Storm Forecast
Valid: Thu 23 Mar 2017 06:00 to Fri 24 Mar 2017 06:00 UTC
Issued: Wed 22 Mar 2017 23:33
Forecaster: GATZEN
A level 1 was issued for eastern France mainly for severe wind gusts, large hail, and tornadoes.
A level 1 was issued for western Romania to south-western Ukraine, eastern Hungary, and northern and central Serbia mainly for large hail.
SYNOPSIS
The amplified west-European trough cuts off over Iberia. A Scandinavian trough slowly progresses east. Between both features, a frontal boundary stretches from southern France to southern Belarus. South of this boundary, diurnal heating and cold mid-levels cause steep lapse rates. Close to the ground, dewpoints are in the range of 5 to 10°C along the frontal boundary that will spread northward, associated with low-level warm air advection.
DISCUSSION
Eastern and northern France
Ahead of the forming Iberian cut-off trough, a strong mid-level jet points towards southern France. Some moisture with surface dew points around 10°C is advected into south-eastern France where a convergence zone develops between easterly winds across southern Germany, southerly winds from the Mediterranean, and a slowly approaching cold front from the west. This moisture overlaps with steep lapse rates originating from the Pyrenees and east Iberia. CAPE in the order of 500 J/kg is expected over eastern France in the afternoon hours. Initiation of thunderstorms is expected in the late morning hours close to the cold front over southern France. Further storms could evolve later on also along the cold front over northern France.
Strong deep layer vertical wind shear in excess of 25 m/s overlaps with the region of instability so that thunderstorms can rapidly organize into multicells and supercells that move north-eastward into the eastern France region. Severe wind gusts are forecast with these storms that may evolve bowing segments associated with some larger wind events. Furthermore, large hail is forecast what is also supported by low wet-bulb zero heights. Finally, tornadoes are possible, especially if storms can form close to the cold front where stronger dynamics exist. If diurnal heating leads to CAPE in this area, also strong tornadoes are not ruled out. Current thinking is that stratiform precipitation and clouds will limit this potential, though. Given the limited instability, a level 1 is issued for this activity at this time.
Balkans to Ukraine and southern Belarus
Latest soundings indicate steep low-level lapse rates over the Balkans and south-western Ukraine. These are expected to advect north on Thursday. At low levels, rather moist air masses are situated along the frontal boundary that moves north. Along this front, thunderstorms have developed on Wednesday. This activity is expected to redevelop on Thursday afternoon, supported by a mid-level short-wave trough that enters the Balkans from the west in the evening hours. Main focus is Hungary and north-western Romania, but isolated storms can also form over the western Ukraine, southern Slovakia, northern Croatia, Bosnia, and Serbia. Due to snow layers in higher mountains, thunderstorms are not too likely given the reduced surface heating.
Storm organization is forecast to be rather weak as deep-layer vertical wind shear is limited to 10 to 15 m/s. Single cells and brief multicells could form. However, close to the warm front, low-level vertical wind shear is much better developed and could exceed 15 m/s in the lowest kilometre with favourably veering profiles from eastern Austria to southern Slovakia western Ukraine and southern Poland. In the case of some stronger diurnal heating in this region, this could allow for rotating updrafts capable of producing large hail. Main severe threat is forecast further south, though, where better instability and low wet-bulb zero is also expected to favour some large hail.
