Storm Forecast
Valid: Thu 18 May 2017 06:00 to Fri 19 May 2017 06:00 UTC
Issued: Wed 17 May 2017 23:35
Forecaster: GATZEN

A level 1 was issued for parts of central Germany mainly for excessive precipitation.

A level 1 was issued for northern central France mainly for severe wind gusts.

A level 1 was issued for eastern Spain mainly for large hail.

A level 1 was issued for Turkey mainly for large hail.

SYNOPSIS

The central European ridge slowly weakens as troughs enter western Europe. However, larger-scale flow pattern is not progressive and main features, a polar cut-off across the north-eastern Ukraine / south-western Russia, the ridge axis, and another polar trough across north-western Europe will hardly move east. However, a strong vort-maxima rotating around the base of the west European trough will cross Spain and parts of France during the period. Ahead of this impulse cyclogenesis is expected across France. Furthermore, a weak trough over the Aegean Sea will slowly move east as well.

At lower levels, a plume of warm air has entered France and Germany and will continue to spread north into the Baltic Sea and southern Sweden with strong low-level winds. To the west, a cold front crosses large portions of Spain and France during the day. Cold air advection is also present in the Black Sea region and parts of the Balkans.

DISCUSSION

Central France into Germany

A cold front is expected from northern Germany into eastern France on Thursday morning. It will only slowly move east over northern Germany whereas flow backs across France in response to the approaching vort-max further south and associated cyclogenesis. Given limited lapse rates in the warm air mass, CAPE builds especially across central and eastern Germany where diurnal heating will be strongest and low-level air is also rather moist. Main initiation is expected along the frontal boundary over eastern France / western Germany that spreads north-east with the main flow. Storms with sideward propagation and movement along the cold front will likely merge to clusters of storms. Locally excessive precipitation and severe wind gusts cannot be ruled out especially when storms move into the warmer air masses across northern central Germany, but overall threat is limited, especially since vertical wind shear is weak and storm organization is low.

Additional storms can form over southern Germany during the day along local convergence lines / mountains. Again, weak vertical wind shear limits storm organization. Clusters are most dominant along with isolated cells. Pulse storms may produce locally large hail and excessive precipitation. Activity will slowly spread east while cold pools will lead to decay of storms in the western portions later on.

Over central / eastern France, warm air advection sets in during the day ahead of the approaching strong vort-max. Some remaining moisture can be expected in the wake of the cold front that increases once warm air advection has started. Main limiting factor for CAPE evolution is the lack of mid-level lapse rates and uncertainty of low-level diurnal heating as many clouds can be expected along the frontal zone.

However, if larger periods with sunshine can cause CAPE ahead of the new frontogenesis across western France, thunderstorms could initiate over central France. With stronger vertical wind shear especially at low levels, supercells cannot be ruled out, capable of producing tornadoes. Overall chances are low, but the situation has to be monitored closely. Furthermore, given strong low-level convergence and rather high moisture content, clusters of storms that organize into bow echoes pose a threat of severe wind gusts. These clusters are expected to move northward rather quickly before cold air advection sets in the from the west, limiting any new initiation.

Eastern Spain

Cold air spreads into Spain from the west. Strong DCVA sets in during the day ahead to the approaching trough axis. Together with diurnal heating, lapse rates are expected to increase over eastern Spain. At the same time, some backing of the low-level flow can advect moisture back onshore, especially over north-eastern Spain. Models suggest some weak CAPE in the afternoon. With lift along the front as well as over the mountain slopes, local initiation cannot be ruled out although CIN is rather large due to latest model output. If storms manage to form, strong vertical wind shear can allow for well-developed multicells or even supercells capable of producing large hail. Excessive precipitation is also possible in north-eastern Spain where strong forcing and rich moisture are situated.

Turkey

Latest Izmir sounding indicates steep low-to mid-level lapse rates and some CAPE. Expect that this air mass will remain across Turkey allowing for another round of storms. Unorganized clusters are most likely given weak vertical wind shear. Large hail is the main threat especially with pulse storms. Convective activity will likely decay after sunset.

Ireland

A sharp trough axis spreads across Ireland on Thursday. CAPE will build in response of steepening lapse rates and rather moist low-level air mass. Scattered showers and thunderstorms are forecast. Weak vertical wind shear will limit storm organization, but tornadoes are not ruled out along sea breeze convergence zones.