Storm Forecast
Valid: Fri 22 Jun 2012 06:00 to Sat 23 Jun 2012 06:00 UTC
Issued: Fri 22 Jun 2012 02:19
Forecaster: TUSCHY
A level 1 was issued for NE Italy, S-Austria, the Balkan States, Romania, Ukraine and parts of Belarus mainly for large to very large hail, excessive rainfall amounts (locally life-threatening flash flood events possible), and an isolated severe wind gust and tornado event.
A level 1 was issued for NW-Belarus and parts of Latvia and Lithuania mainly for heavy rainfall amounts.
A level 1 was issued for Benelux and extreme NW-Germany mainly for an isolated tornado event.
A level 1 was issued for E-UK/far SE Scotland mainly for an isolated tornado and flash flood event.
SYNOPSIS
A deep cyclonic vortex still affects most parts of W/NW Europe with diffluent streamline pattern ongoing over E-Europe. Stable and hot conditions prevail over the Mediterranean.
DISCUSSION
... SE Europe...
At least two SE-ward surging convergence zones (weak cold fronts) will serve as foci for scattered TSTM initiation. Another interesting feature will be a weak PV streamer, which moves eastwards betimes. Also, active overnight MCS activity (e.g. Hungary) will push outflow boundaries to the south, which also serve as foci for enhanced CI (e.g. S-Hungary). Hence, we're not sure if current model suite is too far north with DMC activity, as overnight thunderstorm activity exceeds model forecasts. Finally, a weak impulse at mid-levels also crosses our area of interest during the day from SW to NE.
Despite aforementioned various diffuse forcing mechanism, air mass remains supportive for long lived DMC as inflow parcels reveal high moisture content (e.g. 0-1 km average mixing ratio in excess of 15 g/kg) although highest moisture remains confined to the strongest synoptic fronts (e.g. a gradually SE-ward progressing cold front over S-Hungary and W-Ukraine during the afternoon hours). EML plume remains displaced to the SE, so despite very favorable BL moisture quality, final CAPE magnitude may be lowered by near neutral lapse rate forecasts. Nevertheless, 1-2 kJ/kg MLCAPE evolve and assist in vigorous convection.
Ongoing highly diffluent streamline pattern persists, so DLS remains weak with 10-15 m/s within lowest 6 km. Modest anvil-layer shear and marginal stronger flow next to the mid-level impulse may support well organized multicell clusters and isolated supercells.
This forecast remains persistent to the past 48 h further to the west, where slow moving storm clusters produced torrential rain and large to isolated very large hail (e.g. over Austria). Expect similar risks with today's activity. An extreme hail event is possible mainly during the initiation/discrete stage of any storm. Storm clustering however boosts the significant rainfall risk and locally dangerous/life threatening flash flooding is possible. Strong to isolated severe wind gusts accompany stronger storms and also an isolated tornado event is in the realm of possibility (e.g. mesoscale accidents due to clashing outflow boundaries within high-CAPE environment).
Thunderstorms continue well into the night.
A broad and high-end level 1 was introduced as confidence in a confined level 2 event is too low that far out (as severe coverage will be dictated by the mesoscale). Current thinking is that a few events may match our level 2 on the local scale and an upgrade may become possible later-on.
We expanded the level 1 far to the NW over NW-Belarus and parts of Latvia and Lithuania to account for some elevated thunderstorm activity during the night with northward advecting tongue of modest MUCAPE beneath diffluent flow regime and cooling mid-levels. A few slow moving storm clusters may result in heavy rain over the area of interest.
... N-Italy ...
A stalling frontal boundary over N-Italy may result in isolated CI, despite lack of forcing and weak to modest capping. Numerous subtle short waves cross N-Italy during the forecast period, which may induce enough background lift for isolated CI along the S-Alps next to the aforementioned stalling boundary further to the south. Any storm will find itself in a favorable environment for rapid storm organization. 20-25 m/s DLS and 500-1000 J/kg MLCAPE partially overlap (mainly over NE Italy) , so an isolated multicell/supercell event is possible. We issued a level 1 for NE-Italy as models agree quite well for isolated CI. Large hail and a few severe downbursts will be the main risk. Confidence in CI further to the west (e.g. from Bologna to Genua) is too low for a level 1, so only a 15-% thunderstorm area was introduced. Level 1 conditions may easily evolve that far to the W/SW, if the surface front can initiate a few storms (also large hail and severe downbursts the main risk). Storms gradually decay after sunset.
...E-Spain...
A very sharp dryline (up to 15K/100 km dewpoint gradient) is currently present over far SE Spain (01Z). Placed on the anticyclonic side of a 30 m/s high-level jet over N-Spain, stable geopotential height fields at 500 hPa are forecast with good diabatic heating forecast. Hence, a strong thermal low (onshore) may result in an inland moving sea breeze front, which advects moisture from the Mediterranean to the west (dewpoints in the upper tens/lower twenties). Hence, isolated thunderstorm may evolve mainly along the rough topography over far E-Spain. We used the high-resolution HIRLAM for highlighting the final thunderstorm probabilities. Final probabilities were expanded a bit more to the north to account for a weak mid-level thermal trough, which affects NE Spain around peak heating. 20 m/s DLS with locally modest CAPE (500-800 J/kg MLCAPE) may assist in an isolated well organized multicell/supercell with large hail and strong wind gusts. The risk diminishes after sunset.
... Benelux and far NW-Germany ...
Very mixed signals are present for today's forecast. Any forcing remains weak/diffuse, but favorable mid-level jet configuration may temporarily assist in better lift during the afternoon hours (with strongest subsidence just to the east). Another limiting point will be quite thick cloud cover within the moist postfrontal air mass. However, a few high resolution models indicate some breaks in the cloud fields, so at least low-end diabatic heating is possible (and proably already enough for initiation). Right now, we expect showers and isolated thunderstorms during the afternoon and evening hours, affecting Benelux and far NW Germany. Main concern right now will be a few shallow supercells with warm EL temperatures forecast. Enhanced LL CAPE and 15 m/s LL shear (200 m^2/s^2 SRH-1) will overlap and hence any stronger updraft may be able to rotate with an isolated tornado risk. We issued a level 1/15-% lightning probability overlap for this event. Convection rapidly vanishes after sunset.
... UK ...
Placed beneath the deep cyclonic vortex and with adequate BL moisture present for roughly 500 J/kg SBCAPE, a few thunderstorms are forecast, mainly over E-UK. With augmented LL CAPE and weak background shear, a few funnels/short lived tornadoes are possible, next to marginal hail. Slow storm motion and enhanced inflow from offshore areas may also increase a localized flash flood risk. The risk diminishes after sunset.
