Storm Forecast
Valid: Sun 21 Jun 2020 06:00 to Mon 22 Jun 2020 06:00 UTC
Issued: Sat 20 Jun 2020 20:25
Forecaster: TUSCHY
A level 2 was issued across SW-Sweden for excessive rain.
A level 1 was issued for parts of E/SE Europe mainly for excessive rain, hail and isolated severe wind gusts.
SYNOPSIS
Ongoing split-flow / high-over-low regime over N/CNTRL/E Europe diverts the polar front jet to N-Scandinavia and S-Italy. Both synoptic features maintain a broad fetch with easterly winds over E-Europe where moisture advection continues with ML mixing ratios in excess of 12 g/kg over a broad place (Poland to the Black Sea).
Ridging over SW-Europe builds in and strengthens during the night while expanding towards France.
Quasi-stationary or slow moving warm/cold fronts frame the moist air mass over E-Europe with no substantial motion forecast until 06Z (excluding stronger displacmenet with convetive modulation).
DISCUSSION
...E/SE Europe...
Despite weak mid-level lapse rates due to ongoing diurnal/extensive convective activity, rich BL moisture pushes MUCAPE in the 1-2 kJ/kg range with higher peaks over NE Poland, W Belarus and NW Ukraine, where even MLCAPE exceeds 1.5 kJ/kg. Soundings from yesterday confirm those values and also highlight the anomalous moist troposphere with TPWs in the mid to upper 30 mm range.
DLS remains very weak but 0-3 km flow looks a bit better with slightly curved hodographs mainly along the synoptic-scale fronts, so temporal updraft organization is possible. Main risk will be heavy to excessive rain with a deep warm column in the lowest 3.5 km AGL. However this amount of CAPE also adds an isolated large hail risk with initiating/more discrete storms. Betimes, clustering occurs with rain/hail threat ongoing but also with a micro to mesoscale cold pool driven wind threat. Confidence increases in a larger convective cluster over CNTRL/S Poland to Moldovo within a slow moving confluence zone.
A broad level 1 area covers the general threat. For sure there will be spotty extreme/level 2 events driven by the mesoscale within the broad level 1, but substantial uncertainties/nowcasting were there reason why no level 2 areas were added.
The activity continues well into the night and probably all night long along the confluence axis over SE Poland to Moldova.
... CNTRL / E Mediterranean ...
Placed along the S fringes of an extensive upper low somewhat better CAPE/shear overlap exists with roughly 10-15 m/s DLS and 800-1500 J/kg MLCAPE. Regional synoptic-scale lift (short waves) and the orography add enough forcing for scattered CI. Heavy rain and isolated rain/severe downdrafts occur. Local level-1 conditions can occur but overall the risk/ingredient overlap does no justify an upgrade for now.
... S-Norway/SW-Sweden...
Along a deep confluent channel, which extends from the S Baltic Sea to S Norway, rich BL moisture with ML mixing ratios in excess of 10 g/kg and weak mid-level lapse rates create 500-1000 J/kg MUCAPE. Enhanced synoptic forcing ahead of an approaching through and the mentioned confluent flow regime create a favorable environment for repeated shower/thunderstorm development with rapidly clustering convection. Overall DLS is very weak, but forecast soundings show a kink in the lowest 2 km AGL, which indicates a chance for effective moisture advection into the convection and augmented temporal backbuilding probabilities.
Global models show a broad swath with 24h QPFs in the 40-60 l/qm range, but with expected backbuilding convection, substantially higher peaks on the local scale are well possible (probably in excess of 100 l/qm/24h). A broad level 1 was added. We deciced to upgrade to a level 2 for a small area, where latest data inidcates a high chance for flooding caused by convective activity.
... UK and Scotland...
Weak shear in the 1-6 km layer and 400-800 J/kg MLCAPE support scattered pulsating thunderstorms with bursts of heavy rain and graupel/small hail. Don't want to rule out an isolated tornado event with low LCLs and somehwat better SRH1 (50-100 m^2/s^2) but overall risk remains too marginal for any level issuance.
