Storm Forecast
Valid: Tue 13 Jun 2006 06:00 to Wed 14 Jun 2006 06:00 UTC
Issued: Tue 13 Jun 2006 02:11
Forecaster: VAN DER VELDE
SYNOPSIS
A large upper low resides over Eastern Europe, creating a deeply unstable airmass throughout a large area. Most of central and western Europe is under the influence of an upper ridge and stable conditions.
A warm front early in the period from English Channel into western France and northwestern Spain and its associated moisture plume will remain fairly stationary, while the northern part extending from the occlusion point over southeastern UK towards Norway will push southeastward into the European mainland at the end of the period, with winds over the North Sea turning to the northwest/north.
A cut-off low approaches the Iberian peninsula from the Atlantic, and will continue to generate an unstable airmass by low-level warm air advection and upper level cold air advection and strong synoptic lift.
DISCUSSION
...UK/FR Channel area...
Along the front moisture is measured of about 10 g/kg averaged over the lowest km. GFS model keeps this fairly constant, but with maxima up to 13 g/kg which may be too optimistic. Yesterday 12Z/ recent 00Z soundings showed elevated instability present in Brest, so the area is likely to already see storms overnight into the morning. This will at most yield some gusts and mostly sub-severe sized hail, and may produce a messy convective environment. In the afternoon the present SREH (>200 m2/s2 in GFS, double even in NMM) may be used in boundary layer-based updrafts and potentially cause supercells, thus a chance of large hail, severe gusts, and just a marginal risk of a tornado given not very low LCLs and not very high 0-1 km shear.
Eastward the initial conditions were rather dry and suspect GFS is too positive about moisture. NMM model suggests some precip and instability but less than GFS. The area doesn't profit much from good shear conditions which reside to the west, so expect a few clustered storms, initially perhaps on the sea beeze front, that may just be able to form an MCS into the night with some (severe - dry midlevels) gusty outflow as the front pushes southeastward.
...western Iberian peninsula...
GFS keeps producing deep convection with large precip quantities in an area with more than 15-20 m/s deep layer shear and >150-250 m2/s2 SREH (confirmed by very strong veering of wind with height in Gibraltar). NMM is less positive about SREH. Expect clustered storms, a few storms may be supercells and drop large hail and severe gusts. Convergence in SW Spain and divergent upper flow create the required lift. A very strong cap was however present in the Gibraltar sounding the previous day at 12Z, but expect abundant lift to erode it.
...NW Algeria...
Another area where strong SREH and DL shear will be present. Dry Sahara air with steep lapse rates above forms a boundary with the moister Mediterranean BL air. If flow manages to turn inland, upslope severe storms/supercells could form with large hail and severe gusts. Convection could become elevated after dark with somewhat lower threat.
...Turkey...
Very strong 0-6 km shear vectors (>30 m/s) under the upper jet overlay an area of marginal CAPE. Moisture and lapse rates should improve from the west, slightly lagging the shear area. The area is under divergent upper flow near the jetstreak and will probably see a number of severe storms incl. supercells, mostly with large hail, and/or severe gusts. Low-level shear (mostly <7 m/s 0-1 km) isn't that strong to promote tornadoes.
...Mediterranean south of Greece...
Near the upper cold pool and vort max, waterspouts are likely with the remaining convection due to fairly steep low-level lapse rates and weak winds, if Brindisi 12Z of the previous day can be maintained in terms of low-level CAPE as well (GFS suggests lesser conditions, 00Z Brindisi equal)
...Balkan area...
isolated landspouts may be possible given very weak wind and strong low-level lapse rates (dry to superadiabatic) going into seamlessly into CAPE, as in Sofia 12Z sounding yesterday.
