Storm Forecast
Valid: Tue 05 Aug 2008 06:00 to Wed 06 Aug 2008 06:00 UTC
Issued: Tue 05 Aug 2008 02:33
Forecaster: VAN DER VELDE
SYNOPSIS
An intense but aging low pressure system is situated over the southeastern Baltic Sea. Its strong pressure gradients yield high wind speeds around its core. The occluded frontal zone has created many thunderstorms the past two days, and will now be situated over central Ukraine and western Russia. The jetstream follows the back side of the front over 500 km width.
Elevated instability is forecast by GFS model to be present over Ireland and the UK, into Belgium and Netherlands later, associated with a warm front from an Atlantic low. Saturated airmass (water loading) and low CAPE should keep updrafts weak, while also rather warm cloud tops (most unstable EL) of -15°C should keep thundery activity limited by lack of graupel growth. These EL heights lower in GFS while the front approaches the UK. Have chosen to draw only any areas with strongest deep convergence (0-2 km).
As with SSW-erly winds hot Saharan/Spanish plateau elevated mixed layer is advected over eastern Spain, very steep lapse rates develop. Over the lower terrain of northeastern Spain BL mixing ratios rise above 6 g/kg, north of the Pyrenees 10-12 g/kg. Local convergence zones associated with orography will likely trigger a few thunderstorms with high cloud base >2500 m.
The unstable airmass will be advected northward during the next days as a Spanish Plume, at the arrival of an Atlantic upper trough.
DISCUSSION
...N Poland, Kaliningrad Oblast, Lithuania...
GFS predicts a bubble of unstable airmass to advect from the Baltic Sea into coastal areas of this region. Problematic are the strong low level winds and its vertical shear: >20 m/s density weighted mean 0-3 km winds, >17 m/s 0-1 km shear vectors. Current 00Z soundings along the Baltic coast show potential for good 0-3 km CAPE along with 50 kts observed winds at 900 hPa (!)
Some synoptic scale lift is forecast due to the SE-ward motion of the mid level vorticity maximum. Veering winds with height result in SREH 0-3 km values of 200-350 m2/s2. The prevailing types of thunderstorms resulting from such conditions include supercells and bowing segments or larger scale lines (in case of a convergence line). The main threat is severe gusts, and support for tornados is much enhanced given strong LL shear and very low LCL heights.
...Ukraine into Russia...
Within range of the jet, 20-25 m/s deep layer shear is present, and a few hundred J/kg MLCAPE should be in place. LL shear is 8-12 m/s over a large area as well. While this is less impressive than the day before, it is often sufficient for multicell clusters and a few supercells (probably splitting into small left and rightmovers as SREH is not much enhanced... probably straight hodograph) carrying large hail and strong gusts, and some isolated tornado chance. Towards the Russian occlusion point shear gets impressive but instability and its release questionable.
...western Balkan...
Coping with similar wind shear and instability condition as the Ukraine, but deep layer shear is somewhat lower (-5 m/s) and SREH possibly a bit higher, in the GFS12Z/18Z model. As the cold pool should be stronger with drier mid levels, but LL shear weak, a multicell or isolated supercell is the expected mode with large hail and isolated strong/severe gusts possible. The storm should move southeastward from Slovenia into Croatia.
...Spain, extreme southern France...
GFS produces no convective precipitation signal over Spain during the afternoon except for the Pyrenees... however a considerable uncapped parcel layer depth points out the environment is eager to initiate storms. Focus for initiation are the Iberian mountain ranges SW of the Ebro river, and the central Pyrenees, and perhaps just north of it. GFS lets this evolve into a small MCS moving eastward. Some of the Spanish convection would move into NE Spain, likely deactivating when carried away from convergence zones, and at nightfall.
Threats associated with the storms include isolated strong to severe downdraft winds, and large hail, which is favoured by >8 K/km lapse rates, moderate CAPE, high LCL, and moderate vertical wind shear: 15 m/s 0-6 km, and >150 m2/s2 SREH 0-3 km, sufficient for slightly rotating updrafts. Only the limited forcing factor keeps this level 1, but an upgrade may be considered.
...SW Norway...
A stationary strip of slightly unstable air locked in a convergence/confluence zone near a very shallow and quiescent low offshore seems to offer favorable conditions for a few spout-type tornadoes or funnel clouds. Storm motion will be very slow and locally large amounts of rain could accumulate. 00Z Stavanger sounding shows good 0-3 km LLCAPE.
