Storm Forecast
Valid: Sun 16 Sep 2018 06:00 to Mon 17 Sep 2018 06:00 UTC
Issued: Sat 15 Sep 2018 09:11
Forecaster: TUSCHY

A level 1 was issued across far E Spain and the far W Mediterranean mainly for excessive rain and isolated severe wind gusts.

A level 1 was issued for a confined area north of the Greater Caucasus mountain range mainly for excessive rain and a low-end tornado risk.

SYNOPSIS

An initially amplified frontal zone over far N-Europe aligns in a more zonal fashion with the approach of a strong extratropical cyclone over Iceland, which translates east during the day. Meanwhile, an upper trough over NE-Europe chokes off of the westerlies and crosses the far N Ukraine during the end of the night.
Further south an overall weak steering flow regime persists. Numerous progessive mid-level waves are embedded in this flow regime and shift east over CNTRL Europe. A quasi-stationary weak upper trough over the Iberian Peninsula continues to clamp a SW-erly flow regime over the SW Mediterranean, while a flat upper trough impacts the far SE Mediterranean.

A diffuse LL pressure channel (NW-SE aligned) covers the Iberian Peninsula and features weak pressure gradients. Overall no synoptic-scale front impacts today's thunderstorm forecast.

DISCUSSION

... Spain and the W Mediterranean ...

Responding to the LL pressure channel a persistent easterly boundary layer flow continues over the far W Mediterranean. Not much onshore advection is anticipated with very weak LL flow away from the sea. Nevertheless the BL flow establishes a broad plume with augmented LL moisture (LL mixing ratios in excess of 14 g/kg) along the east coast of Spain due to LL convergence (by friction along the coast and by moisture pooling along the LL pressure channel). This moist plume starts to mix out onshore with diurnal heating and a decrease in moisture depth. Mid-level lapse rates of 7 K/km atop the BL moisture result in 800-1200 J/kg MLCAPE onshore and in excess of 1500 J/kg offshore.
Weak mid-level geopotential height gradients cause a mostly faint mid-level flow regime with DLS around 25 kn. This shear increases towards the N African coast with peaks aoa 40 kn.
The SW-erly flow regime advects a warm air mass from N-Algeria to the N (850 hPa temperature in excess of 20 C) and supports strong capping south of the Balearic Islands.

Therefore the main focus for CI exists between the Balearic Islands and Spain, where repeated CI is forecast all day long. Anticyclonic mid/upper flow regime keeps thunderstorm probabilities on the isolated to scattered scale and in fact the LL forcing (e.g. next to numerous LL convergence zones) plays the main role in regionally augmented CI. Models diverge in timing and placement of the thunderstorm activity and hence we decided to highlight a broad area with most favorable ingredients.

Clustering and slow moving thunderstorms pose an excessive rainfall risk. Onshore activity should start and decay along the mountains (due to weak CIN and near non-existent flow). Offshore, magnitude of CAPE and somewhat better deep layer flow increases chances for upscale growing thunderstorms with excessive rain and isolated severe wind gusts. South of the Balearic Islands, cap and weak synoptic-scale subsidence should suppress CI for most of the time. Nevertheless weak short-waves circling the ridge may support a few elevated thunderstorms south of the Balearic Islands and hence the 50%-lightning area was expanded far south.

... Other lightning/level areas ...

Moderate CAPE of 400-800 J/gk with local peaks around 1 kJ/kg, weak forcing and poor DLS result in isolated slow moving thunderstorms with a local flash flooding and a marginal hail threat (an isolated large hail event is possible with maximized CAPE).

A marginal level 1 was issued for a region north of the Greater Caucasus mountain range. Enhanced LL flow, elongated thin CAPE of 500 J/kg and LL mixing ratios around 10 g/kg indicate a chance for slow moving/clustering thunderstorms with temporal training and heavy rain. In addition regionally stronger curved LL hodographs combined with low LCLs point to an isolated tornado threat.