Storm Forecast
Valid: Fri 26 Feb 2010 06:00 to Sat 27 Feb 2010 06:00 UTC
Issued: Thu 25 Feb 2010 18:57
Forecaster: TUSCHY
A level 1 was issued for extreme E-France and SW-Germany mainly for severe wind gusts.
A level 1 was issued for the E-Adriatic Sea mainly for severe wind gusts, large hail and tornadoes.
SYNOPSIS
Upper streamline pattern becomes more zonal over W/central Europe before winds back somewhat during the end of the forecast, as constantly amplifying Rossby wave approaches from the west. Some showers/thunderstorms are forecat in the zonal-flow regime over central Europe. Further to the NE and N, the winter-like air mass precludes deep convection.
DISCUSSION
... E-France, the Netherlands and parts of Germany ...
Forecasting winter time thunderstorms is not always fun, which is the case in the following outlook. From a synoptic perspective, good forcing enters E-France and SW-Germany during the morning hours as an approaching short wave still features a strong cyclonic kink. However, this feature leaves the deep baroclinic zone (placed over central France and Switzerland) and enters a weakly sheared and diffluent coined environment over Germany, so gradual lessening is forecast already during the late morning hours. Further down in the lower troposphere, no real frontal system accompanies the upper vorticity lobe (besides a diffuse cold front). Some wrap around moisture seeps in from the west just behind the cold front, but the best moisture remains north of the area of interest (e.g. Luxembourg and N/central Germany). In addition, LL trajectories feature a long track over the Iberian Peninsula and France, where nocturnal cool down helped to stabilize the eastward moving air mass, so we see no reason for good PBL air mass quality. To summarize all that, we may see some sort of forced line of showers and isolated, embedded thunderstorms over E-France and SW-Germany between 06-12Z. The wind field is strong with 850hPa winds running well above 20m/s, so severe wind gusts will be a good call. A level 1 for severe wind gusts was issued with low lightning probabilities, as not much activity is expected.
Further east over SE/E/central Germany, no thunderstorm risk is forecast as overall conditions become too obstructive for deep/long-lived convection. The wind gust risk remains, but marginal convection won't justify any level area.
Another story for NW-Germany and the Netherlands during the afternoon hours. Although weakening, a disorganized mid-level jet streak covers N-central Germany, causing some low-end upper divergence over the area of interest. Adequate LL ML mixing ratio and cool mid-levels may support enhanced convection with an isolated thunderstorm event possible. Marginal hail and gusty winds will be the main hazard during the limited time-frame around peak time heating.
... France and Switzerland ...
In my opinion, EZ and GFS are quite optimistic with thunderstorm probabilities, showing an elongated belt with enhanced convection, running from the Bay of Biscay to SE-France. Upper flow becomes more zonal with no real good forcing embedded, which could have served as focus for storms. NVA increases during the afternoon hours and later-on, geopotential heights rise from the SW as another round of WAA starts. We went with isolated thunder along the W-coast of France, where LL moisture influx will be the best, but also over SE-France and SW-Switzerland, where blocked LL flow results in moisture convergence and a stripe with somewhat better SBCAPE build-up. Strong wind gusts and marginal hail remain the main hazard due to low-end CAPE and modest shear. After sunset, thunderstorm chances decay.
... N-Italy and the E-coast of the Adriatic Sea ...
A broad LL vortex evolves over N-Italy and N-Adriatic Sea during the day. This keeps better LL moisture in place during the daytime hours, as mid-levels cool down constantly. Therefore, a prolonged period with enhanced convection is forecast during the daytime hours, before the LL depression and attendant cold front departs to the east/southeast with drier postfrontal conditions. The main question will be, how much those storms profit by better DLS. An isolated large hail event is possible, but the overall risk seems to be too marginal for a broadly brushed level . However, those set-ups have to be monitored for better CAPE build-up over N-Italy compared to what current model data indicate.
Otherwise, the thunderstorm risk over the Adriatic Sea increases from north to south during the day and also the severe risk, as DLS increases to 25m/s. A level 1 for the E-Adriatic Sea was issued for large hail and isolated tornadoes. An isolated tornado event is also forecast over parts of W/SW Slovenia and W-Croatia. Storms over the Adriatic Sea may last well into the night hours.
... Extreme NW-France and the W-English Channel ...
A weakening depression enters the highlighted area after sunset with isolated thunderstorms possible. Strong to severe wind gusts are possible until 06Z, but no level 1 was yet introduced.
... Portugal 21 UTC onwards...
The main attention has a developing depression off the W-coast of Morocco, approaching Portugal from the SW during the night. A warm front consolidates over S-Portugal after sunset, moving to the north and it will be placed somewhere over N-Portugal at 06 UTC. The warm sector air mass is unusually moist but the mid-levels remain warm, so CAPE will be the main issue. Latest GFS runs came up with a swath of some elevated CAPE along the southern fringe of the northward pushing warm front and we don't want to exclude an isolated thunderstorm. Shear will be impressive to say at least, but confidence in anything more than an isolated thunderstorm is still too low for any level/thunderstorm probabilites. If storms evolve, large hail would be the main hazard. We will monitor the developing depression for required updates.
