Date:       9/30/02 2:36 PM
To:         Bulletins
From:       Luis Soltero
Copy:       
Subject:    Weather Bulleting 4 for your review



Hello All,

A while ago I posted a series of 3 bulletins that described how you
might receive weather via e-mail.  The third bulletin of the series
was an introduction to grib files and their use.  In that bulletin I
gave instructions on retrieving grib files via saildocs.com and 
displaying them with raychart.


Grib files that mariners are interested in originate from computer
models created by NOAA.   NOAA has several computer models
that it uses for forecasting but only two of these model output
data that we can use readily.  The two models are "Global 
Forecasting System GFS" and "MMAB Operational Wave Model (wwave3)".

GFS is a monster.  The program models everything from ice concentrations
in the upper atmosphere through pressure gradients, temperature, 
thunderstorms, tornados,  predicts weather for both ocean and
land masses, etc.  Basically every thing.  It's an extremely complex
model that creates several output forecast files divided into segments.
Some of these include

* Short   and extended range Alaska weather
* Short  and extended range thunder storm activity
* Short  and extended range Global forecast
* Medium range guidance
* Short range NGM (Whatever that means)
* + lots more...

In short it 's a **VERY*** complex model.  For people in the
business of providing weather to cruisers the GFS extended
forecast is really nice.  It contains, pressure, temperature, 
wind dir/speed information all in one file.  The grib file contains 
multiple forecasts computed in 24 hour time slices.    

The second model (MMAB or wwave3) is a computer program specifically
designed to predict wave height, direction, and time period.
That is all it does.  As a by product it forecasts the surface
speed and direction of the wind since it needs this information
for computing the wave parameters.  Unlike GFS, wwave only
computes wind speed over water.  wwave generates a 5 day
forecast with time slices computed every 3 hours.

As far as I have been able to determine there are 5 internet weather providers
that extract forecasts from these models and repackage them
for the mariner.  These providers are:
* saildocs using extended GFS
* Raymarine using extended GFS
* navcenter.com repackaging wwave
* MaxSea (I have no idea what they do... Any one have a grib
file I could look at?)
* and us (Global Marine Networks) using wwave (More on this later).

wwave only predicts wind/wave conditions and
does not yield any information on surface temperature or
atmospheric pressure.

My recommendation is for you to use wwave3 data
when available.  As I mentioned before the only Internet
provider of this data packaged in a way that is useful to
you is Navcenter.com.  The problem with their data is
that it's too large for wireless access.  Typical data sets
consists of a 5 day weather file containing 42 forecasts
computed in 3 hour intervals that are about 100-200 Kb
compressed.

So this is where we come in...  I have released our
grib mail robot which repackages wwave data in
a format that can be transmitted readily via wireless
modes.

This program is a small portion of a comprehensive
weather package we plan to release at the end
of this quarter.  (More on this subject in a future 
bulletin).  

gmngrib@globalmarinenet.net is a straight forward robot that takes 
requests for data in the Subject line and returns the requested grib
information as an attachment.  The data can be viewed and 
animated with both maxsea and raychart.

To request information on it's use simply send e-mail to 
gmngrib@globalmarinenet.net and put the word "info"  or "help" in the
Subject line.

To receive a 3-5K 24 hour forecast of the Atlantic
region, for example,  you would enter "Atlantic 24" in the
subject line.  And to receive a 10K 1 day forecast ( 4 forecasts in 6 hour intervals )
that can be animated for the same region enter "Atlantic 1day" on the subject line.

The info file that is returned to you has detailed information 
on the command structure and use of the robot.

Here are few comments for viewing Grib files with the Free DEMO version
of RayChart.

RayChart demo does not display all the information stored in a grib file.
It looks at the date of the data plus some other parameters (I haven't been
able to figure out what) and then decides if it wants to display the weather
chart or not.  So, if you download a 24 hour forecast and it fails to display
I suggest you download the 48 hour forecast and it will display.   Of course
you could always pay the $500 they want for the product to fix the problem.

To animate the data in a grib series such as the 1day forecast mentioned
above you would use the "animate" tool bar.  If "Animate" is not displayed on the
tool bar simply do a right mouse button click on the gray area to the right
of the bar and check "Animate".  Animate has a speed control, start/pause
button and a fast forward button.  Once you have loaded the grib file
push the start button and then adjust the speed control to your liking.

As I mentioned before the grib data provided only contains wind speed
and direction information.    You can adjust the "look" of the wind arrows
by pushing the "layers" tool bar button, then double click on weather and
select wind.  "Barbs" give you same type of arrows you are accustomed
to viewing on your weather faxes.  The contour lines are nice since
they give you regions of iso strength much like pressure gradient information
obtained by isobars.

Details on the use of RayChart to view grib files is discussed in 
greater detail in Weather Bulletin 3.

One last note on the grib robot.  Single weather forecasts are delivered
as attachments in uncompressed format (3-5K).  multi day forecasts (1day,
2day,...) are compressed in bzip2 format.  MaxSea can read these directly.
RayChart cannot read these directly so you will have to get a bunzip2 
program.  The grib robot info file gives instructions on where to obtain
these.

The grib technology is very interesting and is worth experimenting with.

Download the demo version of raychart from www.raymarine.com then
send mail to gmngrib@globalmarinenet.net and give the technology a 
whirl.

--luis