Storm Forecast
Valid: Tue 28 Jun 2011 06:00 to Wed 29 Jun 2011 06:00 UTC
Issued: Mon 27 Jun 2011 21:34
Forecaster: GROENEMEIJER
A level 2 has been issued for parts of northern France, Belgium and the Netherlands mainly for large hail, severe wind gusts and extreme precipitation.
A level 1 has been issued for an area surrounding the level 2 area including the remainder of the Benelux countries, western parts of Germany and northern France, mainly for severe wind gusts, large hail and heavy rain.
A level 1 has been issued for central and southern France and for northern Spain mainly for severe wind gusts and large hail.
A level 1 has been issued for the western and central Ukraine, Moldova and northeastern Romania mainly for weak tornadoes and heavy rain.
A level 1 has been issued for Belarus and southwestern Russia mainly for heavy rain and hail.
SYNOPSIS
East of an amplified trough over the Atlantic, a very warm air-mass is advected northnortheastward over western Europe. A shortwave trough at its base will lift northward producing surface pressure falls ahead of it, resulting in a slack surface low that will be the focus of convective storm activity.
Across eastern Europe, covnective storms are expected within a fairly cold air-mass in vicinity of a cold-core low over sourthern Ukraine, as well as within a warm advection regime over southwestern Russia and Belarus.
... Northern France, Benelux countries, extreme western Germany ...
Considerable CAPE has developed through an elevated mixed layer/moist boundary layer combination that is sustaining several mesoscale convective systems. The influence of their remnants will be relevant to the convective scenario to unfold on Tuesday.
Model guidance has not been consistent during Monday with regard tot eh location of the surface low that should develop over France, and move northward over the Benelux or southern North Sea during the day.
Greatest severe potential appears likely in case the low moves slow and a flow with an easterly component is maintained ahead of it, as this should result in the best overlap of instability and deep-layer shear possibly including sufficient helicity to support storm rotation. It is of concern that none of the available models simulates the MCS that is developing over the Bay of Biscay.
Will sketch a scenario here that some convection will be ongoing over the North Sea, English Channel, (north)western parts of France at daybreak and that mid and upper-level clouds to the east of this activity will have enough breaks for considerable surface-based instability (around or above 2000 J/kg) to redevelop. This should not be a problem courtesy of climatologically very high low-level moisture with dew points of 20-23 forecast in the presence of steep mid-level lapse rates.
The combination of such instability with moderate wind shear (around 15 m/s in the 0-6 km layer) should allow a couple of severe multicell storms to form during the afternoon with large hail and damaging wind gusts.
During the mid- to late evening, as forcing related to the approaching mid-/upper-level trough sets in, storm coverage should increase and develop eastward. The high moisture (40 mm of precipitable water) favours locally high precipitation accumulations and an attendant flash flood threat.
... Central and southern France, Pyrenees ...
Further south, forcing for upward vertical motion and wind shear should be weaker. Yet isolated storms are expected to develop in a large CAPE environment during the late afternoon and evening. The storms will pose a threat of large hail and some localized severe gusts.
... Western and central Ukraine, northeastern Romania, Moldova ...
In vicinity of the upper level low, scattered to widespreas storms are expected. These will be strongly diurnally controlled. Stretching of vertical vorticity along convergence lines rotating around the low centre may prove sufficient for a few weak tornadoes. Upslope flow against the Karpathian mountains may additionally lead to some high local rainfall accumulations, despite modest precipitable water values. Additionally, some small hail will be possible.
... Central and eastern Belarus, central Russian Upland ...
Within a weak warm conveyor belt, widespread convective intiation is expected during the day. The resulting storms will have weak storm motions which may lead to some flooding problems. Additionally some small to marginally severe hail will be possible.
