NAM (COMPUTER MODEL) SURFACE Click to print in a new window
How do I read the map above and what does it all mean? First of all, this is a computer SURFACE model forecasting future weather. In this case, it is forecasting the weather 6 hours in advance. The forecast is valid 10/26/2009 at 12 UTC. How do we know this?
At the bottom of the map see:
10/26/09 06 UTC 006 HR FCST VALID 10/26/09 12 UTC NCEP/NWS/NOAA.
This is what the numbers mean.
10/26/09 06UTC = the date and time the map was produced.
006 HR FCST= This indicates how many hours out the forecast is valid. (6 Hours)
VALID= Thu 10/26/09 12 UTC. This map is valid for 10/26/2009 at 12 UTC.
NCEP, NEW, NOAA= list the branches of the government responsible for the forecasts.
PRECIPITATION: The first thing that pops out are the green areas. Once such area is approaching Western Canada, while another is located in the central part of the United States. You notice there are different shades of green. LIGHTER GREENS indicate light precipitation. The DARKER GREENS indicate heavier precipitation. BLUE indicates very heavy precipitation. Look to the left side of the map. There you will find a scale indicating the intensity of the precipitation. ***This scale indicates how many inches of precipitation will have fallen by the time the map is valid. In other words, the dark blue on the map indicates between .50 and .75 inches of precipitation will already be in the rain gauge by the time 10/28/09 18 UTC occurs.
ISOBARS: Isobars are lines on a weather map, connecting all areas that have the same air pressure at the same time. In the center of the United States see an “L” surrounded by dark, solid lines. These are isobars. They are labeled every 4 millibars. (1000,1004,1008 etc.) The lowest pressure reading is located in the center of the low pressure area.
WIND: When the isobars are packed close together, the winds are strong. Notice the closed isobar (1024) over the northeastern part of the United States. Although this not indicated with an “H”, it is a center of a High Pressure system. Notice there is no precipitation in the center of the High. Winds are also nearly calm. There is no isobar packing. There is also a large high pressure system well off the California coast in the Pacific Ocean. (1028 mb.) Do you see it?
THICKNESS LINES: Look carefully. You will see dashed lines (both red and blue). These lines are good indicators of the temperature of the atmosphere. Generally the RED dashed lines indicate warm weather while the dashed BLUE lines indicate cold weather. Notice these lines have numbers such as 546, 552, 558, 564 etc. in the western section of Texas. The higher the number, the warmer the weather.
You will notice a large high pressure area in north central Canada. (1032 millibars) Do you see the dashed blue lines labeled 528, 534 etc.? These dashed lines indicate cold weather. The lower the number, the colder the weather.
SNOW VERSUS RAIN: The dashed lines on the map help us predict the type of precipitation we can expect. (Rain, snow or mixed precipitation) Generally the 540-dashed line when it passes through areas of precipitation on the map separates rain from snow. Numbers lower than 540, such as 534 would indicate frozen precipitation or snow while numbers above 540, such as 546 would indicate melted precipitation or rain. Right along the 540-dashed line expect a mixture of very wet snow and rain or even ice pellets called sleet.
NOW LET’S GO TO REAL TIME WEATHER FORECASTING.
MODEL ANALYSIS AND FORECASTS (SURFACE)
CLICK ON THIS:
1. Go to left hand column labeled “NAM. Select time. Use latest time possible. See time chart provided. 06Z UTC (Universal time) = 1 AM Eastern Standard Time (EST) during the winter season, and 2AM Eastern Daylight Times during the summer season. Usually 06Z is available for use by approximately 7 or 8 AM. (EST)
2. Click on “FINE”.
3. Go down to “4 Panel Charts”. (hours). Here you have many choices. Let’s select the second column from the right (MSLP-1000-500MB).
4. SURFACE WEATHER FORECASTS
5. Click 000-018. The four panel map that appears is A) the initial state of the atmosphere. It is not a forecast but the actual condition of the atmosphere that existed at the time of observation.
6. Next is a 6 hour forecast from the initial time.
7. Next is a 12 hour forecast from the initial time.
8. The 4th panel is an 18 hour forecast from the initial time.
9. Now click on the 024-042.
10. The first panel is a 24 hour forecast from the initial panel.
WHAT DO YOU DO WHEN THE MAPS COMES INTO VIEW?
1. Can you find an “H”. That is a High Pressure area. High pressure usually means fair weather.
2. Can you find an “L”. That is a Low Pressure Area. Low pressure usually means storms and precipitation.
3. Can you find a light green area? (Light green usually means an area where some light precipitation is expected to fall. Look to the extreme left side of the chart to see how much precipitation is expected to fall in a 6 hour period ending at the time of this map. (In other words, by the time indicated on this map, (last 6 hours) the precipitation has already hit the ground.
4. Can you find a darker green area? (Dark green usually means an area where some heavier precipitation can be expected. (See left side of chart to indicate how much precipitation is expected. Notice if you see “Red Areas” of precipitation it means 4-5 inches of melted precipitation can be expected to hit the ground during the previous six hours.
5. Can you find “Dotted Lines(…….)? Notice some are Red and some are Blue. The “first” blue line is labeled “540”. The higher this number goes, the warmer the weather will be in that area. If the 540 line “cuts through a precipitation area (green) it many time divides snow areas from rain areas. The value 540 and below (colder) normally indicates precipitation in the form of snow. For example, a value of 546 generally indicates rain. A value of 534 usually indicates snow.
6. Can you find solid black lines? These solid black lines on a weather map are called isobars. They are lines on a weather map that connect places that have the same air pressure at the same level of the atmosphere at the same time. These lines are placed around HIGH and LOW pressure areas and are always spaced 4 MB. apart. For example, here are the isobars drawn on a weather map. 1000 mb, 1004 mb, 1008 mb. 1012 mb, 1016 mb, 1020 mb, 1024 mb. 1028 mb, 1032 mb, 1036 mb. 1040 mb, 1044 mb. Around LOW pressure areas usually find the following isobars. 1000 mb, 996 mb, 992 mb, 988 mb. 984 mb, 980 mb, 976 mb, 972 mb, 968 mb, 964 mb, 960 mb.
7. On a weather map (Around a station model) air pressure would be recorded as follows: 1000.0 mb. = 000, 1004.0 mb. = 040. (Cross off the “10” and remove the decimal point. 1008.0= 080, 1012.0= 120, 1016.0=160, 1020.0=200, 1024.0=240, 1028.0=280, 1032.0=320, 1036.0=360, 1040.0=400, 1044.0=440.
8. Around low pressure areas: 1000.0=000, 996.0=960, 992.0=920, 988.0=880, 984.0=840, 980.0=800, 976.0=760. (Cross off the first 9 and eliminate the decimal point)
9. Do you see any black lines (isobars) packed close together? When isobars are “packed” it indicates STRONG WINDS. The closer they are packed, the stronger are the winds.
10. From the first set of maps, you now have an indication if the weather will be: a) dry or wet. (green areas) b) If the precipitation will be in the form of rain or snow (look for green areas and the location of the …..(dotted lines) Below 540 = snow, above 540 = rain. c) You will know whether it will be cold or warm. (By looking at the dashed lines….The higher the number, the warmer the weather. D) You will know whether it will be windy or not windy. (Packed isobars).
11. You will also be able to forecast the direction of the winds from these maps. After you read the chapter on winds, surface wind direction and resulting weather will become quite clear.
12. From the “NAM” YOU WILL BE ABLE TO FORECAST THE WEATHER OUT 84 HOURS FOR YOUR AREA OR ANY AREA IN NORTH AMERICA.. TRY IT! It takes a little practice.