CONVECTIVE FORECAST - UPDATE
VALID 14Z SAT 26/07 - 06Z SUN 27/07 2003
ISSUED: 26/07 13:49Z
FORECASTER: DAHL
There is a slight risk of severe thunderstorms forecast across south-central France.
General thunderstorms are forecast across southwest...central and east Europe.
SYNOPSIS
See yesterday's outlook.
DISCUSSION
...Central and W Europe....
Conditional severe-thunderstorm potential appears to exist across western and central continental Europe. Shear profiles are more than adequate for severe evolution ... but it appears that SFC-based deep convection will not evolve within favorably sheared environment.
Large shield of stratiform precip ahead of the developing frontal wave is covering mainly the northern portions of France ... while weakening occluding frontal system is extending from Germany into S Scandinavia. Some insolation has allowed the warm-sector airmass over E Germany to warm into the low to mid 20 C range ... modifying 06 Z ascents results in neutral lapse rates for a mixed boundary-layer parcels. ATTM weak convection is present along the cold front over central Germany. Given minimal/nonexistent instability ... storms could fail to initiate. Shear will increase during the day to about 30 to 40 knots in the lowest 6 km ... which does not suggest much of a severe threat given the weak thermodynamic support. PRIND that TSTMS will struggle to initiate ... if an updraft can be maintained ... likelihood that it fortuitously encounters a regions of large low-level SRH remains rather low ... and the organized-severe thunderstorms threat appears to be quite low.
Only chance of TSTMS in association with the frontal wave appear to be associated with weak sheets of elevated potential instability ... which may result in some weak shallow elevated TSTMS embedded in the stratiform precip. Given rain-cooled boundary-layer air ... and generally weak thermodynamic properties of the subtropical-Atlantic airmass ... TSTM threat with this system appears to be rather low ATTM. Concern exists with respect to the strong shear profiles ... if deep convection managed to form ... kinematic environmant would be more than adequate for severe storms especially ahead of the warm front where strong veering is present. However ... SFC-based storms are quite unlikely ahead of the warm front but region should still be monitored for possible TSTM development.
Strong DCVA-related lift will overspread W France and NW Iberia late tonight. Again ... shear will be on the order of 60 knots ... and would be conductive to severe evolution ... if storms formed. Possibility would then exist for splitting storms/supercells ... with enhanced tornado potential given very low LCL heights/strong storm-relative winds at low levels. Moist environment should limit large-hail threat but vertical momentum transport could well promote damaging wind gusts. Indications are that airmass across the southern half of France will become marginally unstable amidst upward vertical motion regime ... where fair chances for convective development appears to exist late Saturday night. Negatives with respect to this scenario are again the lack of knowledge of the depth of the SFC-based moisture and MLCAPE ... the bad timing of the upper trough ... and extensive stratiform precip along the front ... and TSTMS may fail to initiate. Nonetheless ... a slight risk is marginally warranted.
Over S France ... deep/dry convective boundary layer is developing ... but only minimal CAPE. Confidence that TSTMS develop is rather low ATTM given lack of upward vertical motion. If TSTMS develop ... thermodynamic environment will be conductive to strong outflow winds ... aided by vertical momentum transport associated with increasing mid-level flow. Low confidence that TSTMS initiate ... and generally marginal setup preclude a SLGT ATTM.
...Spain...
A few high-based nocturnal storms may develop ahead of the upper trough. Around 40 knots deep shear and deep/dry subcloud layer suggest that potential for a few damaging wind gusts exists. PRIND that TSTMS will be quite isolated ... and may fail to develop given the unfavorable time of the arrival of the upper trough ... and a SLGT is not necessary ATTM.
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