Storm Forecast
Valid: Mon 27 Oct 2008 06:00 to Tue 28 Oct 2008 06:00 UTC
Issued: Sun 26 Oct 2008 20:24
Forecaster: TUSCHY

SYNOPSIS

A glance on latest nordhemispheric analysis and forecast maps reveals the development of five dominant longwave troughs with the three strongest ones over the NE-Pacific, NE-USA and NE-Atlantic, so both parts of North-America and parts of Europe face the first outbreak of markedly cooler air.

A strong and constantly amplifying upper trough shifts rapidly to the south over NW and W-Europe for the next 24 hours. Surface depressions should start to evolve along the tip/SE - quadrant of the main upper trough, but they stay in their organisation stage at least until 06Z. Warm and stable conditions persist over far east/southeast Europe with more unsettled conditions over Turkey.

DISCUSSION

... N of Scotland, 00Z onwards ...

Impressive outbreak of polar air is underway from Greenland to the S/SE. Modification of the lower column will be slow east of Greenland due to already extensive ice coverage all the way down to the Jamson Land / extreme northwestern tip of the Street of Denmark. Temperatures at 850hPa stay around or below -20°C quite far to the south (affecting Iceland during the midday hours onwards), and hence a large NNW-SSE aligned baroclinic zone becomes established. A short-wave, rounding the main cyclonic vortex west of Norway, crosses this zone during the morning hours, while racing southwards. The model pool has a bad handling on the track and strength of this feature although GFS and GEM constantly had a rapidly developing surface depression in the past runs, placed just north/northeast of Scotland during the end of the forecast. Due to the very cold mid-levels, a plume of low-end instability should accompany this feature and as the surface pressure gradient tightens, severe wind gusts should be a high risk ( e.g. keeping severe freezing spray conditions for boats in mind ). The evolution of this depression will be monitored and an update may be issued, if the depression moves faster than currently outlined and exhibits signs for enhanced deep convection.

... Highlighted areas over W/NW-Europe...

Broad-scale CAA will be underway and the main regions to look for thunderstorm development will be offshore, e.g. Bay of Biscay. Beside the fact that there will be an extensive area with low-end to moderate SBCAPE release under the base of the upper trough, it is more tricky to determine the spots, where electrified convection will indeed occur. It makes no sense to get into too many details as uncertainties of smaller-scale features in such a CAA-regime are high, which could locally enhance convective activity and hence the chance for thunderstorms. The North Sea does not look too promising for much activity until about 21Z, but then GFS continues to show a short-wave, dropping south over the SW North Sea, enhancing the thunderstorm chances along the coastal areas of the Netherlands and N-Germany. The English Channel was excluded due to persistent offshore flow of dry air, but otherwise, large thunderstorm areas were drawn. Small hail and strong wind gusts will be the main risk.

...Tyrrhenian Sea and parts of the western Mediterranean ...

As upper trough approaches from the NW during the evening hours, conditions over the highlighted area become fine for scattered to widespread thunderstorm development. At the surface, large-scale pressure fall goes hand in hand with the evolution of a still weak and ill defined surface pressure channel, centered somewhere over the Balearic Islands. A plume of EML overspreads a region mainly between Sicily and Sardinia and in accord with rapidly steepening mid-level lapse rates, instability release will increase markedly during the night hours with MLCAPE of well above 1000 J/kg and probably capped SBCAPE values in excess of 2kJ/kg ( ICAPE values of locally up to 3kJ/m^2). DLS is on the increase as mid-level speed max approaches from Algeria during the night hours and readings of 15-20m/s should be available. Directional shear is not yet very strong but combined with such robust instability release, organized multicell storms / supercells are likely with a large hail and strong wind gust threat. Regarding the tornado risk, warm LL should keep the tornado risk quite low although SSTs of 22°C and anticipated temperatures at 850hPa are a borderline case for more surface based storms, so we want to include at least an isolated tornado risk. No level-2 was yet issued as strongest wind field should remain in the capped airmass until 06Z.

The risk for thunderstorms also increases over the Balearic Islands and eastwards, as the surface cold front approaches from the NW. Thunderstorms will take profit of strengthening shear mainly along the frontal boundary and a few organized thunderstorm clusters are anticipated to develop during the evening and night hours.Isolated large hail/ severe wind gusts are possible and hence a level area was drawn. LL CAPE is on a rapid increase during the night hours mainly over the Balearic Islands and to the south/east, so the risk for isolated tornados should increase beside very limited LL directional shear.