Tuesday, October 13, 2020

Can You Be Considered for a State Record? Siting and Observer Practices are Important!

The following is a Twitter thread posted by the National Centers for Environmental Information- Climate (@NOAANCEIClimate). It reviews the verification of state record precipitation for Virginia in 2018 by the State Climate Extremes Committee (SCEC), and a CoCoRaHS station was in the mix. CoCoRaHS measurements can be considered for records, and are subject to the same review as Cooperative stations in this regard. The Twitter thread starts below the cut. Links to the reports about the record are at the end of the thread.


October 13, 2020

In 2018, buckets of rain fell on Virginia. Not just one but several weather stations reported annual precipitation that exceeded the state record. Which would take the prize? Only NCEI’s State Climate Extremes Committee could decide. And it got complicated!

In October 2018, the remnants of Hurricane Michael soaked Virginia. Michael followed Florence (September) and Alberto (May), and these storms arrived amid a persistent wet weather pattern in the East. Nine states from TN to MA recorded their wettest year on record in 2018. 


Three stations reported annual precip totals that bested Virginia’s previous (unofficial) record of 86.06", set in 1996: 1) Montebello, 2) Cave Spring, near Roanoke, and 3) Sperryville. Only one would emerge on top. The SCEC was convened to test the claims.


First contender: Montebello! Claimed precipitation total: 104”. But SCEC found some measurements were wrong, knocking the total down to 89.90”. Other problems: missed observations and failure to record liquid-equivalent measurements for snow.  


Here’s the kicker: Montebello’s gauge was too close to a fence, and thus might have collected extra drips, calling into question the accuracy of all station measurements. SCEC verdict: Rejected!


Next contender: Cave Spring, near Roanoke! Claimed precipitation total: 87.33”. Once again, there were problems: snow-water equivalents were not recorded for several days. (The ‘M’ in the table means ‘missing.’) 


Even worse: the station had its gauge mounted level with a deck rail, making it “highly likely that raindrops hitting the deck railing would enter the gauge,” thus undermining all measurements. SCEC verdict: Rejected!  


Next contender: Sperryville! Claimed precipitation total: 94.43”. This station was operated by a music teacher who had been observing for more than two decades. A staffer from the NWS Baltimore-Washington visited the site and found that “his paper forms are extremely meticulous.”


The gauge was well positioned, without a fence in sight (bonus: it was in a beautiful vegetable garden!). SCEC verdict: Approved, unanimously! The Sperryville annual precipitation total of 94.43” became the official record for Virginia.


The moral of this SCEC story: Record observations daily, don’t forget snow-water equivalents, and keep your rain gauge away from fences and railings! Read the SCEC reports on Virginia: bit.ly/2ZZWF9V, bit.ly/3mLlyQr.

And don't forget to read our story on the SCEC! bit.ly/32SvCPJ


Monday, June 8, 2020

The Importance of Observation Time

This weekend there was a long discussion on the CoCoRaHS group page on Facebook (not the HQ page) regarding observation time. After reading through this discussion it was clear that many observers may not realize why things are done the way they are, so here is an attempt to try and clarify things a bit.

We ask observers to measure their precipitation in the morning, typically 7:00 a.m. There are a number of reasons for this, but the primary one is so that CoCoRaHS observations are consistent with other precipitation observations made in the U.S. Starting in the early 1960s the (then) U.S. Weather Bureau requested that U.S. Cooperative Observers start taking their measurements in the morning as that would minimize the amount of evaporation from rain gauges and result in more accurate precipitation measurements. Prior to this both temperature and precipitation measurements were taken in the late afternoon, typically 5-7 p.m.. The current instructions for U.S. Cooperative Observers states:

"Observations at precipitation stations should be taken at 7 a.m. local time, although you may usually choose any time between 6 a.m. and 8 a.m. Be sure, however, to take observations at the same time everyday throughout the year if at all possible. Continue observing at the same time whether standard or daylight saving time is in effect; i.e., convert from 7 a.m. standard time to 7 a.m. daylight saving time when the latter takes effect." 

CoCoRaHS follows this same guideline. However, because the morning is not the best for everyone who wants to participate, we allow observers to choose an observation time that is more convenient.
Many National Weather Service precipitation products are based on these observations. A map showing 24-hour precipitation amounts, such as this one from the Advanced Hydrologic Prediction Service (AHPS), uses a combination observed precipitation from Coop, CoCoRaHS, first-order stations (any meteorological station that is staffed in whole or in part by National Weather Service , FAA, or civil service personnel), and radar to map the precipitation. You can read a more thorough description here.

Some argue that a calendar day (midnight-to midnight) total is more representative of daily precipitation. That may be true, but precipitation measurement is still by and large a manual process performed by humans, many of whom don't want to or can't wait until midnight to make an observation. The 7:00 a.m. time also closely aligns with 12:00 UTC, one of the standard synoptic hours for weather measurement. These synoptic hours (in Universal Coordinated Time, or UTC), based on international agreement through the World Meteorological Organization (WMO), are hours which meteorological observations are made simultaneously throughout the world at three or six-hourly intervals. The primary synoptic hours are every six hours, commencing at 00:00 UTC.  12:00 UTC also marks the end of the "hydrologic day", a standard used by hydrologic modelers and the River Forecast Centers.

You may hear of other networks making weather measurements at one-minute or five-minute intervals. These are automated stations, such as those located at airports or the Climate Reference Network. However, for the purposes of climate, data from these stations can be aggregated and summarized on a daily basis or longer.

The CoCoRaHS Precipitation Map

The CoCoRaHS precipitation map displays all reports of precipitation with observation times within 2.5 hours of 7:00 a.m., i.e. 4:30 a.m. to 9:30 a.m. local time. Any observations made outside of this window will not appear on the daily map, but remain in the database for users. This data is still needed and valuable for time periods longer than a day, such as weekly, monthly and longer precipitation summaries. Our map represents 24-hour precipitation amounts, in line with the synoptic time, the end of the hydrologic day, and represents most of the CoCoRaHS observations made that day. For example, on Friday, June 5, out of nearly 13,000 CoCoRaHS observations only 340, about 2.5 percent, were made outside of the 4:30 a.m. - 9:30 a.m. window.

The take-way from all of this is pick an observation time that is convenient to your schedule. If 9:00 a.m. or 5:00 a.m. or 4:00 p.m. work better for you - no problem. The important thing is to be consistent. Don't switch observation times from day to day (for example,  from 7:00 a.m. on one day to 11:00 a.m. the next to 8:00 a.m. on the next). If on one day you take your observation time earlier or later than usual, that's OK - be sure to enter that time in the Observation Time Field.

Here is one more thing to remember. It's not about when you enter your data (the map itself updates all the time to reflect late entries), rather it's the time you actually look at your gauge - and what is entered on the form - that is important. Time of observation is the time you make your measurement, not the time you submit your observation to CoCoRaHS. If you make your observation at 7:00 a.m. but aren't able enter it until 11:00 a.m., it is still a 7:00 a.m. observation.

If you have been taking your observation at one time, say 6:00 a.m., and you want to change it to 8:00 a.m., contact your state coordinator or headquarters to make that change. You can't change your default ob time using the My Account menu.

If you have any questions about observation time, let us know!