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Thunderstorm Outlook - Wed August 6th

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martinastro
Martin Mc Kenna
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« on: August 06, 2008, 10:13:22 am »



From Tony Gilbert....

Slight Risk Gen Tstorms Ireland 12Z-21Z

Slight Risk Strong Convection 15Z-00Z Central East UK regions

Moderate Risk of Tornado Wash & surrounding Counties 18Z-00Z blue box

Ireland's outlook remains poorly sheared, though mid level dry air will in fact utilise potential instability and overrun a moist surface layer. Sferics at this stage look more moderate risk. Cannot rule out the odd funnel given the positive strong confluence expected at surface though lapse rates seem rather weak for me today.

Central & Central East UK have a potentially explosive environment, given enough surface heating. Heavy cloud cover remains the main suppressive issue here ATM. Loaded moisture at surface creates a perfect environment to fuel any building storm. LCL drops to 600m by 18Z (hense the potential for storms to 'drag their feet on the ground';..expression!). Based on the current model output the suggestion is that storms could in fact become surface based by late in the day. If this can occur, given the overlay of very strong upper shear and low level jet with very weak surface winds we have the full potential for tornado development and possibly catagorised severe thunderstorms.

It is mentioned earlier that very dry air overruns moist at low level and hinders vertical cloud growth. This would be a serious issue if the moisture was weak at surface. This is not the case today. The main issue regarding vertical moisture profiles is the heavy moisture at 500mb which can act as a lid by suppressing cooling at this level and weakening lapse rates. Shear profiles are pretty much straight lined, though I see exceptional speed shear in the lowest layer. Such conditions will commonly create very tight roles of vorticity at surface and actually improve the risk of rather strong narrow tornado genesis.

This is all pretty much like a puzzle where certain pieces need to fit in at certain times to create the picture. The overall opinion for me today so far is moderate risk. The blue box is based on current models and may need shifting closer to the time.


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