CONVECTIVE FORECAST
VALID 06Z SAT 29/11 - 06Z SUN 30/11 2003
ISSUED: 28/11 22:20Z
FORECASTER: DAHL

General thunderstorms are forecast across the central and northern Mediterranean regions.

SYNOPSIS

Primary feature remains the quasi-stationary Mediterranean upper low ... which is expected to weaken substantially over the forecast period. Associated SFC low anchored over central Italy/Adriatic Sea ... is progged to fill until early Sunday morning. Band of intense upper SWly flow will affect the NW portions of Europe at the E periphery of a broad N Atlantic long-wave trough. Intense embedded vort max will cross the British Isles and the North Sea on Saturday ... and is coherently progged to promote the development of an intense SFC low over the E Atlantic ... which will rapidly cross the British Isles during the day to be located over the northern North Sea towards the end of the FCST period.

DISCUSSION

...Central Mediterranean...
Plume of slightly unstable Mediterranean airmass is present E of the upper low ... 12Z soundings across Greece reveal nearly neutral lapse rates in this airmass ... and quite dry mid/upper levels. Although QG forcing for upward vertical motion is progged to weaken substantially compared to what is present ATTM ... lift/mid-level cooling ahead of the upper low should steepen the lapse rates somewhat and a few 100 J/kg CAPE should be realized. Latest satellite imagery and lightning network suggest that current TSTMS are quite shallow ... and are limited to the W slopes of the Dinaric and the Pindos Mountains within upslope flow regime. Current thinking is that a few shallow TSTMS will continue during Saturday primarily over W Greece ... and possibly over the Ionian ... spreading east into the Aegean Sea late in the period. Deep-layer shear is expected to be on the order of 40 knots ... and given the weak thermodynamic support ... organized severe storms appear unlikely. However ... dry mid levels could promote very gusty outflow winds. Also ... mountainous terrain over Greece could affect near-surface SRH field such that brief updraft rotation could be promoted. In this case ... marginally severe hail could additionally occur. However ... confidence that severe TSTMS occur ... and coverage of severe TSTMS in case they do occur ... are too low for a SLGT ATTM.

Beneath the upper cold pool ... TSTMS will likely continue primarily over the sea and over coastal regions. Chance for a few watersouts appears to exist given relatively warm sea waters ... But weak thermodynamic support precludes significant organized severe TSTM threat ATTM.

...British Isles...
Cold front of the developing E Atlantic cyclone is expected to stretch from the N British Isles across the Biscay into NW Iberia towards Saturday afternoon. Pre-frontal airmass will likely not be particularly unstable ... but narrow lines of rather shallow ... maybe forced convection cannot be excluded. Kinematic setup would promote bowing segments ... and some rotating updrafts if isolated cells were to form ... and brief/isolated severe TSTM events such as severe straight-line winds ... marginally severe hail and even a brief tornado or two could occur. This scenario appears somewhat unlikely ATTM but situation needs to be monitored for possible severe TSTM development. Refraining from including this area in a GEN THUNDER area ATTM ... but an upgrade may be necessary if scenario indeed verifies.

In the post-frontal environment ... field of enhanced Cu's will likely develop ahead of CVA max expected to reach the SW British Isles late Saturday evening/night. Vertical momentum transfer should support strong wind gusts with this activity ... but given expected dearth of lightning ... and marginal kinematic setup for severe wind gusts ... a GEN THUNDER area does not appear to be needed ATTM.

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