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General Category => Weather & Atmospherics => Topic started by: martinastro on October 19, 2008, 11:36:20 pm



Title: Thunderstorm Outlook - Mon 20th (Level 1 Risk)
Post by: martinastro on October 19, 2008, 11:36:20 pm
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Storm Forecast
Valid: Mon 20 Oct 2008 06:00 to Tue 21 Oct 2008 06:00 UTC
Issued: Sun 19 Oct 2008 22:29
Forecaster: SCHLENCZEK
SYNOPSIS

A very strong occluding low pressure system northeast of Scotland will move towards northern Norway / Arctic Sea during the period. Its cold front should stretch from the Irish Sea via southern Scotland towards the northern North Sea at Monday 06 UTC. Later in the period, this frontal boundary will cross southern Scandinavia while weakening. A well-defined polar jet stream with 60 - 80 m/s at 300hPa stretches from Newfoundland via Ireland towards Scandinavia with its left exit region northeast of Scotland and will translate northeastward. In the wake of the large upper trough, cold air overspreads large portions of western and northern Europe. Some "warm air convection" can only be found in the western / central Mediterranean region where low to moderate instability is present ahead of a weak upper low.

DISCUSSION

...Ireland, NW UK...

In the early morning, the cold front of the aforementioned large low pressure complex will cross the British Isles. A tongue of rather high Theta-E (40C) is present ahead of the front and vertical cross sections by GFS indicate a great probability for a potentially unstable stratification close to the front. Very strong QG forcing in that region will likely allow some linearly organized convection that may also contain bowing segments. About 200 J/kg SRH1 combined with more than 15 m/s LLS and 40 m/s DLS will support some short-lived mesocyclones and a few tornadoes may occur. About 30 m/s at 700hPa can easily be transferred downward if convection develops and some damaging gusts are possible as well. Most of the measured gusts should be rather non-convective but the risk of isolated very strong downbursts and possibly some tornadoes warrants an upper LVL1 threat.

...S Norway...

The upper trough should overspread southern Norway in the late afternoon. Some low-end instability is forecast and a few storms may develop along the coast and also further inland. LLS around 15 m/s and SRH1 around 200 J/kg may support an isolated tornado. Severe gusts are also possible as 850hPa winds exceed 25 m/s. A marginal LVL1 threat should be warranted.

...Iberian Peninsula, Morocco, N Algeria, Tyrrhenian Sea...

Some weakly organized multicells will develop in an environment with moderate instability and weak vertical shear. Severe weather is not likely but a funnel / waterspout is not ruled out, though.