms with heavy rain and gusty winds are forecast. We included some of this activity in a level 1, where rich BL moisture and slow storm motion increase the flash flood threat.

DLS increases towards the coast with 20-25 m/s forecast (even 30 m/s offshore). Another round of onshore moving thunderstorms is expected with all kind of hazards (given enhanced speed/directional shear) including a tornado risk, excessive rain and hail. However, the latest indication is that strongest LL confluence remains offshore lowering the risk of repeatedly onshore moving storms. Early CI and a cooling LL troposphere could also increase capping issues along the coast (f.ex. limiting the tornado threat).

Numerical models did a bad job in CI over N-Algeria during the past 48 h and once again, an underestimation is forecast for this forecast period. Forecast soundings show little CIN, a mixed BL and ample LL flow normal to the Atlas mountains (from the north and south), including the passage of subtle mid-level waves from the SW. Hence scattered CI is forecast during the afternoon. This activity grows upscale into a large cluster, which shifts offshore/north. Onshore risk includes severe wind gusts, large hail and heavy rain while offshore activity - more elevated - probably favors the hail threat. A broad level 1 was issued and expanded far north.

... Far E-Sweden to Estonia/S-Finland into Russia ...

Eastbound surging occlusion will be overrun by cold mid-levels and a broad IPV anomaly. Combined with prefrontal diabatic heating, 200-500 J/kg SBCAPE seems to be a reasonable forecast. DLS in the 20-30 m/s range with 20 m/s 0-3 km shear and straight hodographs point to a severe wind gust risk in case of a temporarily growing convective line of showers/thunderstorms. With some LL shear in place, an isolated tornado even can't be ruled out. We expanded the level area far east to account for the chance of a longer-lived convective line due to the strong environmental synoptic-scale forcing. Overall this threat diminishes however with the loss of daytime heating.

Thought about upgrading Estonia and the Gulf of Finland due to a more widespread severe wind gust risk, but ongoing uncertainties about CAPE magnitude precluded an upgrade for now. However the offshore evolution of a potential convective line should be monitored closely.