In this section we will go over the different maps you can view that help to further understand weather forecasts. Remember, the weather isn’t simply what is happening at the surface, it is a function of the larger pattern at hand. This pattern can be best viewed in the upper air plots. With advances in technology, many of these upper air plots can be over-layed with different parameters such as precipitation, winds, temperature, temperature advection, so on and so forth.
In this section we will review charts related to 850 mb (5000 ft), 500 mb (18,000 feet), and 250 mb (35,000 feet). These charts can all be accessed via the model links in the menus above. These layers are not static, as the air warms or cools based on the seasons, the pressure changes, leading to changes in where the 850 mb pressure layer is, the 500 mb pressure area, and the 250 mb pressure area. With these maps, pressure is in mb, which equals millibars. You may also see maps that refer to 850 hPa or 500 hPa, with hPa being hectopascals and they are equal pressures.
So we begin with…
The 850 mb level, which corresponds to about 5000 feet. Note, if you live 5000 feet above sea level, this one is right on top of your head, so it would be better to move up to the 700 mb charts. The 850 mb is used frequently in storm mode. Snow, hail, severe weather, location of the low level jet, all are dependent on this region from a forecasting standpoint. But even if you aren’t looking for severe weather, the 850 mb region can tell you all about the set-up of the surface weather. It is considered to be the outside boundary of the lowest level of the troposphere (known as the planetary boundary layer in some circles). The 700 mb level, if you choose to view that, is the location where many of the clouds form and thus is used for relative humidity and also location of high and low pressure.
Depending on the website you use, the map can have several features. I have chosen the 850mb heights, temperature, and wind chart provided by the NWS. The forecast models have come a long way. It used to be that you could only get the 500 mb charts, and with that, attempt to discern where your front will set-up and where the action is. In today’s model outputs, everything is right there for you. DAM stands for decameter. So if you want to know the height of the pressure in meters, place a 0 behind the 3 digits on the countours. For example, the 148 dm height contour slices through Philadelphia, Pennsylvania (purple line). If you were standing in Philly and let loose a weather balloon, it would take it approximately 1480 meters to reach a pressure of 850 mb. And the conversion to feet is calculated by multiplying the meters by 3.3.
Following the contours on the map, you can see a low pressure in the deep south (highlighted in Navy blue, with low pressure extension shown in light blue) and a high pressure area well off the east and west coasts and hardly on the map at all (clockwise winds). Off the southeast coast near the Bahamas there is a weak low pressure area (highlighted in Navy blue with counter-clockwise winds). What is not well defined is the precise location of troughs, but with the help of other overlays (for example: theta-e advection), you can have a pretty good idea of where it may be. That and combined with the temperature charts you can locate the general vicinity of a cold front.
Temperatures are contoured with red lines (>0C) and blue lines (<0C) with shadings representing higher levels of warmth or cold. For example, much of the SW US is shaded orange, which is indicative of temperatures >21C but less than 27C (which would be in pink). Remember, this is 5000 feet up, so the ground temp is significantly warmer than that.
For more in-depth discussion about the 850 mb level go here
The next level up is 500 mb…
The 500 mb is the gold standard of forecasting. If you asked a meteorologist which forecast map they would prefer, they would say the 500 mb layer. This layer coincides with the middle of the troposphere and serves two main purposes: height contours which show you where high pressure and low pressure are, and locations of vorticity, which helps determine the location of rising air and fronts.
NAM 500mb chart: Source
In the above image, we have the 500 mb pressure heights, the vorticity, and the winds. Similar to the 850 mb chart, the heights here are in decameters, so if you place a 0 at the end of the 3 digits you get the height in meters. As you can tell, to reach 500 mb requires you to go much higher than 850 mb of pressure.
Areas with more oranges and greens and reds have higher vorticity, and as such, more air rising. The “x” on the chart marks shortwave troughs and are associated with these areas of increased vorticity, which are small-scale perturbations in the atmosphere. They are embedded within the long-wave pattern (think big ocean swells vs small white caps). So here we again see low pressure in the deep south, readily identifiable with the contours. Off the SE coast there is another counter-clockwise spin regarding the winds, indicating another low pressure (but not contoured).
Another thing to note is that as the height contours get closer together, the pressure gradient gets tighter. If you look at the Pac NW, you can see the height contours getting closer together as opposed to off the Southern California coast. As a result of the tighter pressure gradients off the coast of Oregon, air molecules are being squeezed which results in higher winds. If you take a look at the wind barbs, the closer the contours are together, the more barbs you have. Over Oregon, Washington, Idaho, Wyoming, the winds are raging at 50+ knots, but off the SE coast with high pressure, winds are light (10-20 knots).
Intro to 500mb charts: University of Arizona Atmospheric Science -> Intro 500 mb chart
Video Courtesy of Steve DiMartino
And Lastly…
The 250 mb chart, which is approximately 35000 feet up. Does it make that much of a difference between the 200 mb, 250 mb, or 300 mb charts? Not necessarily all that much. The 300 mb chart would be closer to the mid of the troposphere, influencing more of the 500 mb pattern. But, overall, there isn’t a tremendous difference. When you are looking for some slight nuances in a mesoscale event, then it may come into play.
These upper level charts are significant for determining where the upper level trough, upper level high pressure, and upper level winds are located. It’s important to know where the maximum air flow is occurring. During different events, where the maximum air streak occurs can kick off or support lower level environments because the winds help blow and ventilate the tops of the clouds. These upper level charts are associated with the jet stream and local areas of wind max.
250 mb chart
You can see from the chart a deep trough extending into the Pac NW US. The south US has a trough as well. Off the coast closer to the Bahamas you can still make out a weak low pressure area with the counter-clockwise spin almost to the edge of the page. If you go down into the 850 mb – 500 mb level you can see some consistency with the location.
Over the Pac NW you have a fairly substantial jet streak of over 110 knots. The deep south has another jet max of about 90 knots. These areas are associated with diverging and converging winds…..suffice to say they even have a circulation of there own!
Schematic of a jet streak:
Without getting too technical, the jet streak is divided into 4 quadrants, 2 in the front and 2 in the back. These are further subdivided, 1 is rising air, and one is sinking air, known as divergent and convergent air aloft.
And as the 2nd diagram states, these local maximum regions help kick up smaller scale events. Today, on 5/21/19, it will contribute to severe weather and flooding over the Southern Plains. These jet streaks allow storms to breathe.
But that is a topic for another day!
Philadelphia Weather Authority 