Storm Forecast
Valid: Fri 28 Mar 2014 06:00 to Sat 29 Mar 2014 06:00 UTC
Issued: Thu 27 Mar 2014 18:04
Forecaster: TUSCHY

A level 1 was issued for parts of Bulgaria mainly for large hail.

A level 1 was issued for W-Turkey mainly for large hail.

A level 1 was issued for the E-Aegean Sea and extreme W/NW Turkey mainly for an isolated tornado, severe wind gust and large hail risk.

SYNOPSIS

A blocking pattern with a complex distribution of cyclonic/anticyclonic vortices exists over Europe. The most dominant feature during the forecast is a Scandinavian high pressure area, which expands south, fostered by latest positive 500 hPa height tendencies over C-Europe. This high is surrounded by numerous cyclonic vortices, which bring unsettled conditions to many parts of Europe. A retrograde moving vortex over the W-English Channel affects Ireland and UK, whereas a quasi-stationary trough over the open waters of the E-Atlantic gradually approaches Portugal and Spain during the forecast. A third mid-layer vortex over Greece and Bulgaria continues its eastbound motion while weakening.

At the surface, all aforementioned vortices push occluded fronts ashore which cause confined areas with enhanced BL moisture content due to persistent onshore advection of a marine layer. Thunderstorm chances will limit themselves to those postfrontal sectors.

DISCUSSION

... Balkan States ...

A complex behavior of a weakening/flattening upper low and a reforming vortex in the lower troposphere with pure eastbound motion cause yesterday's progressive front to become quasi-stationary in an E-W fashion over Serbia and Bulgaria. Yesterday's moisture advection from the Aegean Sea now assists in LL mixing ratios to exceed 5 g/kg over most of the Balkan States with peaks of 7 g/kg along the stationary front (along the Romanian/Bulgarian border). The upper low continues to expand east and therefore interacts with the rich BL moisture. Steep mid-layer lapse rates and sporadic diabatic heating during the daytime hours should help to push MLCAPE up to 500 J/kg with local peaks of 800 J/kg over E-Bulgaria and along the front itself. Residual moisture from yesterday's front-coast interaction of W-Turkey also approves MLCAPE to achieve 800 J/kg just along the coast with rapidly decreasing values over land.

Weak upper divergence and numerous mid-layer impulses induce adequate lift for scattered initiation during the forecast. The airmass is weakly capped and hence supportive of early and widespread initiation.

Weak low/mid-tropospheric shear north of Greece lowers probabilities for organized updrafts, but forecast soundings offer somewhat fatter mid-layer CAPE profiles. Combined with a constant cooling-down at mid-levels and the forecast of a few strong pulsating storms indicate a risk of large hail on an isolated and temporary scale. Rapid and disorganized upscale growth probably results in complex thunderstorm interaction, so numerous mesoscale outflow boundaries in combination with some LLCAPE build-up also point to an isolated funnel and short-lived tornado risk. Heavy rain is possible due to slow storm motion and clustering storms, but rather meager effective PWs lower that risk. We issued a level 1 for the front and E-Bulgaria, where confidence in a few severe events will be enhanced. Further west, less mid-layer CAPE limits the severe risk, although robust LLCAPE may still cause a few funnel/isolated tornado events over Albania, Serbia and far NW Greece. The same for the coastal areas of the SE Adriatic and Ionian Sea, where waterspouts are possible next to heavy rain.

Along the E-coast of the Aegean Sea, ongoing organized convection probably lasts until noon, before BL moisture lowers from NW to SE. DLS of 30 m/s and augmented LL shear indicate an ongoing severe wind gusts, isolated tornado and large hail risk. Further inland, rapidly waning CAPE but strong DLS may support organized but more elevated convection with large hail the primary risk. During the afternoon hours onwards, thunderstorm probabilities decrease from W to E.

During the overnight hours, the eastward swinging upper trough crosses the warm Aegean Sea and induces another round of DMC. Weak shear precludes organized convection, but an isolated waterspout event is well possible.

... SW-UK, NW France and Ireland ...

Retrograde westbound moving upper low affects those regions and induces adequate forcing for scattered initiation. A well mixed marine layer beneath steep mid-layer lapse rates offers widespread 300-600 J/kg weakly capped MLCAPE. Forecast soundings indicate a buoyant BL, so the main risk will be a few funnel/isolated short-lived tornado events. Marginal hail/sleet and gusty winds accompany strongest storms. The activity decays after sunset with isolated ongoing storms over the offshore areas.

... Portugal ...

Another confined area with modest MLCAPE build-up beneath an onshore moving upper low exists over Portugal. A tight DLS gradient exists, but expect most active convection to evolve beneath the cold-core center and therefore displaced from strongest shear. Scattered thunderstorms are forecast with marginal hail/sleet and gusty winds. LLCAPE in excess of 100 J/kg and a few strong updrafts may help to produce a few funnel/an isolated short-lived tornado event. Thunderstorms diminish after sunset with ongoing convection south of Portugal.