Astronomy, Photography and Weather
October 16, 2019, 12:07:14 pm
Welcome, Guest. Please login or register.

Login with username, password and session length
News: IAA lecture programme continues alternate Wednesdays from September - an excellent programme of lectures- Queens University Belfast - Bell Lecture Theatre. Also keep an eye out for the Summer Events
 
  Home Help Search Gallery Staff List Login Register  

Thunderstorm Outlook - Wed August 6th

Pages: [1]
  Print  
Author Topic: Thunderstorm Outlook - Wed August 6th  (Read 332 times)
martinastro
Martin Mc Kenna
Global Moderator
Hero Member
*****
Posts: 5182


Maghera, N. Ireland


View Profile WWW
« on: August 05, 2008, 07:59:11 pm »



Extended Forecast
Valid: Wed 06 Aug 2008 06:00 to Thu 07 Aug 2008 06:00 UTC
Issued: Mon 04 Aug 2008 23:08
Forecaster: TUSCHY
SYNOPSIS

A strong upper trough just west of Europe shifts eastwards with intense WAA on its downstream side, overspreading most parts of central Europe. The ridge is not too strong and numerous disturbances circle around the ridge during the forecast period with various areas having thunderstorm chances. Strong CAA downstream of a broad cyclonic vortex over W-Russia prevents deep convection to form over most parts of NE and E-Europe.

DISCUSSION

.... Parts of France, Belgium and the Netherlands ....

Ahead of a strong upper trough just west of France, a hot and humid airmass spreads NE-wards during the forecast period with dewpoint calculations in the upper tens over France and Belgium / Netherlands. On the 4th of August, dewpoint readings over S-France were between 16C and 19 C, so those values are not too unrealistic. Atop of this favorable BL airmass, a Spanish Plume overspreads the area from the SW with very steep mid - level lapse rates so we can expect very unstable profilers over France, although mostly capped during the day due to very warm lower levels ( e.g. 850hPa temperatures between 20C and 25C over E / S-France ). MLCAPE values should reach widespread 1000-2000 J/kg and SBCAPE/MUCAPE values well above 3000 J/kg. Digging into details that far out is not useful as models like GFS try to develop numerous more or less potent short-waves, which will serve as foci for thunderstorm initiation, but model consistency regarding timing / exact track and strength of those lifting mechanisms is still considerably. It looks like storms will start over NW-France and later in the night also over SW-France, moving east-/northeastwards just west of the capped airmass over central / eastern France.

We are concerned about the extent of the 20-25 m/s 0-6km bulk shear and very high instability overlap, but also about a broad area with strong veering throughout the atmosphere, resulting in widespread strong helicity values (e.g. SRH 3 between 200 and 400 m^2/s^2). Each thunderstorm will have a good chance to rotate, producing large and possibly significant hail, but also strong to severe wind gusts. LL shear increases during the night hours mainly over E/NE France and a tornado threat could evolve for storms, moving into this environment but later outlooks will include new model informations to evaluate the final tornado risk.

Strong DLS and 500-1000 J/kg MLCAPE also overlap over Belgium and the Netherlands during the evening and night hours and storms will pose a large hail / strong to severe wind gust risk. We did not include those areas in the level-2 area as overall risk for significant events looks less likely.

... N - Italy ...

10-15 m/s DLS and strong instability release with up to 1000 J/kg MLCAPE will be present as a short-wave crosses the area from the west during the midday hours. Large and isolated very large hail, but also severe wind gusts due to strong downdrafts will be the main threat from those storms. For now, an high-end level-1 is enough, but if confidence in a more robust hail risk increases, an upgrade could become necessary.


From Tony Gilbert

Ireland has in fact improved its chances for 'general' thunderstorm activity. With some very promissing convergence zones. Central west Ireland currently looks best . Though, I would prefer to look at both Ireland & UK in the morning for a more accurate opinion
Report Spam   Logged

martinastro
Martin Mc Kenna
Global Moderator
Hero Member
*****
Posts: 5182


Maghera, N. Ireland


View Profile WWW
« Reply #1 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.


Report Spam   Logged

martinastro
Martin Mc Kenna
Global Moderator
Hero Member
*****
Posts: 5182


Maghera, N. Ireland


View Profile WWW
« Reply #2 on: August 06, 2008, 04:33:27 pm »

Interesting update from Tony Gilbert...



Moderate Risk of Gen Tstorms & Slight Risk Funnel Obs Ireland & N.Ireland 12Z-21Z

06Z model output pretty much lowers any earlier risk over UK to negligible.

Moist surface flow tends to stay further west than earlier anticipations. Dry surface plume pushes in from east during period all but cuts off any chance of true convection initiating. The earlier mid level dry air incursion is reduced to 100mb layer thickness only,  with saturation from 600mb upwards. This will cancel out any potential instability from this level upwards. Given that this whole outlook was based within the warm sector the advance of potential instability was crucial. The end result is almost zero instability at mid levels.

I still feel the need to view the 12Z run regarding the UK to be 100% sure that nothing convective will occur overnight!

Ireland as per earlier forecast and as per convective map below. Though, prime instability will not be utilised till quite late in the day and so too goes for the risk of funnel obs. Whilst surface confluent vector is classic for producing the right vorticity for funnels. I am rather uncertain about the overall lapse rates so have suggested a Slight Risk only.


« Last Edit: August 06, 2008, 04:34:58 pm by martinastro » Report Spam   Logged

martinastro
Martin Mc Kenna
Global Moderator
Hero Member
*****
Posts: 5182


Maghera, N. Ireland


View Profile WWW
« Reply #3 on: August 06, 2008, 04:40:25 pm »

TORRO CONVECTIVE DISCUSSION 2008/019

A TORRO CONVECTIVE DISCUSSION has been issued at 12:40 GMT on Wednesday 6th August 2008
Valid from/until: 12:40 - 18:00 GMT on Wednesday 6th August 2008 for the following regions of the United Kingdom & Eire:

Portions of central and north-eastern Eire

SE Northern Ireland

THREATS

Isolated tornadoes, gusty winds, hail, CG lightning, torrential rain.

SYNOPSIS

Convergence zone has developed in SW Eire, and will continue to develop north-eastwards this afternoon. Dry slot aloft should allow some cloud clearance, and further surface heating. Showers and thunderstorms will continue to develop and INVOF convergence zone, a few weak tornadoes are possible.

* keep an eye out for funnels/tornadoes this evening guys.
Report Spam   Logged

martinastro
Martin Mc Kenna
Global Moderator
Hero Member
*****
Posts: 5182


Maghera, N. Ireland


View Profile WWW
« Reply #4 on: August 06, 2008, 07:34:54 pm »

Severe weather warning from Met Office for NI this evening...

http://www.metoffice.gov.uk/weather/uk/ni/ni_forecast_warnings.html?day=1

Radar and sferic show storms moving into NI now. Eastern areas may get the best of it.
Report Spam   Logged



Pages: [1]
  Print  
 
Jump to:  

Bookmark this site! | Upgrade This Forum
SMF For Free - Create your own Forum

Powered by SMF | SMF © 2016, Simple Machines
Privacy Policy