Storm Forecast
Valid: Mon 13 Mar 2023 06:00 to Tue 14 Mar 2023 06:00 UTC
Issued: Mon 13 Mar 2023 01:46
Forecaster: BEYER

A level 2 was issued for parts of C and E France mainly for large hail, severe wind gusts and to a lesser extent for tornadoes and heavy precipitation.

A level 1 surrounds the level 2 area, including parts of Germany and Switzerland for the same hazards but with a lower probability.

A level 1 was issued for Cyprus and parts of Turkey and Syria mainly for heavy precipitation and locally large hail and to a lesser extent for waterspouts.

SYNOPSIS

A long wave trough situated over W Europe at the beginning of the outlook period is moving eastward while amplifying. Its main axis will be present over E France/W-Germany at the end of the forecast. On its forward flank, unusual and record warm airmasses will be advected into many parts of SW, W, and C Europe with a strong SW flow.

The main focus of interest will be the cold front situated just to the east of the sharp UL trough axis and accompanied by an UL jet maximum. Ahead of the cold front, moist and unstable airmasses are present, especially over C and E France.

An UL low is situated over the Gulf of Antalya and will be quasistationary during the outlook period. A moist and humid airmass is present over the E Mediterranean and will also affect the coastal regions. In addition, the region of interest is situated on the left exit region an UL jet maximum. That supports synoptic scale lift thanks to UL divergence.


DISCUSSION

... Parts of C and E France, (SW Germany and Switzerland) ...

A cold front situated over NW France in the morning hours will slowly move southeastward during the forecast period. Ahead of the front low-level moisture will increase during the day and can overlap with steep lapse rates. Instability is additionally increased due to Foehn winds north of the Pyrenees. Between cold front and Pyrennees, the best overlap of both ingredients is resulting in CAPE values of 1000 to 1500 J/kg. When approaching the Pyrenees these high CAPE values are capped due to dry airmasses in lower layers (Foehn).

In the region further north, marked by the level 2 area, CIN is low and first storms are forecasted to develop in the afternoon hours. These storms can benefit from strong DLS with values around and above 30 m/s. Not surprising, LAMs indicate the development of several rotating storms (supercells), as can be seen by simulated UH tracks and reflectivity. Those supercells are capable to produce large hail of up to 5 cm and severe wind gusts. The latter is supported by a significant spread in the lower troposphere (inverted-V), especially to the south of the LVL2 area. Enhanced DCAPE values illustrate the risk of dry downbursts.

Further north LCLs are lower. Since LL shear is also quite high (10 to 20 m/s) this supports the possibility of tornadoes. However, LL hodographs look less promising and most of the shear in that region is due to speed shear. Local variations due to orography may alter the low level wind field in a way that tornadoes cannot be ruled out.

During the late afternoon and night hours, the storms should organize along a line thanks to the linear forcing of the cold front. Locally heavy rain is possible especially when interacting with the local orography. Later, convective activity will decrease.

In the area surrounding the LVL 2 area, the overall threat of severe convection is lowered. This is mainly due to much less CAPE. However, DLS is also high in those regions. Thus, severe convection is probably not that frequent but could nevertheless still occur with similar accompaniments.

... Cyprus and parts of Turkey and Syria ...

On the forward flank of the UL low moist airmasses over the E Mediterranean can overlap with steep lapse rates. This results in CAPE values of up to around 500 J/kg. With a southerly LL flow this moist and unstable airmass is advected towards the coast of Turkey and the other E Mediterranean littoral. Synoptic scale lift is provided by the left exit region of the UL jet. In addition, low-level winds show convergence around the Turkish coastline.

First storms will be already active in the morning hours. Convective activity will further increase during the day. Convection can overlap with enhanced values of DLS (15 to locally 20 m/s). Thus, locally large hail is forecasted with isolated storms. Further in time storms will cluster and strengthen the probability of heavy rain. Convective activity will persist during the following night and can also extend further inland.

It should be added, that to the wester parts of the level 1 area and also further west (approaching the center of the UL low) the possibility that waterspouts can form, increases. This is mainly due to steep LL lapse rates and weak mid to upper-level winds and can occur along local LL convergence zones.