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How to predict overnight thunderstorms


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Posted
  • Location: Norwich, Norfolk, East Anglia
  • Weather Preferences: Sunny, stormy and I don't dislike rain only cold
  • Location: Norwich, Norfolk, East Anglia

    I know how to get a reasonable idea on how to forecast if a storm may develop during the day but how can we tell if a thunderstorm may arrive during the night?

     

    What charts do we look at to determine a boundary layer?

     

    and how can we determine if there is enough moisture in the atmosphere to aid storm development?

     

    Thanks in advance :good:

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    A question for Nick F I suggest. Maybe pm him as he may not have seen this. If not pm me and I'll try and rack my decaying brain cells to give sensible answers.

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    Posted
  • Location: Norwich, Norfolk, East Anglia
  • Weather Preferences: Sunny, stormy and I don't dislike rain only cold
  • Location: Norwich, Norfolk, East Anglia

    Ok John thanks a lot! I will send him a pm.

     

    Quick question, to find out if there is enough moisture for storms to develop in a given area is it just based on how much relative humidity there is?

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    Ok John thanks a lot! I will send him a pm. Quick question, to find out if there is enough moisture for storms to develop in a given area is it just based on how much relative humidity there is?

    Only just seen this-sorryI suppose you could say yes but it is fairly complex, need to know temp and humidity from surface through atmosphere up to tropopauseIs that any help?
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    Posted
  • Location: The Netherlands
  • Location: The Netherlands

    Ok John thanks a lot! I will send him a pm.

     

    Quick question, to find out if there is enough moisture for storms to develop in a given area is it just based on how much relative humidity there is?

     

    In addition to what John said, a good guide for a humidity profile through the complete height of the atmosphere is a Skew-T diagram. I think Nick was going to create a nice guide about this shortly. Of note is that reading the humidty profile from such a diagram is rather complicated.

     

    Furthermore, John made a very informative guide about Skew-T diagrams a few years ago. It can be found in the link below:

     

    http://forum.netweather.tv/topic/16002-a-simple-guide-to-understanding-skew-t-diagrams/

     

    I hope this helps!

    Edited by Vorticity0123
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    Posted
  • Location: Norwich, Norfolk, East Anglia
  • Weather Preferences: Sunny, stormy and I don't dislike rain only cold
  • Location: Norwich, Norfolk, East Anglia

    Thanks guys I'm reasonably familiar with skew-t and thought that was a good way to check before I saw your post vorticity.

    I know that if both the dewpoint and temp ELR are at the same temperature a good way along it means the air is saturated and will indicate non-convective rain but how close in temperature do the ELR lines need to be to indicate cloud formation?

    More to the matter in question, how close do the ELR lines need to be to confirm enough moisture in the atmosphere for storm development? Is their a formula to calculate an overall percentage of relative humidity in the atmosphere?

    Thanks for your help :-)

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    Posted
  • Location: The Netherlands
  • Location: The Netherlands

    I know that if both the dewpoint and temp ELR are at the same temperature a good way along it means the air is saturated and will indicate non-convective rain but how close in temperature do the ELR lines need to be to indicate cloud formation?

     

    In general, the real temperature and dewpoint temperature need to be equal in order to have stratiform clouds (or rain, as you stated). The same accounts for convective precipitation. However, it is important to note that convective precipitation develops due to rising air parcels. This means that the relative humidity does not have to be 100% at any point (in other words, the temperature- and dewpoint lines do not have to be at the same value at any point) in order to have convective activity. 

     

    For elaboration, check the sounding of Peachtree City, Georgia, below:

     

    Posted Image

     

    Sounding of Peachtree, Georgia, April 30 00Z (about afternoon in the US). At the time of the sounding, a severe weather outbreak was occurring in the US (some tornadoes also developed close to Peachtree City). Courtesy: University of Wyoming. 

     

    The grey line indicates the temperature of an air parcel if it would lift adiabatically from near the surface. Initially, the grey line follows the dry adiabat line up to when it hits the moisture content of the surface (this can be traced back by tracking the bend in the grey line back toward the surface via the moisture lines; the dew point temperature at the surface should then be reached). The air parcel becomes saturated at this point. This point is also known as the LCL, or lifted condensation level.

