How To Read a 1000mb to 500mb thickness chart.

I've been posting a lot of 1000-500mb thickness surface charts lately. I sure at least a few of you don’t know how to read it. So, I thought I would post in the blog  how heck you read all of this stuff. I grabbed one of the model runs I posted on my Facebook page earlier today.


This particular model is called the Global Forecast System or GFS model. The GFS is run four times a day at 00UTC, 06UTC, 12UTC, and 18UTC. The GFS can look out in the future for 384 hours. It's used for short range and medium range forecasting. It is just of many models that we can use, others are the NAM which stands for North American Mesoscale, the RUC which stands for Rapid Update Cycle, and ECMWF which stands for The European Centre for Medium-Range Weather Forecasts (It is more commonly called just the EURO), and many more.    

The Weather models look different from each other, but they work and show more or less the same things. Models show you different layers of the atmosphere; the 250mb, 500mb, 800mb, and many others. The one I will be talking about is the 1000-500mb Thickness Surface Chart.

How do I read the above chart and what does it all mean?

First of all, this is forecasting future weather. In this case, it is forecasting the surface 1000-500mb thickness for 114 hours in advance. The forecasts is valid for Thursday, Nov 11, 2012 at 06 UTC.

How do we know this?

At the top of the chart we see:   11/03/20 12UTC 114 HR FCST VALID THU 11/08/12 06 UTC NCEP/NWS/NOAA

This is what these numbers mean:

11/03/20 12UTC = The date and time the chart was produced.

114 HR FCST = This indicates how many hours out the forecast is valid (114 hrs).

VALID THU 11/08/12 06 UTC = This map is valid Saturday, 12/26/2009 at 06 UTC.

NCEP/NWS/NOAA = Who produced the chart.

What do we see on the chart?

Precipitation: The first thing that pops out are the colored areas. Once such area is multicolored area approaching the Northeast (This is the Nor'easter, I've been discussing on my Facebook weather page). The different colors indicate different levels of precipitation. the lighter shaded green areas indicate light precipitation. The darker greens indicate heavier precipitation, The blues and purple areas are showing very heavy precipitation.

If you look to the left side of the chart. You will see a scale; this indicating the intensity of the precipitation. What it's telling you is how much precipitation will have fallen by the time the map is valid. For example, the light green area over West Virginia on the chart indicates between 0.01 and 0.10 inches of precipitation will have fallen over West Virginia by the time 11/08/12 06 UTC occurs.

Isobars: these are the dark solid lines on the chart. They represent areas of equal atmospheric pressure. Each line passes through a pressure of a given value. There are certain rules you most follow when dealing with isobars

 1) isobar lines can never cross each other.

2) they are labeled every 4 millibars (millibar = 0.02953 inches of mercury). So allowable lines would be 992, 996, 1000, 1004, 1008, and so on.

3) isobar lines are generally corrected for sea level so you can ignore differences in pressure due to altitude.

It's easy to locate high and low pressure zones as a result of the lines on the chart. Furthermore since winds flow from high pressure to low pressure you can predict what the local wind pattern will be when the chart is valid.

Wind: the isobars can also give you an idea how strong the winds will be. When they are very close together; you know the winds are going to be strong. If you look at the area between British Columba and the Yukon you will see a closed isobar with an (H) and the number 1036 under it. This an area of high pressure. Notice there is no precipitation in the center of the High. If you go east or northwest of that area of high pressure you will see the isobars are far apart; this means the winds are light or even calm. However, if you go south of the area of high pressure the line will get closer together. therefore the winds are stronger. The same thing applies to the area of low pressure approaching Long Island, NY. The isobars are very close. So, the winds will be very strong, with the strongest winds at the center. One other thing you could tell, would be the winds would be coming out of the NW over the Great Lakes.

Thickness lines: these are the blue and red dashed lines. The red dashed lines show where the warmer air is and the blue dashed lines show where the cooler air is. Notice these lines have numbers such as 546, 552, 558, 564 etc. The higher the number, the warmer the air temperature.

During the cold season thickness lines can aid in forecasting where frozen precipitation may fall. Forecasting precipitation type during the cold season, is one of the most challenging types of weather forecasting there is. Forecasting precipitation across the United States during the seasons of late Spring, Summer, and early Fall is usually simple. The primary precipitation type in those seasons is rain. However, during winter, the primary precipitation types are rain, snow, sleet, and freezing rain.

There are several ways to forecast precipitation type.

1) using the 1000-500mb thickness method.

2) the 1000 - 850 and 850 -700mb partial thickness method.

3) using soundings method.

Since we're talking about the 1000-500mb thickness chart. That is the method I will explain. But I will answer any questions you may have on the other two.

The 1000-500mb thickness method is a very general method. It can be used to give you an idea of precipitation type over a given area (method#2 and # 3 are more accurate).   In the Northeast and northern Mid Atlantic states, the critical 1000-500mb thickness value that represents the cutoff between liquid versus solid precipitation is 540 line. Other locations across the country could have a slightly different value. For the Northeast lines that have numbers lower than 540 should see frozen precipitation while regions that have a thickness higher than 540 should see liquid precipitation. The 540 line shows more or less where there is a 50% chance for rain versus or a 50% chance for frozen precipitation.

Well, you should know what your looking at and how to read it. I hope you learned something and found this interesting.



Rebecca