Storm Forecast
Valid: Fri 23 May 2008 06:00 to Sat 24 May 2008 06:00 UTC
Issued: Thu 22 May 2008 18:20
Forecaster: TUSCHY

SYNOPSIS

Long advertised by global models and now underway is a major upper - level cold pool, dripping off south of Iceland. Geopotential height gradients between this feature and an eastward building high pressure area over the Azores strengthen considerably, resulting in a persistent 50m/s upper - level speed max on its upstream side pulling the whole feature southwards. This compact upper -level low will reach the western part of the Bay of Biscay during the forecast period with widespread surface pressure fall over Spain. Geopotential heights rise downstream of this system including the central Mediterranean and rapidly steepening mid - level lapse rates SW of Sardinia hint on a developing EML, spreading slowly northwards after 00Z. An intense frontal boundary has established during the past 24 hours or so over far western Russia and the temperature gradient along this front increases during the forecast period. Cold and dry conditions prevail over far northern Europe.

DISCUSSION

... Portugal and Spain ...

An interesting forecast is in store for most parts of Spain with both an excessive rainfall and strong to severe thunderstorm threat.

22th May, 14Z IR images show the southward digging upper - level feature just west of Ireland and this overall trend will continue during the next 1 - 2 days or so. Constellation with persistent speed max along its upstream side result in a sharp upper - level trough axis west of Portugal and a highly diffluent streamline pattern futher eastwards including Spain and Portugal. At lower levels a somewhat diffuse and very slowly eastward moving cold front ( due to near parallel alignment to the background flow ) will cross the area from the west, reaching a line Faro, PT - Madrid, ES - Toulouse, FR during the evening hours ( ~ 18Z ). We have to split the forecast area as there exists two different risk areas, arising by this constellation :

... Portugal, SW / W / NW and Central Spain and extreme SW / W France ...

Slow moving front will be the focus for scattered to widespread showers and thunderstorms. Upper - levels favor persistent re - development of convective activity during the day as atmosphere along and west of the front stays mostly uncapped. Strong DLS of 15 - 20 m/s will be present and a few strong thunderstorms with marginal hail and strong wind gusts will be possible but overall messy setup should exclude discrete storm mode and favor rapid clustering of thunderstorms with an excessive rainfall threat for the area of interest.

... E / NE Spain and parts of SW France ...

An environment conducive for severe thunderstorms arises over those areas east of the eastward trailing cold front at 12Z onwards . Some warming at low / mid - levels is forecast but axis of strong thermal ridging should stay to the east of the area of interest so capping should supress activity until the midday hours before breaking. Dewpoints over NE Spain are already in the lower tens and latest synop data reveal a pool of mid to upper tens just east of Spain ( e.g. Balearic Islands ). As the cold front approaches during the afternoon / early evening hours, wind field should back and help to pull this moisture northwards, also affecting NE Spain. Mid -level lapse rates, although somewhat weaker compared to the past few days, are still steep enough for up to 500 J/kg MLCAPE release. The main area of concern is NE Spain as front arrives during peak time heating. Some veering in the lowest 3km is present ahead of this boundary with SRH 1 and 3 values of 200 m^2/s^2 and 250 m^2/s^2 respectively and 10m/s / 20m/s LLS / DLS. The LCL should be around 1000 m at 12Z but is forecast to decrease to below 500m between 15Z and 18Z with parcel layer depth increasing during the same time. All this points to an enhanced tornado risk especially when more discrete thunderstorms could evolve. We decided to stick with a high - end level -1 as strong meridional component of mean flow provides some uncertainties regarding the final storm mode. At the moment NE Spain is the most likely region which could be upgraded to a level -2 in later updates. Large hail and severe wind gusts as well as torrential rain due to very slow frontal movement will be all possible, too. Thunderstorms over NE Spain and SW France will keep going during the night hours with an upscale growth to one or two clusters, which move northwards while weakening as BL stabilizes and instability decreases.

Latest GFS runs develop another area of convection SW of the Balearic Islands around 03Z but confidence in this scenario is too low for the moment. Shear and instability would be adequate for organized thunderstorms and an update may be needed later - on.

... Eastern Belarus and parts of western Russia ...

Strong thermal gradient was already mentioned in the synoptic discussion and this slowly northward pushing warm front will be the focus for a few organized thunderstorms. Airmass south of this front gets characterized by quite steep mid - level lapse rates, dewpoints at the surface between 10 and 14°C and a large T - Td spread due to strong diabatic heating. Yesterday's 12Z sounding of Suhinici had 1.5 kJ/kg CAPE and there are no reasons why this same region should not get such nice thermodynamic profiles again. LL Cape release south of this front is impressive also indicating that only low - end dewpoint variations could lead to significantly augmented instability release at all levels. Hodographs along this boundary are strongly enlarged and curved and LCLs should stay quite low just south of the front. Any thunderstorm developing in this environment would be up to produce tornados and large hail. To the north of the front, thunderstorms should tend to be elevated in an environment with rapidly weakening lapse rates / instability but still an isolated large hail risk can't be excluded as extreme helicity values persist. Thunderstorms persist in the level areas until 06Z although severe weather risk should diminish after 00Z.

... SE Europe, the Alpine region and parts of NW Europe ...

Favorable atmospheric stratification supports widespread thunderstom development in a weakly sheared - modest instability environment. Pulsating thunderstorms can be expected with mostly sub - severe hail and strong wind gusts. As typical for such broad - scale events we can't exclude an isolated pulsating thunderstorms approaching the severe limits but overall environment would not support an extensive level - 1 area. Although mainly diurnal driven, thunderstorms should keep going well into the night with a significant lull in activity after 00Z. SW - UK and Ireland could see an isolated funnel report due to low LCLs and steep 0 - 3km CAPE release but the threat is too conditional to issue a level area.

... S - Norway and parts of SW Sweden ...

A pool of cold mid-levels is placed over the region and should stay there for the next 24 hours. This results in steep lapse rates with some modest instability release. Like yesterday the lowest 2km reveal quite steep lapse rates and strong updrafts could result in a few funnel / short - lived tornado reports in the level - 1 area. This level was also issued due to very strong DLS values between 25 and 30 m/s resulting in potential updraft / downdraft separation which in fact supports a few longer lived multicell storms with a local large hail threat. This activity is diurnal driven and thunderstorms should diminish in intensity and strength after sunset.




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