CONVECTIVE FORECAST
VALID 06Z SUN 27/06 - 06Z MON 28/06 2004
ISSUED: 26/06 13:51Z
FORECASTER: DAHL
General thunderstorms are forecast across SW and S-central Europe.
General thunderstorms are forecast across the UK.
General thunderstorms are forecast across E Europe and Scandinavia.
SYNOPSIS
Upper vort max ejecting from the quasistationary Atlantic upper low ... will flatten the central European ridge during Sunday. E European upper trough will attain increasingly negative tilt as it is rounded by a vort max ... This will result in a broad band of fast WSWlies stretching across central portions of Europe towards late Sunday. At low levels ... extensive Atlantic SFC low is progged to fill during the period ... with weak high pressure area persisting across the central Mediterranean regions into W Europe ... plume of weakly unstable air mass is lingering SE Spain and S France ... which may spread into S Germany ... Switzerland ... Austria and N Italy during Sunday.
DISCUSSION
...SW and south-central Europe...
A thorough analysis is hampered by limited availability of rasiosonde data ... and much of the forecast will be based on model fields. If availability of data improves until Sunday mid day ... and the situation unfolds differently than anticipated today ... an update will be issued.
Indications are that weak instability will persist over SE Spain and S France. SFC OBS reveal dewpoints on the order of 12°C with temps above 30°C within the air mass which originated from the Iberian Peninsula. Farther N and W ... subtropical/Atlantic air mass is present ... and advecting eastwards ... with somewhat lower SFC temps ... and possibly weaker lapse rates ... unlikely to allow for positive CAPE. Current thinking is that weakly unstable ... originally Iberian ... air mass will be the only focus for convective development over this region. During the day ... large-scale forcing for UVVs will be weak/non-existent ... and only isolated TSTM development is anticipated ... mainly along orographic and other small-scale features.
Models coherently simulate quite favorable deep-layer shear profiles over central France ... with shear decreasing towards S France and E Spain. Central France will likely see 20+ m/s 0-6 km shear ... with 15 m/s or less over S France.
Low-level thermodynamic and kinematic filds appear to be rather weak ... with less than 10 m/s 0-1 km shear ... rather high LCLs ... and weak low-level CAPE debsities. However ... a few large hail and severe wind events may occur ... primarily with multicellular storms ... though a few rotating updrafts may occur as well.
Farther downstream over S Germany... Switzerland ... parts of Austria and N Italy ... kinematic profiles will be quite strong per GFS and MM5 output ... with more than 20 m/s 0-6 km shear and up to 10 m/s 0-1 km shear. This in conjunction with the complex terrain may support supercellular storms capable of producing large hail and damaging wind gusts. A brief tornado cannot be ruled out either ... though this will likely be conditional on the presence of locally rich/trapped low-level moisture.
Uncertainty exists with respect to TSTM coverage ... Given lack large-scale vertical motion ... it appears that TSTM coverage will remain arther low. However ... situation will be monitored with the option of upgrading to SLGT if TSTM coverage is greater than currently assumed.
Air mass over SE Iberia is expected to advect NWWD late in the period ... and a few elevated TSTMS may develop late Sunday evening/night. Shear in the cloud-bearing layer will be marginal ... and severe TSTM threat should be limited. However ... an isolated marginally severe hail/wind event may occur.
...E Europe ... Scandinavia...
Organized severe TSTM threat appears to be limited owing to generally weak shear profiles. N Turkey ... parts of the Black Sea as well as S Finland will likely continue to see about 15 to 20 m/s deep shear ... suggesting that a few marginally severe wind/hail events may occur. Thermodynamic support and marginal kinematic parameters should limit allover severe threat and a SLGT is not necessary ATTM.
...UK...
Strong shear will be present in the post-frontal environment over the British Isles. Also ... strong DCVA-related lift will overspread the region during the day. Big qestion appears to be how deep the mixed polar air mass will become. Models are not quite in agreement with respect to the depth of the convective layer ... also ... it looks that deppest convection will occur outside regions of best shear ... and allover severe weather threat may be somewhat limited. Later observations and model data have to be awaited to specify severe threat.
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