Storm Forecast
Valid: Thu 27 Apr 2017 06:00 to Fri 28 Apr 2017 06:00 UTC
Issued: Wed 26 Apr 2017 22:40
Forecaster: GATZEN

A level 1 was issued for north-eastern Algeria and northern Tunisia.

A level 1 was issued for north-eastern Italy, eastern Croatia and northern Serbia into Hungary, western Romania, and south-western Ukraine mainly for large hail and tornadoes.

SYNOPSIS

The positively-tilted northwest-European trough starts to weaken and separates into two vort-maxima over Scandinavia and southern France. Downstream of the southern vort-max, flow backs to the south from the north Mediterranean to the Balkans and east Europe.

DISCUSSION

North Mediterranean, Balkans into west Ukraine

Ahead of the trough, steep lapse rates are advected north-east. Overlap with sufficient moisture is questionable given limited moisture over most places. Latest GFS points towards the region from northern Italy to the northern Balkans, where a tongue to better moisture is situated. However, GFS overestimated low-level moisture on Wednesday already, so that CAPE could be below the rather high values in the model (exceeding 1000 J/kg locally). Main source of upward vertical motion is topographic lift and the frontal circulation, but quasi-geostrophic omega will be rather weak ahead of the French trough that causes weak mid-level ridging during the day.

Frontal precipitation is expected on Thursday from the Alps towards Poland and western Russia. The southern edge of the stratiform rain could see some stronger diurnal heating what increases the low-level lapse rates over the central Balkans. Some overlap with the moist air mass further north can be possible, but weak forcing gives only little chances of initiation.

Current thinking is that a few storms could form especially close to the frontal boundary. Given strong vertical wind shear (0-3 km bulk shear about 15 m/s) multicells and mesocyclones are not ruled out, and large hail and tornadoes are not ruled out. Main activity may be embedded in the stratiform frontal rain, adding to high precipitation amounts.

Northern Algeria and Tunisia

The axis of the west-European trough passes the region in the afternoon. Affected air mass is characterized by steep lapse rates advected north-east from the Atlas mountains. At low levels, sea breeze flow causes Mediterranean moisture to spread onshore, and moisture convergence could lead to rich low-level moisture as indicated by latest GFS. Although 800 J/kg as indicated by GFS may overestimate the real CAPA and also CIN might be quite large, a few storms are expected to form over the mountains and move north-east. Strong vertical wind shear and favourably veering profiles indicates quite good potential for mesocyclogenesis, and right-moving supercells could be capable of producing large hail and severe wind gusts. Storms that move offshore will quickly become elevated given the stable maritime low-level air masses.