CONVECTIVE FORECAST
VALID Fri 24 Jun 06:00 - Sat 25 Jun 06:00 2005 (UTC)
ISSUED: 23 Jun 21:54 (UTC)
FORECASTER: DAHL
There is a moderate risk of severe thunderstorms forecast across northern France.
There is a slight risk of severe thunderstorms forecast across Spain ... France ... S UK ... S Benelux ... W Germany ... NW Switzerland.
SYNOPSIS
High-amplitude upper flow pattern continuing across Europe ... with large-scale ... quasi-stationary upper trough present over the E Atlantic ... which is expected to continue to amplify and close off into a cutoff low late in the period ... promoting SWLY mid/upper flow over the western portions of Europe. Vort maxima over Spain and N France ... are merging with the amplifying long-wave trough ... and are progged to weaken while being advected northeastwards. Another ... rather strong vort max is expecte to travel SEWD along the E periphery of central-European large-scale upper ridge ... affecting the SE Balkans and Turkey on Friday. Models advertise development of SFC low over NRN France/the Channel in the afternoon hours ... though evolution of this feature is quite incoherently progged ... which lends some uncertainty to the convective scenario on Friday.
DISCUSSION
...France ... Benelux ... W Germany ... Switzerland...
Thursday's 12Z ascents from Spain ... France and N Italy suggest that the temperature profiles are rather homogeneous above the boundary layer ... mainly featuring an EML from the Iberian Peninsula ... and varying BL temps/mixing ratios ... yielding CAPEs in the range of a few 100 J/kg to more than 2000 J/kg. Modifying these ascents with anticipated SFC highs in the low 30's ... and assuming up to about 15 g/kg mean mixing ratios ... MLCAPE may locally exceed 2500 J/kg. However ... variability of MLCAPE is expected to persist also on Friday. Also ... convective debris may linger across much of France ... the SW UK and parts of SW Germany through the morning/early-afternoon hours ... which may delay the build-up of large CAPE somewhat.
Deep shear is expected to be marginal ... with 500 hPa flow of 10 to 15 m/s across Iberia ... France ... the UK and western Germany. Low-level shear will strongly depend on the evolution of the SFC low over NRN France ... but SFC flow could become strong enough/sufficiently backed to become supportive of supercells. Also ... numerous outflow boundaries and orographic features should locally augment low-level kinematic fields.
Some DCVA-related ascent associated with ill-defined/small vort maxima should overspread France and Spain during the day ... though the foci for convective evolution will likely be tied to mesoscale boundaries mentioned above. Relatively weak capping ... and increasingly convergent SFC flow associated with the cyclogenesis should promote quite widespread convective development again.
Thunderstorms should have potential to become severe ... especially with mesocyclones that manage to form. These will have the potential to produce large hail ... damaging wind gusts ... and depening on the evolution of the low-level shear ... also a few tornadoes. Moreover ... very moist low-level air ... anticipated clustering of the storms in the evening hours ... and rather slow storm motion suggest that local flash flooding may also occur.
Storms should spread into WRN Germany ... NRN Switzerland and SRN Benelux in the late-afternoon/evening hours.
...Spain ...
Weakly unstable/weakly capped inverted-V profiles should develop over Spain towards early afternoon. Rather widespread convective evolution is expected again given numerous outflow boundaries laid out by Thursday's convection and weak large-scale ascent. About 15 m/s deep shear ... and deep CBL's will likely support severe outflow winds ... also ... isolated large hail should be possible.
...S UK...
It seems that the thermodynamic profiles across the UK may be somewhat weaker than over France ... though this also depends on the evolution of the SFC low over the Channel region/N France. BOLAM offers the northernmost evolution of this feature ... still only allowing the unstable air from France to grace the British Isles. Solutions with more southerly position of the SFC low ... or those that do not assume cyclogenesis at all (like GFS 12Z) would likely favor even less favorable thermodynamic fields across the S UK. However ... at least several hundred Joules of CAPE should be available ... and about 15 m/s deep shear should promote a few severe TSTMS ... mainly producing severe wind gusts and large hail. Again the tornado threat is strongly depending on the low-level wind fields/strength/location of the cyclogenesis.
...N Italy...
Strong thermodynamic profiles will likely be in place again ... and isolated/scattered TSTMS should form during peak-heating hours. Though shear should be quite weak ... strong outflow winds and some hail may occur ... possibly briefly reaching severe limits with the most intense cells. Coverage of severe should be too low for a SLGT though.
...N Sweden ... Finland...
NMM and GFS advertise some CAPE across NRN Sweden and Scandinavia ahead of vigorous vort max/cold front. Thursday's 12Z ascents do not suggest that much instability across this region ... but a few hundred J/kg of CAPE may develop. 500 hPa flow of 20 to 25 m/s ... and 850 hPa flow of 10 to 15 m/s should support short lines of convection and maybe a mesocyclone or two ... posing some threat of severe wind gusts and some hail. However ... weak thermodynamic and marginal kinematic setup suggests that allover organized-severe TSTM threat should be rather low.
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