Storm Forecast
Valid: Sat 20 Jul 2013 06:00 to Sun 21 Jul 2013 06:00 UTC
Issued: Fri 19 Jul 2013 20:57
Forecaster: TUSCHY

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

A level 1 surrounds the level 2 mainly for the similar risks but with no extreme events expected.

A level 1 was issued for N-Morocco, N-Algeria and N-Tunisia mainly for an isolated but very large hail and severe downburst event.

A level 1 was issued for parts of France, Switzerland, parts of Austria and Italy mainly for large hail, strong to severe downbursts and heavy rainfall amounts. An isolated excessive rainfall event is possible. An isolated tornado event is possible, too.

A level 1 was issued for parts of Moldova, parts of the Ukraine into extreme W-Russia mainly for a few large hail and strong to severe wind gust reports.

SYNOPSIS

Another day is in store where a pronounced blocking takes place in the steering flow regime over Europe. A strong deep and warm high pressure area over the North Sea remains essentially unchanged regarding strength and placement compared to yesterday. The only sign of movement/expansion is seen along its southern fringe, where rising heights are forecast over Benelux and Germany. Along its southern fringe, the leftovers of the previous blocking event still meander to the ESE over S/C France and Italy. In fact, the strongest vortex will be the one over N-Italy with a closed circulation and a cold thermal print at 500 hPa, whereas the rest over S-France resembles more an elongated channel with cooler mid-levels (east-est aligned).

The strong upper trough over E/NE Europe continues to amplify and it approaches the northwestern part of the Black Sea during the overnight hours. This feature tries to constrict into a cut-off low, but another impulse approaches Norway/Sweden from the NW and probably delays any cut-off process at least beyond our forecast period.

An upper level ridge over far N-Africa expands a bit to the north and covers parts of the W-Mediterranean. A very hot and well mixed continental/desert air mass remains in place with convection once again being confined to the mountain areas along the coast.

At the surface, the southward blasting cold front gradually approaches Romania/Bulgaria with significantly drier postfrontal air affecting most parts of E-Europe. More synoptic fronts enter the scene from Norway/Sweden like a warm front, which will cross the Baltic Sea around midnight. Ongoing prefrontal low/mid tropospheric CAA and approaching WAA from the west result in more stable conditions spreading E/SE-wards (convective-wise).

DISCUSSION

... N/NE Spain and SW France ...

The basic set-up remains more or less unchanged with minor nuances, which however induce more noticeable changes regarding thunderstorm development/coverage.

The low/mid-tropospheric vortex just offshore of NW Portugal/far NW Spain, which was well seen in yesterday's VIS loops and which sent a cooler and more stable maritime air mass far inland, is forecast to weaken a bit while drifting to the N. However this features probably still influences far NW-Spain with the aforementioned cooler marine air mass and hence CI probabilities will stay low. Further east, along the coastline of N-Spain, another round of both daytime driven onshore flow (see-breeze event) and synoptically induced onshore flow by a daytime driven thermal low (C-Spain) is forecast. This thermal low also induces a persistent SE-erly flow over NE-Spain, so moist air from the W-Mediterranean spreads far inland. This air mass however will be a bit less moist (especially regarding BL depth of moisture) due to some drying/subsidence of the BL air mass beneath the surface high over the W-Mediterranean. However, we expect surface dewpoints in the mid to upper tens east of the Sistema Iberico, along and south of the Pyrenees and north of the Cordillera Cantabrica (excluding Galicia). Mid-level lapse rates of 7 to 8.5 K/km will be placed atop that BL moisture, so we will likely see another day with 1500 to 2500 J/kg MLCAPE build-up. A few forecast soundings once again show the chance for extreme CAPE build-up in the order of 3000 to 3500 J/kg MLCAPE in certain regions, where deeper BL moisture reduces diurnal mixing. Of interest is the increase of the temperature at 700 hPa by about 1 to 2 K, which causes a stronger cap.

Shear weakens a bit with a 6 km bulk shear decrease of 5 m/s forecast in all models. Therefore, expect around 10 m/s DLS over N-Spain and far SW-France and 10 to 15 m/s over NE Spain and further south. Of interest is a stronger 1-8 km shear pocket over NE Spain with 15 to 20 m/s shear. A few forecast soundings from this area show a reversed shear profile with E/SE-erly flow below 800 hPa, changing to westerlies above. Keeping capping in mind, updrafts might experience mainly speed shear, although any storm which manages to root into the BL will once again see an increase in veering/directional shear.

Compared to yesterday, initiation will be a bigger issue and probably more on an isolated scale. Thunderstorms evolve along the mountain areas with slow storm motions to the east. Expect rapid to explosive development due to the CAPE magnitude and rapid growth into well organized multicells or supercells.

