Mesoscale Discussion

Mesoscale Discussion
Valid: Sat 09 Mar 2019 12:00 to Sat 09 Mar 2019 18:00 UTC
Issued: Sat 09 Mar 2019 09:54
Forecaster: TUSCHY

This Mesoscale Discussion was issued to highlight an augmented severe risk from the Netherlands, N-Germany to NW-Poland including the risk of severe to isolated extreme wind gusts and a few tornadoes - a strong event is possible.

Latest remote-sensing data places the advertised low-/mid-tropospheric wave currently (10Z) over UK in form of an expanding cloud shield and a sharpened WV gradient. In addition most models now converged with respect to timing and stength of that wave. Realtime surface data matches most of the model data/near-term forecasts, so confidence in a robust severe event has increased. Lingering uncertainty remains a cap, analyzed in latest AMDAR data at around 800 hpa over the Netherlands but incoming forcing and temporal diabatic heating should be strong enough for CI.

This wave is about to enter the S North Sea and is forecast to approach the Netherlands between 13 and 15Z (still a marginal time lag present in DMOs). It is forecast to impact NW and N-CNTRL Germany until 18Z and E-Germany/far NW Poland between 17-19Z onwards.

As this wave moves ashore it interacts with a moist and unstable air mass. Latest dewpoint readings show 5-8 C over Belgium and far W-Germany and this air mass advects into the open warm sector of this wave. This LL moisture beneath cold mid-levels yields widespread 200-400 J/kg SBCAPE with some high resolution data even supporting up to 500 J/kg. Strong forcing and nearly uncapped conditions result in scattered thunderstorms, which spread rapidly east during the forecast.

The CAPE plume overlaps with impressive shear values, including 20 m/s LL shear and SRH well in excess of 200-300 m^2/s^2. Forecast soundings show a deeply mixed BL up to 900/850 hPa and hence taps into 60 kn winds with elongated hodographs due to DLS in excess of 30 m/s. In addition, strongly looped hodographs are forecast especially beneath 700 hPa, which matches the area of maximized CAPE release. Hence well organized mesocyclones are well possible.

The main risk will be severe wind gusts, especially if forcing manages to cause small line-ups/bowing segments. A few extreme severe wind gust events are well possible.
Beside that risk, a tornado risk is present and with the amount of LL CAPE/shear and fast storm motion in excess of 20 m/s a strong and long-tracked event can't be excluded. This is especially true for the western part of the MD, where the greatest CAPE exists.
Strongest cores may also be accompanied by hail approaching/locally exceeding our severe criterion.

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