Storm Forecast
Valid: Sat 15 Jun 2013 06:00 to Sun 16 Jun 2013 06:00 UTC
Issued: Fri 14 Jun 2013 22:16
Forecaster: TUSCHY

A level 1 was issued for C-UK mainly for an isolated tornado and scattered strong to isolated severe wind gust risk.

A level 1 was issued for far N-Germany and Denmark mainly for an isolated tornado and scattered strong to isolated severe wind gust risk.

A level 1 was issued for extreme NE Bulgaria into C-Ukraine and adjacent areas into Russia mainly for a few large hail and severe downburst events.

SYNOPSIS

A robust low with strong thermal anomalies especially at mid/upper levels advances east and crosses Ireland/UK during the forecast from west to east. An unseasonably intense wind field along its southern fringe spreads east and pulls the complete feature all the way to the North Sea until Sunday 06Z. It's not common to see such an healthy and progressive feature during June but luckily its wind field does not overspread any serious CAPE plumes over NW/N Europe.
Eastwards building westerlies with that feature assure height rises over the W/C Mediterranean with mostly stable conditions. A broad but ill defined upper trough over SE Europe lifts slowly to the N/NE during the forecast, so most of SE/E Europe will see another round of active thunderstorm development.

At the surface, an extensive W-E aligned boundary becomes reinforced by a SE-ward moving cold front from France...at least its western fringe. This boundary will be the focus for isolated to scattered CI.

DISCUSSION

... Parts of UK ...

A severe risk could arise for a few hours during the afternoon due to perfect matching of modest (300-600 J/kg) SBCAPE, strong shear and some forcing. A plume of wrap-around moisture spreads east and covers parts of C-UK during peak time heating. Mid-level lapse rates remain steep due to cold mid-levels and with some temporal diabatic heating expected, SBCAPE build-up of 600 J/kg looks reasonable for a confined region. A weak impulse, embedded in this brisk westerly flow regime, crosses the area at the same time and inserts enough lift for isolated to scattered CI. DLS rapidly increases from N to S from 20 to 35 m/s and LL shear peaks at 10-15 m/s. Forecast soundings reveal mainly straight wind fields and therefore quite unidirectional hodographs. However, incoming impulse/wave also induces some slight backing of the LL winds, so directional shear in the lowest 1000 m should increase to 100-200 m^2/s^2. Also, with expected shear in mind, splitting storms may occur with deviant storm motions also increasing helical inflow. LCLs remain at or below 800 m, so there is a risk for an isolated tornado event. Beside that, downward mixing of 20 m/s at 850 hPa may also produce a few strong to severe wind gusts next to an isolated large hail risk. The main thunderstorm risk will be confined to the 12-18 Z time frame. Beyond sunset, thunderstorms rapidly weaken while moving offshore.

... The Netherlands, NW Germany and Denmark ...

A wave rotates rapidly around the main cyclonic vortex over UK and acquires a negative tilt. This trough spreads rapidly east and causes a strongly diffluent mid/upper flow pattern to overspread the area of interest during the daytime hours from the Netherlands to the E/NE. Such a pattern is not favorable for healthy and compact UVV maxima and current data also points to a gradual weakening of the forcing. Some better BL moisture becomes advected into the NE-ward shifting trough/wave and a pool of marginally more favorable mid-level lapse rates crosses the region .. with N-Germany/Denmark probably being affected during peak heating. Also with such patterns, mid-level cooling won't play any serious role until the wave exits to the north with better/depper CAA expected by then. Hence, EL temperatures remain quite warm and if DMC can evolve is still a bit unclear. The risk however increases from south (N-Germany and the Netherlands) to north (Denmark). We issued a 50-% lightning area, where DMC is expected right now.

Ingredients-wise, strongest flow starts above 700 hPa, where convection might already top out, so how much effective DLS will become available is difficult to pinpoint. However, any stronger and longer lived convection will evolve in an environment, featured by 300-700 J/kg SBCAPE (increasing to the north), 15-20 m/s DLS and veering profiles. Marginal hail and strong wind gusts will be the main hazard. In addition I don't want to rule out an isolated tornado event mainly in the level 1 area with low LCLs and better shear. In fact the upgrade to a level 1 was performed due to forecast soundings along the German/Danish border, which show some curvature in the lowest 3 km, strong speed shear, veering in excess of 45° between LL and mid-levels in addition to 400-500 J/kg CAPE. The risk rapidly vanishes after sunset.

... S-France to the Alpine region to W-Ukraine ...

The frontal boundary will be the focus for at least isolated CI. Numerous weak mid-level impulses cross that boundary during the forecast and they, combined with the rough orography, should overcome mainly weak capping. Yesterday's sounding from S-France to Austria already featured ill mid-level lapse rates due to deep WAA from 700 hPa upwards. Lapse rates increase somewhat towards the W-Ukraine but still remain marginal. Local BL dewpoints in the mid tens with averaged BL mixing ratios exceeding 10 g/kg (on a regional scale) could offset those negative effects of the lapse rates. Hence, numerous patches with 300-800 J/kg MLCAPE probably evolve. CI over S-France into Switzerland will be questionable due to very weak dynamics but most models show at least some very isolated initiation and we therefore issued a 15-% lightning area. Confidence in initiation increases somewhat from E-Switzerland towards Austria due to the passage of a weak impulse during the late afternoon and evening hours from W to E. We issued a 50-% lightning area, where initiation looks most likely and a few strong to temporarily severe storms might evolve as DLS increases to at or above 15 m/s with higher 1-8 km shear expected. Isolated large hail and strong wind gusts are forecast with those storms which rapidly decay after sunset.
Similar confidence in CI and expected severe probabilities exists further east over Slovakia into W-Ukraine.

... Bulgaria to C-Ukraine into parts of W-Russia ...

A weakening frontal boundary enters the forecast area and is expected to dissolve until 06Z. Nevertheless, some moisture pooling along that convergence boundary occurs beneath moderately steepened lapse rates. Hence, 800 - 1500 J/kg MLCAPE likely evolve with virtually no shear present at all levels. Numerous intense pulsating storms likely pose a large hail and severe downburst risk before upscale growth into numerous ill defined clusters is forecast. Somewhat stronger convergence with higher effective PWs are forecast from the C-Ukraine to the N/NE. Slow moving or training storms might pose a flash flood risk in those areas due to heavy rainfall amounts, but neither the moisture nor the strength of convergence nor the strength of the inflow look healthy enough to expect any serious flood concerns. However the strongest storm clusters evolve from the C-Ukraine to the N/NE with more ill defined clusters further to the south into Bulgaria. Especially along the boundary, storms continue well into the night with a deacreasing severe risk.