CONVECTIVE FORECAST
VALID 06Z SAT 26/06 - 06Z SUN 27/06 2004
ISSUED: 26/06 00:11Z
FORECASTER: DAHL

General thunderstorms are forecast across the British Isles.

General thunderstorms are forecast across E Spain ... S and central France.

General thunderstorms are forecast across E Europe and the central and E Mediterranean.

SYNOPSIS

Strongly meandering upper frontal zone is present over central and NRN portions Europe ... with a vigorous perturbation crossing the British Isles during the period. E European upper trough will dig SEWD and reach the Black Sea/Turkey late in the period. At low levels ... strong/extensive cyclone is present W of the British Isles ... which will affect the British Isles on Saturday.

DISCUSSION

...British Isles...
Substantial uncertainty exists with respect to the shape of the thermodynamic profiles within the Atlantic theta-e plume E of the large Atlantic SFC low ... Based on climatology as well as on NMM and GFS output ... this air mass should be characterized by too weak lapse rates to be supportive to deep convection. Also ... current SFLOC data don't indicate TSTMS amidst this air mass.

Present thinking is that a sheets of weak elevated instability may be imbedded in the WAA regime at the nose of the theta-e plume ... and an isolated lightning strike or two cannot be discounted. At the W edge of the plume along the cold front ... chances of deep convection appear to be rather meager ATTM. However ... low-level shear will be quite strong ... and in addition to low LCL heights ... kinematic and low-level thermodynamic environment look to be favorable for rotating updrafts ... conditional on the presence of deep convection. Currently this condition is not expected to be met ... and a severe TSTM outlook is not necessary.

In the post-frontal environment ... models coherently simulate deep enough convective mixing for a few TSTMS to occur ... mainly over Ireland and over extreme SW England. 0-1 km shear on the order of 10 m/s ... and deep shear of 15 to 20 m/s may prove sufficient for a few rotating updrafts ... posing threats for large hail and an isolated tornado or two. Very gusty winds will likely accompany any convective cloud in this environment.

...France...
As Spanish air mass is advected into France E of the Atlantic low-pressure system ... weak instability will likely spread into France. Large-scale upper support will be quite meager ... but isolated TSTMS could form during peak heating hours ... primarily along orographic fatures. During the afternoon ... deep shear will increase from about 15 m/s to more than 25 m/s from S France to central France ... and any TSTM forming in this environment may become multicellular ... with increasing threat for rotating updrafts towards the north. Main threat with such storms would be large hail and severe wind gusts. However ... uncertainty exists with respect to the degree of instability ... also lack of forcing may limit ... or completely suppress ... convective development. An upgrade to SLGT may be necessary on Saturday if TSTM coverage will be greater than currently anticipated.

...E Europe...
Mostly weak instability will persist across large portions of E Europe within modified subtropical air mass E of the upper long-wave trough ... Several perturbations are expected to overspread the frontal boundary ... stretching from central Finland SWD into the S Balkans by 12Z per GFS. TSTMS will likely develop along and E of this boundary during the day. Especially over Finland and the S Balkans ... about 15 m/s 0-6 km shear may support a few marginally severe wind/hail events ... but allover organized severe TSTM threat appears to be rather limited and a severe TSTM outlook is not necessary ATTM.

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