Storm Forecast
Valid: Fri 02 Aug 2013 06:00 to Sat 03 Aug 2013 06:00 UTC
Issued: Thu 01 Aug 2013 23:47
Forecaster: TUSCHY

A level 2 was issued for parts of France mainly due to severe to damaging wind gusts, large to very large hail, heavy rainfall amounts and an isolated tornado event.

A level 1 was issued for parts of the W-Pyrenées, France and Benelux mainly for large hail, strong to severe wind gusts, heavy rain and and isolated tornado event.

A level 1 was issued for the North Sea and W-Norway mainly for severe wind gusts, heavy rain, large hail and an isolated tornado event.

A level 1 was issued for Scotland and Ireland mainly for an isolated tornado and heavy rainfall risk.

SYNOPSIS

An unseasonably intense upper trough for that time of year just west of Europe reaches its highest north/south extension during the forecast with its tip affecting the Canary Islands. Pronounced and deep WAA downstream of this trough reinforces a ridge over S/C Europe with 500 hPa heights exceeding 590 gpdm over the far SW Mediterranean. A compact upper level low grazes Belarus and the Ukraine while moving to the SE.

An extensive but slowly weakening frontal boundary (cold front) will be the main focus for organized DMC. This boundary runs from SW Norway all the way down to the Pyrenees during the overnight hours.

DISCUSSION

... Ireland and Scotland ...

Behind the eastward departing cold front, the post frontal air mass still features reasonably high BL moisture. This moisture also increases from the west before noon as a wrap-around occlusion approaches from the west. With cooling mid-levels atop, some modest MLCAPE build-up becomes likely in the order of 500 to 1000 J/kg. With surface pressure still below 995 hPa in the vortex's center, expect an ongoing brisk gradient flow in the LL with 850 hPa winds of 15 to 20 m/s. LCLs remain at or below 800 m and with some anticipated breaks in the clouds and attendant diabatic heating, it seems to be reasonable to expect modest LL buoyancy (as forecast by GFS). Final geometry of the cyclonic vortex in the low to mid-troposphere dictates strength of directional shear and therefore magnitude of SRH. However, there are signs of a broad overlap of 100 to 200 m^2/s^2 SRH-1, modest LLCAPE and adequate mid-tropospheric CAPE for longer lived and rotating updrafts ... mainly from Ireland to Scotland. Isolated tornadoes , marginal hail and strong wind gusts accompany strongest storms. Thunderstorms gradually decay onshore after sunset but keep going all night long south of Ireland beneath the base of an eastward racing mid-level trough with 500 hPa temperatures below -20 °C (SSTs around 16 to 17°C).

... Extreme N/NE France, N-Belgium/Netherland, parts of UK, the North Sea and W-Norway ...

Model data indicate a more or less undisturbed SW-erly flow regime over the area of interest. In different model runs/model data however, subtle waves were seen and one of those waves could indeed support enhanced CI probabilities on a regional and very isolated scale. With strong prefrontal WAA however (in 850 hPa, the 20 °C isotherm being advected as far north as the Netherlands), the air mass will stay capped from C-France all the way to the Netherlands. Foci for CI before 18Z will be areas like:

- extreme N-France, the E/SE coasts of UK and the coasts of Belgium and the Netherlands, where diurnal driven sea-breeze fronts may serve as trigger for initiation on an isolated scale.

- the ESE-ward shifting cold front itself. Despite ongoing weakening of this boundary, lift should still be adequate to break the cap in a few areas. This might be fostered by complex interaction with aforementioned mesoscale convergence zones along the coasts.

- a weak surface low, seen in a few high resolution model runs, which exits NE France during the afternoon hours to the NE, affecting the coasts of Belgium and the Netherlands. Enhanced BL convergence with that feature might also induce isolated CI.

Those storms evolve within a volatile kinematic environment of 25 to 30 m/s more or less unidirectional 0-6 km DLS, so this activity is forecast to speed to the NE over the offshore areas of the North Sea. Strongest shear will be displaced from the maximized and mostly capped CAPE plume along the front with up to 2000 J/kg MLCAPE forecast by GFS/EZ. Still 500 to 1000 J/kg will be available for this coastal/offshore activity, which is enough for long-lived and organized DMC. This activity is forecast to grow upscale into numerous storm clusters over the S-North Sea. Severe wind gusts and large hail will be the main hazard.

