Storm Forecast
Valid: Tue 07 Aug 2007 06:00 to Wed 08 Aug 2007 06:00 UTC
Issued: Tue 07 Aug 2007 10:47
Forecaster: VAN DER VELDE
SYNOPSIS
A cold front has stalled over western Germany, southern France and northern Spain. To its east and south a warm unstable airmass is present. A cold upper trough enters the continent, becoming a cut-off upper low. A surface low over the Iberian Peninsula maintains the supply of warm air over eastern Spain.
A major low pressure system moving northward affects eastern Romania, Moldova and Ukraine with large amounts of mostly convective precipitation.
DISCUSSION
... Romania, Moldova, S Ukraine...
Large instability has developed north of the Balkan upper low, in the 1000-2000 J/kg MLCAPE range. While deep layer shear can be called at most moderate (10-15 m/s), the best shear is found in the lowest 4 km, with the best values bounding the level 2 area to the west and north. Where surface-based instability is found values are a bit lower, but still 0-3 km SREH possible up to 300 m2/s2, 0-1 km shear up to 12 m/s. Continuous large scale instability release may limit severe potential to a degree, but overall, supercells may be possible and long-lived multicells, having a chance of producing large hail, severe gusts and perhaps an isolated tornado. The latter is limited by insufficient low level CAPE where low level shear is strongest.
...N Scandinavia...
Some instability is found in GFS and a band of QG lift and 850 hPa speeds of 15 m/s. With enhanced SREH (150 m2/s2) and deep layer shear (20 m/s). Such values would permit well-organized storms, possibly with rotating updrafts, capable of producing large hail and severe gusts, but only on an isolated scale as instability is rather modest.
...S United Kingdom, NW France...
00Z soundings of Valentia, Camborne and Brest have indicated good low-level CAPE, moist air and weak winds in the lowest layers, classic for spouts. As the airmass is not that cold, the low level lapse rates (and also convergence zones) are best over land, which will be the favoured area of development.
...E Spain....
Strongly veering wind profiles with height are found over eastern Spain in 00Z balloon ascents, though not with great speed shear. With strong convergence of moist air in low levels, the present instability will be released into abundant thunderstorms, which have a tendency to be long lived and possibly mesocyclonic... especially in the level 1 area where SREH is up to 250 m2/s2 and deep layer shear up to 15-20 m/s. The storms are likely to produce large hail, and severe gusts are possible, especially in the south where delta-theta-e is progged to reach values over 20 degrees.
...N Africa...
20m/s deep layer shear and <200 m2/s2 is forecast by GFS to be collocated with instability and convergence. 00Z DAAG soundings shows indeed rather deep moist low levels, steep mid level lapse rates and good speed shear. Storms may develop rotating updrafts producing large hail. Severe gusts are possible as well.
...Central Europe...
A low pressure area sits in the unstable airmass. Steepest lapse rates are indicated by the model over the Czech/Austria/Hungary region, while convergence zones seem complex but persistent in the GFS model. Weak QG lift occurs through this area as well. Chances for widespread storms are good, though GFS shows somewhat poor signals, and some of them may produce marginally large hail, especially when situated on convergence zones for a long period. Deep layer shear is poor, under 10 m/s, with the possible exception of northern Czech Republic with better 1-4 km shear vectors. As 00/06Z soundings have not indicated very steep mid level lapse rates and relatively dry boundary layers, a level 1 has not been issued.
