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A number of flood forecast guidance products are available from the National Weather Service and the River Forecast Centers. The following is a guideline for reading and interpreting some of these products. Taken together, these products will allow you to understand the River Forecast Center's assessment of flood risk: how much rain is needed, how much is expected, and overall conditions in the affected counties. Return Period rainfall totals may be used to estimate how much rain is needed to produce flooding when no other information is available. To learn more about determining the return period for rainfall, click here. Note that the risk of flooding is elevated in urban areas. Because of all the asphalt and concrete, more rain runs off into streams instead of soaking into the ground. You might notice that areas which have been built up in recent years seem more susceptible to flooding. For these areas, you might want to excercise more caution than indicated by the risk maps and perhaps use a lower rainfall threshold than indicated on the return-period maps.
Twice a day, the Arkansas-Red Basin River Forecast Center (ABRFC) issues Flash Flood Guidance products. You will notice a number of links labeled like "OKCFFGABQ. 10209714z". This label tells you (1) the office issuing the product, (2) the type of product, (3) the warning area covered, and (4) the date/time at which it was issued. In this example, OKC is Oklahoma City - the area of responsibility. FFG means Flash Flood Guidance. ABQ is Albuquerque, which tells you that information from New Mexico will be in that file. The first two digits of the date/time stamp are the month (10 = October), the next two digits are the day (20th), the next two digits are the year (97 = 1997) and the last two digits are the time at which the product was issued (14z = 1400 GMT). There are several older reports along with the current report on-line for a single station at any time. For most purposes, the ‘OKCFFGOKC' or OKCFFGTUL' labels will contain products for your area. Each of the products contains a listing of the amount of rainfall required to produce flooding in any given county (less in urban areas). Rainfall totals for 1-hour, 3-hours and 6-hours are listed for each county. The 3-hour total is the most useful for predicting stream flooding. Note that this is an assessment based on current conditions. If a risk of flooding is expected at a time beyond the next six hours, conditions may change which could affect these thresholds. The thresholds can be used to guage the needed rainfall, but you should check back to this page as you get closer to the event.
Quantitative Precipitation Forecasts (QPF) Now that you know whether your area is in risk of flooding, as assessed by the River Forecast Center, you can take a look at some products to provide information regarding how much rainfall is expected. The Quantitative Precipitation Forecast (QPF), also issued by the ABRFC, is a product which combines computer-model rainfall forecasts with the human forecaster's judgement. There are two sets of QPF maps each day - one issued in the morning and the other in the evening. Each set of QPF forecasts contains four six-hour periods. In the lower right-hand corner there is a date/time stamp for when the product is valid. This is read the same as the time stamp on the Flash Flood Guidance files. You need to make sure that the time on the QPF map matches the current forecast period, in case a product update was not properly received. Across the bottom of the chart is a color-coded bar. The colors identify rainfall increments expected across areas of the map. Light blue is rainfall of 0.05 - 0.10 inch. Dark blue is 0.50 to 0.75 inch. Yellow is 2.00 to 2.50 inches and red is 5.00 - 6.00 inches. Find your area on the map and match the color to the key on the bottom. If the expected rainfall amount is greater than the 6-hour rainfall total from the previous flash flood guidance product, you may have a flooding problem. Even if it does not exceed the guidance threshold, if it seems close it is best to keep an eye on rainfall as the system moves through because heavier bands of rainfall may develop.
Hydrometeorological Discussions The final step in the analysis is to look at the Hydrometeorological Discussions. These are text products, with names similar to the Flash Flood Guidance. They are issued once a day. HMD stands for Hydrometeorological Discussions. TUR stands for the River Forecast Center in Tulsa. The first part of the discussion lists an overview of flood potentials. This is followed by a discussion of the past 24-hours. Current and future hydrologic (river) conditions are then discussed. Finally, flood (river) and Flash Flood outlooks are presented.
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Climatological Survey, 100 East Boyd, Suite 1210, Norman, OK 73019
Copyright © 1996-2005 Oklahoma Climatological Survey. All Rights Reserved. Send comments or questions concerning OK-FIRST to okfirst@mesonet.org. |
Flood Forecast Guidance