Storm Forecast
Valid: Mon 22 May 2023 06:00 to Tue 23 May 2023 06:00 UTC
Issued: Sun 21 May 2023 21:58
Forecaster: SCHLECK

A level 2 was issued across parts of southeastern Spain for extreme rainfall.

A level 1 was issued across parts of Algeria and Tunisia for hail and wind gusts.

A level 1 was issued across a large area of Central Europe for hail and wind gusts.

A level 1 was issued across Turkey around the surrounding countries to the east for hail and wind gusts.


DISCUSSION


Southeastern Spain: Level 2, and the surrounding level 1 area.

A cut-off upper low/vortex/cold pool is centred across the extreme southwest of the Iberian Peninsula with a fairly strong northern branch of the subtropical jet running west to east across northern Africa. As a jet streak runs through this portion of the jet, a marked left jet exit / development area lies across the Alboro Sea, with a surface low developing here.

This leads to strong and humid low level northeasterly flow impinging on the high topography of southeastern Spain, this results in the repeated triggering of deep convection beneath the overlying cold pool. Convection will be triggered through a mixture of frictional convergence, orographic uplift through the 24 hour period, and by day on Monday some diurnal heating over the land.

Profiles show fairly deep skinny MLCAPE reaching mostly of the order of 200-600J/Kg, within an environment of increasingly high tropospheric relative humidity, and the warm portion of the updraughts extending to around 3.5 KM in altitude. At low levels within the 600-1000 M deep Effective Inflow Layer there is a near 15ms northeasterly jet. A elevation the wind direction switches to a westerly with the subtropical jet towards the higher part of the updraughts. This allows Effective Bulk Wind Difference (EBWD) to exceed 10ms, with shear in the entire cloud layer exceeding 30ms.

All this favours very high precipitation efficiency of often multicell convection in the region, with such cells generally being steered towards the southwest at only 5-10ms, but at the same time the axis of instability lying to the northeast, favouring the propagation vector of the convection to opposing the advection vector, perhaps causing net slow system motion. There will often be a substantial stratiform element to the precipitation in this area too.

As such an exceptionally wet period is likely across southeastern Spain, with the entire level 1 area expected to see 25-50mm by 06:00 UTC on Tuesday, and parts of the level 2 area seeing 100-150mm, and isolated peak totals within the level 2 area perhaps exceeding 200mm.

Algeria and Tunisia: Level 1

This region lies beneath the west to east orientated northern branch of the subtropical jet. At low levels the flow is often east of southeasterly (due to the developing low in the Alboro Sea), with this flow drawing reasonably high dewpoints inland across the northern Atlas Mountains.

Strong diurnal heating over the Atlas Mountains will result in the development of high based deep convection on Monday afternoon, with the Lifting Condensation Level (LCL) often more than 3 KM amsl, providing a very deep Effective Inflow Layer. With the low level easterly flow, and strong westerly flow at upper EBWD exceeds 20ms in places, supporting supercell as a convective mode.

The high LCL plus modest low level shear will limit tornado potential, but large hail is probable (especially over mountains where melting is less of a factor), with the potential for isolated very large hail >5cm diameter. Inverted “V” type profiles will support strong convective downdraughts / wind gusts capable of lifting sand and dust.

Large area of Central Europe: Level 1

An upper ridge extends from the northern Bay of Biscay to Slovakia and edges slowly southwards during this time. At upper levels there is a reasonable westerly flow to the north of the ridge (Netherlands and northern Germany).

Active afternoon thunderstorms are likely to develop across much of this area by day, then decaying in the evening. In this region, fairly fat MLCAPE is abundant, reaching between 500-1000 J/Kg, with a 1-2 KM deep Effective Inflow Layer. However, vertical wind shear is often limited to 5-10ms. As such much of the convection is expected to be of a single cell mode. However, given the high CAPE updraught strength is likely to be sufficient to support some large hail >2cm diameter. It varies markedly across the region but some areas see inverted “V” type profiles suggesting an isolated strong wind gust remains probable.

Across the far north of this region in the extreme northeast of the Netherlands and northwestern Germany, there is a small region where the fairly fat and abundant MLCAPE, and reasonable vertical windshear overlaps. Here conditions are similar to further south, with the exception of the EBWD locally reaching 15ms. As such here there is the reasonable chance that some of the convection will be of a supercell nature, with straight hodographs favouring left and right splits.

In association with any such supercells 2-4cm diameter hail is possible, as well as strong wind gusts. Although limited wind shear in the lowest 500-1000M means that the risk of mesocyclonic tornadoes in this area remains very low.

Turkey around the surrounding countries to the east: Level 1

An broad upper low/vortex/cold pool lies from the Black Sea to Crete with a strong southwesterly subtropical jet ahead of this across this region. Deep energetic convection will be already underway by the morning in the eastern Mediterranean (just off the Turkish coastline), but will then develop widely over land area during the day. Into the evening convection will mostly decay, but will continue in association with a northeastward moving shortwave trough, and likely continue across the Gulf of Iskenderun due to anabatic flow into the horseshoe shaped bay overnight.

Wherever convection forms by day the EBWD will pretty much exceed 15ms, and be more than 20ms in some locations. Hodographs often show a cyclonic curvature in the lowest few KM which would favour right moving supercells. Large hail >2cm and strong wind gusts are supported. Although generally the tornado risk is low, where the LCLs are lowest close to the Mediterranean Coastline and complex flow exists around topography, there is a small chance a tornado could be seen.