Storm Forecast
Valid: Fri 08 May 2015 06:00 to Sat 09 May 2015 06:00 UTC
Issued: Thu 07 May 2015 17:31
Forecaster: TUSCHY
A level 1 was issued for S-France, N-Italy and parts of the Alps mainly for isolated large hail, severe wind gusts and excessive rain.
A level 1 was issued for parts of S/CNTRL Italy mainly for large hail and an isolated severe wind gust report.
A level 1 was issued for W/CNTRL Turkey mainly for large hail, severe wind gusts and excessive rain.
A level 1 was issued for parts of Ireland and CNTRL-UK mainly for an isolated tornado event.
SYNOPSIS
Split flow pattern continues over Europe. One branch detours northeast over UK towards Finland. Another one branches off to the southeast and grazes the N-African coast. Steering cold-core vortex over the southern Artic Ocean/N-Norwegian Sea remains in place and pushes numerous low-amplitude waves east. With those waves, many parts of Europe will see some kind of impact. Regarding dynamics, the northern ones feature best structure with distinct stamps (complex pattern of various low pressure areas of weak waves) in the lower troposphere, whereas waves to the south start to weaken with only diffuse signals in the surface pressure fields (e.g. W-Mediterranean).
DISCUSSION
... France, N-Italy and Alps ...
Rather compact mid-level wave enters the area of interest from the west during the start. While moving east, decoupling is underway with one part lifting to the NE while the other one turns SE towards Sicily.
The wave interacts with an EML plume over S-France. No significant moisture recovery is forecast with weak response to the approaching disturbance in the lower troposphere (marginal pressure falls). Increased cyclonic flow down to 850 hPa still ensures a few hours of backed/more southerly flow and moisture advection from the NW Mediterranean. Moisture recovery beneath EML plume and approaching forcing will set the stage for rather widespread CI.
Expect ongoing but temporarily weakening cluster of elevated thunderstorms over SW France during the forenoon hours. Shear/CAPE fields continue to segregate, so severe risk should be low (despite locally marginal hail and heavy rain). 60kt SW-erly anvil-layer flow however may support thick cirrus shield to fan east, which could impact initiation there.
Debris of morning convection could inhibit diabatic heating over S-/CNTRL-France, which could delay CI there. Still, most models try to initiate storms around noon and embedded pockets with enhanced sunshine could lead to enough heating to surpass convective temperatures. Beyond noon, the focus for scattered to widespread CI turns to SE France and the Alps. A mixture of orographic initiation and forced convection is forecast. DLS remains in the 10-15 m/s range, so combined with 400-800 J/kg MLCAPE a mixture of multicells and isolated supercells is expected. Widespread CI should limit discrete storm structures, but during initiation and also interaction with mesoscale boundaries could lead to temporarily organized updrafts with large hail and severe wind gusts. In general, moderate steering flow exists over France, which weakens to less than 10 kt over Switzerland and Austria. Hence excessive rain is possible from SE France to S-Switzerland to parts of W/CNTRL Austria. Forecast soundings show deep/moist profiles with storm motions in hodographs of less than 10 kt. Elongated/thin CAPE profiles alsos support storms with copious amounts of small to isolated large hail. Modest PWATs however should keep the risks in check and hence we kept a broad level 1.
Storms continue well into the night although decreasing in coverage and intensity. Best moisture support exists over N-Italy, where a few storms might continue until 06 Z. Weak shear and diminishing CAPE should keep the nocturnal activity non-severe ... especially beyond midnight. A few elevated storms may also impact S-Germany and the Czech Republic as the wave lifts NE...nothing severe is anticipated.
... S-CNTRL Italy ...
SE-ward moving energy of aforementioned wave impacts the area of interest during the afternoon/overnight hours. Cap remains strong with warm temperatures up to 3 km AGL and substantial cool-down first starts during the night. Hence orographic storms and isolated CI along onshore moving sea breeze fronts will be the main foci for thunderstorm development. Pockets with up to 800 J/kg MLCAPE and 15-20 m/s 6 km bulk shear inidcate a large hail threat with any storm and a significant hail event can't be ruled out...next to severe wind gusts. Divergent model signals regarding QPF kept this in a level 1 right now...mainly due to limited thunderstorm coverage. The risk continues during the night.
... Ireland to CNTRL-UK...
During the daytime hours a warm front lifts north/northeast. Postfrontal warm sector remains mainly cloud covered, but a few breaks in the clouds could lead to some diabatic heating and to a few hundreds J/kg of SBCAPE. Forecast soundings would support favorable shear for rotating storms, especially for any storm next to the warm front. In case a (low-topped) thunderstorm manages to form and organize, an isolated tornado event will be possible. A level 1 was added mainly for the time period of 12-18 Z.
... Turkey ...
And yet another wave impacts parts of our forecast area. W/CNTRL Turkey resides beneath a gradually SE-ward moving trough with cool mid-levels. Steep mid-level lapse rates atop seasonably moist low-levels result in moderate MLCAPE of up to 800 J/kg during the day. Weak synoptic forcing and weak capping ensure scattered to widespread CI in an environment of 10 m/s DLS or less. Rapidly clustering and slow moving storms will be the result with excessive rain the main risk. During initiation, storms may temporarily become severe with large hail and a few severe wind gust events. In case GFS verifies with somewhat stronger DLS of 15 m/s, even a few supercell events with large to isolated very large hail will be possible. Nevertheless, any storm will rapidly grow upscale into slow moving clusters with an excessive rain threat. The risk gradually diminishes during the overnight hours with the main focus over S-Turkey (next to the warm/moist air mass over the E-Mediterranean).
... Further lightning areas ...
More regions with lightning activity exist, but either weak shear/CAPE or both should keep thunderstorms non-severe. The activity weakens rapidly after sunset.
