Storm Forecast
Valid: Fri 03 Jun 2022 06:00 to Sat 04 Jun 2022 06:00 UTC
Issued: Thu 02 Jun 2022 11:52
Forecaster: TUSCHY
Numerous level 2 areas were issued across parts of France, Germany and Austria mainly for large hail and severe wind gusts.
A level 1 surrounds all level 2 areas mainly for similar hazards but with lower coverage. In addition, an isolated tornado event is possible over NW France and excessive rain is forecast over CNTRL Germany.
A level 1 was issued for W-Turkey mainly for hail/severe wind gusts and excessive rain.
A level 1 was issued for parts of SE/E Europe mainly for heavy rain, hail and a few strong/severe wind gusts.
SYNOPSIS
High-latitude blocking (going hand in hand with an AO way below zero) causes slow Rossby-wave propagation at least for the trough over NW/W Europe. Pattern recognition ensures an active round of convection over W/CNTRL Europe with this geopotential height configuration.
Downstream ridge over the W/CNTRL Mediterranean merges with a strong ridge over Near East with not much change of the geopotential height geometry troughout the forecast period.
Deep layer flow turns out to be a bit more progressive over NE Europe as numerous low-amplitude waves circle a decaying upper trough, centered over Scandinavia. This vortex is present all the way down to the surface and attendant broad LL vortex pushes a cold front E into Russia. However its southern part seems to stall over SE Europe as this synoptic-scale front moves into the ridge. It bends NW towards the Alps/S-Germany and connects to a wavy (meridionally aligned) boundary over France into Spain. This extensive boundary separates a warm/hot and unstable air mass to its south from a less unstable one north of the front.
DISCUSSION
An extensive area over W/CNTRL Europe will see an ingredient overlap, supportive for a mixed mode of multicells/isolated supercells within the level 1 area. Orography and cell interactions dictate the mesoscale risk for a regionaly more enhanced severe risk. In the following lines, these regions will be highlighted.
... France ...
A leading low-amplitude wave exits the base of the trough and crosses SE France before noon. Initially elevated 400-800 J/kg MUCAPE and 15-20 m/s (non effective) DLS favor an isolated large hail and heavy rain risk mainly over W/SW France. Departing wave with a temporal reinforced ridge including subsidence should lower the thunderstorm and severe risk until noon. The geopotential height gradient including DLS relax somewhat, before increasing again during the afternoon hours from the S. So overall a lull in activity is forecast around noon.
Beyond noon, rapid clearing behind the leading wave assists in adequate diabatic heating for surface based CI. Although CI is possible anywhere in the highlighted area, the highest probabilistic risk exists over NW France (interacting slowly lifting warm front with another weak wave) and (orographically induced) from SW France towards CNTRL France.
Area-averaged forecast soundings over NW France offer up to 1kJ/kg MUCAPE with steep LL lapse rates and some dry air at 700-600 hPa so initiating storms pose a large hail and severe wind gust threat. As storm ride along the warm front and with a slow increase of LL shear during the late afternoon/evening due to a decoupling BL, an isolated tornado event is possible.
With signals of good cold pool developments, upscale growth into an organized cluster with severe wind gusts, large hail and heavy rain is possible, probably affecting the area of Paris and areas further E well into the night. This scenario is fostered by increasing 3km/6km shear during the evening hours. Gradually diminishing CAPE will lower the severe risk towards Luxembourg. Due to the expected high storm coverage, a level 2 was added.
A second level 2 was added for SW France, where beyond noon, scattered cells evolve - probably more discrete with weak background lift. Area-averaged soundings for SW France show steep mid-level and low-level lapse rates and up to 1300 J/kg MLCAPE (with regional MUCAPE peaks in excess of 2kJ/kg). DLS of 20 m/s and 0-3km shear around 15 m/s is enough for long-lived multicells and a few supercells with large to very large hail, severe wind gusts and bursts of heavy rain. Dependant on the coverage of storms, a temporal line-up is possible, which would support swaths of severe wind gusts including large hail. The coverage and severe risk diminishes gradually during the overnight hours.
... S Germany and E Austria...
A bimodal risk event is forecast over this area. Slow moving convection, probably anchoring along the warm front/orography over W-CNTRL Germany, evolves around noon in a moderate DLS environment (10-15 m/s) with very weak 0-4km shear. Enhanced high-level shear should help to evacuate updrafts from precipitation, so many ingredients point to slow moving and longer-lived storms with excessive rain. Main caveat will be well mixed/dry/rather deep subcloud layer, which could support stronger cold pools and moving cells. Nevertheless, excessive rain on a local scale is forecast where numerous storms cross the same area/merge or interact with the orography. We kept this event in a level 1 due to expected coarser coverage of excessive rainfall events and the unpredictable impact on the mesoscale.
Over S Germany, DLS and 0-3km shear increase during the afternoon and a mix of multicells/ a few supercells is forecast. Severe wind gusts and (wind driven) large hail is possible and a confined level 2 was added, where current risk seems to be the highest for a longer tracked supercell event.
The level 2 was connected with another 2 over E Austria. DLS is borderline with 15 m/s but enhanced BL moisture along the Alpine Rim pushes MUCAPE in excess of 2kJ/kg over parts of E-CNTRL Austria. With CAMs indicating a few long-lasting and organized storms, an upgrade was performed despite the overall borderline setup.
In addition the level 2 was expanded towards SW Germany and Jura mountains, as some CAMS indicate a chance for a nocturnal cold pool driven MCS event, emerging from E France. Not sure about how fast this line decouples from the BL so expect a risk of severe wind gusts and hail with such an event.
... W-Russia to SE Europe ...
A broad area with up to 1kJ/kg MUCAPE exists ahead of the eastbound moving cold front. Near front-parallel alignement of the shear lowers the overall severe risk next to weak CIN and widespread/early CI. A broad level 1 was issued for a few severe wind gusts, large hail and heavy rainfall events.
We expanded the level 1 to Romania for scattered convection along the orography. Weak shear and moderate CAPE assist in a rainfall/isolated hail threat.
...W-Turkey...
A level 1 was issued for slow moving thunderstorms with excessive rain in the western part of the level 1 (beneath an upper trough) and large hail/severe wind gusts within its eastern part due to increasing DLS.
