Convective Outlook Tues 6th - Weds 7th Dec 11
Posted 06 December 2011 - 10:22
Slight Risk of Thundersnow/ Hail Showers Northern Eire, West and central regions of the UK 09Z Mon- 06Z Tues
Slight Risk of an Isolated Tornado 21Z-05Z
Weak low pressure centre moves from northern Atlantic through Scotland. UKMO Fax chart currently shows a cold front with Triple point traversing UK overnight with a post frontal surface trough likely to bring another run of snow showers to the regions within white box, below. Deep upper long wave trough remains in place enhancing lapse rates primarily across west facing coastal regions where SST temps remain relatively mild. Strong upper jet stream runs from NW to SE and marginal overruns the best instability fed inland from the Irish Sea. The TP is currently predicted to track from NW England eastward through Humber region late evening. The triple point becomes the main focus for the strongest convection and a marginal risk for an isolated tornado (red box). The triple point will commonly serve as a small portion of the cold front where dynamic lift is enhanced as the boundary is lifting the intersection to the warm front most efficiently. This zone will also likely produce the strongest directional shear at low levels. Strongest convective gusts are calculated at around 55kts (severe level) Though the risk for this level of gust is likely to be confined only to the strongest convective cell. Most gust reports will be closer to 35kts (non convective)
Later on there is some focus regarding the post frontal trough which looks to follow a similar path during the early hrs of Weds morning. By this time, both GFS and NMM show enhanced potential instability as a very sharp dry incursion above 600 mb overspreads the region. During this period the upper vertical shear is actually increased but low level veer looks to be less by comparison the the TP. So whilst an isolated tornado event still looks possible it would appear slightly less than earlier in the period.
The broader risk for isolated sferics remains in place over the broader expanse of the white box, though looks to be more sporadic in nature.