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  • 1 Post By GeorgeB
  • 1 Post By RichH
  • 1 Post By AdamLein
  • 1 Post By krazykoozak
  • 1 Post By Tempest
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  #1  
Old 08-02-2012
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Help with reading a weather chart

This is today's upper atmosphere North Atlantic NOAA chart:


http://weather.noaa.gov/pub/fax/PPAA10.gif

I am about 1/3 through "mariner's weather". It is interesting to read, but I often have to read a paragraph several times to really understand it so it is slow going. The author suggests reading the weather charts every day, making predictions based on those charts and then seeing how your predictions do. I want to use the same charts that I would be able to receive via weatherfax.

However, I am not even sure that I am using the correct chart!

Is this what I should be looking at to try and predict storms/winds/etc? Why are the pressures not in millibars?

Here is what I see. There is a huge low just west of Ireland. The topographic lines are relatively close to one another, so this indicates strong winds. In fact the wind barbs indicate between 30 and 40 knots wind strength. I suppose this is a storm system. Is it possible to figure out the sea state from this?

Meanwhile, in New Jersey where I live, there should be light southwestsouth wind. The lines are well spaced apart, indicating the pressure differential is slight, and the high is in the south so the wind should be running to the north, more or less perpendicular (slightly to the right because it is the northern hemisphere) to the pressure line.

Am I in the ballpark here based on the chart?
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Old 08-02-2012
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Re: Help with reading a weather chart

A couple real quick things: Get the chart for the western Atlantic. Also, look for the lower atmosphere chart. You are getting into some esoteric forecasting when you start predicting how an upper atmosphere high is steering the lower atmosphere weather. These charts are from NOAAs big, world wide weather model and you are looking at the model's prediction of current conditions. You can down-load the forecasts (accuracy goes way down after five days out) Your TV weather man is using the same forecasts to predict the five day forecast. Weather fax is so yesterday. Those guys are downloading the GRIB files and are using thier computer software to construct the chart. GRIBs are much quicker and easier to receive (or so said our weatherman-navigator on the Hawaii race).
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Old 08-02-2012
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Re: Help with reading a weather chart

What you are looking at is a 500 mb chart (above the earth's surface at a geopotential PRESSURE of 500 millibars), or the upper level atmosphere at approx. 18000 to 20000 ft. Essentially this is a 'jet stream' chart, since 'storm tracks' usually follow the jet stream pathways. Very different charting from 'surface' charts.
Since you have the 500 mb chart, now go and compare or overlay this on a 'surface' chart ... and you will then get the 3 dimensional perspective of the weather.
:-)
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Old 08-02-2012
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Re: Help with reading a weather chart

And note that local features such as terrain and heating will affect the weather very strongly and probably won't show up much on these charts.

I sail in a NW-SE straight that is bordered by high mountains on either side. We have lots of fjords. In the summer, inflows dominate in the later afternoon. You would never guess that looking at the surface analysis.

See if you can find a book about weather patterns specific to your area. We have a great book out here called The Wind Came All Ways or some such thing. It has eight chapters, one for each major pressure gradient direction (N, NE, E, etc), which can be read off a surface analysis. Each chapter then describes what you can really expect from such a large-scale atmospheric configuration, given the season and specific location.
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Re: Help with reading a weather chart

RichH's comments are correct.
While your general observations on the chart are pretty right on, i'm pretty sure you are thinking in terms of surface based features. But since it is a 500mb chart you can't make too many inferences to the surface based weather on what you see here.
Highs and lows on this chart determine whether there is upper support or not for surface based weather systems. Surface based systems will typically move in the direction of the parallel isobars. That is why they call the 500 chart the driving chart, because it drives the surface based systems.

That upper low over ireland you mentioned is connected to a surface low and also has upper air support at the 250mb level. This is called a cold low, and tends to be associated with continued bad surface weather (ie. thunderstorms, heavy precip, high winds). the surface conditions on the western coast of ireland today are low stratus ceilings with rain and drizzle and winds gusting 25 kts from the south east. sig weather charts show layers of cloud to 24000 ft with embeded cb's to 32000. the thing is it's not always as easy to see significant surface weather by just looking at a 500 chart. there is a trough line depicted along the eastern seaboard of the US that would tend to support instability aloft giving rise to thunderstorms, yet if you look at the surface weather there today, virginia is really the only area encountering sig thunderstorms.
I might suggest that you start with surface charts, and learn cylogenesis theory first. once you have a full grasp of how systems develop, move and die along with the associated cloud types and weather, then you can start to think in terms of the air column and how the upper atmosphere contributes to the surface weather's evolution.
A meteorology instructor once told me to think of the atmoshere like a river with rapids in it. The eddies around rocks acting much the same as air around lows. it helped me try and visuallize the larger picture of why weather does what it does.
cheers.
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Re: Help with reading a weather chart

Peter, you can Go here:

Radiofax Charts - Boston



and take a look at this chart:

48HR Surface Forecast VT00/12Z 10E-95W NW Atlantic
Size: 24.8K - Updated: Thu Aug 2 18:20:00 2012 UTC
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Re: Help with reading a weather chart

"Why are the pressures not in millibars?"

Because they are. It is a 500 MB (millibar) chart. This is about 1/2 atmosphere of pressure so for an altitude of about 20,000 feet, or about where the jet stream runs.

I also find the 500 MB charts interesting for longer term forecasting but the surface
charts are much more illuminating.

Also of interest for surface weather conditions are the NOAA buoys that are offshore
found here: NDBC - New York Recent Marine Data
Check conditions at Ambrose Light (12 mi. offshore NY/NJ) in nearly real time.
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Old 08-03-2012
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Re: Help with reading a weather chart

It is a good idea to keep track of both the upper level and surface forecasts. The upper level is really handy for predicting the weather patterns that will develop. For instance, if you see that there is a big upper level low setting itself into the Canadian Maritimes, you pretty much know that the winds in the NE will be changing to predominantly north or west. If you keep track of these over a long period of time, the changes in seasons become obvious. These winds also affect tropical storms in predictable ways. Of course it's all guesswork but if trying to guess where to point to stay on the navigable side of a hurricane, it could prove to be useful.

Surface forecasts show the lower level pressure systems and reflect what NOAA figures might happen at ground level over from one to four days in the future. When cruising, I religiously tune in via Pactor to download these charts onto the computer, as well as the TS map. I have a schedule posted right next to the SSB and try to time it just right so that in five minutes or so I can turn the radio and computer on, get a weather map on the laptop via Airmail/GetFax, then shut everything down (SSBs are power hogs).
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Re: Help with reading a weather chart

has anyone read Chris Parker's book on weather ? Have searched for reviews and find none. Am trying to get a handle on all this....
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Re: Help with reading a weather chart



This is interesting stuff. Look at the two big 500mb low pressure systems that are controlling the North Atlantic weather. A hurricane (Gordon) just developed which is forecast to head almost due east, toward the Azores, because it will follow the upper level wind pattern from these lows. When these lows are in place, hurricanes stay out to sea, when the highs move north, that's when hurricanes track west and up the US coast.




It will be interesting to watch the next low, Invest94, which is predicted to track really far south, toward the Grenadines/Trinidad like the one did a couple of weeks ago.
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