Storm Forecast
Valid: Tue 02 Jul 2013 06:00 to Wed 03 Jul 2013 06:00 UTC
Issued: Mon 01 Jul 2013 22:26
Forecaster: TUSCHY
A level 2 was issued for N-Algeria mainly for large hail and severe downbursts.
A level 1 was issued for the C-Ukraine and adjacent areas mainly for large hail and heavy rainfall amounts.
A level 1 was issued for E/NE Germany and W/NW/N Poland mainly for an isolated large hail and strong to severe wind gust event. A very isolated tornado event is possible in Poland.
A level 1 was issued for parts of N-Finland mainly for an isolated tornado and severe wind gust event.
A level 1 was issued for the Pyrenees mainly for an isolated large hail event.
SYNOPSIS
A strong vortex over Iceland steers numerous smaller scale vortices/troughs around its southern/eastern fringe to the east/north. The first crosses Finland from the south to the north/northwest, the second one enters the Baltic Sea and moves due north during the forecast while a third impulse evolves just west of Scotland during the overnight hours. Further south, a flat upper trough affects most of SW Europe, whereas the long-lived cut-off low over the Black Sea gradually shifts east out of our forecast area.
All those features support various spots with enhanced thunderstorm probabilities, although the severe risk remains modest at best (excluding N-Algeria).
DISCUSSION
... NE/E Germany, parts of Poland into S-Lithuania ...
A disintegrating and dragging front and abating QG forcing both support convection which continues from the overnight hours (e.g. from NE/E Germany). Convergence is still pronounced but this parameter becomes blurred during the afternoon hours.
Current thinking is that a cluster of showers and isolated thunderstorms emerges from the night/E-Germany and enters NW/NPoland. Those clusters move off to the NE while weakening (e.g. moving offshore). Not much thermal modification behind those clusters is needed for CI next to the aforementioned forcing mechanism, so expect a rapid increase in coverage of showers and thunderstorms within the 50-% lightning area. The main uncertainty is the placement of that area due to position issues of the front in models. Also, if convection of the night before becomes more active than currently anticipated, the front might find itself further east/southeast than currently shown in numerical data. Hence the 50-% lightning area was expanded far west into NE/E Germany but also far northeast/east well into Poland.
A shear/CAPE overlap exists from NE Germany into far N-Poland and all the way to Lithuania. Forecast soundings in this corridor reveal bad kinematic profiles in the lowest 1 to 2 km with a rapid improvement above as a 15 m/s (3 km AGL) speed maximum gradually exits the area to the NE. The wind speed atop at 5 km reveals similar strength, so hodographs show only modest length and no curving. A mix of NE-ward moving mutlicells/pulsating storms is expected. Forecast soundings support the idea that storms become surface based before noon with a better downward transport of stronger winds, so strong to isolated severe wind gusts and a few large hail events are likely. Despite meager looking LL wind profiles, NW Poland has a short overlap of 100 J/kg LL CAPE, LCLs below 800 m, some helicity and good background shear, so a very isolated tornado event can't be ruled out in case a storm exhibits temporal supercellular structures. The level 1 was expanded into E-Germany where stronger veering may compensate somewhat weaker shear. This part of the level area is a low-end one however. Thunderstorms gradually decay after sunset (excluding NE-Poland into Lithuania where an isolated thunderstorm may survive until midnight due to better MUCAPE and some lingering forcing).
...Alps and parts of France until 21 Z...
Beneath a weak upper trough, daytime and orographically driven thunderstorms are forecast. Even EZ points to local MLCAPE build-up in excess of 800 J/kg, so an isolated strong to temporarily severe thunderstorm is possible from the Alps towards S/C France. Slow storm motion increases the heavy rainfall risk, but strong wind gusts and isolated large hail are possible, too. Most models also agree in clustering storms over the N/C part of the Massic Central, but we think that the air mass will already be worked over by storms before shear strengthens during the afternoon/evening hours. Hence we did not upgrade to a marginal level 1. The activity weakens around sunset (excluding the one over France ... see second section below for more information)
... Pyrenees ...
