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Convective Outlook N UK - Wed March 9th

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Author Topic: Convective Outlook N UK - Wed March 9th  (Read 136 times)
martinastro
Martin Mc Kenna
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« on: March 08, 2011, 10:08:50 pm »



Storm Forecast
Valid: Wed 09 Mar 2011 06:00 to Thu 10 Mar 2011 06:00 UTC
Issued: Tue 08 Mar 2011 21:50
Forecaster: PUCIK
A level 1 was issued for Nortwestern Morocco mainly for large hail.

SYNOPSIS

A blocking pattern, which has been prevalent over Europe for the past days, will eventually let go as a well defined short-wave trough will move into NW and N Europe. Previously dominant ridge will weaken significantly and lose its "control" over Central Europe. The above-mentioned short wave is simulated with an embedded jet-streak (windspeeds over 40 m/s) at its base at mid-levels of troposphere, resulting in a strong QG forcing ahead of the trough. Other important mid-level features will include a stagnant shallow cut-off low centered to the west of the Iberian penninsula and another, this time deeper cut-off situated over Turkey and filled with very cold airmass with respect to the latitude. Situation will be similar close to the surface with a system of lows dominating weather over NW and N Europe and other two lows are also simulated in conjuction with the cut-off lows at mid-levels mentioned above. The high pressure system will remain over Eastern Europe and the Central Mediterranean.

With each of the above-mentioned disturbances, isolated to scattered thunderstorms will be possible in certain areas. Probably the highest coverage of thunderstorms will be associated with the cut-off low to the west/southwest of Iberia.

DISCUSSION

... Northwestern/Northern Europe ...

As the short-wave associated with the frontal system passes over the place in the strong westerly to northwesterly flow, GFS simulates some low-end CAPE values in the post-frontal polar maritime airmass (ECMWF and WRF remain more sceptical, especially with CAPE values inland). Wind shear values look potent for a well organised convection (most probably in the form of a quasi-linear MCS) with DLS values reaching 30 m/s and LLS inland values over 10 m/s. The strongest forcing, associated with the left-exit region of an embedded jet-streak will be observed on the forward flank of the trough, resulting in a stratiform rain shield. It seems that the forcing will be most favorable over Denmark, but still, models simulate instability built-up after the vort max passes over the place with little overlap. It is therefore highly questionable, whether a strongly forced convective line could develop behind the main stratiform rain shield (talking about Denmark and NW Germany in this case). In case of Germany next con in this situation would be the fact, that only after the frontal passage will the advection of moister airmass with higher dewpoints begin. Speaking of northern half of Great Britain and SW Norway, convection will be clearly post-frontal with little forcing from synoptic-scale system. Too many uncertainities remain to warrant a Level 1 counting with the development of a strongly forced line close to or low-topped supercells behind the front with attendant marginally severe wind gusts and weak tornadoes risk. Nevertheless, these areas might experience isolated thunderstorms and will be monitored for a possible update.

... Northwestern Morocco...

On the forward flank of the cut-off low, warm air advection at lower levels with cool mid-levels above should contribute to the development of latent instability (GFS hints around 500 J/kg in this area). With gradually strenghtening wind (20 m/s at 500 hPa and over 30 m/s at 300 hPa), sufficient bulk-wind shear values for well organised multicells and supercells will likely be observed. Low-end instability will be a limiting factor in this case, but few low-topped supercells might develop, posing a marginally severe hail risk and a low-end Level 1 is introduced for this area.
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