Ws20070613

COPS Weather Summary UPDATE
13 June 2007
Issued: 13:15 UTC Valid: Wednesday 13 June 13:15 UTC to Saturday 16 June 13:15 UTC Forecasters: Pieter Groenemeijer, Johannes Dahl The GFS 06 UTC run delays the passage of the cold front on Friday compared to the 00 UTC run. This scenario is supported by GME 00 UTC and yesterday’s ECMWF 12 UTC run as discussed in the text below. Also, the GFS now simulates a large MCS developing in the late afternoon over southwestern Germany. The strength of the upper trough remains unchanged, so that widespread organized severe thunderstorms still appear likely. Synoptic overview
As the large-scale upper trough currently located over the eastern Atlantic makes some eastward progress,
mid- and upper-level winds will back to southwesterly directions. Several short-wave troughs will be
embedded in this flow. The main vorticity maximum associated with the large-scale trough is expected to lift
northeastwards during Friday, providing vigorous synoptic-scale ascent overspreading an unstable air mass.
This in addition to the strong vertical shear associated with this system will provide favorable conditions for
severe convective storms over western Europe. The exact location and timing of the cold front associated
with the long-wave trough is not simulated coherently by the various models, lending some uncertainty
especially to the forecast for Friday.
Analysis and forecast of synoptic controls in the COPS region
Wednesday . Thermodynamic profiles suggest that weak instability will evolve during the day, so that a few
isolated thunderstorms may form again in the afternoon hours. Given a lack of large- and mesoscale forcing
for ascent, convection should diminish with the loss of daytime heating in the evening hours.
Thursday . The first trough ejecting from the Atlantic upper low is expected to affect the COPS area quite
early on Wednesday, possibly before 06 Z. Associated upward vertical motion should sustain some
mid/upper-level clouds, which may be of convective nature, with some chances of elevated showers, maybe
even brief thunderstorms. After temporary subsidence, the next upward motion regime is expected to
overspread the area towards the afternoon. Though mid/high-level cloudiness will likely exist, surface-based
convective initiation is anticipated, in the late afternoon and evening hours. Given about 10 to 15 m/s 0-6 km
shear, dominant mode should be multicellular storms, the strongest of which could produce some hail and
strong outflow wind gusts.
Friday . Scenarios mainly differ in the different timing of the passage of the cold front. The ECMWF 12Z
and GME 00Z models are somewhat slower in the eastward progression of the cold front, while the GFS 00Z
is quite fast. At the moment, no scenario can be singled out as most likely, so further model runs will have to
be awaited. Whenever deep convection develops in either scenario, intense shear would be in place to
organize the storms. This includes linear/bowing storms mainly producing damaging wind gusts and
supercells, posing an additional large-hail threat.
Saturday . The cool post-frontal air mass will likely be quite shallow. Moisture accumulation beneath the
frontal inversion may still allow for some CAPE and isolated thunderstorms in the afternoon. Given that
strong shear is still in place, chance of well-organized storms with an attendant severe weather threat will
persist. However, the previous GFS runs and also the available ECMWF do not suggest appreciable
convective precipitation, but Saturday should be monitored for additional interesting convective evolution.
Extended outlook
Through the next week the low-level frontal boundary should remain just south of the COPS area, stretching
from southern France into the pre-Alpine region. Also, the strong southwesterly mid-/upper level flow will
remain in place. This suggests that the allover severe weather threat is to continue as soon as the front
either makes some northward progress, placing the COPS area in the unstable subtropical air mass, or if
frontal waves develop, which would result in elevated storms affecting the region, some of the capable of
tapping boundary-layer air.
Today, Wednesday 13 June
Time/location of
Chance of a couple of isolated convective storms in the mountains during the afternoon, especially over the southern Vosges. Surface-based single cells and weakly-organized multicells. From around 1200 m in the south to 1600 m in the north.
Tomorrow, Thursday 14 June
Time/location of
Some mid-level cloudiness (including altocumulus castellanus/floccus) is likely, particularly in the morning. Some elevated showers are possibly associated with this cloudiness. Especially in the afternoon, surface-based convection is likely to develop, first over the Vosges, then over the Black Forest and later over the Swabian Jura. A few elevated storms throughout the day. Some isolated surface-based storms during the afternoon and especially early evening are likely. These will be multicells that may become rather well-organized. It is possible that a larger convective system that has developed further west or remnants thereof affect parts of the COPS area late Thursday and the following night. Cloud bases of surface-based storms rising to around 1400 m. Otherwise, higher bases. Rain, locally 20 – 40 mm with isolated storms. Some hail possible. Some hail, likely below 2 cm in diameter, may occur. Any convective gusts will likely remain below 20 m/s.
Friday 15 June
Time/location of
Extensive cloudiness and – partly convective – rain will likely be present over parts of the area, most likely in the west. Storms are expected to redevelop during the morning and early afternoon over the COPS area (according to ECMWF and GME), or over the extreme eastern part of the area and even further east (according to GFS 00 UTC and UKMO). Rather strong shear suggests any developing storms will become rapidly organized. Development of a squall line is likely. Shear is sufficient for some storms to develop rotating updrafts (supercells). Cloud bases of surface-based storms around 1200 m. Otherwise, higher bases. From the west, at 15 m/s, faster when the convection organizes into a squall line. Rain, locally 20 – 40 mm with isolated storms. Some hail possible. Some hail, locally in excess of 2 cm in diameter. Convective gusts stronger than 25 m/s are possible, especially if a bowing squall line
Saturday 16 June
Time/location of
Convective clouds are expected to develop across the COPS area, that may especially later on the day evolve into isolated storms. Shear should be strong so that briefly rotating updrafts are possible. Isolated storms should move from the west at about 15 m/s. Some hail possible with storms that manage to rotate. Suggestions for IOP’s and down days
Weatherwise, IOP’s are strongly recommended on Thursday and Friday. Currently, Saturday appears to be
calmer. Sunday will likely be more interesting again and an IOP could be worthwhile.

Source: http://www.cops2007.de/html/daily_reports/ws/ws20070613.pdf