Storm Forecast
Valid: Wed 17 Jul 2024 06:00 to Thu 18 Jul 2024 06:00 UTC
Issued: Tue 16 Jul 2024 09:28
Forecaster: TUSCHY
A level 2 was issued across parts of S Austria mainly for severe gusts and large hail.
A level 2 was issued across the W-Ukraine and surroundings mainly for heavy rain, large hail and severe gusts.
A level 1 surrounds both level 2 areas with similar hazards but lower probabilities.
A level 1 was issued across parts of Sweden mainly for heavy rain.
A level 1 was issued for parts of Turkey mainly for hail and heavy rain.
SYNOPSIS
The main driver for today's forecast is once again extensive blocking over many parts of Europe. The long-lasting subtropical ridge over S/SE Europe, which continues to deliver a prolonged heatwave with record breaking peaks, remains in place, as well as positive geopotential height anomalies over the Greenland into the Barent Sea.
The dominant low pressure area over NW Europe shifts temporarily a bit W and assists in building heights also over CNTRL Europe. A residual trough gets framed by rising heights and shifts slowly N into Sweden/SW Finland.
Its trailing cold front is forecast to run from the Alps into Belarus and further N into SW Finland (changing to an occlusion). An extensive prefrontal convergence zone is forecast to evolve within the broad warm sector and this area serves as the main focus for DMC activity (next to the cold front itself).
DISCUSSION
... Austria ...
The quasi-stationary cold front nestles up along the Alpine rim and squeezes out enough moisture for modest instability build-up along the inner alpine regions (roughly 1000 J/kg MUCAPE), increasing to 1500-2000 J/kg MUCAPE into S/SE Austria, where deeper BL moisture persists.
A modest westerly flow overspreads that area, as an upper trough passes E with 500 hPa flow hovering around 30-40kt and pushes DLS in the 15-20 m/s range with lower values in the 1 and 3km layers (although Alpine pumping may regionally enhance those values). Forecast hodographs indicate a favorable shear distribution for a few organized multicells/supercells, especially when maturing storms start to deviate.
Expect agile CI along the mountains around noon with growing convection moving E/SE betimes. One or two longer tracked supercells are possible into Carinthia and Styria with large hail and severe gusts. As cells move into Slovenia/far NE Italy during the evening, they start to outrun more favorable conditions with a gradual weakening trend forecast.
A few more orgnized cells are also possible into Lower Austria with isolated hail/heavy rain issues before rapid weakening occurs beyond sunset.
... W Ukraine into Romania and E Hungary into the Balkans...
Both the cold front itself but also a prefrontal convergence zone cause a more robust BL moisture depth, which lower the risk of diurnal mixing, keeping MLCAPE in the 2000-3000 J/kg range from E Belarus into W Ukraine and E Hungary and further S into Serbia/Bosnia and Herzegovina.
Overall weak background forcing is forecast with ongoing capping issues especially in the Balkan area. A few mountain storms are still possible, which could become temporarily severe with that amount of CAPE, producing large hail and a few severe downbursts next to heavy rain. Nearly non-existent shear values keep convection rather disorganized and short lived although a temporarily better organized cluster is possible in case of cold pool growth.
Further NE, the cold front and the convergence zone support scattered CI (next to the orography). DLS of 5-10 m/s (combined with that amount of CAPE) assists in a few strong/severe storms with large hail and heavy rain, including a few severe downburst events. However the convection should stay mostly disorganized with only a few locally/temporarily better organized cold pool driven small clusters.
A confined level 2 upgrade was performed over far W Ukraine into E-Slovakia, where NWP guidance indicates a growing cluster of storms. CAPE magnitude and high moisture content in the troposphere support a larger scale cold pool event with swaths of strong to severe gusts, large hail and heavy rain. This activity weakens during the night while moving into SE Belarus and/or CNTRL Ukraine.
... Belarus N into W Russia ...
A similar CAPE/shear space results in a few strong storms with heavy rain/hail and strong gusts the main hazard.
... S Finland into SE Sweden ...
We extended the level 1 far W into the N ward pushing vortex, which lifts N into the Gavleborg County before stalling and drifting E during the night.
The main issue becomes heavy to excessive rain just N/NW of the vortex' center with some models indicating 24h QPFs in excess of 80-100 l/qm. This area is also highlighted by the EFI precipitation with anomalous high values. The main issue for not upgrading to a convectively enhanced heavy precipitation event is ongoing uncertainty with how far N the vortex drifts and how its geomtery looks like, causing issues with the placement of the QPF maxima.
Most of the activity stays elevated despite over SW Finland, where N/NE ward moving convection poses a low-end tornado risk with better LL directional shear and neutral/slightly unstable LL conditions.
... Turkey ...
A broad and weakening upper trough over Turkey causes NE-erly onshore flow from the Black Sea and inland moving sea breeze fronts around the Gulf of Mersin. Ongoing marine heatwave (SST anomalies in excess of 3K) cause dewpoints in the lower twenties which sneak beneath steeper mid-level lapse rates more inland. MUCAPE along the coast resides in the 2000-3000 J/kg range with somewhat lower values more inland (e.g N Turkey into W Georgia). Very weak shear in all levels brings a risk of heavy rain and large amounts of small hail (although some top-down weighted CAPE profiles support some large hail events as well).
