NOAA/NWS Frequently Asked Questions (FAQ)

1.      What exactly is the NOAA/National Weather Service?

The National Weather Service is an agency of the federal government.  It is actually part of the Department of Commerce, with its parent organization being the National Oceanic and Atmospheric Administration.  You may be familiar with its acronym NOAA, which is pronounced “Noah”.

Our office is staffed 24 hours a day, every day of the year, protecting life and property of Iowa citizens residing in our 51 county area of responsibility. 

The website for the Des Moines office can be found at

2.      What is the difference between Partly Cloudy and Partly Sunny?

They essentially mean the same thing.  Partly Cloudy can be used during the day or night, while Partly Sunny should obviously only be mentioned in the forecast during daylight hours.  They are both supposed to represent sky conditions where between 32% and 69% of the sky is predominately covered by clouds.  This is more of a generalization for the time period in question, and does not mean that skies couldn’t be nearly clear or totally cloudy for brief periods.

3.      What are heating and cooling degree days? 

Heating and cooling degree days are a measure of how much the average daily temperature deviates from a baseline of 65 degrees.  The average temperature is crudely computed by averaging the high and low temperatures for the day, so a high of 70 and a low of 50 would result in an average daily temperature of 60 and 5 heating degree days.  Conversely, a high of 90 and a low of 65 ((90 + 65)/2 = 78 rounded up) results in 13 cooling degree days for that particular day.  These degree days are sometimes used by utility companies to get a rough idea of how much you may by using your heating or air conditioning.

4.      Does the NOAA/National Weather Service activate the tornado siren in my town?

The NOAA/National Weather Service does not have direct control over any tornado or severe weather sirens.  They are typically activated by city or county officials, usually police, fire department or emergency management personnel.   Some officials wait for the issuance of a tornado warning, while others wait until a tornado has been identified by spotters.  Other activate based on high wind speeds in Severe Thunderstorm Warnings.  The NOAA/National Weather Service in Des Moines recommends siren activation with the first occurrence of either a Tornado Warning with favorable path-cast references, or an impending tornado identified by trained, credible spotters.

 5.      The forecast says there is a 30 percent chance of rain.  That means it probably won’t rain, or if it does it will be light, right?

 Well, not really.  Probability forecasts cannot be used to predict the location, intensity, or amount of precipitation.  A 30 percent chance of rain only means that there is a 3 in 10 chance of seeing measurable precipitation somewhere in the forecast area (Measurable means 0.01” or greater).  If it does rain, it does not necessarily mean it is a bad forecast either.  It means that if we issue that forecast 100 times, there will be measurable rain in 30 of them.  Of those 30 instances, some could be just a few hundredths of an inch, while other could be over an inch.    The few hundredths could accumulate in 30 minutes or take the entire day. Conversely an inch of rain could fall in less than an hour, with the remainder of the day dry.   It all verifies the same way.

6.      Can the NOAA/National Weather Service in Des Moines provide certified records regarding weather phenomena that occurred on a particular day?

 The NOAA/National Weather Service in Des Moines does not provide certified records for court cases or any other reason.  We have a limited amount of archived data available to office staff members, and would be glad to share that with you over the phone (270-2614), but we cannot certify any observations or records.  We provide unofficial information on our webpage for 8 sites in our forecast area (Waterloo, Mason City, Estherville, Ames, Marshalltown, Des Moines, Ottumwa and Lamoni), but certified information has to come from the National Climatic Data Center (NCDC) in Ashville, NC.  They are the national repository of all weather related records, and can be reached at, or National Climatic Data Center, 151 Patton Avenue, Asheville NC 28801-5001 (828) 271-4800 then press '2'.   Local unofficial data can be found on the Des Moines NOAA/National Weather Service website (click here).

7.      We had storm damage and large hail at my house a few weeks ago.  Where can I find out information about past severe weather events? 

For information from October 2006 up to a few months ago, please visit NOAA’s National  Climatic Data Center (NCDC) website at .  Previous data can be found here.

For more recent information, please visit the NOAA/Storm Prediction Center (SPC) website at  Click on the “Storm Reports” link on the left hand side under Weather Information.

8.      Are tours of the Des Moines NOAA/National Weather Service office available?

Yes.  Tours can be arranged by calling 270-2614 weekdays between 700 am and 300 pm.  Large group tours of 15 people or more can be conducted weekdays.  Tours larger than 25 will need to separate into small groups.  Minimum tour age is ten years old.  More information can be found here.

9.      Can the NOAA/National Weather Service please remove those watches and warnings from my favorite broadcast and cable TV station?  They keep interrupting my shows.

 Graphics or scrolls along the bottom of the screen may appear when the National Weather Service issues watches, warnings, or advisories, but we have no direct control of what appears on television.  Your local broadcast station or cable company provides a valuable community service helping relay these important weather bulletins to the public, often times using automated systems that generate scrolls or graphics.  The National Weather Service does not have control of how long they appear or when they are removed.

10.  I’ve heard that an inch of rain would fall as 10 inches of snow.  Is that always true?

 That may be the case sometimes, but it cannot be used as an absolute conversion.  The ratio of liquid or melted precipitation to the amount of snow received can vary quite a bit depending on vertical profiles of temperature, moisture, upward vertical motion, and instability.  A common liquid to snow ratio in central Iowa is 13:1 (13 inches of snow for every inch of liquid precipitation) with research showing about half of our snow events occurring with ratios between 9 and 15 to 1.  However outlier events have occurred with ratios in the single digits and well into the 20s.  Heavy wet snows typically have lower ratios, while higher ratios are more favorable during fluffier snows when the atmosphere is relatively cold.

11.  Your forecast has periods that begin Today, Tonight, Tomorrow night, etc.  When do those periods begin and end exactly?

 All of those periods begin or end at 600 am or 600 pm CST.  Today would be 600 am to 600 pm, while Tonight would go from 600 pm to 600 am.  Thus a 30 percent chance of rain in the Tonight period means that there is a 3 in 10 chance that measurable rain will occur between 600 pm and 600 am.

12.  Where is the best place to get information on current river stages and forecasts?  Does this have anything to do with AHPS?

 The National Weather Service website ( includes all the latest river stages and forecast information for over 40 sites in our forecast area, including all or portions of the Des Moines, Raccoon, Skunk, Cedar and Iowa river basins, as well as several others.  AHPS stands for Advanced Hydrologic Prediction Service and delivers all the above information plus weekly and 90 day exceedance probabilities for stage and flow, and weekly exceedance probabilities for volume.

13.  We had torrential rains last night with frequent lightning and a severe thunderstorm warning wasn't issued.  Why is that?

The issuance criteria for Severe Thunderstorm Warnings is soley based on wind speed or wind damage potential and hail size.  The intensity of rain or frequency of lightning are not factors.

Severe Thunderstorms are defined as those producing wind damage, having wind speeds of 58 mph or greater (50 knots), or containing hail one inch diameter or greater (quarter size).  A storm meeting any of these critiera, even if there is little rain or lightning activity, is categorized as a severe thunderstorm.

In situations where very heavy or excessive rains are expected to cause flooding, Flood Warnings or Flash Flood Warnings would be issued.

14. Does the National Weather Service have any educational materials that I can use in the classroom or even just for my own personal interest?

Yes, there is a very detailed educational webpage with information on multiple topics.   This online weather school, called JetStream, can be found at is the U.S. government's official web portal to all federal, state and local government web resources and services.