Precipitation Forecasts from the
NCEP Global Forecasting System (GFS) Ensemble

Typical ensemble size: 20 forecast members.
Animated sequences below contain 30 images, each covering a 6-hour period.

Note regarding Google Earth graphics in column 2 below: To view any of the graphics in the second column below, you must first download and install Google Earth, which can be downloaded for free here. Google's Chrome Web browser is not required; any Web browser supported by Google Earth, such as Firefox or Internet Explorer is fine. After installation, click on any of the links below in the second column. The first time you do this, a window will open asking what to do with a file ending in KML. Select "Open with" instead of "Save." If necessary, select Google Earth as the software to open the file. For Firefox, select "Do this automatically for files like this from now on." For Internet Explorer, uncheck the box that would always ask about opening this type of file.

Click here for a primer on how to use Google Earth's animation controls to view our graphics.
SEE SECTION 2 ON PAGE 3 OF THE PRIMER FOR A TRICK TO LOAD THE IMAGES.
Without the trick, the images usually do not load. Even with the trick, loading is slow for Windows, Mac, or Linux.
You will need Adobe Acrobat reader to view the primer. Click here to download Acrobat Reader.

Click on the product below
whose maps you want to see
. These maps are simpler and load much faster than those in the second column. Individual maps cover various land areas of the world.



Ensemble-average 6-hour precipitation

Ensemble-median 6-hour precipitation

Ensemble-generated probabilities for 6-hour precipitation amounts

 

Ensemble "spaghetti" contours for 6-hour precipitation amounts

 

Ensemble-average accumulated precipitation "t" hours into the forecast


Google Earth version. First, install Google Earth. Second, download our primer; see the trick on page 3 about getting images to load. Lastly, click on a link below. Image loading and rendering is slow. Each 30-image animated sequence occupies roughly 6MB.

Ensemble-average 6-hour precipitation

Ensemble-median 6-hour precipitation

Ensemble-generated probabilities for 6-hour precipitation exceeding thresholds in inches of:
0.01 0.10. 0.25 0.5 1.0

Ensemble "spaghetti" contours for 6-hour precipitation exceeding thresholds in inches of:
0.25 0.5 1.0

Ensemble-average accumulated precipitation "t" hours into the forecast

 

 

 

 

 

About These Products

The plots on this web site are generated using the raw (unadjusted) output from the Global Forecasting System (GFS) Ensemble run at NOAA's Environmental Modeling Center (EMC).. Four times each day, EMC produces this ensemble forecast using a single computer model that is run multiple times with slightly different initial conditions. These various initial states approximate the uncertainties inherent in the initial atmospheric state. As the forecasts progress in time, the slight discrepancies in the initial conditions will, in general, grow until the forecasts in the ensemble become quite different. As the differences in the ensemble members evolve, they provide a measure of the forecast uncertainty. The more disagreement there is among the ensemble members for a particular region, the less certain the forecast is for that region. Note that this uncertainty arises through the use of a single model.

Additionally, the ensemble itself has been shown to have deficiencies in describing the forecast uncertainty. Studies of the ensemble's performance indicate that the ensemble tends to underestimate the extremes. That is, a forecast for a little rain is often wetter than it should be, and a forecast for a lot of rain should often be for even more rain.

Research in this lab and elsewhere is being conducted to develop post-processing techniques to reduce the error of individual forecasts within the ensemble and to adjust the statistical distribution represented by the ensemble to represent the forecast probabilities more accurately. Even in their present form, the plots on this web site can be used to provide an idea of both the forecast and its uncertainty along with the range of possibilities that exist. Remember, though, the ensemble is typically a little too wet for small amounts of precipitation and not wet enough for large amounts of precipitation.




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Jon Ahlquist. My email address is the reverse of these words: edu.fsu@ahlquist
Department of Earth, Ocean, and Atmospheric Science, The Florida State University
Tallahassee, FL 32306-4520, (850) 644-6205