Page 1 of 3 123 LastLast
Results 1 to 10 of 26
  1. #1
    Join Date
    Oct 2005
    Location
    Yorkshire England
    Posts
    875

    Default Hurricane Lorenzo

    Why is it when I go to
    https://www.nhc.noaa.gov

    to track Lorenzo, I'm told max sustained winds of 120knts, however, when I go to

    https://www.windy.com/?19.878,-44.42...sure,m:edSaeXO

    I struggle to see anything much above 69knts?

    Thanks

  2. #2
    Join Date
    Feb 2010
    Location
    Bristol
    Posts
    2,147

    Default Re: Hurricane Lorenzo

    Quote Originally Posted by Dave100456 View Post
    Why is it when I go to
    https://www.nhc.noaa.gov

    to track Lorenzo, I'm told max sustained winds of 120knts, however, when I go to

    https://www.windy.com/?19.878,-44.42...sure,m:edSaeXO

    I struggle to see anything much above 69knts?

    Thanks
    The models used for large scale forecasts like gfs and Ecmwf are designed to predict winds over relatively large areas but the hurricane forecasts have different models and predict max winds at the pressure wall to the bottom of the hurricane. So as I believe windy will be accurate when looking out 50km or so away from eye but closer then more specific hurricane models will predict the speeds.

    Just don’t get near the eye!

  3. #3
    Join Date
    Jul 2001
    Location
    Lorient, just back from a second round Atlantic trip
    Posts
    3,587

    Default Re: Hurricane Lorenzo

    Quote Originally Posted by Dave100456 View Post
    Why is it when I go to
    https://www.nhc.noaa.gov

    to track Lorenzo, I'm told max sustained winds of 120knts, however, when I go to

    https://www.windy.com/?19.878,-44.42...sure,m:edSaeXO

    I struggle to see anything much above 69knts?

    Thanks
    It's mainly because of GFS (and ecmwf to a lesser extent) model physics: hurricanes are basically enormous heat pumps where convection plays a fundamental role, convection and related processes are rather poorly modeled by those type of models, as often integrated as parameter.

    Read a few of the NHC "Discussions" to have an idea of the additional data forecaster use to integrate and "tweak" the various models output to provide a more accurate forecast.
    Actual wind speeds are usually provided by scatterometry satellites.
    oh no, yet another sailing blog
    http://sybrancaleone.blogspot.com/

  4. #4
    Join Date
    Oct 2002
    Location
    UK when not sailing
    Posts
    3,924

    Default Re: Hurricane Lorenzo

    Northcave and Roberto have got it broadly right. I hope that does not sound too condescending and would like to add a few comments. All numerical weather models use the laws of physics. Limitations to what models can do arise from several factors. The use of a finite grid determines the size of detail tat can be handled by models to about 5 grid lengths. The two best NWP global models, the ECMWF and UK, use grid lengths of 0.1 degree lat/long so can, at best only, predict detail of about 50 km size. Secondly, there are always uncertainties due to uncertainties in the initial data analysis. Other limitations are due to the difficulties in calculating how heat is transferred throughout the atmosphere.

    As Roberto says, convection plays a large part on tropical storm development. Until recently, global models were "hydrostatic". that means that convection effects were dealt with crudely. Now, ECMWF, UK, DWD ICON and, perhaps, the GFS are all “non-hydrostatic." Modellers say that 10 km is about the limit of grid length below which models can be non-hydrostatic. So, even though some global models are non-hydro9static, I doubt that convection effects are realistically calculated
    L
    https://www.bas.ac.uk/data/our-data/...limate-change/ lists the various model inputs to the NHC and, as you will see, meteorology is truly international. In addition to global model data, NHC also uses finer scale , non-hydrostatic models fed by input from global models and more detailed data nalyses. The global models are used for the longer term evolution and track prediction. Fine scale models help with short term detail. However, in the final analysis, NHC, forecasts are a combination of model outputs plus real time data from aircraft and whatever else is available. The worded output and hurricane track predictions that you see are a man-machine mix. Even the fine scale models will under-estimate the strongest winds. Getting back to the OP, it is inevitable that strong winds generally will be under-predicted. Given the nature of hurricanes, that will be even more so.
    Last edited by franksingleton; 28-09-19 at 08:46.

  5. #5
    Join Date
    Feb 2010
    Location
    Bristol
    Posts
    2,147

    Default Re: Hurricane Lorenzo

    Thanks Frank. Very interesting.

    For the same reasons, I might also add, that this is why we often find wind speed greater around coastal areas. Often when windy says 20kts we might be getting 25-30. The models can’t accommodate for acceleration zones and local
    Pressure differences.

    Offshore however I find the models very accurate in terms of average wind speed.

    However for the UK the paragliding RASP model is highly accurate. Go to the link below and selecting BL average wind. Compare it to the wind model on windy.

    http://rasp.stratus.org.uk/app/viewer.php

    A22E07BC-30B4-4EA3-B0BE-B0AAAAE3462B.jpg

  6. #6
    Join Date
    Nov 2016
    Posts
    2,741

    Default Re: Hurricane Lorenzo

    Last edited by zoidberg; 28-09-19 at 10:22.

  7. #7
    Join Date
    Oct 2005
    Location
    Yorkshire England
    Posts
    875

    Default Re: Hurricane Lorenzo

    Thanks all, very informative.
    Cheers
    Dave

  8. #8
    Join Date
    Oct 2002
    Location
    UK when not sailing
    Posts
    3,924

    Default Re: Hurricane Lorenzo

    Quote Originally Posted by northcave View Post
    Thanks Frank. Very interesting.

    For the same reasons, I might also add, that this is why we often find wind speed greater around coastal areas. Often when windy says 20kts we might be getting 25-30. The models can’t accommodate for acceleration zones and local
    Pressure differences.

    Offshore however I find the models very accurate in terms of average wind speed.

    However for the UK the paragliding RASP model is highly accurate. Go to the link below and selecting BL average wind. Compare it to the wind model on windy.

    http://rasp.stratus.org.uk/app/viewer.php

    A22E07BC-30B4-4EA3-B0BE-B0AAAAE3462B.jpg
    Agree about average wind forecasts over the sea. They are generally pretty good. I always assume that I will get spells about one force stronger and one force less. Occasionally, they can still go wrong. A couple of years ago on passage Ile de Brehat to St Peter Port we were expecting F5-6, possibly a touch of 7. From west of Roches Douvres to St Martins we had a sustained F8-9.

    The kind of detailed forecasts that you show are alway fascinating both for the detail that they show and what they do not show. Most “unofficial” models that I see have unnaturally uniform wind patterns. This one looks better than most. I find it galling that we cannot see the Met Office output from their detailed model. It really should be the best given their modelling capability and data analysis schemes. You can see their 4km model but only with a degraded resolution and rather too late to be useful via Jersey Met for the English Channel. Of course, one problem with such detailed models is that the details predicted have such short lifetimes. For use such as gliding they are clearly useful. As a Cruising sailor, I do not find them any more help than global models. They may be indicative but not better in a deterministic sense,
    Last edited by franksingleton; 28-09-19 at 18:59.

  9. #9
    Join Date
    Nov 2016
    Posts
    2,741

    Default Re: Hurricane Lorenzo

    'Incoming...'





    Anyone think this will just fizzle out? A damp squib?

  10. #10
    Join Date
    Sep 2017
    Posts
    1,854

Page 1 of 3 123 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Latest YBW News

Find Boats For Sale

to
to