Storm Forecast
Valid: Fri 31 Jul 2015 06:00 to Sat 01 Aug 2015 06:00 UTC
Issued: Thu 30 Jul 2015 18:41
Forecaster: TUSCHY

A level 2 was issued for E/NE Spain and parts of the W-Mediterranean mainly for very large hail (hail diameter of more than 5 cm), severe to damaging wind gusts, excessive rain and an isolated tornado event.

A level 1 surrounds the level 2 for similar risks but with less coverage.

A level 1 was issued for S/SE France and parts of CNTRL Italy mainly for large hail, severe wind gusts and excessive rain.

A level 1 was issued for parts of the CNTRL Ukraine mainly for large hail, severe wind gusts, excessive rain and an isolated tornado report.

A level 1 was issued for Bulgaria and Romania mainly for large hail and excessive rainfall amounts. A few severe wind gust events are also possible.

SYNOPSIS

Progressive pattern continues over Europe. Most of N-Europe resides beneath a pool of cold mid-levels, arching westwards and opening out into a deep vortex just south of Iceland. Progressive troughs circle that area and therefore affect most parts of N-/NE-Europe.

Another feature of interest is an amplifying trough west of the Iberian Peninsula, which shifts east and enters the far W-Mediterranean during the end of the forecast. This feature interacts with a volatile air mass surrounding the Balearic Islands and enables organized DMC development.

At the surface, an E-W aligned frontal boundary stretches from far W-Russia to Italy to Portugal. Air mass to its south features high low-tropospheric moisture content beneath steep mid-level rates and therefore acquires daytime driven strong to extreme CAPE values. To its north, meager BL moisture precludes the development of substantial CAPE.


DISCUSSION

...Parts of the Ukraine ...

A wavy/quasi-stationary front acquires more cold front characteristics while starting to sag southwards. Mid-level lapse rates remain weak as low-amplitude mid-level wave just grazes the area of interest as it lifts NE. However, moisture pooling along that front should decrease diurnal mixing and hence 800-1500 J/kg MLCAPE is forecast. The passage of that mid-tropospheric wave will be accompanied by a strong wind field with 20-25 m/s SW-erly winds at 500 hPa. Hence, 25 m/s DLS and 15-20 m/s 0-3 km shear produce a kinematic setup, which is favorable for organized/rotating updrafts with a large to very large hail, severe wind gust and isolated tornado risk. The latter may be augmented in the vicinity of the BL convergence zone, where backed surface flow and rich BL moisture keep low LCLs in a enhanced LL shear regime. Heavy to excessive rain is also possible during the clustering stage of storms, as shear vector and front align nearly parallel to each other (combined with 40 mm or more of effective PWs!). Storms probably tend to build south/southeast into the more unstable air mass, but then also move away from stronger forcing and better shear. The activity gradually diminishes during the night and shifts to the E/NE.

Further southwest over Bulgaria/Romania, 10-15 m/s DLS, roughly 1kJ/kg MLCAPE and the passage of a weak upper-level trough cause scattered DMC with large hail and excessive rain the main risk. Weak steering flow may push the rainfall risk to a level, where locally serious flash flooding could occur with slow moving storm clusters. Mesoscale determines where an isolated tornado or severe downburst risk may occur. The activity gradually diminishes during the night.

... Spain and the W-Mediterranean ...

As the upper-level wave approaches, diffuse LL depression is forecast to evolve over Spain. It remains challenging to determine, where its center will be, which also determines degree of return flow for NE Spain. Latest idea is that prolonged period of synoptically driven onshore flow exists ove NE Spain, which brings a moist air mass onshore and beneath steep mid-level lapse rates. Model guidance agrees well in the development of 1-2 kJ/kg MLCAPE over the coastal areas of NE Spain with decreasing values onshore. Offshore areas may see substantially higher CAPE values, which remain capped until the night.
A speed maximum of 25 m/s SW-erly winds at 500 hPa overspreads E-Spain and the W-Mediterranean during the forecast and as the wave shifts east, a tightening baroclinic zone ensures also an increased shear environment for the W-Mediterranean in the lower troposphere. 20-30 m/s DLS and 15-20 m/s 0-3 km shear overlap with significant CAPE, so the stage is set for an outbreak of severe thunderstorms.

During the daytime hours, the focus for CI resides over parts of N/E Spain and repeated thunderstorm development is forecast. Very large hail is likely over E/NE Spain next to excessive rain and severe wind gusts. We're concerned that persistent low-tropospheric SE-erly onshore flow could induce a training cluster of well organized thunderstorms over the mountains of NE Spain (e.g. the Pyrenees or the Sistema Iberico and the far E-Cordillera Cantabrica), which would increase the risk of extreme and flash flood producing rainfall amounts. Very high BL moisture along the coastal regions and lowering T-Td depressions over higher terrain may induce a tornado risk, especially for more discrete thunderstorms.

Further to the W/NW, shear and CAPE decrease, but an ongoing risk of multicells with large hail and excessive rain exists. A level 1 was expanded far west.

During the night, an eastbound shifting cold front over Spain gradually confines the thunderstorm and severe risk to E-/NE Spain but also increases BL flow convergence east of Spain where very high BL moisture is present. This moisture, combined with approaching lift by subtle short-waves and later-on also by the main trough axis but also with gradually cooling mid-levels and an eroding cap will increase probabilities for CI around the Balearic Islands. 2-3 kJ/kg MLCAPE, 25 m/s DLS, 15-20 m/s 0-3 km shear and enhanced helicity assist any thunderstorm to acquire rotation in a short amount of time with very large hail possible (in excess of 5 cm in diameter), severe to damaging wind gusts, excessive rain and an isolated tornado risk. With 0-3 km flow becoming aligned near parallel to a strengthening baroclinic zone over/just east of the Balearic Islands, a NE-ward racing MCS/bow echo is possible. It remains unclear, where CI finally occurs, but confidence in initiation somewhere in this area urged us to issue a very large level 2 for now. We also expanded the level areas more to the warm/unstable air mass, as models tend to underestimate this process.

More elevated convection occurs over S-France, but increasing BL moisture and shear towards the coast also increases the severe risk. Large hail and severe wind gusts next to heavy rain are possible all night long.

CNTRL Italy was added to the level 1 with a few severe thunderstorms during the afternoon/evening hours. 15-20 m/s DLS and more than 1 kJ/kg MLCAPE offer enough support for large hail, excessive rain and a few severe downburst reports.
Storms decay during the first part of the night.

... From Lithuania to Finland to W-Russia...

Diurnal driven SBCAPE build-up of 600-1000 J/kg with 5-10 m/s DLS results in less organized convection, which tends to grow upscale rapidly. A few large hail and/or heavy rainfall reports are possible, but with such weak shear and ongoing model discrepancies about final CAPE amount preclude a level 1 for now.