CONVECTIVE FORECAST
VALID 06Z SUN 08/08 - 06Z MON 09/08 2004
ISSUED: 07/08 17:36Z
FORECASTER: DAHL
There is a slight risk of severe thunderstorms forecast across France.
General thunderstorms are forecast across most of the European Continent and teh British Isles.
SYNOPSIS
Omega-type mid-level flow pattern is expected over Europe on Sunday ... with upper high centered over S Scandinavia ... and a weak upper trough covering SE-central and SE Europe ... and quite an intense upper low over E Atlantic. Weakly meandering SLY upper frontal zone is stretching across the Mediterranean Sea. At low levels ... intense cyclogenesis has accompanied strong vort max pivoting about the main Atlantic upper low ... this SFC cyclone is progged to stay off-shore over the E Atlantic ... but will strengthen SLY low-level flow over the W portions of Europe ... advecting weak EML from Iberia into France ... and Saharan EML into the SW Mediterranean. Weak high pressure is persisting over N Europe ... with quescent conditions elsewhere.
DISCUSSION
...France...
It appears that steep mid-level lapse rates will be advected across France in response to strengthening SLY low-level flow ... with SSELY boundary-layer winds advecting Mediterranean moisture underneath the EML. However ... depth of this moisture is somewhat uncertain ... but soundings across the SW Mediterranean suggest that at least patches of deep 14+ g/kg mixing ratios are in place. This suggests that thermodynamic fields may improve substantially over France towards late Sunday afternoon ... and MLCAPEs ranging from 1500 to 2000 J/kg may be realized.
SLY low-level flow is expected to increase over S France towards late Sunday afternoon/evening ... with 10+ m/s 850 hPa flow and about 15 m/s at 500 hPa ... yielding 0-3 km SRH in excess of 150 m2/s2 over S France late in the day.
Potential problem may be the strong cap ... fostered by weak mid/upper ridging. Models are reluctant to show a coherent signal in QPFs ... and it is somewhat questionable whether widespread convection will occur. However ... Low-level WAA and weak patches of DCVA are expected to overspread France ... with GFS 06Z showing a maximum of Q-vector convergence over central France towards 18Z.
All this suggests that at least scattered TSTMS may form ... possibly increasing in coverage towards late Sunday evening/night. Depending on cap strength and degree of large/mesoscale UVV's ... one or more MCSs may form late in the evening/night. Especially over S France ... supercells will be likely ... capable of producing large hail and damaging winds. Also ... a brief tornado or two may be possible late in the day as LCL heights decrease and low-level shear increases.
...E Spain...
Scattered SFC-based TSTMS are expected over E Spain mainly in response to diabatic SFC heating ... Saturday'S 12Z launch from Zaragoza as well as SFC OBS suggest that SFC-based moist layer has not been mixed out everywhere ... yielding CAPEs on the order of 1000 J/kg. Given sustained influx of Mediterranean moisture into N and E Spain ... similar thermodynamic setup like that on Saturday is expected on Sunday.
About 15 m/s deep-layer shear should be in place ... and an isolated severe straight-line wind and/or large-hail event could occur ... especially if mesocyclones manage to form. Current thinking is that SVR TSTM coverage will be fairly low ... not necessitating a SLGT.
...NW Benelux...
A couple of 100 J/kg will likely develop after convective debris from Saturday have mixed out ... outflow boundaries ... weak baroclinity and orographic features will likely serve as foci for initiation of scattered TSTMS over W Germany and Benelux. Shear will be quite marginal with 10 to 15 m/s at 500 hPa ... but an isolated marginally severe hail/wind event or two cannot be ruled out.
...England...
NMM and GFS 12Z agree in the evolution of weak CAPE over England during the day. Given strong large-scale UVVs ... TSTMS will likely initiate. Uncertainty exists with respect to TSTM coverage ... which may be limited as widespread stratiform precip overspreads the UK ... suggesting that TSTMS could be imbedded/elevated. However ... if SFC-based TSTMS should form ... kinematic setup would be quite favorable for supercells ... which may produce large hail ... damaging winds and tornadoes as LCL heights will likely be rather low and 0-1 km shear of 10+ m/s will be in place.
Confidence in isolated/SFC-based TSTMS is rather low ATTM ... and will refrain from a SLGT. However ... an upgrade to SLGT may be required on Sunday.
...Italy...
Midlevel flow of 15 to 20 m/s is expected to be present over Italy late in the day. Thermodynamic profiles will likely vary substantially actoss the peninsula ... but locally CAPE in excess of 1000 J/kg should be present. Expect an isolated large hail/wind event or two ... but marginal setup suggests that allover severe TSTM threat is rather limited.
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