Storm Forecast
Valid: Sat 22 Dec 2012 06:00 to Sun 23 Dec 2012 06:00 UTC
Issued: Fri 21 Dec 2012 18:16
Forecaster: DAHL

A level 1 was issued across Ireland mainly for severe straight-line wind gusts.

SYNOPSIS

The dominant features in the surface analyses are an extensive, cold high-pressure area covering the eastern half of the forecast area, and a large surface low over the eastern Atlantic, which is making slow eastward progress. At upper levels a northwesterly jet is stretching from western Europe into the Mediterranean. A series of short-wave troughs is embedded in this jet, which will become unstable, leading to the development of a pronounced upper trough/low over the northern Ionian Sea by Saturday morning. While digging southeastward into the southern/easten Mediterranean, this upper trough is expected to continue amplifying through the period. At the low levels, this development will be accompanied by cyclogenesis, which will be the main focus for convective development on during this forecast period.

DISCUSSION

... southern Mediterranean ...

Ahead of the developing Mediterranean upper trough, large- and mesoscale ascent should aid in destabilization, leading to increased chances of scattered showers and thunderstorms. It appears that the air will be seasonably CAPE-starved and that the regions of appreciable deep-layer shear will be decorrelated with the regions of nonzero CAPE. This implies that any convective development will likely be on the shallow and weak side, though the storms should be deep enough for scattered lightning. The organized severe threat with this activity seems to be rather small, though a brief waterspout cannot be ruled out given rather steep near-surface lapse rates.

... British Isles ...

Around 00 UTC on Sunday, a rather vigorous vorticity maximum and an associated SFC cold front will cross Ireland and the westen UK. The cold front may be accompanied by a narrow and shallow line of strongly-forced convection, capable of mixing some 30 m/s winds (present at 850 hPa) to the ground. WRF supports this scenario by simulating a band of weak CAPE near the location of the cold front. Will place a LVL1 threat where forcing for upward motion will be the strongest (and presumably, convection the deepest), which should be the most likely location for effective vertical momentum transfer.