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NAM (COMPUTER MODEL)

500-MILLIBAR MAP (18,000 FT.)

How do I read the map above and what does it all mean?  First of all, this is a computer 500 MB.model forecasting future weather.  In this case, it is forecasting the weather 12 hours in advance.  The forecast is valid 10/26/2009 at 18 UTC. How do we know this?

At the bottom of the map see:

10/26/09 06 UTC 012 HR FCST VALID 10/26/09 18 UTC NCEP/NWS/NOAA.

This is what the numbers mean.

10/26/09 06UTC = the date and time the map was produced.

012 HR FCST= this indicates how many hours out the forecast is valid.

VALID= Thu 10/26/09 18 UTC. This map is valid for 10/26/2009 at 18 UTC.

NCEP, NEW, NOAA= list the branches of the government responsible for the forecasts.

WHAT DO WE SEE?  It looks like there is a “mountain” off the West Coast of the United States and another “mountain off the East Coast.  We call these mountains, RIDGES. They are pockets of warm air. In the central part of the United States there appears to be a deep valley.  We call these valleys in the upper air, TROUGHS. These are pockets of cold air.

WINDS: Winds on this map are indicated by a “shaft” which points into the direction from whence the wind comes. On the shaft, find lines and flags. A line represents a wind speed of 10 knots.  One half line represents a wind speed of 5 knots.  A flag represents a speed of 50 knots. For example, the upper level winds to the right of the “valley or trough” are blowing from the south or slightly southwest.  The winds along the west coast are coming from the north.  Notice the winds in the center of the troughs and ridges are very light and variable, meaning they can come from any direction.

SURFACE STORM MOVEMENT:  Storms and precipitation (we call this weather) are carried by these upper air winds. We call these winds the “steering winds”. Weather on the surface moves “with” the winds at about one half the speed of the 500-millibar winds. For example, if the 500 millibar winds are blowing from the south at a speed of 80 knots, the storm and the accompanying precipitation will move towards the north at a speed of 40 knots.  (VERY IMPORTANT)

VORTICITY:  Notice the yellow and red coloring on the map?  This indicates vorticity.  Vorticity is a spi8nning, upward motion of air. Vorticity supports the intensification  (strengthening or deepening) of storms. Yellow is moderate vorticity while red is strong vorticity.  The brighter the red, the stronger the vorticity and the greater will be the intensification of the storm.

NOW LET’S TRY SOME REAL TIME FORECASTING.

MODEL ANALYSIS AND FORECASTS (UPPER AIR)

500 MB (18,000 FT)

 Click on this:   http://www.nco.ncep.noaa.gov/p/   ``` 1. CLICK ON www.nco.ncep.noaa.gov LINK ABOVE. 2. CHOOSE MODEL-NAM 3. SELECT UPPER AIR PARAM.- 500_rh_ht 4. CHOOSE FORECAST HOUR- 000 5. CLICK ON THE "NEXT" BUTTON, THE MAP WILL ADVANCE 3 HOURS, CLICK NEXT AGAIN, THE MAP WILL ADVANCE ANOTHER 3 HOURS. ```

4.     DO YOU SEE SOLID BLACK LINES?  They are “height lines”.  Oddly enough, they actually indicate the temperature of the atmosphere.  The lower the number, the colder the air at 18,000 feet.  For example, a reading of 570 over an area indicates it is much warmer than an area that has 540 over it.

5.    DO YOU SEE WIND BARBS? These barbs indicate the wind speed and the direction from which the wind is blowing.

6.     The flag indicates 50 knots, while each line represents 10 knots.  A half flag equals 5 knots. If there were one flag and two lines, it would indicate a speed of 70 knots.( 50 knots for the flag and 20 knots for the two lines. (See wind chapter for further details).  STORM SYSTEMS ON THE SURFACE WOULD MOVE AT A SPEED OF 35 KNOTS (1/2 THE SPEED OF THE 500 MB. WINDS.   IF THE WINDS WERE FROM THE WEST, THE STORM WOULD BE MOVING TOWARDS THE EAST BEING PUSHED BY THE WESTERLY WINDS.

DO YOU SEE RED OR YELLOW AREAS? These areas indicate VORTICITY. (Vorticity is actually a spinning motion and supports the development of storms.  Storms become much stronger & more severe when areas of vorticity approach them.  Red areas of vorticity are stronger than yellow areas and thus would cause a storm to deepen much more rapidly. When vorticity is increasing, it is called Positive Vorticity Advection. (PVA)