Thunderstorm/Convective Watch: Autumn 2010

I was amazed at the storms we were getting in Brisbane last year when I was over there. 3600 CAPE and -11 LI!

Roll on the THUNDERSNOW!!!!!!!:D

Causes of Thundersnow

What causes thunder and lightning during some snowstorms? Those who study the formation and life cycle of thunderstorms forming during the warm season understand that a strong, warm and moist updraft is a necessary condition.

In the warm season, these updrafts generally arise from the surface, but they need not. It appears that only those thundersnows developing over bodies of warm water arise from warm surface air. Most others, particularly those observed in the US central states, develop when the necessary convective process starts at higher altitudes than the surface air layer.

The two most important mechanisms for producing most thunderstorms are layers of instability — a region of the atmosphere with a tendency to promote upward air motions — and strong dynamic lifting — factors other than buoyancy that move air upward. Working in tandem, these two mechanisms provide that lift necessary to turn a precipitating cumulus cloud into a towering, lightning-producing cumulonimbus.

In a warm-season, airmass thunderstorm, the atmospheric instability needed to start the process begins at the sun-heated ground. The moist air warmed at the surface becomes less dense (more buoyant) and begins to rise high into the sky, a process known as surface convection. The less dense the air is relative to the surrounding air, the higher and faster it will rise; in such cases we have a deep layer of surface instability.

But in the cold season, and particularly in air conducive to snowfall, the lower troposphere is dry and cold which makes it relatively stable and inhibits parcels of surface air from rising. However, the troposphere can have many layers of distinct airs with differing vertical thermal gradients (called lapse rates), and when a layer of warmer, moist air wedges its way between a surface cold layer and colder air above, a region of elevated instability forms that may give rise to elevated convection, the convective rise of air beginning at an altitude above the surface. The effect is enhanced when there is cooling in the middle troposphere layers above the warm air wedge to enhance the convection potential.

An example of elevated convection at an occluded front resulting in thundersnow.

There are several mechanisms that can cause strong dynamic lifting of air (which I look at in more detail in What Goes Up: Introduction to Updrafts ). When one of these occurs in an atmosphere that has layers of instability, it becomes a trigger for air to begin rising and continue to do so. In this manner, strong updrafts are formed. The lifting can work on elevated layers as well as surface layers and thus produce elevated convection. In the most intense cases of dynamic lifting, such as air forced over a mountain range, the resulting cumulus clouds have all the characteristics of purely convection cloud formation.

In late fall and winter, when cold air moves across the relatively warm waters of the Great Lakes and other large water bodies, intense convective storms often develop that drop snow upon reaching the downwind shores (see Lake-Effect Snowfall on this site). When the waters are significantly warmer than the air, they act similar to the solar-heated ground in the summer and form surface convection. Under these conditions, intense cumulonimbus cloud development may arise that induces lightning flashes. When these snow squalls strike with heavy snows and gusty winds, they are punctuated with flashing lightning and booming thunder, i.e., thundersnow

skipz wrote: »

I mostly think of thunderstorms as a summer time thing, surely we wont get to much action in the cooler months?

You are right, thunderstorms are more frequent* during the summer months, but thunderstorms in the Autumn/Winter time can happen. Normally, they bring nothing more than a few flashes and a couple of bangs, but very occassionally, something much more intense will pass through. Some of the most dramatic lightning strokes I have ever witnessed have occurred during the winter months.

Roll on the squalls!!!

*a strictly relative term...

Was reading through the met eireann monthly bulletins archive earlier on to see if there was any notably thundery spell in the autumn months.

Nothing much, but the 21st to 23rd October 2003 seemed to be one exception. Thundery showers affected the east coast, and in particular the greater Dublin region, for a period of 36hrs almost non-stop!!!

Here is how the event is described in the October 2003 monthly bulletin

"Low pressure to the southeast of Ireland between the 21st
and 23rd generated a strong north to northeasterly airstream
over the country. These very cold winds picked up moisture
as they moved over the relatively warm waters of the Irish
Sea, generating thunderstorms over the Dublin area in
particular. Between 1900 on the 21st and 0700 on the 23rd,
thunder was reported during 22 hours of the 36-hour period.

Many of the showers were of hail and with
temperatures just above freezing for part of the period, snow
also fell over higher ground. Casement Aerodrome’s fall of
51.6mm on the 22nd was the highest midnight-to-midnight fall
for October at the station since records began there in 1964".


Screen grab of rain radar from the same met eireann PDF for the morning of the 23rd of October 2003:


pretty nasty looking by all accounts!



As descibed in the met summary, the cause of this almost continous series of thunder showers in the region was the result of very unstable airmass that followed on from cold front that moved down across the country late in the evening of the 20th of Oct:



It is interesting to note that despite the passage of the trough, pressure contiuned to fall, and quite rapidly, in its wake, which is a good sign that the instability is increasing rapidly within the cool sector. Chart for the 22nd 00z shows just how much the air pressure fell behing the cold front:



showery trough developing over the relatively warm waters of the Irish sea.



Satalite pics of the event:

The evening of the 21st Oct:



showers begin to impinge on the east coast.



By the small hours of the 22nd, organised and intense showers are buffeting much of the east coast into the east midlands:





Showers continue to move in over the east on the afternoon of the 22nd:





It is not until the morning of the 23rd that the air begins to stablise:






UK Met Office recorded strikes for the month of October 2003 showing the extent of the lightning over the east from these showers:





Lets hope for a similar event this autumn!


Data sources:
Met Eireann,
UK Met Office
Dundee University.

Thanks for the post Deep, I remember that night well. Some very nice purple lightning, of which I got some video, but of course lost it when my other laptop packed it in on me! :mad:

I wonder if we'll see something in southeastern parts Tuesday night/Wednesday morning. Models have pretty heavy rainrates near the triple point of the frontal system as it moves eastwards across the country. At mid-upper levels the trough becomes negatively tilted, maximising lapse rates, and with decent deep layer shear present, there could be some very heavy pulses as the occulsion clears the southeast and east coast, p[ossibly giving rise to embedded thunderstorms.

Behind the system Wednesday the airmass is a lot cooler and drier, with single digit dewpoints and low level cold air advection putting a strain on deep convection, except maybe over the western half of the country, where showery troughs will probably move in off the Atlantic.

Fantastic post Patrick - thanks!!
The thunder and lightning was discussed on boards at the time
http://www.boards.ie/vbulletin/showthread.php?t=121267
but unfortunately the link to some video footage no longer works

Fionagus wrote: »

Fantastic post Patrick - thanks!!
The thunder and lightning was discussed on boards
http://www.boards.ie/vbulletin/showthread.php?t=121267
but unfortunately the link to some video footage no longer works

Interesting blast from the past there! I didn't even know boards.ie existed back then!

Thought one post in particular summed up the actual specifics of the event nicely:

"It still works on the same principles, but at the moment we have really cold arctic air coming down over relatively cold seas and convecting upward really strongly.

That it lasted so long is weird, though it's more loads of seperate ones than one big one sticking around for ages".


I think he/she/it meant to say 'warm' rather than 'cold' seas though.

Pretty impressive shower here (N Louth) just now, came in with the underside of the cloud lit up pink and orange by setting sun.
Heavy, looked like it had thundery potential...looks good on the radar too at 6.45...

Good shots John, well spotted. You should report that on the ESWD or TORRO sites