Storm Forecast
Valid: Fri 12 Apr 2013 06:00 to Sat 13 Apr 2013 06:00 UTC
Issued: Thu 11 Apr 2013 22:03
Forecaster: TUSCHY

A level 1 was issued for NE France, Benelux and far W-Germany mainly for an isolated large hail and a funnel/tornado event.

A level 1 was issued for NW Italy mainly for large hail.

SYNOPSIS

The synoptic flow regime turns westerly over C-Europe with the approach of a long-wave trough from the west. This trough, placed over UK/Bay of Biscay at 06Z translates east, crosses France until 18Z from W to E and lifts ENE over Germany during the overnight hours. Downstream ridging over the Balkan States shifts east conform to the displacement of the mid-level warm temperature anomaly and both features affect E-Europe during the overnight hours. Caused by a temporarily established zonal flow pattern over C-Europe, geopotential heights somewhat increase over the Mediterranean due to strengthening anticyclonical relative vorticity along the southern fringe of the frontal zone and weak subsidence. This set-up (incoming upper wave over C-Europe and somewhat rising heights over the Mediterranean) results in strengthening gradient flow with the main shear axis running from S-France to N-Italy to the N-Balkan States during the day.

At the surface, a wavy N-S aligned cold front is placed from NE Poland to the Alps and all the way towards the C-Mediterranean at 06Z with a slow eastward motion expected during the day. Despite supportive BL moisture present with this front, placement within a mid-level thermal ridge keeps lapse rates way too weak for any CAPE build-up, so non-thundery stratiform rain continues to accompany that front.
Postfrontal cold front sector then becomes the focus for daytime driven thunderstorm activity, which extends from France to Germany, bound by an occlusion to its north, which itself runs from C-UK to the S-North Sea while losing frontal characteristics. Some models depict the chance that this bounday might re-organize into a leisurely northward moving warm front, but models still diverge significantly with what to do with that boundary. However, all models agree that in the postfrontal sector, numerous more or less pronounced convergence zones will move from W to E. One mesoscale convergence zone (consolidating warm front/moisture boundary on the mesoscale) evolves over N/C Germany just beneath a low tropospheric thermal ridge axis with a slow eastward motion expected. Signals for numerous convergence zones over Frances exist, but they diverge too much regarding strength and placement to define their expected positions right now.


DISCUSSION

... S/SE UK ...

The vertically stacked vortex (most healthy in the lowest 3 km) won't see any serious strengthening as it crosses S-C UK during the daytime hours from W to E. At 21Z (11th April), BL moisture already features dewpoints up to 8°C over S-UK which continue to spread north beneath cold mid-levels. Diurnal driven showers and thunderstorms are forecast with a prolonged period of CI expected due to the proximity to the low and the presence of the nearly uncapped air mass. However, early departure of a weak vorticity lobe and gradually increasing NVA from the west may induce a decrease in thunderstorm activity during the afternoon hours and therefore during the maximized activity for diurnal driven convection. Marginal hail/sleet and gusty winds accompany stronger pulsating thunderstorms although an isolated funnel/short-lived tornado report can't be ruled out with improving LL CAPE, some weak background vorticity and low LCLs. The overall coverage of storms and expected severe probabilities remains too low for either a 50 % lightning area or a level 1 (although SE-UK might come close to a 50-% lightning verification with best parameters/timing for thunderstorm activity).

... France, Benelux and parts of Germany ...

A weak mid-level vorticity lobe traverses the highlighted area from W to E during the forecast period, inducing some lift next to the cyclonically curved mid-level streak, which places most of the area beneath the favorable left exit region. Also, numerous weaker pieces from this jet loosen and result in areas with regionally enhanced lift. Aforementioned mesoscale convergence zones but also the orography itself result in regionally enhanced thunderstorm activity. This messy picture makes it hard to highlight the hot spots for lightning activity and therefore a coarse lightning area was added.

A combination of mid-level polar air atop well mixed maritime Atlantic air results in modest MLCAPE build-up on the order of 300 - 600 J/kg with EZ being once again on the lower-end side. The air mass will be weakly capped and therfore prone to early initiation.

DLS remains weak during the forecast, although latest model outputs increased the 6 km bulk shear to 15 m/s, which would be enough for a few better organized thunderstorms. Placed in the SE-quadrant of the UK low, a broad area with deeply veered flow is expected, nicely reflected in SRH fields, which yield values of 100-150 m^2/s^2 around noon, increasing to 200 m^2/s^2 during the evening hours. Given complex surface boundary pattern, we would not be surprised to see a few storms with a more pronounced deviant storm motion, which would locally augment helicity values significantly.

Expect scattered to widespread storms to evolve over France until noon from W to E with increasing activity trends thereafter over Benelux and Germany. Given broadening CAPE profiles within the hail growth zone and 15 m/s DLS, marginal hail is likely and an isolated large hail event (1.5 to 2 cm) can't be ruled out. Gusty winds also accompany strongest storms. The highest probabilities for an isolated tornado event arise from C France to Benelux and probably into W-Germany, especially during the late afternoon hours, when still available and nearly uncapped SBCAPE and increasing LL directional shear overlap. A level 1 was issued mainly for those areas, where most of the aforementioned parameters overlap.

After sunset, rapid BL cooling should bring CAPE values down and also the thunderstorm activity. However, stronger forcing and a lingering MUCAPE plume may support isolated and short-lived thunderstorms over Benelux and NW/N-Germany until midnight. This activity will be sub-severe.

... N-Italy, NE-Croatia and S-Hungary ...

Eastward moving mid-level trough overspreads leisurely eastward moving and wavy surface cold front. Along and ahead of this boundary, 6-7 g/kg mixed-layer mixing ratios spread north and beneath cooling mid-levels. A belt of 300 - 600 J/kg MLCAPE is forecast to evolve in those areas althoug highly variable BL moisture distribution also results in a patchy CAPE field. The highest values probably will be found over N-Italy and NE Coratia, where initiating storms during the afternoon hours will pose a large hail and strong wind gust risk. Main problem for widespread CI is the trapped cold front along the Alps, which results in cloudy skies and lots of precipiation, also limiting heating during the day. Thunderstorms rapidly decay after sunset or transform into a messy cluster of showers/stratiform rain, locally heavy over S-Austria/N-Slovenia.