Storm Forecast
Valid: Tue 15 Aug 2023 06:00 to Wed 16 Aug 2023 06:00 UTC
Issued: Mon 14 Aug 2023 22:24
Forecaster: PUCIK/KUZMENKO
A level 2 was issued across central/NE Germany and NW Poland mainly for large to very large hail, severe wind gusts and heavy rainfall.
A level 1 was issued across Denmark and S Sweden mainly for severe wind gusts and to the lesser degree for heavy rain.
A level 1 was issued across W Latvia and Lithuania mainly for large hail and tornadoes.
A level 1 was issued across N Spain and central/E France mainly for large hail and severe wind gusts.
A level 1 was issued across Alps mainly for heavy rainfall and to the lesser degree for large hail and severe wind gusts.
A level 1 was issued across S Germany and W Czechia mainly for heavy rainfall and large hail.
SYNOPSIS
A broad belt of mid to upper tropospheric southwesterly flow covers large part of Europe from Iberia through France into Germany, Scandinavia and the Baltics. No major change in the synoptic-scale pattern is forecast on Tuesday with exception being a slow filling of a low residing over SE Europe. Closer to the surface, a wavy cold front will remain across France and Germany. A warm front is forecast to lift across Latvia and Estonia in the morning hours towards Finland. A belt of rich low-level moisture is associated with the frontal boundaries with dewpoint observations showing values up to 20 deg C.
DISCUSSION
... Central Germany to NW Poland ...
Models consistently simulate a belt of MLCAPE between 1000 and 2000 J/kg across the area, combined with 0-3 and 0-6 km bulk shear between 15 and 20 m/s, straight hodographs and rather low LCLs. CAPE will increase from W to E. The corridor of the highest risk may be influenced by early morning convection, which may lay outflow boundaries. Besides these, local orography and the cold front will serve as triggering mechanisms. High-resolution, convection-allowing models agree on an initial development of some supercells, capable of large to perhaps even very large hail. Later on, upscale growth is forecast, yielding one or two convective systems that will move NE with threat of severe wind gusts and heavy rainfall. The wind gust risk will be mitigated to some degree by low LCLs and will increase towards E, where deeper boundary layer will be present.
... France ...
Numerous storms will be present over the area already in the morning. These storms may pose a risk of heavy rainfall or large hail. Further development is forecast especially over Massif Central in the afternoon hours. Isolated to scattered multicells and/or brief supercells are forecast with 0-6 km bulk shear around 15 m/s with threats of large hail and isolated severe wind gusts.
... Alps ...
Scattered to widespread storms are forecast to initiate over the Alps during the day. Mean wind close to 0 will guarantee a number of stationary storms capable of localized very heavy rainfall. The storms will spread over S/SE Germany during the late afternoon and evening hours towards higher CAPE, with increasing risk of isolated large hail and severe wind gusts, besides localised flash flooding.
... N Spain ...
A highly questionable situation is forecast across N Spain. Some models simulate a chance of convective initiation in environment supportive of large hail. While Lvl 1 has been issued, confidence in whether any storms will form at all is low.
... Denmark and S Sweden towards Finland ...
Scattered to widespread storms are forecast to form along the frontal boundaries. Over Denmark and S Sweden, surface-based storms are forecast in the environment of straight hodographs and 0-6 km bulk shear around 20 m/s. Some well-organised linear segments may produce isolated severe wind gusts. Storms will rapidly spread NE and will move towards Finland, supported by a strong warm air advection regime, causing isentropic lift. Abundant MUCAPE will be present, but the effective shear above the stable boundary layer will be quite low, reducing the severe weather threat. Nevertheless, very isolated large hail or heavy rainfall events can't be ruled out.
... Lithuania, Latvia ...
Models consistently show around 1000 J/kg of MLCAPE over the area combined with curved hodographs and 10 to 15 m/s of 0-6 km bulk shear and even around 10 m/s of shear in 0-1 km layer. This suggests a non-zero threat of severe weather given that storms initiate, including a risk of tornadoes. However, majority of models also don't simulate convective initiation. For this reason, decided to stay with a low prob of lightning line over most of the area, but also introduced a Lvl 1. The area needs to be monitored for convective initiation in the afternoon hours. While it currently seems that the storms are not very likely, if they form, they will pose a risk of tornadoes and large hail.
