MARYLAND/DC COCORAHS Volume 1, Issue 3 June 2009

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1 MARYLAND/DC COCORAHS Volume 1, Issue 3 June 2009 More on Cleaning Your Rain Gauge One of our observers, Stan (MD-MG-52) has had success in cleaning with a 10 pct solution of bleach with water and that should work for the algae in your rain gauge. As for getting down deep into the tube, he also mentioned that aquarium stores sell a siphon tube cleaner on a flexible wand that can get to those hard to reach deposits. Birds can also be a problem with your gauge and you can check Nolan s latest version of The Catch on the main Cocorahs home page for tips on how to prevent them from messing up your gauge. May Weather Review May turned out to be another wet month, making it the second month in a row of above average rainfall for most of the state. Temperaturewise, May was just about normal though there were several periods of both above and below average temperatures. Parts of Central Maryland were particularly hard hit with heavy rain from May 3 rd thru May 8 th and again from May 25 th thru May 26 th. Some locations across Eastern Montgomery, Northern Prince Georges, Southern Howard, and Northwestern Anne Arundel Counties, including parts of Washington DC saw anywhere from 2 to 4 plus inches from a round of showers and heavy thunderstorms that fell across the region during the afternoon of May 25 th through the morning of May 26 th. A line of strong thunderstorms in advance of a cold front produced some rather large hail across parts of southern Maryland and the southern Eastern Shore on Fri May 29 th. Nickel size hail was reported in Port Tobacco in Charles County (MD-CH-13), while half dollar size hail (1 ¼ inch) was submitted in an intense hail report near Parsonburg in Wicomico County (MD- WC-9). Berlin (MD-WR-2) reported pea size hail from the same series of storms.the highest 1 day rainfall total was 4.10 inches at MD-MG-52 near Silver Spring, MD,. (Continued on page 5) How Frequent Are 1 Inch Rains in Maryland? One of the neat things about the Cocorahs web site is the ability to look up past weather events and from this, one is able to collect statistics on those events. Say for instance you want to find out how many days in the past year you had over an inch of rain or melted snow. Using the station precipitation summary under view data, you can compose a list of reports from any station based on dates that you input, say from 1/1/2008 thru 12/31/2008 if you wanted a whole year. Now for fun, I calculated both the greater than 1 inch days and greater than 3 inch days for 5 stations in Maryland for 2007 and One station in Garrett County, two stations in Allegany County, one in Baltimore County and my own in Prince Georges County were used. Those results can be found on page 2. One of the first things you will notice is that for all stations, the number of 1 inch plus rains is very infrequent, varying not only from year to year but also varying by location. In most instances, 1 inch or greater rains occurred less than 13 times a year. The 3 inch or greater rains occurred even less frequent at most only once or twice. What is interesting to note is that although Garrett County typically has higher rainfall amounts than the rest of the state, its frequency of 1 inch or greater rains is about the same or slightly less than in the central portion of the state where 1 inch plus rains were reported on over 10 days a year. Allegany County, which lives in a rain shadow east of Garrett County, did not have any days of greater than 3 inch rains, while the other stations did. Newsletter 1

2 Frequency of 1 inch rains..contd from page 1 In general the normals or averages that are used in station climate reports are generally based on 30 year averages that get updated about every 10 years or so. So, while two years of data is not enough time to make generalizations about ones local climate, it is still fun to look at and in the long run may be a valuable source of data when those state and county climate normals are updated Heavy Rain Frequency #of occurences/yearly pcpn MD-GR- 4 MD-AL- 2 MD-AL- 3 MD-PG- 1 MD-BL- 9 # of days > 1" # of days > 3" YEARLY PCPN Cocorahs ID 2007 Heavy Rain Frequency #of occurences/yearly pcpn MD-GR-4 MD-AL-2 MD-AL-3 MD-PG-1 MD-BL-9 # of days > 1" # of days > 3" YEARLY PCPN Cocorahs ID Newsletter 2

3 Understanding Stream Response to Rainfall Michael J. Brayton Hydrologist and CoCoRaHS volunteer One of the more interesting aspects of measuring precipitation is to try and understand how a stream responds to rainfall. CoCoRaHS data provide a daily snapshot of precipitation distributed throughout Maryland, but what actually happens to this water that has fallen from the sky? The relationship between rainfall and stream flow is not always simple due to many factors such as the size of the watershed, land cover, soil saturation, time of year, and other antecedent (prior existing) conditions. Some of the water will infiltrate and recharge our aquifers as groundwater; some will be used by vegetation; some will run off over land and directly into streams and rivers; and some will simply evaporate back into the atmosphere. The timing, duration, and total amount of rainfall can also affect how a stream responds. We can, however, make some inferences. Take a look at the CoCoRaHS rainfall data below in Figure 1. You will notice the higher rainfall totals north and west of Washington D.C. as reported on May 27 (dark green and orange squares). Figure 1. CoCoRaHS 24-hour accumulated rainfall data. Now compare this to Figure 2, which is from the U.S. Geological Survey real-time water data network for rivers ( as of the afternoon of May 27. Stream-flow conditions are presented in percentiles as compared with long-term records at a stream-gauging station (green is a normal flow range). You will notice higher than normal stream-flow conditions at several locations (light and dark blue circles), that generally correspond to the rainfall data. You may not always see a direct relationship because multiple rainfall events in succession will generally lead to sustained higher stream flow (higher percentiles), but keep checking to see how this changes throughout the year for your local stream or river. More information about water resources in Maryland, Delaware, and the District of Columbia can be found at the following link: (Continued on page 4) Newsletter 3

4 Explanation High > 90th percentile 75th - 89th percentile 25th - 74th percentile 10th - 24th percentile < 10th percentile Low Not ranked The colored dots on this map depict streamflow conditions as a percentile, which is computed from the period of record for the current day of the year. Only stations with at least 30 years of record are used. The gray circles indicate other stations that were not ranked in percentiles either because they have fewer than 30 years of record or because they report parameters other than streamflow. Some stations, for example, measure stage only. Figure 2. U.S. Geological Survey real-time stream-flow conditions. Maryland/DC CoCoRaHS 4

5 May Weather Review (continued from page 1) while 4.01 was reported at MD-AA-3 near Laurel, both ending on May 26th. Monthly rainfall totals were quite impressive and riveled last May s high numbers for portions of the state. Jay (MD-AA-2) topped the list with a whopping10.07 inches for the month while several other stations in Montgomery, Prince Georges, and Anne Arundel counties received over 9 inches for the month. In general, every reporting station in Maryland and Washington DC had over 4 inches for the month, with Sharon in Cecil county (MD-CC-9) reporting the lowest amount in the state with 4.25 inches. If you have any interesting weather photos or information to share with your fellow observers, please forward them to bruce.sullivan@noaa.gov or bruce.sullivan@cocorahs.org Maryland/DC CoCoRaHS 5