Storm Forecast
Valid: Fri 19 Jul 2013 06:00 to Sat 20 Jul 2013 06:00 UTC
Issued: Fri 19 Jul 2013 01:23
Forecaster: TUSCHY

A level 2 was issued for NE Spain mainly for large to very large hail, severe downbursts and heavy rainfall amounts. An isolated tornado event is possible.

A level 1 surrounds the level 2 for a similar risk but with no extreme events forecast.

A level 1 was issued for parts of France, Switzerland, parts of Austria and Italy mainly for large hail and strong to severe wind gusts. Excessive rainfall amounts are possible.

A level 1 was issued for N-Algeria and N-Tunisia mainly for large to isolated very large hail and severe downbursts.

A level 1 was issued for Latvia, Lithuania, Belarus and parts of extreme W-Russia mainly for severe wind gusts, isolated large hail and an isolated tornado event.

SYNOPSIS

The big picture reveals no change in the blocking pattern over Europe although the type of blocking changes from an high-over-low blocking to an evolving messy looking omega-like situation. The high pressure area over Scotland/Ireland shifts further east , while a pronounced upper low over Finland amplifies further to the south. This pattern supports very warm and dry conditions over NW Europe with a more serious cool-down forecast over the Baltic Sea and areas to the east.

Leftovers from the dissolving high-over-low pattern, featured by numerous negative mid-tropospheric thermal anomalies, move either to the west (caught in the steering flow of the strong high pressure area to the north) or drift to the east and affect France, Italy and the far western Balkan States during the forecast. Hot and stable conditions continue over the Aegean Sea, Turkey and the far eastern Mediterranean.

At the surface a strong cold front pushes south from Latvia and Lithuania (06Z) to far S-Poland and the NW Ukraine (06Z, 20th July). Despite that synoptic front, various mesoscale foci (e.g. sea-breeze fronts or outflow boundaries from the overnight activity) will be in place to assist in an active convecive weather day over numerous parts of Europe.

DISCUSSION

... N-Spain into SW-France ...

Rising mid-level heights due to the retreating high pressure area to the north and falling pressure/heights west of Portugal allow a shift of the EML plume a bit to the north/northeast ... now also covering all of N-Spain and parts of extreme SW France with 7.5 to 8 K/km mid-level lapse rates. A mixture of onshore flow due to the diurnal peak strength of the Spanish thermal low over NE Spain and inland moving sea breeze fronts once again results in high BL moisture over most of N/NE Spain. Surface dewpoints in the mid to upper tens will be common with regional peaks in excess of 20 °C. This combined with the steepening lapse rates results in moderate to high MLCAPE values with most models showing 1000 to 2000 J/kg over NE Spain into the Pyrenees and 2000 to 3000 J/kg from the western Pyrenees to the Cordillera Cantábrica and to the Sierra de la Demanda. Higher values /extreme CAPE in excess of 3000 J/kg MLCAPE / can't be ruled out.
The exact temporal and regional shear distribution is a bit hard to determine due to the passage of at least two mid/upper waves. The first one induces 15 to 20 m/s 0-6 km shear over NE Spain during the daytime hours with some weakening forecast around sunset to 10 to 15 m/s. With the approach of the second impulse during the night, DLS remains constant at 10 to 15 m/s, so plenty of shear to work with. Forecast soundings reveal good veering with height and nice shear profiles within the hail growth layer.
The main uncertainty will be the magnitude of initiation, as models like GFS and EZ still show ongoing QPF differences. Nevertheless, we expect at least isolated to scattered thunderstorms either due to strong diabatic heating or due to the passage of those mid/upper waves. Any storm which evolves will see rapid to explosive development into well organized mutlicells or supercells. Fat CAPE profiles within the hail growth layer in combination with aforementioned shear profiles and steep lapse rates favor large to very large hail with any mature thunderstorm. Hail size might exceed 5 cm in diameter for a few events. Especially when moving a bit inland, an increase in LCLs is noted and deeply mixed subcloud layers (locally up to 600 hPa) also show an enhanced risk for severe downburst events. Also, given amount of CAPE and regional enhanced LL moisture depth/low LCLs, an isolated tornado event can't be ruled out despite weak LL shear profiles. Enhanced helicity along either mesoscale boundaries or due to local mountainous wind regimes might induce enough shear for an isolated event.
During the evening hours, thunderstorms grow upscale into a few clusters. A movement to the east might be possible given the predominant westerly flow regime, but strong cold pools could drive those clusters in various directions ... that's why we added SW-France also into a level 1 despite very low-end QPF signals from most models. Large hail and strong to severe wind gusts next to heavy rainfall amounts can be expected.