A marginal level 1 was added for those storms which evolve along the Pyrenees. Despite weakening mid-level lapse rates / mid-level warming and height rises, orographically forced thunderstorms likely evolve during the afternoon and evening hours ... probably on an isolated scale. Shear profiles improve somewhat during that time mainly within the 0-3 km layer (15 to 20 m/s), so some temporal updraft organization is possible. MLCAPE up to 800 J/kg and aforementioned shear may support an isolated large hail and strong wind gust report. Thunderstorms rapidly decay after sunset.
... SE-France, W-Switzerland into W-Germany beyond 18 Z ...
Despite dampening effects due to increasing WAA of an upstream trough, an impulse is forecast to cross N-France while moving rapidly to the E/NE. Weak PVA ahead and the right entrance of a 20 m/s mid-level jet next to favorable cross-barrier flow towards the W-Alps and the Vosges support an increase in showers/thunderstorms around sunset. A lot depends on how far north the CAPE plume will be drawn, so a region between Strasbourg and Frankfurt am Main might take profit of 15-20 m/s DLS. There, an isolated better organized thunderstorm might evolve between 15-21 Z, but the main activity will be weakly organized ... the same for storms further south, which decay before midnight. Strongest storms might produce an isolated large hail and strong wind gust event. During the overnight hours, NE-ward fanning MUCAPE plume nuzzles along the vorticity lobe, so an isolated thunderstorm event is possible over all of SW/C/NE Germany. Nothing severe is anticipated. A few night time thunderstorms may evolve outside of our 15-% lightning area (N-France into Belgium/the Netherlands), but confidence remains too low for anything bigger than a few short-lived thunderstorms. In case a more substantial surge of the moist/unstable air occurs to the north, an update might be needed later-on.
... N-Algeria ...
A flat upper trough approaches the area from the west with some forcing affecting the coastal area of N-Algeria. With cooling at mid-levels, a few elevated thunderstorms likely evolve after sunset. This activity continues well into the night. Large hail will be the main risk due to 20 m/s DLS and healthy looking CAPE profiles in the hail growth layer. A few very large hail events are very well possible. High-based storms might also produce a few severe downbursts. With past model runs showed a constant sign of lowering surface pressure and enhanced coastal convergence, confidence in higher thunderstorm coverage (e.g. a cluster of elevated storms) is high enough to issue a level 2.
... Parts of N-Sweden and N-Finland ...
A very complex forecast unfolds for those areas. A low with a strong vertical tilt affects the area and shifts only slowly to the N/NW. Despite an heavy rainfall event for N-Sweden (not included into our probabiities due to missing/very isolated thunderstorms activity), a concentrated area over N-Finland might see a few severe thunderstorms during the afternoon. Models still diagree with the structure of that low and therefore also with the dispersal of moisture, CAPE and shear, but a few forecast soundings show impressive shear profiles with at least some low-end CAPE. As cold front pushes north, a moist post frontal sector covers the level 1 area around noon onwards. Winds at low/mid-levels increase again due to strengthening upstream ridging, so showers/isolated thunderstorms migh see a favorable environment for organization ... at least for a few hours during the afternoon. Isolated large hail and strong wind gusts will be the main risk but I don't want to rule out an isolated tornado event, too. The risk rapidly diminishes after sunset.
... C-Ukraine ...
Moisture becomes squeezed due to strong convergence and results in a NW-SE band of enhanced CAPE. No serious shear is forecast, so slow moving thunderstorms with rapid upscale growth into numerous clusters are forecast. MLCAPE up to 1500 J/kg may support a few organized and severe storms with large hail and heavy rainfall before upscale growth occurs. Thereafter, cold pool driven storm clusters might produce concentrated swaths with strong wind gusts. Otherwise isolated large hail and widespread heavy rain will be the main hazard. Thunderstorms continue well into the night.
