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!

Wednesday, September 11, 2019

The 8-inch Standard Rain Gauge

The 8-inch standard rain gauge hereafter referred to as SRG) is the workhorse of the National Weather Service Coop Program. In fact, this gauge is the world-wide standard for measuring precipitation. The gauge is your basic straight-sided cylinder and is the big brother of the 4-inch rain gauge we use in CoCoRaHS. The components are the same. Each has and outer cylinder which catches overflow from the inner measuring tube. Both rain gauges have funnels which direct the precipitation into the inner measuring tube. The SRG includes another component, a measuring stick graduated to hundredths of an inch.

For a long time the SRG outer cylinder (aka "the can") and funnel were made of copper, and the inner tube was made of brass. Better and less expensive materials have for the most part replaced copper. The outer cylinder of the SRG pictured here is stainless steel, the inner tube is made of poly carbonate plastic, and the funnel is made of fiberglass. The outer cylinders are also made of aluminum. The measuring stick is a laminated fiberglass that "wets" so that you can read the precipitation measurement. Even with these newer materials there are issues. The outer cylinders can develop leaks along the seams on the bottom, and those can be hard to detect. Both the brass and plastic inner measuring tubes can be damaged and develop leaks in freezing weather.

Funnel and inner measuring tube of 8-inch SRG. Credit: NWS
One of the advantages of the SRG is its capacity. The inner measuring tube holds two inches of rain, and the outer cylinder holds 20 inches of rain. However, that advantage is also a big disadvantage. The 8-inch outer cylinder holds about 4.3 gallons of water, and a gallon weighs 8.3 pounds. An observer would have a difficult time lifting 35 pounds of water and even a more difficult time trying to pour anything into the inner measuring tube. Four inches of water in the outer cylinder would be about a gallon, and that plus the weight of the cylinder itself can be cumbersome to handle. It is definitely not a one-person job. My personal experience with the 8-inch "can" and pouring led me to build a stand to hold the inner tube while pouring from the can. The funnel is placed on the tube before pouring.

Once I became a CoCoRaHS observer I designed and built a smaller version for the CoCoRaHS gauge. You can find the plans to build one at this link.

The 8-inch SRG is the standard at NWS primary Coop sites, but states "The four-inch plastic rain gauge is a suitable substitute for the eight-inch standard rain gauge because it meets the accuracy requirements". That accuracy is specified as ±0.02 inches. (National Weather Service Instruction 10-1302: Requirements and Standards for NWS Climate Observations, April 2018) 

Tuesday, September 10, 2019

The Rain Gauge - How Can Something So Simple Be So Complex?

The rain gauge. At its most basic it is just a straight-sided cylinder, with a bottom, of course. Like the mousetrap, someone is always trying to build a better one. A simple web search of "rain gauge" will display a gallery of images of different types of rain gauges.

While the basic concept for a rain gauge is simple, the complications start to come in with calibration, measurement, and siting/exposure. Add to that the increasing complexity when mechanical and electronic components become part of the measurement process and the potential for measurement errors greatly increases. There are weighing bucket rain gauges, which measure the amount of precipitation by weighing the water and recording it on a revolving chart (and now digital storage). There are also weighing rain gauges that utilize more complex technology and software to measure precipitation.

An older weighing bucket rain gauge. The recording drum is located behind the door in the bottom.

Tipping bucket rain gauges utilize a small "bucket" that tips and triggers a signal every time one one hundredth of an inch of rain is collected. Optical rain gauges utilize photo diodes, laser, or infrared to detect drops and measure rainfall rate and intensity. Acoustic rain gauges have sensors that detect the acoustic signature for each drop size on the sensor surface. From this data the drop size distribution can be determined, and from the the rainfall rate and accumulation.

