Storm Forecast
Valid: Tue 31 Mar 2015 06:00 to Wed 01 Apr 2015 06:00 UTC
Issued: Mon 30 Mar 2015 16:19
Forecaster: TUSCHY

A level 1 was issued for many parts of NW/CNTRL Europe mainly for severe/damaging wind gusts and an isolated tornado threat.

SYNOPSIS

Diversified/progressive weather pattern continues over many European regions with one trough exiting towards E-Europe while another system will be close on it's heels. Latest model guidance places this new feature over the northern North Sea around 06 Z and crossing the Baltic Sea during the overnight hours.
This deep vortex is more or less vertically stacked and hence won't move that fast to the east. Latest analysis would favor a so called flat trough cyclogenesis in a diffluent flow regime. The low/mid-tropospheric vortex remains rather stable regarding intensity fluctuations despite being placed in the left exit of a 80 m/s polar jet streak. In fact, strongest upper divergence becomes displaced to the east betimes. Still, ongoing moderate pressure fall occurs with aforementioned dynamics and as the Azores high moves east with slight intensification expected (1035 hPa plus), intense pressure gardient will be in place over a rather broad region.
In addition, this vortex comprised a plume of modified subtropical air over the E-Atlantic, which becomes advected towards its center and betimes also wraps around the core of the strong/deep vortex. Latest phase diagrams reflect that scenario with a well structured and rather deep shallow warm-core structure. This will bring the focus for intense gradient flow to the SW-part of its center.

A highly meridional trough over E/SE Europe shifts east as deep WAA of the N-European low spreads east. Rising mid-level heights ensure stable conditions for SW Europe.

DISCUSSION

... Benelux, Germany, the Czech Republic, Poland, Slovenia, Slovakia, Hungary and the Alpine regions ...

Similar cyclogeneses in the past featured well structured split fronts due to potential instability build-up with interacting high-level dry air, low/mid-tropospheric boundaries and deep lift. Right now the dry mid-level air is rather ill defined in forecast cross sections. BL moisture will reach accetable values along/ahead of the cold front, but postfrontal air mass suffers from a well mixed and dry BL air mass. Hence, numerous foci for somewhat augmented thunderstorm probabilties exist over an extensive area, but right now no region can be detected, where substantial thunderstorm probabilities exist. Hence despite aggressive gradient flow, only a broad level 1 area was issued. Please keep in mind that ESTOFEX does not issue level areas for non-convective straight-line wind events. Please refer to your local weather agency for more information about that threat.

... N-/E-Germany, the Czech Republic and parts of Poland 06 Z-18 Z ...// ... Hungary, Slovakia and W-Romania during the night ...

First focus for deeper convection will exist along the eastbound moving cold front, which affects N/E Germany until noon and Poland/the Czech Republic thereafter. Despite mixed signals from the mid/upper tropospheric humidity in model guidance, quite robust prefrontal moisture advection in a well defined warm sector and strong/deep lift should ensure at least the development of a broken line of strong showers/isolated thunderstorms. Placed beneath the left exit of a 50 m/s mid-level streak with some high-level divergence present, there is no reason to doubt the development of at least isolated thunderstorm activity. Forecast soundings and EL temperature forecasts indicate the chance for some updrafts to reach 2.5-3 km AGL. They therefore tap into 25-30 m/s westerly flow, which could be partially mixed down to the surface due to rather steep LL lapse rates. Severe to damaging wind gusts accompany not only electrified convection but also stronger showers. Another concern will be low LCLs and strongly looped hodographs along the cold front, which indicates a chance for an isolated tornado event especially over Saxony/N-Czech Republic and far SW Poland due to favorable timing.

As a side-note:
Later on, N-Germany will probably face a period of very strong wind gusts along the SW-quadrant of the vortex. A well mixed air mass and strong pressure rises ensure downward mixing of up to 40 m/s LL winds between 12-21 Z. Damaging winds are forecast but without DMC this risk won't be reflected in our risk scheme.

Beyond noon, the thunderstorm risk from N/E Germany spreads further E/SE, but at the same time BL moisture continues to decrease (due to mixing). However, similar events in the past revealed a weakeness in model forecasts, which tended to overestimate BL mixing. Combined with eastward spreading intense deep forcing, we expect a broken line of strong showers/isolated thunderstorms to spread southeast with probably enhanced lightning activity along mountainous areas due to additional lift. We therefore expanded the risk level far east. Severe to isolated damaging wind gusts and an isolated tornado risk continue well into the night. We expect the highest probabilities over Hungary, Slovakia and W-Romania for the night. Further E/NE, the overall risk should be very low with this front.

SE Poland and W-Ukraine may see an upswing in isolated thunderstorm probabilities during the end of the forecast as another front surges east. We expect the highest severe threat to occur beyond this forecast range and hence we don't cover it right now.

... Benelux and Germany during the late afternoon and overnight hours ...

Despite a rather dry postfrontal air mass, rapidly steepening mid-level lapse rate due to the advection of very cold mid-/upper level air assists in neutral forecast profiles. Some models indicate weak mid-level disturbances, which cross the area and result in an increase of showers/isolated thunderstorms. Even enhanced friction along the coasts of the Netherlands my assist in spotty CI. Any deeper updraft may bring strong winds down to the surface and with isolated thunderstorm activity forecast, we added this activity to our level area. Severe wind gusts will be the primary threat, but once again an isolated tornado event can't be ruled out due to very favorable LL shear.

...N-Italy and the N-Balkan States ...

SE-erly racing front decouples during the passage of the Alpes which results in a broad area of low-end MLCAPE build-up along /south of the Alpine crest. Far NE Italy, W-Slovenia and SW/W-Croatia will have a good chance for an isolated low-topped supercell event due to very favorable kinematic flow and better CAPE. Isolated large hail, severe wind gusts and an isolated tornado event will be possible. The low-end thunderstorm risk spreads far SE, but decreasing BL moisture should gradually limit the thunderstorm risk.

... UK and Ireland ...

Active marine convection affects the forecast area. 25 m/s near BL flow will result in an severe wind gust risk with strongest cells (beside graupel). Despite marginl SRH-1, an isolated funnel/short-lived tornado event can't be ruled out especially for deviant moving convective cells. The risk diminishes during the night as mid-levels start to rise a bit from SW to NE.