CONVECTIVE FORECAST
VALID 06Z SAT 22/11 - 06Z SUN 23/11 2003
ISSUED: 21/11 20:24Z
FORECASTER: DAHL

General thunderstorms are forecast across the central Mediterranean regions.

General thunderstorms are forecast across the western Iberian Peninsula.

SYNOPSIS

Upper flow across W and central Europe will back to SSWLY directions As deep E Atlantic long-wave trough slowly progresses east with the southern portions beginning to tear off. Weakening upper low ATTM over the central Mediterranean ... will continue east and reach the E Mediterranean Sea towards the end of this period. At low levels ... main frontal zone will be stretching from W Iberia across the Biscay ... S Great Britain and the S North Sea into the Baltic States. Extensive low-pressure system is associated with the Atlantic long-wave trough over the northern North Sea and the eastern Atlantic ... with weak cyclogenesis progged over SW Europe ahead of the main vort max/upper low ... frontal wave is expected to develop over the S British Isles/SW North Sea as NRN stream short-wave overspreads the region.

DISCUSSION

...Central Mediterranean...
Central Mediterranean vort max should continue to be the focus for convective development ... 12Z Soundings confirm the GFS solution which assumes CAPE on the order of 1000 J/kg ahead of and beneath the upper low ... where shear profiles are expected to be quite weak. Mesoscale mid/upper wind band accompanying the vort max will be strongest at the E periphery of the feature per MM5 solution. On or more larger clusters may be present over the central Mediterranean/S Ionian Sea at the beginning of the period ... and spread east during the day. Given expected weak low-level shear ... severe threat appears to be quite low with this activity. However ... there appears to be a slight threat of waterspouts ... also ... favorable interaction of TSTM updrafts with orography/outflow boundaries may promote brief updraft rotation with a concomitant threat of some hail ... but chances for this are considered quite low ATTM and a categorical risk is not necessary.

...Iberia...
Ahead of the main vort max affecting Iberia late in the period ... SLY/SWLY low-level flow will advect tongue of subtropical-Atlantic airmass across the Iberian Peninsula. Indications are that extensive cloudiness/precip and generally weak lapse rates will suppress deep SFC-based convection. Sheets of weak elevated instability could be embedded in the subtropical plume ... and a few brief/isolated TSTMS may be possibe. Better chances for TSTMS will exist in the post-frontal environment over Portugal and W Spain late in the day. Shear is expected to be quite meager ... and organized severe-weather threat should be quite low.

A few isolated lightning strikes will be possible withing mesoscale fields of deep Benard-type convection over the E Atlantic and the northern North Sea ... this activity may affect the NRN and WRN parts of Great Britain ... but TSTM coverage is expected to be too low for a GEN thunder outlook.

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