Storm Forecast
Valid: Fri 25 Dec 2009 06:00 to Sat 26 Dec 2009 06:00 UTC
Issued: Thu 24 Dec 2009 21:52
Forecaster: TUSCHY

A level 2 was issued for the Strait of Gibraltar and surrounding areas mainly for excessive rainfall.

A level 1 was issued for S-Portugal, SW-Spain and NW-Morocco, mainly for tornadoes, severe wind gusts and excessive rainfall.

A level 1 was issued for Slovenia, Croatia, parts of Hungary, Bosnia and Herzegovina, northern Serbia, Montenegro and extreme west Romania mainly for severe wind gusts and large hail and to a lesser extent for tornadoes.


SYNOPSIS

A potent 980-hPa depression over the Low Countries moves eastnortheastwards, reaching the western Baltic Sea during the night. Gradients along its southern edge increase as high pressure builds in from the Alps, so despite some gradual filling, wind speeds increase during the forecast with strong to severe wind gusts possible along its southern quadrant. Its warm front stalls somewhere over the central Baltic Sea and eastwards with a prolonged period of strong, isentropic upglide with an augmented freezing rain and heavy snowfall risk. Further south, in the warm sector, impressive WAA over E/SE-Europe will be underway with some thundery showers possible east of the Adriatic Sea. Most of the WAA regime remains capped and stable.

Another upper wave and attendant surface depression over the far eastern Atlantic approaches S-Portugal and Spain during the forecast with some weakening during the night hours and onwards.

DISCUSSION

... Slovenia, parts of Austria, Hungary, Croatia, Bosnia and Herzegovina, Serbia and Montenegro and parts of Romania ...

The cyclonic vortex over extreme NE-France/Belgium, the Netherlands and Luxembourg caused a broad swath of warm air advection, visible in local sounding data. Surface dewpoints next to the Adriatic Sea are well above 10°C with a gradual decrease more inland but still quite high for this time of the year. Surface pressure is on a constant decrease south of the Alps, due to the progressive nature and favorable coupling between the northeastward moving surface depression over the Low Countries and a potent upper trough, entering the central Mediterranean during the start of the forecast. This pattern is quite favorable for a broad sector SE of the Alpes to experience some instability build-up, as forward movement of the surface cold front along the Alpes temporarily slows down, whereas mid-level CAA overspreads the warm/moist prefrontal air mass. GFS and ECMWF both agree in quite significant destabilization for this time of the year with 300-500 J/kg MLCAPE possible. Eastward advancing upper vort max pushes eastwards over the area of interest, intersecting the SE-ward moving surface cold front. In respect of thermodynamics and dynamics, thunderstorms are probable well inland, despite the lightning climatology, being shy above zero east of the Adriatic Sea for Dezember/January.

The attention turns to the SE-ward pushing cold front, with overall thunderstorm probabilities increase from NW to SE in the highlighted area. At least isolated showers and thunderstorms already evolve over Slovenia, S/SE-Austria and W-Hungary. As the front speeds up during the afternoon/evening, prefrontal air mass becomes more unstable, and thunderstorms fill in, so even a squall line is possible in the area with highest thunderstorm probabilities. The complex terrain to the south and east and diminishing moisture advection ahead of the cold front prohibit further organization, so cold front becomes less organized with isolated activity still anticipated over west/central Romania.

The main risk will be a severe wind gust and large hail risk, maximized in the region with most robust instability. However, the environment is also prime for a few tornado reports with low LCLs and strong BL shear although the LL CAPE is not too impressive. No prefrontal activity is expected (in respect of discrete thunderstorm evolution) , as the warm sector is at least weakly capped.

Despite the broad nature of this level 1, the maximized risk will probably reside in those regions, where high thunderstorm proabilities were introduced.

... Parts of Portugal and Spain ...

The past few days resembled a merry-go-round with the next feature of interest already approaching from the west. Due to the far southward displaced polar front jet, another upper wave/surface depression is in full progress and taps into tropical moisture. Again, this system features an extensive warm sector with SBCAPE in excess of 1000 J/kg possible due to the combination of a tropical LL air mass beneath a cooler, extratropical layer, bumping up mid-level lapse rates significantly. Forecast warm front progress in respect of timing and placement is a blend of EZMWF and GFS, both placing the front well inland, with EZWMF being more aggressive. The warm sector features high BL moisture with excessive rainfall possible along the warm front itself and next to the high theta-e tongue, affecting mainly the Strait of Gibraltar and eastwards. As the upper wave also approaches from the west, the unstable and uncapped warm sector experiences a prolonged period of lift, so scattered thunderstorms south of the warm front are forecast. High PWAT values point to excessive rainfall and hence a level 2 was introduced. Another reason for issuing a level 2 is the high, positive rainfall anomaly, measured by TRMM for the past few days over the Strait of Gibraltar and surrounding areas, so flooding problems could be more easily arise even when forecast rain amounts reside on the lower-end side of our level 2 criterion.

A larger level 1 covers the warm sector due to the augmented tornado risk as LL shear and DLS in this warm sector increase significantly. Next to the tornado risk, severe wind gusts accompany stronger storms, as winds at 850hPa increase to 20m/s.

Again, another strong baroclinic zone evolves over N-Spain although air mass north of this boundary is not as cold as in the past event. However, some MUCAPE is forecast all the way up to N-Spain where freezing rain and snow are possible, locally enhanced by stronger convection. Overall risk seems too low for now to go with a low probability thunderstorm area, but a very isolated event can't be ruled out north of the 15-% thunderstorm line..