A concern right now is the chance of an isolated storm (e.g. tail-end Charlie of any cluster) with strong deviant storm motion to affect parts of the Netherlands and Belgium during the afternoon hours. A 18Z forecast sounding from the W-Netherlands show more than 2500 J/kg MLCAPE, strongly veered profiles, 25 m/s shear already in the lowest 3 km and a rich helical environment. In case initiation along the coast verifies, expect any storm to reveal explosive development into a mature supercell. Large to very large hail (hail diameter in excess of 5 cm), severe wind gusts and very heavy rainfall amounts likley accompany that activity. Combination of SRH-1 above 200 m^2/s^2 and LCLs around 1 km along the coasts also indicate an enhanced tornado risk in case this activity will be surface-based. CI has to be monitored closely and an upgrade may be needed if confidence in CI has increased.

Beyond sunset, a rapidly intensifying 850 hPa jet (15 to 20 m/s in its core) and increasing QG forcing from a NE-ward racing upper level short-wave all support constant upscale growth of the convection over the North Sea into an extensive thunderstorm cluster. With ongoing WAA, capped/elevated 500 to 1000 J/kg MUCAPE are forecast, but maximized BL moisture along the leisurely eastward moving cold front also supports less capped MLCAPE build-up over the E-North Sea into W-Norway. An organized line of storms should cross the North Sea during the night from SW to NE in response to the forcing and scattered strong to severe wind gusts can be anticipated over most parts of the C/E North Sea and over W-Norway. With rapidly increasing directional shear along the coast of W-Norway, embedded rotating cells with an isolated tornado risk can be anticipated. Beside that, isolated large hail will be possible, too. Effective PWs approach 40 mm, so flash flood producing rain will be another concern over SW/W Norway.
The southern fringe of that convective cluster has to be monitored in case of some nocturnal back-building towards the Netherlands and far NW Germany occurs. Despite a thinning CAPE plume and some BL stabilization, the risk for large hail, strong to severe wind gusts, heavy rain and an isolated tornado threat would still be present.

A broad level 1 was issued for that activity although confined areas might see level 2 conditions especially in case of an organized MCS (wind) event over the North Sea or isolated supercells over parts of Belgium/the Netherlands. An upgrade might be performed later-on.

... France between 18Z to 06 Z ...

A pronounced short-wave circles around the base of the E-Atlantic trough, enters the Bay of Biscay during the afternoon/evening hours and rotates to the N/NE thereafter. This feature crosses France within less than 12 hours.

Convection develops during the late afternoon/evening hours over the SE Bay of Biscay either due to offshore moving mountain convection from N-Spain or due to coastal convergence zones. Another possibility of WRF f.ex. with CI over C-France was dismissed for now due to ongoing capping issues. In case this verifies however, an increase of severe probabilities over Benelux has to be expected.

Anticipated thunderstorm activity over the SE Bay of Biscay is forecast to spread rapidly to the E/NE ahead of the approaching vorticity maximum while growing upscale into a large thunderstorm cluster. The SW/NE aligned cold front likely serves as focus, where storms race to the NE. With 30 to 35 m/s DLS and 20 to 25 m/s 0-3 km shear vectors aligned nearly parallel to the boundary, either numerous small line segments of organized and deep convection (in case of discrete CI ahead of the cluster) or a sole MCS event (in case of no further CI ahead of the cluster) are forecast to race to the ENE and NE ... probably as bow echoes or forward propagating MCS events. Swaths of severe to damaging wind gusts are likely with that activity. Beside the wind risk, large to isolated very large hail, heavy rain and an isolated tornado event are all possible.

A level 2 covers the area, where confidence in initiation is most likely. We might need to expand the level 2 more to the NE into Belgium and the Netherlands later on, in case of CI more to the east (e.g. C-France).