Storm Forecast

Storm Forecast
Valid: Tue 16 Jul 2013 10:00 to Wed 17 Jul 2013 06:00 UTC
Issued: Tue 16 Jul 2013 10:08
Forecaster: PISTOTNIK

A level 1 area was issued for N and E Spain mainly for large hail and severe wind gusts.

Level 1 areas were issued for S France, NW Italy and for parts of Romania and Bulgaria mainly for excessive precipitation and to a lesser extent for large hail and severe wind gusts.

SYNOPSIS

Weak geopotential gradients still prevail across most of Europe. Weak upper-level lows are present over Western Russia, Romania/Bulgaria and the Bay of Biscay. Near the surface, the circulation is governed by thermal heat lows over Iberia, Northern Africa and Turkey, as well as by local wind systems in response to diurnal heating. In general, the lapse rates have weakened across the Southern half of the continent compared to most of last week, which is why particularly rich low-level moisture (e.g. in Southern France, the Po Valley, and in general along the Mediterranean coasts) is necessary to create at least moderate CAPE values.
A strong frontal zone is only present over Northernmost Europe, where the most prominent cyclone crosses Finland. However, rather low-topped convection is thought to be confined to the weakly sheared low pressure center.

DISCUSSION

... Spain ...

The thermal heat low will again advect maritime moisture inland. Some of this moisture will be mixed out with daytime heating, but the last days have shown again that even inland stations reported fairly high dew points around 15C, notably higher than forecasted by any model. At least locally, CAPE on the order of 1000 J/kg or even higher should be realized again, though it will continue to be strongly capped in coastal areas.
Since the upper-level flow has relaxed a bit, 0-6 km shear has decreased to 10-15 m/s and storms will experience not so much "anvil venting" any more. However, the pronounced sea breezes / upslope flows and their counter-current in the free atmosphere concentrate much of this shear within the lowest 3 km, where also storm-relative helicity may be enhanced to 100-150 m^2/s^2 in places.
Scattered to widespread thunderstorms are expected again over the Northern third of Spain, and kinematic conditions still seem favourable for multicells and isolated supercells. Compared to last week with its impressive elevated mixed layer, profiles have moistened but lapse rates have decreased somewhat. This means that the threat for large hail and severe wind gusts has decreased a little bit, but a level 1 still seems to be adequate. On the other hand, localized flash floods now pose an additional threat with slow-moving storms and a persistent moist inflow.

Further South, deeper boundary layer mixing and entrainment will keep thunderstorm coverage low in inland areas, and a strong cap is still in place along the coasts. However, a short-wave trough spreads across Southern and Eastern Spain in the course of the day, and deep-layer shear increases to 25 m/s beneath the subtropical jet stream. In case the lift by sea breezes and the support by the trough is strong enough to break the cap and feed the coastal moisture into the updrafts, strong multicells and supercells with a more pronounced threat of large hail and severe wind gusts are expected. If confidence in convective initiation were higher, a level 2 would be required for Eastern Spain. However, with only rather isolated storms expected, it was decided to stay with a level 1.

... other 50% lightning areas ...

Diurnal heating will result in low to locally moderate CAPE under weak vertical wind shear. Scattered to widespread afternoon thunderstorms are forecast over orographic features and may move some distance into flatter terrain in case better organized outflow boundaries form. Strong pulse storms pose a threat of heavy rainfall plus isolated events of marginally large hail and severe downbursts. However, in general their density will not be too high.
Level 1 areas were issued for Southern France, Northwestern Italy, and parts of Romania and Bulgaria where the lift support of travelling vorticity maxima overlaps with rich low-level moisture and the highest precipitable water, and where thunderstorm coverage will likely be maximized.

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