An Instructional Guide to Forecasting the Weather
Justin McCoy
English 202 C
4/15/14
I) The Importance of Forecasting
The weather is an extremely influential force that impacts our everyday lives. From freezing rain
to severe thunderstorms, meteorological events can ultimately endanger the lives of many
uninformed people. It is for this reason that forecasting the weather is exceptionally important: to
protect life and property. This instruction set is designed to teach, guide, and lead you through
various meteorological approaches to forecasting the weather. Keep in mind that this is a
beginner’s guide and will only teach you basic forecasting skills. Successful forecasting is
contingent upon the understanding of various models and websites. These tools will be
introduced throughout the instruction set, as well as step-by-step instructions on how to use each
website and model run. We will begin by looking at temperature, one of the most important
meteorological measurements. The instruction set will be divided into categories (purple) that
will explain the important features of forecasting. Sub-categories (blue) may also be defined in
order to further explain various meteorological phenomena. As you will see below, temperature
is the main category. The sub-categories will include how cloud cover and wind affect
temperature forecasting (as well as the introduction of pressure fields and high and low pressure
systems). With this layout, let’s begin your forecasting lesson.
II) Temperature
Temperature is often the first thing people want to know about in a weather forecast. How cool
or warm is the air temperature going to be tomorrow? Should I really wear a t-shirt or is that
jacket hanging in my closet the best bet? Whatever it is, the first thing you need to do is check
out a model run. In this case, you will be checking out temperature outlooks from one of the
most helpful models, the Global Forecast System (GFS). Take a look below.
I)
II)
III)
IV)
Visit http://weather.cod.edu/forecast/.
You should notice a variety of tabs at the top (HRRR, RAP, NAM etc). Click on the
GFS.
Click on the surface tab located on the left-hand side of the page.
Click on 2m Temperature. It is located halfway down the scroll drop menu.
Congratulations, you now have a very powerful tool in forecasting expected temperatures across
the United States, out to 10 days!
Disclaimer: Although you have access to this model output, this does not necessarily mean these
temperatures will actually occur. We will discuss other factors that can change surface
temperatures in the sub-categories below.
To begin viewing the projected temperatures across the contingent United States, put your mouse
over the small rectangle boxes located underneath the HRRR, RAP, and NAM tabs. As you
scroll across the interface, you will notice slight changes in the temperature field across the U.S.
Each rectangle you move across will advance the map 3 hours. At any time, you can view the
temperature for any particular location by decoding the color shown on the map with the legend
provided at the bottom of the map. Keep in mind that the time is in UTC (a meteorological time
that keeps time consistent across the globe). To convert to regular time, simply subtract 4 hours
from the UTC time. This will then put the time into military time.
Example:
Let’s say it’s 18z (the UTC time). Subtract 4 hours and we obtain 14 (in military time). 14 in
military time is 2 hours past noon, so 18z is 2 pm in the afternoon! You will get the hang of it.
II.2 Pressure Fields
Notice the funky looking lines you see scattered about the map. These are called isobars or in
layman's terms, lines of equal pressure. These lines can help you determine the location of a high
or low-pressure system, which ultimately drives the temperature changes you see as you scroll
across the top of the interface. To locate an area of high or low pressure, simply look for an area
of enclosed circular isobars like the one pictured in
Figure 1. You can see an area of low pressure situated
in northern Nebraska. Read below to learn what else to
look for when dealing with high and low pressure
systems.
Determining High and Low Pressures:
I) Look for circular isobar enclosures like the ones
pictured in Figure 1.
II) Numbers are assigned to each drawn isobar.
III) If the numbers decrease as you move away from
the center, it is an area of high pressure.
IV) If the numbers increase as you move away from
the center, it is an area of low pressure.
V) Notice that the circular isobar in northern
Nebraska says 998. As you move away, the numbers
slowly increase. We know then that this is an area of low pressure.
Figure 1: The temperature field of the United
States. Lines of equal pressure are drawn to
scale. Low pressure is to the west, while high
pressure is to the east.
weather.cod.edu/forecast
But What Does This All Mean?:
All you need to know about high pressure is that it rotates air clockwise around its center.
Therefore, any area ahead of a high-pressure system will experience cold air advection (cold
Canadian air will be brought southward) which will likely decrease surface temperature (Figure
2). As you would then expect, warm air advection (warm
tropical air will be brought northward) is experienced behind
the high-pressure system, resulting in a likely increase in
surface temperature. A low-pressure system rotates air
counterclockwise around its center. So cold air advection
occurs behind the low, while warm air advection occurs
ahead of the low (just the opposite of the high pressure
system). It is important to watch the movement of these high
and low pressure systems, as they tell the story about how
Figure 2: A typical set-up for an area of
high pressure with clockwise flow
about its origin.
http://ww2010.atmos.uiuc.edu/guides/
mtr/fcst/tmps/gifs/hl1.gif
temperature will change with respect to time. It may seem confusing at first, but with a little
practice, you will begin to notice when a significant warming or cooling event will occur. Put
this in your forecasting toolbox!
II.3 Cloud Cover and Wind
So now you have a basic understanding of how to determine projected temperatures across the
United States. But you cannot simply peek at the GFS temperature outlook and call it a day.
There are two more factors that significantly impact temperature, and those two factors are wind
and cloud cover. After you get a sense about what you think the temperature will be in a
particular area, examine cloud cover.
I)
II)
III)
Visit http://mp1.met.psu.edu/~fxg1/SAT_US/anim8ir.html.
The image that loads is an infrared satellite depiction of cloud top temperatures.
