CONVECTIVE FORECAST
VALID 06Z TUE 12/08 - 06Z WED 13/08 2003
ISSUED: 11/08 21:44Z
FORECASTER: HAKLANDER
General thunderstorms are forecast across the Benelux, Germany, France, northwestern Iberian Peninsula, the Alpine countries, and Italy.
General thunderstorms are forecast across southern Sweden and the Baltic Sea.
General thunderstorms are forecast across European Russia and northern Ukraine.
SYNOPSIS
During the forecast period, a well developed upper low over Ukraine will rotate northeastward into western Russia. Meanwhile, a longwave Atlantic trough at 40°W is cut off and moves eastward, reaching the Faroe Islands (5-10°W) at 13/0600Z. Between these two features, a sharp upper ridge over western Scandinavia weakens and migrates southeastward.
South of the Atlantic trough, a strong southwesterly upper jet - with maximum winds exceeding 100 kts near the tropopause - moves eastward across the northern British Isles. Upper air flow is weak over the western en central Mediterranean, with a strong upper high present over the Iberian Peninsula.
DISCUSSION
...Benelux, western Germany and north-central France...
A weak and shallow low level frontal zone moves southward over northern/central France and southeastward over northwestern Germany. Ahead of the associated low level trough, SBCAPE should reach 1500-2000 J/kg late Tuesday afternoon, according to Monday's GFS 12z. Monday's BOLAM 00z shows a ill-defined theta-e tongue at 850 hPa just downstream of this feature. Although SBCAPE should become moderate, low level moisture will be very shallow yielding only low MLCAPE. Associated with the aforementioned frontal zone, a region of enhanced low level vertical wind shear is present. During late afternoon, a southwesterly jet streak at 700 hPa with wind speeds > 30 kts will be present from the extreme southern British Isles over the Netherlands into northwestern Germany. Since the baroclinic zone is only developed in the lower levels, deep layer shear is found along the polar front further north. In the positive CAPE area, only 25 kts 0-6km shear is expected over the extreme northwest of Germany, yielding only marginal support for organized convective activity. Supercells are unlikely to form, but are not ruled out across northwestern Germany, where moderate SBCAPE and marginal deep layer shear will be present, while 1000-700 hPa SREH will attain values > 100 m²/s², according to GFS 12z. Considering Monday's 12z Herstmonceux "inverted-V" sounding, it is believed that convective wind gusts are the main threat, due to evaporative cooling of relatively dry air below the LCL.
...northwestern Iberian Peninsula...
Day-time airmass storms are expected to form as solar heating creates CAPE, especially in the elevated areas. Along with coastal convergence due to see breeze effects, the latently unstable air should be able to reach its LFC. Since midlevel moisture is also present, current thinking is that thunderstorm activity might become quite widespread over the area.
...southern Sweden...
Strong lifting combined with low to moderate SBCAPE and a deep moist layer could support the development of several strong thunderstorms. Cells are expected to be outflow dominated, owing to the lack of deep layer shear (approx. 15 kts). However, deep convection might be fueled and sustained by low level moisture convergence, modeled over the area by Monday's GFS 12z.
...Italy...
Expect thunderstorms to initiate early in the afternoon over the Apennines mountain range, due to insolation and coastal convergence. No significant vertical wind shear will be present, implying outflow dominated storms. According to GFS forecast soundings for north-central Italy, vertical temperature profiles should become favorable for strong downdrafts due to evaporational cooling of entrained air at midlevels as well as dry subcloud air.
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