Storm Forecast
Valid: Fri 19 Apr 2013 06:00 to Sat 20 Apr 2013 06:00 UTC
Issued: Fri 19 Apr 2013 07:28
Forecaster: PISTOTNIK

A level 1 was issued for Southern Switzerland (Ticino) and Northwestern Italy (Piedmont and Lombardia) for excessive convective precipitation and large hail.

SYNOPSIS

Embedded in a meandering zonal flow, a positively tilted long-wave trough extends from Scandinavia to Western Europe and slowly shifts eastward. A progressive shorter wave overruns the trough axis during the forecast period, which results in a deepening of the trough and later a cut-off of a separate upper-level low over France. High pressure starts to build up from the West in its wake.
Ahead of the trough, a long and pronounced cold front moves southeastward into Central Europe, Southern France and Catalonia. A jet streak runs from France and Germany to the Baltic states and further to Western Russia, but it only overlaps with stably stratified areas in the range of the front. Both prefrontal and postfrontal areas of (limited) instability are placed under mostly weak shear.
Further downstream, a narrowing upper-level ridge with warm and dry conditions extends from the Central Mediterranean to the Ukraine. Finally, Southeastern Europe is still affected by a quasi-barotropic upper-level low with cool and unsettled weather, including isolated to scattered thunderstorms in a weakly sheared environment over Turkey and over the Ionian Sea.

DISCUSSION

... Central Europe, Southern Alpine region ...

The advancing cold front over Central Europe provides the main focus for possible thunderstorm development. Frontal convergence of the low-level wind field results in a "piling" of boundary layer moisture and limits its exposure to daytime mixing processes. Consequently, a belt of 7 to 9 g/kg low-level mixing ratios was observed along the cold front over Germany and France on Thursday and is expected to be in place again on Friday, flanked by otherwise seasonally low moisture ahead of and behind the cold front. However, poor mid-level lapse rates will keep the resulting CAPE marginal even along the moisture tongue. Another concern is that the cold front is outrunning the trough and is already placed far ahead of noteworthy QG lift and vertical wind shear. Having said that, there are strong indications that the cold front will decelerate and attain an inactive "ana front" character over Poland, the Czech Republic and Slovakia with a shallow wind shift line first and only little stratiform precipitation afterwards. Chances for deep convection are slim, though an elevated thunderstorm can't be ruled out with the rearward ascent of warm air along the frontal plane.
Further Southwest, blocking of the low-level cold front by the Alpine chain improves the conditions for deep convection somewhat, though they are not as good as they could be in a setup like this. First, due to extensive cloudiness from the beginning of the day the forecasted instability build-up will have to rely more on upper-level cooling than on strong insolation, keeping CAPE magnitude limited to probably below 500 J/kg. Second, also in these regions better kinematics and dynamics do not arrive before late in the day, which makes the overlap with patches of instability quite small.
Early in the forecast period, debris from overnight convection bring rain and possibly a few embedded (elevated) thunderstorms over Eastern Austria, which will decay while moving into Slovakia. The next round of scattered convection is expected to initiate from the late morning hours onwards over the French Maritimes Alps and the Alpine foothills of Northern Italy and Slovenia. Weak vertical wind shear will keep storm organization low and severe weather unlikely for most of the day.
However, the approach of the French upper-level low finally brings stronger QG lift towards evening, accompanied by a pronounced cyclogenesis over Northwestern Italy. Also, shear conditions will improve with an increase of Southwesterlies at upper levels and the evolution of a moist Southeasterly inflow at lower levels ahead of the forming low. By 18 UTC, deep-layer shear may increase to 20 m/s along the Western rim of the unstable area, while the cold front finally crosses the Alpine chain. Storms will likely become more numerous and better organized with a bimodal severe weather threat: The sustained moist inflow may result in repeated rounds of thunderstorms with excessive convective precipitation in Ticino (Switzerland) and Northern Lombardia (Italy) in the 15 to 21 UTC time frime. Simultaneously, one or two strong multicells or even supercells with a hail threat are possible with "tail-end" storms further South over Piedmont and Southern Lombardia (Italy), which can draw from a larger moisture reservoir. A low-end level 1 was issued to cover these threats.
Thunderstorms will become increasingly elevated while they move northeastward into Austria, where forecast models largely agree on a possibility of still heavy (mainly stratiform) precipitation till the night hours. An outflow boundary will spread into Hungary, Slovenia and Croatia overnight but will remain inactive due to nocturnal boundary layer cooling and decreasing moisture further away from the front.

... Belgium, Northern France ...

In the range of the long-wave trough, postfrontal showers and low-topped thunderstorms are expected to form in response to diurnal heating of the maritime air mass. Weak vertical wind shear, low cloud bases and enhanced background vorticity with a convergence line point to a small risk of "cold air funnels" and an isolated landspout.

... Western Mediterranean ...

The Western Mediterranean region will see a quiescent pattern. Towards the Southwest, a maritime boundary layer (partly capped by Stratocumulus fields) gets increasingly overspread by steep lapse rates from the Sahara. Low-level moisture accumulation is slow at this time of the year, but still patches of instability are forecast to form over Morocco, Southern Spain and along the Apennin mountains in Italy with the ingestion of some maritime moisture into onshore and upslope flow. Confidence of convective initiation is rather low, but isolated cap breakers are possible in the afternoon hours.
QG lift support stays distant to the North and deep-layer shear remains below 15 m/s. Hence, severe weather is unlikely, though an isolated hailfall can't be ruled out.