Valid: Fri 09 Jan 2015 06:00 to Sat 10 Jan 2015 06:00 UTC
Issued: Thu 08 Jan 2015 21:25
A level 2 was issued for parts of Denmark and far N-Germany mainly for very severe wind gusts.
A level 1 was issued for parts of the North Sea, Denmark, N/NTRL Germany and parts of Poland mainly for severe to damaging wind gusts and an isolated tornado threat.
A mature extratropcial cyclone just NE of Scotland features all characteristics of a well developed warm seclusion. The occlusion continues to wrap around its center with a potential sting jet in full progress along its S/SE-ern fringe. Latest forecast soundings place 95 kt winds just shy above sea level in a well mixed air mass, so expect similar gusts over N/NE Scotland during the start of this forecast. This will be a life-threatening and very dangerous situation! Be aware of the fact that our level scheme does not capture non-convective wind gusts.
Later on this depression fills rapidly during its SE-ward motion and the gradient flow weakens. Still very strong winds spread SE-wards towards Denmark, N/CNTRL Germany and Poland. The trough approaches the Ukraine during the end of the forecast.
Another intense extratropical cyclone develops west of Ireland during the end of the forecast and races east. Strongest winds remain confined to the warm sector which should keep most intense winds off the surface over Irleand, UK and Scotland. Nevertheless another strong wind event seems to unfold for those regions during the overnight hours. The focus for enhanced downward mixing of strongest winds will be along the cold front, which is just about to enter the forecast area from the west around 06 Z. Therefore a level 1 was issued for isolated convection embedded in 35 m/s LL flow. Model discrepancies remain high with exact timing of the cold front so confidence in this level 1 placement is low.
... Denmark, N/NE Germany and parts of Poland ...
Rapidly filling/deamplifying depression/trough both push rapidly SE during the forecast. Attendant cold front affects Denmark during forenoon, N-Germany around noon and NW Poland during the afternoon hours. Despite converging model outputs, ongoing timing issue continue during the writing of this outlook.
Regarding the structure of this cold front, the front itself looks pretty healthy offshore (west of Denmark) and also over far N-Germany. Slim prefrontal moisture, cooling and gradually drying mid-levels with weakening but still adequate forcing should support a broken line of enhanced convection. Offshore regions see some chance for deeper updrafts up to 600 hPa which take profit of the mid-layer shear. DLS up to 80 kt with SRH in the lowest 2 and 3 km exceeding 400 m^2/s^2 indicate a chance of bowing line segments, which enhance downward transport of 35 m/s winds from 850 hPa. Severe to damaging wind gusts accompany bowing line segments and affect Denmark and the coastal areas of N-Germany until noon. An isolated tornado risk is also forecast as LL directional shear ramps up due to ageostrophic deflection of the BL flow well onshore.
The further development has to be split into smaller forecast areas.
After the front will come ashore, models inidcate a slow-down of the line segments and forecast soundings indicate a transition towards a broke line of a narrow cold-frontal rainband. The depth of the unstable layer shrinks to 1.5-2 km AGL. The slow-down however places those bands in even stronger shear with SRH increasing to 500 m^2/s^2 and 120 kt DLS. Most of the shear won't be used due to the shallow nature of the convection. Also, those bands often have a hard time to bring winds down to the surface due to a rather pronounced inversion (in this case around 800 hPa). Core of strongest BL winds also shifts east, so right now we don't expect a substantially enhanced gust risk during the cold front passage for those areas. Nevertheless with 25-30 m/s just shy above the surface we still want to keep those areas in a level-1. The focus also shifts to an isolated tornado threat as more southbound movement of any cell will increase SRH even more.
NE Germany and NW Poland:
This area still takes profit of stronger forcing and somewhat colder mid-levels (deeper CAPE profiles up to 3 km). Hence environmental conditions still support bowing line segments with an ongoing severe to damaging wind gust threat. Despite marginal CAPE, an isolated tornado threat continues due to low LCLs and extreme LL shear.
The issuance of the level 2 area was not straightforward due to meager CAPE and the expected broken nature of the convective line. However, favorable placement beneath the left exit of a 65 m/s mid-layer jet, 35-40 m/s winds at 850 hPa and at least some deeper updrafts indicate an adequate chance for a few 33 m/s gusts to go with a level 2.
The lightning area was confined to the regions with strongest CAPE signals, best forcing and where forecast soundings show at least low-end probabilities for graupel development in updrafts/charging regions.