Mesoscale Discussion
Valid: Wed 30 Jun 2021 09:00 to Wed 30 Jun 2021 13:00 UTC
Issued: Wed 30 Jun 2021 08:36
Forecaster: TUSCHY

MESOSCALE DISCUSSION:

The 12Z guidance places an upper low over Rhineland Palatinate with a short wave now crossing E-Germany into W-Poland while evolving into a sharp upper trough.
NWP guidance converged in the past few runs and now indicates a structuring shallow warm core low along the German/Polish border with a drift to extreme NE Germany/NW Poland. In fact, latest surface observations support this idea with a weak vortex roughly 110km ENE of Berlin. Pressure tendencies support a drift towards the Pomeranian Bay or far E Mecklenburg Western Pomerania.

In the past few runs of various models this shallow warm core low improved its structure with a constantly better defined low-to mid-level circulation, yielding 30-40kn in the 700-850 hPa layer and a surface pressure drop down to 1001-1003 hPa until 00Z. During this time, the cross-track spread is around 50-100 km with GFS along the W and IFS along the E fringe. No surprise due to the still ill defined center of the vortex with numerous smaller scale circulations still seen to its N.

With the approach of this low beneath strong upper divergence, a surface trough/convergence zone consolidates in the MD and is already seen in latest surface data with a placement just E of Hamburg. Environmental conditions along this convergence zone will be favorable for widespread slow moving thunderstorms as SBCAPE approaches 800 J/kg with long and skinny CAPE profiles. TPWATs run in the 30-35 mm range with sub-10 m/s shear at all levels. In addition, good 500-300 hPa ventilation to the SW/W is forecast with a speed max over N-CNTRL to NW Germany. A persistent and mostly undisturbed inflow of very moist and unstable air is also expected with forecast soundings showing enlarged 0-3 km hodographs along this convergence zone.
Expect heavy to extreme rainfall amounts with flooding and a non-zero chance of a short-lived/isolated tornado event. Ensemble data is clustered with probabilities of locally up to 40% or 50% for more than 40 l/m in 60min. Local amounts of 100-150 l/qm in 2-3h are possible with training convection. The main time-frame will be 12-18Z with a weakening trend afterwards. However, further CI is possible during the night with an ongoing heavy rainfall threat.

This risk of excessive rain extends just to the N of Berlin, but detoriating inflow with developing convection downstream over W Poland should lower the very heavy rainfall risk during the afternoon hours. In addition, the currently ongoing cluster adds more uncertanties but improving LL convergence with the deepening LL vortex to the E/NE should support more focused LL convergence and bands of deep moist convection (probably evolving out of this cluster).

Another round of very heavy rainfall may evolve over NE Germany during the night with the approaching surface low, but uncertainties with its geometry and final track preclude any detailed discussions for now. However another round of training convection is possible over Mecklenburg Western Pomerania during the night.

Note: The duration of this MD does not reflect the duration of the severe event, which continues well into the evening hours. This MD is just a nowcast update.
Also, we did no include the potential second round of convection for far NE Germany in this MD.