Storm Forecast
Valid: Sun 10 May 2020 06:00 to Mon 11 May 2020 06:00 UTC
Issued: Sat 09 May 2020 18:52
Forecaster: BEYER
A level 1 was issued for Corsica and Sardinia mainly due to large hail and locally excessive precipitation.
A level 2 was issued for parts of Southwestern France mainly due to excessive precipitation.
A level 1 was issued for Northern France into Central Germany mainly due to excessive precipitation.
A level 1 was issued for the Czech Republic and Northern Austria mainly due to locally large hail and severe wind gusts and to a lesser extent due to excessive precipitation.
A level 1 was issued for Northeastern Algeria and Northern Tunisia mainly for severe wind gusts and large hail.
SYNOPSIS
A trough can be found over the Iberian Peninsula. The Western Mediterranean and parts of Western Europe are situated on its diffluent forward flank. With a southwesterly flow, warm and humid airmasses are advected in those regions.
During the day a low-pressure center forms over Southern France. The related cold front crosses the Western Mediterranean eastward during the day.
A pronounced and active trough can be found over Nothern Europe. Its strongly amplified axis is traveling from Northwest to Southeast throughout the forecast period. Airmasses of polar origin will flood North and Northwestern Europe.
With the progressing trough from the North and the diffluent forward flank of the upper level low over the Iberian Peninsula, the air mass differences over central Europe will increase. North of the french surface low a zonal oriented low-pressure trough will reach from Northern France into Central Germany including a low-level convergence zone.
Another upper level low can be found over Western Russia leading to some convective activity and rather cool airmasses from Eastern Ukraine into Russia.
DISCUSSION
... Western Mediterranean ...
This region is situated on the forward flank oft he upper-level trough and thus under good lift conditions. Humid airmasses are present and the values of precipitable water reach 40 mm. This low-level moisture can overlap with some instability. Lapse Rates are steeper to the south due to an elevated mixed layer advected from Northern Africa. This EML is mostly capped by a stable maritime boundary layer.
However, over Sardinia and Corsica the orography can help to overcome this capping and initiate a few storms.
Strong DLS with values of locally more than 25 m/s is present. Large hail should be the main threat. However a strong to severe wind gust cannot be rules out.
The cold front it self will produce a linear oriented convective rain band. Model give hints that this line may become active during the afternoon and evening. Especially to the south. There mid-level wind vectors are forecasted to be rather perpendicular to the front compared to the regions further north. Thus strong wind gust are possible with the passage of the front.
During the night eastward progressing convective activity will become non-severe since it runs out of the strongest forcing (from the trough).
... Southwestern France ...
Quite humid airmasses are present in that region while instability is only marginal. Thus only a few hundred J/kg of CAPE can develop. The developing surface low north of the Pyrenees will increase low-level convergence and thus lift. Along that convergence zone storms can develop during the day and further cluster to a larger convective system.
As a result, longer-lasting and thundery excessive precipitation is forecast in that area during the afternoon and evening hours. It is fed by backbuilding storms from the southeast.
Having DLS values of about 15 m/s there is a certain risk for a few better-organized storms and large hail at the beginning of the development.
... Northern France into Central Germany ...
A zonal organized low pressure trough with increasing low-level convergence is forecasted in this region. Combined with enhanced low-level moisture and instability a few hundred J/kg of CAPE can develop. The highest values of up to 1000 J/kg can be found in Northeastern France.
Several storms will develop along this convergence zone. Since DLS is rather weak they should mainly stay unorganized. However, the can cluster and several storms can move over the same area as mid-level wind vectors indicate. They are oriented along the convergence zone. Also, storms are forecasted to be rather slow-moving, especially to the east of the LVL1 area with 500 hPa winds around 10 kn.
Altogether, the main threat will be excessive precipitation. In the eastern part of the LVL1 area, DLS is a bit higher. Marginal to locally large hail and strong wind gusts can thus not be ruled out.
... The Czech Republic and Northern Austria ...
A few hundred up to locally 1000 J/kg of CAPE are presented. In regions of enhanced low-level convergence are forecasted. Also, DLS shear will be around 15 m/s. In combination with the orography, developing storms may become better organized.
Thus large hail and severe wind gusts are the mains threats. The latter is supported by high LCLs and rather large spread in the lower troposphere, taking the forecast soundings.
... Northeastern Algeria and Northern Tunisia...
An elevated mixed layer can be found in the forecast sounding leading to good lapse ratest that can overlap with rather high values of low-level moisture. In addition, the area of interest is influenced by the diffluent forward flank of the upper-level low over Spain and the cold front will pass during the afternoon.
However, it is not sure if convection can indeed develop since an inversion in the lower troposphere leads to a rather high amount of convective inhibition. Nevertheless, there a few hints from LAM that a few storms may indeed develop. Forecast soundings show some areas where CIN is limited, mainly close to the cold front.
While CI is questionable, high amounts of DLS are present rising the threat for any developing storm to become severe and producing large hail as well as severe wind gusts. Only a LVL 1 was chosen since the probability of developing storms is rather low.
