Mesoscale Discussion

Mesoscale Discussion
Valid: Sun 10 Jul 2016 14:00 to Mon 11 Jul 2016 00:00 UTC
Issued: Sun 10 Jul 2016 13:55
Forecaster: TUSCHY

DISCUSSION about thunderstorm probabilities for NW/N Germany and far S-Denmark.

TIMING covers the time-frame, where CI is possible. A slow increase in coverage is anticipated.

12 UTC surface hand analysis showes plume of better BL moisture from the Netherlands to far NW/N Germany and to the far W Baltic Sea. Surface dewpoints remain in excess of 16 C with highest values confined to a region from Eindhoven to Kiel with values of 18 C. 13 UTC analysis showed some lowering of sfc dewpoints due to diurnal mixing, but the overall shape of the area with enhanced moisture remains more or less the same. Diabatic heating improves from west to east as mid/high level clouds exit east and more widespread clearing sets in from the Netherlands. Trapped gravity waves over N-CNTRL Germany highlight current stout cap due to ongoing WAA from the south with peak inversion strength at around the 750 hPa layer. A gradual cool-down of that layer is expected from the west (the Netherlands), which cannot yet be detected in 12 UTC soundings. However, the clouds over the Netherlands and far NW Germany indicate a less stable appearance (e.g. no wave structure) but appear more convective in nature. This indicates that atmosphere at low/mid-levels becomes gradually more favorable for initiation. Hand analysis also indicates a sharp temperature gradient more or less parallel to the coastlines with rouhgly 5 K between offshore and onshore regions (a bit more inland). No clear signals for land/sea breeze fronts can be seen, probably suppressed either by limited diabatic heating over extreme N-Germany and/or due to the gradually strengthening background flow. Main signals exist over the far W-Netherlands with veering winds along the coast.

A combination of gradually strengthening lift from the west as a weakly amplified synoptic-scale trough approaches, faint cooling of the mid-levels and ongoing diabatic heating plus frictionally induced and somewhat enhanced mesoscale convergent flow along the coast will slowly increase the risk for isolated CI during the upcoming hours. CAMs show first CI at around 15 or 16 UTC over the extreme S-North Sea and further CI to the east thereafter.

12 UTC soundings show effective DLS of roughly 15 m/s, which is on track with model data and a further increase to 20 m/s and more is anticipated during the following hours. In addition, a 25 m/s speed maximum at 700 hPa is forecast to pass far NW/N Germany from W to E during the overnight hours, which boosts 0-3 km shear to 15-20 m/s. These values are more than adequate for well organized DMC. In addition, weak backing ahead of the trough (which is also visible in the surface maps) enhances LL shear. Peak LL shear most likely exists along the coast and along any mesoscale convergence zone, where SRH increases.

12 UTC soundings show in general weak CAPE due to meager mid-level lapse rates but ongoing moistening of the BL and marginal cooling of the mid-levels should support 400-800 J/kg MLCAPE with locally higher peaks.
Mixing shear/instability results in a chance for a few well organized multicells / isolated supercells to affect the immediate coastal areas, Schleswig-Holstein, far S-Denmark and the northern parts of Mecklenburg-West Pomerania during the late evening and overnight hours. A gradual decrease in activity is anticipated at 00-03 UTC from west to east. Strongest storms may contain large hail and produce strong to isolated severe wind gusts. The severe wind gust risk may be enhanced on an isolated scale with aforementiond winds at 700 hPa. Would not be surprised about an isolated swath of severe wind gusts over the N/NE region of my MD. Despite currently unfavorable looking LLs, BL conditions gradually improve during the next few hours with decreasing LCLs and strengthening LL shear, so an isolated tornado event is expected with any mature thunderstorm.

Further south, questions remain about degree of CI. Cooling will also take place from S- Lower Saxony to Berlin, which enhances probabilities for CI during the evening hours. Better CAPE, stronger cap and 15-20 m/s DLS may support isolated to scattered more discrete multicells/supercells, which grow upscale into a small cluster, before weakening during the night over far E-Germany. Large hail and and isolated severe wind gust risk will be the main hazard. Models still diverge with that risk and in case of stronger capping than currently anticipated, the risk of CI decreases substantially. Latest HRV loops indicates growing/cooling Cu fields over W-Lower Saxony, which currently increases confidence in CI and hence the MD area was expanded far south.

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