The level 2 was issued for those areas, where all models agreeded in initiation. The level 1 however was issued far to the south (even further south what models show) due to expected mid-level (capped) elevated instability, which should activate a few storms at least on an isolated scale. Thunderstorms keep going all night long especially along the Pyrenees, where an overnight thunderstorm cluster likely offers ongoing chances for severe thunderstorm reports until 06Z.

... Parts of France, Switzerland, parts of Austria and Italy ...

An extensive WNW-ESE channel with lower 500 hPa heights runs from W-C France all the way to the Adriatic Sea. Somewhat cooler mid-levels overspread a moist boundary layer air mass, which features dewpoints in the mid to upper tens. This rich BL moisture content helps to offset weaker mid-level lapse rates, so we expect a broad area with 1000 to 2000 J/kg MLCAPE ... regionally higher values will be possible. Shear at all levels remains weak (e.g. 0-6 km shear below 10 m/s), but 1-8 km shear offers somewhat stronger signals over C and S-France (15 to 20 m/s shear). Hence, stronger anvil layer shear likely assists in some advection of precipitation away from the updrafts and therefore in longer updraft/downdraft separation ... ergo longer lived thunderstorms. Scattered to widespread thunderstorm initiation is forecast with diurnal weakening cap strength and mesoscale forcing along numerous convergence zones. Rapid thunderstorm development is anticipated and any storm will be able to produce large hail and severe wind gusts, mainly during the discrete storm stage. However, rapid thunderstorm clustering is anticipated fairly soon after initiation. Cold pool driven swaths of strong to severe wind gusts and isolated large hail will be the one risk with numerous smaller clusters/temporal MCS structures. The other hazard however will be excessive rainfall amounts due to slow storm motion and effective PWs well in excess of 30 mm. This risk might be enhanced due to backbuilding/training activity along mesoscale boundaries or due to the complex orography in Switzerland and Austria. A more serious flash flood risk could arise betimes with those storms.

A similar situation is expected for Italy, where weak shear and MLCAPE around 1000 J/kg is forecast. A very moist air mass is in place, so excessive and flash flood producing thunderstorms are very well possible.

Due to the interaction of various outflow boundaries and expected amount of LLCAPE (locally in excess of 200 J/kg), an isolated tornado event is posisble.
Thunderstorms last well into the night, but wind down beyond midnight.

A broad level 1 should cover that event, although a few regions might see level 2 conditions (mainly due to rainfall amounts). It is hard (if not impossible) to pinpoint those areas that far out, especially with so much influence of the mesoscale.

... Dinaric Alps ...

Scattered daytime driven thunderstorms are forecast. Shear will be weak, but 500 to 1000 J/kg MLCAPE still offers a good chance for a few severe pulse storms. Isolated large hail and a few strong to severe downbursts are possible. Heavy rain may be a temporal hazard during the clustering of slow moving thunderstorms, but the activity remains weakly organized, so no level area will be issued. The thunderstorm activity gradually diminishes after sunset.

... Latvia, Lithuania, Belarus, NE Poland and far W-Russia ...

A southbound moving cold front will be the focus for scattered to widespread thunderstorm development. A potent 40-50 m/s 300 hPa jet develops within the western fringe of a potent upper low and moves southbound. A favorable interaction of the surface front, the left exit region of the mid/upper jet and a southbound moving PVA maximum is anticipated. An overlap of 15-20 m/s DLS and 500 to 1000 J/kg MLCAPE should result in rapid thunderstorm organization. Expect early thunderstorm initiation along the cold front with thunderstorms moving rapidly to the S/SE. An upscale growth into a large MCS is forecast with strong to severe wind gusts and isolated hail the main risk. However, forecast soundings show some marginal veering and low LCLs just ahead of the cold front, so an isolated tornado event can't be declined. During the evening hours, a gradual cool down of the prefrontal air mass and the influx of somewhat drier air from the Ukraine should induce a more rapid weakening trend of thunderstorms and no severe risk is expected during the overnight hours.

... N-Algeria and N-Tunisia ...

EZ and GFS both show some low-end probabilities for isolated thunderstorm initiation during the afternoon and evening hours. Forecast soundings show strong capping but initiation along the mountain peaks can't be ruled out. Therefore we expect a few thunderstorms around/just after peak heating. Very steep-mid-level lapse rates and a maritime layer along the coast result in 800 to 1500 J/kg capped MLCAPE with 20 to 25 m/s DLS. Any updraft which survives entrainment could produce large to very large hail and severe downbursts. Expected low thunderstorm coverage however kept us from issuing a level 2. An upgrade may be considered, if confidence in more scattered thunderstorm activity increases with later model runs.