The combination of fat CAPE profiles in the hail growth layer and aforementioned shear point to a large hail risk with any storm and a few very large hail events (e.g. hail diameter in excess of 5 cm) are likely. Beside the hail risk, severe downbursts and heavy rain will accompany those storms, too. Thunderstorms probably grow upscale into a few intense thunderstorm clusters, which could also affects extreme SW-France. An isolated tornado event is possible in case a supercell evolves and takes profit of lowering LCLs during the evening hours with somewhat stronger directional shear.

Despite limited thunderstorm coverage, another level 2 was issued to reflect the high impact potential of any thunderstorm, which evolves. The level 1 was expanded far south over N/NE Spain in case an isolated mountain storm manages to evolve.

For SW-France, thunderstorm initiation seems to be unlikely during the day, although we have to monitor residual outflow boundaries from the convection the night before. Right now, all data indicate strong capping, so confidence in CI is low. In case an isolated storm will be able to break the cap (e.g. due to inland moving see-breeze fronts in W-Aquitaine), explosive thunderstorm development is possible within a high CAPE (2000-3000 J/kg MLCAPE) and weak shear (less than 10 m/s DLS) environment. Large to very large hail, excessive rainfall amounts and severe downbursts accompany any storm. CI becomes more likely after sunset due to approaching clusters either from the Pyrenees or from the extreme SE Bay of Biscay. Large hail, strong to severe wind gusts and heavy rain are forecast with those clusters. The main reason for not expanding the level 2 further north is the capping problem and uncertainty in CI during the daytime hours. Therefore we kept a high-end level 1 for now.

... Rest of S/C France, Switzerland, parts of Austria, Italy and Albania ...

Another round of daytime driven thunderstorm development is anticipated within a belt from NW-France to the Alps. Effective PWs are forecast to decrease somewhat, so the rainfall risk should lower a bit. Also, the cap strengthens a bit and LL convergence signals will be a bit weaker compared to yesterday. Despite those changes, isolated to scattered thunderstorms are forecast to evolve after noon with an increase in coverage and intensity. Literally no shear is seen in forecast soundings, so expect slow moving or training thunderstorms. 1000 to 2000 J/kg MLCAPE support rapid thunderstorm growth and any storm is capable of producing large hail, strong to isolated severe downbursts (LCLs of 1.5 km and deep/dry subcloud layers) and heavy rainfall amounts. A few excessive and flash flood producing events are possible as thunderstorms might train along mesoscale convergence zones (from yesterday's activity) or due to the complex orography in Switzerland and the Alps. With high chances for colliding outflow boundaries and LLCAPE in excess of 150 J/kg, an isolated tornado event can't be ruled out. A few healthy looking clusters probably survive until midnight before the activity diminishes.

A similar risk is forecast for Italy and Albania although Albania was excluded from a level 1 due to lower CAPE and lower thunderstorm coverage.

... Moldova, Ukraine, extreme W-Russia and Belarus ...

The southbound moving cold front affects Moldova and the Ukraine during the daytime hours. This front outruns strongest forcing and enters a somewhat drier prefrontal air mass, so confidence in a robust MCS event remains low for the moment. However, the cold front will probably see an upswing in DMC during the daytime hours and with a combination of 15 m/s DLS and 400 to 800 J/kg MLCAPE, a few organized multicells are possible along the cold front. A broad level 1 was issued for a few large hail and strong to severe wind gust events. In fact I would not be surprised to see a swath of strong to isolated severe wind gusts during peak strength of that cold front (e.g. during the afternoon and evening hours). Thunderstorm probabilities decrease after sunset while the front moves offshore.

Further to the E/NE, the cold front becomes aligned more parallel to the background flow and most models forecast a separation of shear/CAPE. Still, 15 m/s DLS and 500 to 1000 J/kg MLCAPE result in numerous organized multicells along the cold front and ahead (likely along a prefrontal convergence zone) and large hail/strong wind gusts are likely. Upscale growth into numerous clusters is forecast and the main hazard changs to heavy rain during the evening hours. Those clusters gradually decay until midnight.

Finally, we expect scattered thunderstorms beneath the base of the upper level trough. Weak shear but 400 to 800 J/kg MLCAPE support strong to temporarily severe pulse storms with marginal hail and strong wind gusts. Low LCLs and enhanced LLCAPE could result in an isolated tornado event. This activity rapidly diminishes after sunset.

... N-Algeria/Morocco and N-Tunisia ...

GFS once again is the most bullish model regarding CI beneath the subtropical ridge. Steep mid-level lapse rates atop a coastal marine layer result in 1 to 2 kJ/kg capped MLCAPE. 20 to 25 m/s DLS is more than enough for high based/elevated supercells with a very large hail risk (probably in excess of 5 cm) and a few severe downbursts. Uncertainty in CI is high, as yesterday's storms remained south of our forecast area and today's models keep QPF signals on the lower end side. However a level 1 was issued for at least isolated thunderstorm activity... especially for N-Tunisia, which could feel the influence (some forcing) of a weak upper-tropospheric wave, which passes by to the north.