Storm Forecast
Valid: Sat 10 Jan 2015 06:00 to Sun 11 Jan 2015 06:00 UTC
Issued: Fri 09 Jan 2015 23:08
Forecaster: TASZAREK

A level 2 was issued for N Germany, Denmark, and NW Poland mainly for severe and damaging wind gusts and in lesser extend to isolated tornado event.

A level 1 was issued for large part of W and C Europe mainly for severe wind gusts

SYNOPSIS

A large thermal contrast over N Atlantic that persists for several days resulted in strong thermal wind and thus Jet Stream. Exit of jet streak on Friday's sounding performed on the Bergen station at 12UTC measured extreme wind speed value of 83,9m/s at 300 hPa level. Longitudinally arranged exit of jet stream support cyclogenesis and waving of thermal contrast zone that on Saturday will be pushed towards east. Such a significant wave with cold arctic air advection preceded by warm and moist marine air of polar origin will shape the Saturday's weather pattern in Western and Central Europe. Large pressure gradient extending from Azores High (1040 hPa: with ridge up to Italian Peninsula and 1025 hPa) to Icelandic Low (980 hPa) and extratropical cyclones over Scandinavia (955 hPa) will create very dynamic westerly airflow that with instability and frontal forcing will provide conditions for severe wintertime convective storms. Extratropical cyclone "Elone" will stop at the coast of Norway and produce secondary cyclone that center's will move through Stockholm and Helsinki. In the end of forecast period a widespread advection of Arctic airmass will cover C Europe.

DISCUSSION

...N Germany, Denmark, and NW Poland...

Cold front that is associated with extratropical cyclone "Elone" will replace relatively (as for this time of the year) moist and warm air (850 hPa temperature: 7C, mixing ratios: 7-8 g/kg) with cold and dry arctic air (850 hPa temperature: -7C, mixing ratios: 2-3 g/kg). Such a contrast and forcing of moist airmass along frontal boundary will result in instability up to 200 J/kg. Although instability will be marginal, it will occur with very strong and sheared flow. Extreme values of 0-3km ranging up to 30-35m/s and LLS 20-25m/s (we do not consider DLS due to shallow extent of convection) with combination of even small values of CAPE are capable of producing extreme severe weather phenomena with the greatest threat focusing on damaging convective wind gusts. We expect that convection over German and Danish coast may organize into line of storms around 12 UTC and move towards NW Poland where its intensity should decrease. In a such setup, if convection will organize linearly wind speed gusts of convective origin may reach significant severity (> 33m/s). Supportive conditions of low LCL (400-600m AGL) and impressive 0-1km SRH of 400m2/s2 in front of the frontal zone may pose marginal risk for isolated significant tornado event.

Although circumstances seems reasonable to issue level 2 there is large uncertainty whether this situation deserves this level. The biggest inhibiting factors are related to not significant large-scale upward forcing that with such a low values of CAPE should be very strong to force convection, and to marginal values of CAPE in itself. Level 2 zone denote area where left exit of cyclonically-curved jet stream together with frontal-forcing and advection of absolute vorticity will provide the strongest forcing. Although the highest QG-forcing is expected around 12-15UTC in N Germany and should weaken when entering to Poland, it can partially benefit from orographic lifting in NW parts of Poland.

Another limiting factor is also related to vertical extend of convection that according to NWP modes should be around 3km AGL. Such a shallow unstable layer is considerably less capable of producing lightning discharges and its updraft do not profit from DLS.

The issuance of the 50% lightning probability area in this region was also not straightforward due to high uncertainty related to the strength of the convection. However, relatively high boundary-layer moisture content that overlap with QG-forcing indicate and adequate increased probability for lightning. Thunderstorms originating in the cold sector are also expected along Polish coast and around Denmark in the late evening.

... N Sea, Denmark, parts of United Kingdom, Benelux, Germany, parts of France Czech Republic, Poland, Sweden and Norway...

Level 1 threat denote area where organized convection originating from cold front passage will move in the forecast period. In the environment of little CAPE (100-200 J/kg) with shallow extent of convection (up to 3-4 km AGL) and strong pressure gradient, severe wind gusts of convective origin are expected.

Behind the cold front, a tongue of Arctic air with impressive lapse rates (8.5C / km) will provide more instability and thus will be more supportive for producing lightning discharges in the late evening and during the night. Therefore, we issue 50% lightning probability for this area.