     

    Of note is that to reach this LCL, a mechanism (or trigger) should be present to rise the parcel. This has to do with the fact that once the air parcel rises, it has about the same temperature as the surrounding air. As a result, this air parcel won’t rise automatically, and so a trigger is necessary.

     

    Another important thing to realize is that the LCL is in general the point from which clouds can start to develop (including convective clouds). In this example, the LCL was determined by taking an air parcel from the surface.

     

    The saturation described above occurs even though the fact that the dew point line and temperature line do not touch each other (i.e. the relative humidity is below 100%). Because of this saturation, the air parcel now cools via the saturated adiabat. As this air parcel becomes warmer than the surrounding air (unstable conditions, see Skew-T diagram), it starts to rise automatically. This occurs up to when the air parcel becomes cooler than its surroundings, which would in this example be around 13 km. That is theoretically the maximum cloud height of the convection caused by the rising of an air parcel from the surface. 

     

    In short, the closer the lines of temperature and dew point temperature are to each other, the lower (in altitude) the LCL will be located, and thus the more easily convection will form (based on the fact that less of a trigger mechanism is necessary to reach the LCL). However, there isn't a certain fixed distance between the two lines (or in other words, a specified RH), which is necessary for convection to develop. Therefore, the RH can only be used as a general indication. 

     

     

    Is their a formula to calculate an overall percentage of relative humidity in the atmosphere?

     

    An option would be to take all RH values throughout the height profile and average them, but such a value has little meaning (based on the fact that RH can only be indicatively used for assessing chances of storm formation).

     

    Finally, a useful guide about severe convection formation can be found in the link below:

     

    http://www.estofex.org/guide/

     

    I hope this helps a little. If you have any questions, or if something is unclear, just ask  :) .

     

    Sources:

    http://weather.uwyo.edu/cgi-bin/sounding?region=naconf&TYPE=GIF%3ASKEWT&YEAR=2014&MONTH=04&FROM=2800&TO=2800&STNM=72235

    http://en.wikipedia.org/wiki/Tornadoes_of_2014#May_22

    http://en.wikipedia.org/wiki/April_27%E2%80%9330,_2014_tornado_outbreak

    http://www.estofex.org/guide/

    Edited by Vorticity0123
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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    Thanks guys I'm reasonably familiar with skew-t and thought that was a good way to check before I saw your post vorticity.I know that if both the dewpoint and temp ELR are at the same temperature a good way along it means the air is saturated and will indicate non-convective rain but how close in temperature do the ELR lines need to be to indicate cloud formation?More to the matter in question, how close do the ELR lines need to be to confirm enough moisture in the atmosphere for storm development? Is their a formula to calculate an overall percentage of relative humidity in the atmosphere?Thanks for your help :-)

    not easy to answer, there are no solid rules just guidance and experience reallyIf I get time tomorrow I will try and give fuller answer based on the 20 plus years forecasting

     

    sorry not yet had time to do as I promised

     

    Looking again at this and I feel that V has given at least as good an explanation as I would be able to do. Take a look at the skew-t article I wrote several years ago to see if that helps. Look at the values on the right hand side that link data on the skew-t to what so many use directly; things like CAPE etc. They are derived from the skew-t anyway. One thing that does occur re overnight storms is that temperatures at cloud height tops can and do at times decrease. This allows the Cb tops to increase in height and this can sometimes overcome the loss of heat being input at the surface. Say for instance when storms move over the English Channel from France or the Low Countries, with sea surface T obviously lower than surface T was that afternoon.

     

    hope this helps

     

    you may also find this link of use; written initially by a senior forecaster with UK Met, a first class link for most objective definitions and explanations on the weather

    http://weatherfaqs.org.uk/node/180

    Edited by johnholmes
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    Posted
  • Location: Norwich, Norfolk, East Anglia
  • Weather Preferences: Sunny, stormy and I don't dislike rain only cold
  • Location: Norwich, Norfolk, East Anglia

    Ok really very much appreciated for you guys taking the time to explain in such detail  :good: This reply is to signify that I have read them and may post a simple question or 2 if needed :)

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