Storm Forecast
Valid: Tue 18 Jun 2013 06:00 to Wed 19 Jun 2013 06:00 UTC
Issued: Mon 17 Jun 2013 22:25
Forecaster: PUCIK

A level 2 was issued for Northeastern Spain mainly for large hail, tornadoes, severe wind gusts and excessive precipitation.

A level 1 was issued for Southern to Central France mainly for (very) large hail, severe wind gusts, tornadoes and to the lesser degree for excessive precipitation.

A level 1 was issued for parts of BENELUX, Northeastern France, Central to Northern Germany mainly for (very) large hail and severe wind gusts.

A level 2 was issed for Poland and Northwestern Ukraine mainly for (very) large hail and damaging wind gusts.

A level 1 was issued for Alpine and Carpathian regions mainly for large hail, severe wind gusts and to the lesser degree for excessive precipitation.

SYNOPSIS

A plume of very steep lapse rates has been advected over western part of Europe between two major synoptic-scale features: a deep trough centered over Iberia and a large ridge over the Central Mediterranean and Central Europe. This is confirmed by most of the sounding measurements from the area. A wavy frontal boundary stretches around the warm sector with 850 hPa temperatures exceeding 20°C. Such pattern will remain mostly quasi-stationary during the forecast period without any major changes in the position of the main frontal zones. Surrounding the ridge a belt of strong flow is observed with windspeeds between 20 - 40 m/s at 500 hPa.

Steep mid-level lapse rates atop moist boundary layer are expected to contribute to the development of moderate to high CAPE values over a large area. However, strong capping and lack of strong "forcing" will likely prevent explosive or widespread growth of deep moist convection apart from a few locations and this makes the forecast quite challenging.

DISCUSSION

... Northeastern Spain ...

Cooling of 850 to 700 hPa temperatures should lead to at least partial erosion of significant CIN values as revealed by Tuesday 12 UTC Barcelona sounding. At the same time, lapse rates will become less steep, so we do not expect much CAPE to build up over the region. Southeasterly low level flow should advect quite moist airmass from the Balearic Sea, with rather low LCLs likely. With southerly jet-streak at 500 hPa overlying the region, we expect long, curvy hodographs suggestive of backed low level flow and strong deep layer wind shear. DLS values might actually well exceed 30 m/s. Well organised convection, including supercells and "line-echo wave pattern" are expected in cases that storms will be able to root in the boundary layer and utilize the available helicity. Full facet of severe weather is possible - more isolated cells should be capable of large hail, damaging wind gusts or even tornadoes, while a front parallel flow might favour a quasistationary MCS with threat of excessive precipitation. Thus, Level 2 seems to be warranted for this region.

... Southern to Central France ...

Another day with vertical wind profiles very favourable for well organised DMC is expected, with DLS values exceeding 25 m/s, while thanks to the easterly low-level flow SREH is simulated to be locally over 400 m2/s2 as 850 hPa flow reaches between 15 - 20 m/s. However, degree of instability might be not that pronounced, especially over the southern part of the area and again - CIN and lack of strong forcing might preclude surface-based development. A very conditional scenario is forecast - while on one side, isolated supercells capable of very large hail, damaging wind gusts and tornadoes are possible, on other side, convection might stay elevated and confined closely to the frontal zone with only marginal threats of large hail and excessive precipitation. Situation will be monitored for an update especially in case that more widespread initiation in the warm sector would become a distinct possibility.

... BENELUX, Northeastern France, Central and Northern Germany ...

A very isolated convection is possible along the warm front in the environment of very steep lapse rates and moderate to strong vertical wind shear. In such conditions, supercells are possible, capable of (very) large hail and severe wind gusts. Questionable, very uncertain initiation precludes higher level issuance attm.

... Poland to Northwestern Ukraine ...

Warm front is forecast to extend over the area with strong mid-level flow atop of it. Thanks to the steep mid-level lapse rates south of the front as well as moist boundary layer with dew points in the upper teens, we expect moderate to high CAPE values, perhaps exceeding 2000 J/kg. Deep layer shear should reach 15 - 25 m/s, increasing to the north, with veering in the lower levels (easterly low-level flow along and just north of the warm front) which might result in the SREH values exceeding 250 m2/s2 in a narrow belt along the front. NWP simulates scattered to widespread tstm coverage in the region, which might first take the form of isolated supercells capable of very large hail and severe wind gusts. Later on, as mid to upper level flow has a significant front-parallel component, quick clustering might result in a severe quasi-linear MCS surging towards east-southeast. In such case, severe to extremely severe wind gusts would be possible, as well as an isolated report of excessive precipitation with the northern part of the system.

... Alpine region, Southern Germany, Carpathian range ...

Steep lapse rates contributing to the moderate to high CAPE values along with moderate vertical wind shear (featuring small "loop" in the hodograph as winds veer with height) might be well conducive for strong multicells and/or marginal supercells. Large hail, isolated severe wind gusts might occur with the stronger cells and slow storm motion might result in a localised excessive precipitation event.