Storm Forecast
Valid: Sat 22 Mar 2014 06:00 to Sun 23 Mar 2014 06:00 UTC
Issued: Fri 21 Mar 2014 21:08
Forecaster: PISTOTNIK
A level 1 was issued for E Germany and W Poland for severe convective wind gusts.
A level 1 was issued for the Balearic Islands and around for severe convective wind gusts and to a lesser extent for tornadoes.
A level 1 was issued for NW Italy for excessive convective precipitation.
SYNOPSIS
A pronounced long-wave trough stretches from the Faeroe Islands to the Iberian Peninsula and slowly moves Eastward while it further amplifies. Ahead of it, a steepening Southwesterly flow covers much of Europe.
Near the surface, a long and wavy frontal boundary stretches from Belarus across Poland and Central Europe all the way to Spain. It separates very warm continental air to the Southeast from cool maritime air to the Northwest. A broad area in the range of this frontal boundary gets overspread by positive QG forcing, though the quasi-barotropic nature of the long waves means that the positive vorticity advection is largely compensated by cold air advection.
Further downstream of the Western European trough, the remnants of a shallow upper-level low create slightly unsettled conditions over the Central Mediterranean. Sunny and warm weather prevails over Southeastern Europe, where the dominant high pressure system slowly recedes.
DISCUSSION
... Central Europe ...
Friday afternoon's observations showed a deeply mixed boundary layer in the broad warm sector of an open frontal wave. Dewpoints were mostly in the -2°C to 4°C range while the maximum temperatures exceeded 20°C at many places. Low-level moisture was systematically lower than the forecast models suggested, possibly resulting from the recent scarcity of precipitation. Hence, none of the 12 UTC soundings showed positive CAPE despite fairly steep lapse rates in the low- to mid-troposphere. However, a narrow band of better/deeper moisture seemed to exist close to the frontal boundary, and a line of thunderstorms managed to initiate in the late afternoon along the cold front over Northwestern Poland.
The situation on Saturday will closely mimic Friday's pattern, as the first frontal wave will quickly exit into Russia and the next one is forecast to form over Central Germany and Poland. The narrow band of augmented moisture near the boundary will be the site of possible low-end CAPE development in response to diurnal heating again, probably 200 or 300 km to the South compared to Friday. Wind profiles exhibit moderate, mostly unidirectional shear with values around 15 m/s between 0-3 km.
The cold-active arm of the frontal wave will remain rather inert until noon and then eject Eastward in response to strong diurnal heating, resembling a density current. Limited area models remarkably agree on convective initiation over Eastern Germany in the early afternoon, most likely over the Ore Mountains along the Czech/German border. If evaporative cooling manages to create a substantive cold pool, a linear organization with a risk of isolated severe wind gusts can be expected. Despite remaining doubts if instability will grow deep enough to produce thunder, a (rather low-end) level 1 was issued due to the high intra- and inter-model consistency. Small hail is possible as well with more discrete updrafts, but should mostly remain below 2 cm in size. Convection will move Northeastward and decay either when the night falls or when it moves over the Baltic Sea.
Further South and East, the cold front is expected to pass in form of a dry wind shift line over Southern Germany, the Czech Republic and Northern Austria, followed by stratiform precipitation later on.
... Western Mediterranean ...
The tail of the surface cold front will be delayed in the wake of the Pyrenees. Consequently, cooling upper levels are forecast to result in the buildup of CAPE on the order of a few hundred J/kg over the Western Mediterranean towards evening, partly overlapping with increasing shear from the West. Scattered thunderstorms are expected overnight which could bring isolated severe wind gusts, as well as one or two tornadoes in case of favourably colliding boundaries. Due to larger model discrepancies and the difficult predictability of convective initiation over the open sea, no high probability thunder area was issued. However, a low-end level 1 for severe wind gusts covers the area of the highest risk of better storm organization around the Balearic Islands. Convection may also affect Corsica and Sardegna late at night.
... Central Mediterranean, Italy ...
Scattered showers and isolated thunderstorms may form over orographic features in response to diurnal heating, but most areas will still see a rather calm day before a strong Genoa cyclone starts to develop along the advancing cold front in the evening. Prior to its passage, the Ligurian coast and Tuscany will see sustained onshore flow of deeply saturated and neutral to slightly unstable air. A period of heavy rain with embedded convection may result especially in the 18 to 00 UTC time frame. The risk of other severe weather phenomena seems to be limited despite constantly increasing shear, since discrete storms are unlikely.
With cooling upper levels and strong forcing, a few late (and probably elevated) thunderstorms could also move further inland over Northern Italy. They should be well detached from the preceding period of heavy rain- and snowfalls in the French, Swiss and Italian Alps, which is why no level 1 is issued.
... Western Europe ...
Very cold upper levels overspread the British Isles and France in the wake of the cold front, enabling a few hundred J/kg of CAPE under weak vertical wind shear. Scattered to widespread thundery showers are expected. Activity will likely be maximized over Ireland, England and the Bretagne during daytime and over the Channel region overnight. A number of funnel clouds and an isolated land- or waterspout are possible.
