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Convective Potential Ireland & UK - Wed/Thurs Aug 19/20th

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Author Topic: Convective Potential Ireland & UK - Wed/Thurs Aug 19/20th  (Read 720 times)
martinastro
Martin Mc Kenna
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« on: August 20, 2009, 09:55:20 pm »



Storm Forecast
Valid: Fri 21 Aug 2009 06:00 to Sat 22 Aug 2009 06:00 UTC
Issued: Thu 20 Aug 2009 20:28
Forecaster: TUSCHY
A level 2 was issued for parts of Germany and extreme western Czech Republic mainly for large hail and severe wind gusts.

A level 1 was issued for S-Sweden mainly for large hail.

A level 1 was issued for Switzerland, E-France, S-Germany and parts of Austria mainly for large hail, severe wind gusts and excessive rain.

SYNOPSIS

A huge upper trough over NW-Europe continues its slow eastward motion and will be placed over the North Sea during the end of the forecast. Impressive WAA tongue further downstream still covers parts of central and northern Europe. The Mediterranean remains hot and stable with chilly conditions north of the Black Sea.

A north-south elongated cold front is moving gradually to the east, but decelerates during the forecast. Abundant precipitation is forecast along this front and a prefrontal convergence zone.

DISCUSSION

... Most parts of Germany ...

Not much change in the forecast philosophy. A cold front advances eastwards, faster over NE and E-Germany with the trailing part over southern Germany. Next to this front, a prefrontal convergence zone / surface low pressure channel is situated already over the far eastern regions of the level 1 area, still dependent on the magnitude and extend of the overnight activity. Upper trough axis draws near during the forecast from France and Benelux and enhances mid-level forcing, but this may only play a timing issue in respect of initiation.

Marked off by the convergence zone and the cold front, a warm and moist sector covers the level 1 area with surface dewpoints in the upper tens and temperatures between 25C and 30C. Strongest moisture advection and highest averaged mixing ratios are likely along the more active part of the front, over NE and E-Germany. Despite a weakening EML due to active convection the night before and diminishing WAA, a prefrontal tongue of 500 - 1000 J/kg MLCAPE looks still reasonable, probably too narrow to be captured by the sparse sounding mesh. Values, exceeding 1000 J/kg may evolve over SE-Germany, extreme SW-Czech Republic and extreme N-Austria, where mid-level lapse rates remain a tad steeper.

Obviously, shear is the best along the cold front and most parts of the aforementioned sector, between the front and the convergence zone. 15m/s and 20m/s shear in the lowest 3km and 6km respectively cause rapid thunderstorm organisation with large hail and severe wind gusts. Convergence zone further east is a bit displaced from the best wind field and forcing, but still adequate for an augmented risk.

Thunderstorms, probably disorganized and non-severe, continue during the early morning hours along the cold front, but undergow a diurnal minimum in organization and activity. Already during the late morning hours / noon, thunderstorms over NE-Germany re-intensify with isochronic initiation further to the south but also along the convergence zone over far east Germany, west Poland and the Czech Republic. Initiation over SE-Germany holds up until the afternoon and probably even until the evening hours. Models hold firm on the development of two thunderstorm clusters. The first one out of multicells and supercells over E/NE-Germany (afternoon and evening hours) and a second one over S-Germany/W-Austria during the night hours, probably a quite disorganized cluster of storms with a rapidly developing more stratiform rain shield. In-between (e.g. central Germany), neither forcing nor instability impress us, but shear remains strong, so more isolated multicells and supercells are still forecast during the afternoon hours.

The level 2 was adjusted to those regions, where storm mode and environment looks promising for a more widespread large hail and severe wind gust risk. The level 1 was issued for various reasons, which can be seen in the header.

The level 1 was expanded well to the north over S-Sweden to reflect an isolated hail risk with elevated thunderstorms. Favorable overlap of MUCAPE/shear diminishes betimes and so does the hail risk.

Excessive rain is likely with those storm clusters over the western Alps and NE-Germany /NW-Poland, but also over E/SE-Sweden.

... UK and Ireland ...

An active, diurnal driven thunderstorm day is in store, but there are no hints of an augmented funnel/tornado risk. Isolated events can't be ruled out, given some low-end LLCAPE and low LCLs, but not enough for a level 1. Thunderstorms decay after sunset.

... Belgium, Netherlands and the SE-North Sea , 18-06 UTC ...

Beneath the base of the upper trough, conditions improve for a few evening storms over Belgium and the Netherlands with an isolated hail risk. However, the thunderstorm activity shifts offshore and conditions for a few waterspout events improve. Expected risk is not yet enough for a level 1.

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