The colder the cloud top, the higher and more dense the cloud. See the legend on the
left-hand side to determine the cloud top temperature.
Infrared imagery will tell a lot about how air temperature will change during the day or night
(figure 3).
Figure 3: Infrared Satellite Imagery of the United
States. The darker the color, the colder the cloud
tops.
Penn State EWALL
So now you have another tool, this time for
forecasting projected cloud cover for the U.S. If
you expect cloud cover to be significant during
the day (cold cloud tops), then forecast a slightly
smaller temperature value than what the GFS
predicted (roughly one or two degrees smaller).
This is because the clouds effectively block some
of the solar radiation trying to reach the Earth’s
surface, thus resulting in a slightly cooler
temperature. If you expect cloud cover to be
significant during the night, then forecast a
slightly larger temperature value (maybe two to
three degrees warmer). This is because the clouds
act like a blanket at night, and trap solar radiation
from escaping back into the atmosphere (so
temperatures would be slighter warmer).
Getting the hang of it? Let’s take a look at a model run that shows wind. When you visit the site,
it should look like Figure 4.
I)
Visit http://hint.fm/wind/.
II)
Study the map and determine wind
speed for your forecast area.
III)
If little or no wind is shown, ignore
this effect on temperature.
Figure 4: A map of the current wind
pattern and speed in the United States.
IV)
If wind is present, carefully read the following instructions below.
Wind during the day tends to cool things down because it mixes the lowest layer in the
atmosphere (distributes heat). So when forecasting temperature during the day (with significant
wind speeds greater than 8 mph), go with a cooler temperature than what the GFS stated (maybe
one to two degrees cooler). However, significant wind speed at night actually warms up the
lowest layer of the atmosphere (since escaping radiation aloft will mix back down to the surface).
If this is the case, forecast slightly warmer temperatures (maybe two or three degrees warmer).
In Summary (Example Practice):
If you gathered from the GFS that central Pennsylvania was roughly 67 degrees F at 23 UTC
(7:00 pm), you moved on to analyze wind and cloud cover. When you checked the cloud
situation over central Pennsylvania, you noticed that there was substantial cloud cover (as
gathered by extremely cold cloud tops). You knew that this would significantly trap outgoing
radiation from the surface, and keep the lower layer slightly warmer than predicted. You then
added two degrees to your previous temperature forecast (which brings the temperature to 69
degrees F). Next, you looked at the wind situation in central PA and noticed a nice 10-12 mph
southerly wind. You remembered that wind mixes the boundary layer, and brings warmer
temperatures back to the surface. Due to this, you added another two degrees. Therefore your
true predicted forecast temperature was 71 degrees and not 67 like the GFS predicted.
III) Surface Analysis
A surface analysis (Figure 5) is a map of current weather conditions. It provides you with
information about pressure fields (as learned above), temperature advections (also learned), and
precipitation.
Meteorologists typically use surface analysis maps to get
an idea about the general weather set-up for a particular
day. Follow the instructions below to get a feel of how this
is done.
Figure 5: A classic example of a surface
analysis. The blue lines are cold fronts
(associated with cold air advection),
while the red lines are warm fronts
(associated with warm air advection).
I) Visit http://atsc.uga.edu/wx/ncep_loops.htm.
II) Locate the “GFS Forecasts” that is located at the
bottom right section of the page.
III) Choose the 12 z SLP/Thickness/Pcpn loop. It
should look like Figure 6 below.
http://www.srh.noaa.gov
Figure 6: Surface Analysis depicting
precipitation.
NCEP
This map is easy to navigate and understand, much like the one you used for temperature. Using
the arrows on the left-hand side of the screen, click through the timeframes (each advancing
three hours). The solid lines you see are isobars while the dotted lines are isotherms (lines of
equal temperature). By using this site, you can get an idea of the expected precipitation to fall
across the contingent United States (precipitation depiction can be analyzed with the legend on
the left hand side of the screen). Forecasting precipitation can be relatively easy at times,
especially when you have model sites like the one listed above. Use this to your advantage if you
ever want to keep track of future precipitation events.
The weather is only as powerful as you make it out to be.
IV) Conclusion
Forecasting the weather is an extremely tricky and time-consuming process. The most important
features of forecasting are temperature, high and low pressure systems, cold and warm air
advection, and surface analysis features such as precipitation. By learning how to determine what
the temperature will be, based on factors such as cloud cover and wind speed, and when to
expect precipitation, you have already surpassed the forecasting skills of many individuals
(provided that you keep the sites shown above). Forecasting the weather is a powerful tool, and
often an easy conversation starter for almost anyone! Keeping up to date with the weather is
what keeps us safe. It also helps you decide when to have that outdoor barbeque those annoying
neighbors have been pushing you to have.
Relevant Terms (as obtained by the AMS Glossary)
http://glossary.ametsoc.org/wiki/Main_Page
Global Forecast System (GFS): The Global Forecast System (GFS) is a global numerical
weather prediction system containing a global computer model and variational analysis run by
the U.S. National Weather Service (NWS).
Isobars: A line of equal or constant pressure; an isopleth of pressure. In meteorology, it most
often refers to a line drawn through all points of equal atmospheric pressure along a given
reference surface, such as a constant height surface.
Isopleth: A line of equal or constant temperature.
UTC Time (Z time): The basis for civil timekeeping.
Surface Analysis: A special type of weather map that provides a view of weather elements over
a geographical area at a specified time based on information from ground-based weather stations.
Cold Air Advection: Cold advection is the process in which the wind blows from a region of
cold air to a region of warmer air.
Warm Air Advection: Warm advection is the process in which the wind blows from a region of
warm air to a region of colder air.