Tipping bucket rain gauge

As we all know, weather also has an effect on precipitation measurement. Strong winds affect collection efficiency, typically resulting in an under-catch of rain, and even more so for snow. Speaking of winter, rain gauges that rely on collecting liquid water must be heated if mechanical or electronic, or the frozen precipitation must be melted before it can be measured, for example, as with the CoCoRaHS gauge and the NWS 8-inch standard rain gauge. Siting and exposure all can affect the measurement with any of the rain gauges mentioned, not just how close the rain gauge is to nearby objects that might affect the rain gauge catch, such as trees and buildings, but also the height of the opening above the ground.  A rain gauge installed closer to the ground is slightly less susceptible to affects from strong winds compared to one at a greater height above ground.

So, what should be a simple, straightforward measurement (how much water is in the straight-sided cylinder) is affected by many factors. Even manual measurement is subject to observer error. The more complicated rain gauges, largely developed to make measurements where or when manual measurements are not practical, introduce the potential for a variety of other errors even though minimizing the human factor.

Wednesday, September 4, 2019

Dorian Part 1 - Cluster of Thunderstorms to Tropical Beast

Tonight Hurricane Dorian is affecting the southeastern U.S. coast. The center of the eye is roughly 100 miles off the coast, and it's precipitation shield extends inland 30 miles or less from the northern Florida coast up through Georgia and into South Carolina. We have already been following Dorian for 11 days.

The first advisory for then Tropical Depression #5 was issued during the morning on August 24th. Later that day Tropical Depression #5 became Tropical Storm Dorian.

On Wednesday, August 28 Dorian strengthened into a hurricane near St. Thomas in the U.S. Virgin Islands. Dorian steadily strengthened the next 48 hours, becoming a major category 3 hurricane on Friday morning, August 30. Dorian rapidly strengthened to a Category 4 hurricane by Friday evening. Dorian became a Category 5 hurricane sometime near dawn on Sunday, September 1, just 40 miles or so east of Great Abaco island in the northwest Bahamas.

Track of Hurricane Dorian from August 24 to 5:00 p.m. EDT September 4.

Satellite infrared image of Hurricane Dorian at 6:00 p.m. EDT on August 31. At this time Dorian was a Category 3 hurricane and quickly ramped up to a Category 4 within three hours.
Radar image from the Bahamian Weather Service showing Dorian at 4:00 pm EDT on September 1. At this time the eye was moving over Great Abaco with sustained winds up to 185 mph and gusts to 220 mph

For the next 40 hours the combination of 155 to 185 mph+ winds in Dorian's eyewall and storm surge exceeding 20 feet of water produced catastrophic damage across Great Abaco and Grand Bahama islands as the storm virtually came to a standstill. Buildings not damaged or destroyed by the winds were likely "protected" by the deep water that submerged them.

These two radar images, 12.5 hours apart show how little Dorian moved during the time period while the eye was over Grand Bahama Island. During this time sustained winds were sustained around 155 mph with gusts to 195 mph.

As of this writing the death toll in the Bahamas is 20, and is likely to go higher.

Tonight (September 4) Dorian has re-intensified to a Category 3 hurricane and is likely headed to the Outer Banks of North Carolina. 2.3 million people in the Southeast are under evacuation orders. It will be another several days before Dorian is no longer something to worry about. For the latest information on Dorian visit the National Hurricane Center website.

Tuesday, August 27, 2019

A Normal Start to the 2019 Tropical Storm Season

Two years ago this week Texas and the Gulf Coast were dealing with Harvey, which waddled over the southeast Texas and Louisiana coasts for three days after making landfall, dumping record amounts of rain. Harvey was the eighth named tropical cyclone of the 2017 season. Here in 2019 Tropical Storm Dorian is the fourth named storm of the season, and the former Tropical Depression #6, located midway between the U.S coast and Bermuda, became Tropical Storm Erin Tuesday evening. Right now 2019 is mirroring last year with one named storm in June (Andrea), two in July (Barry, Chantal), and two in August.
Atlantic tropical cyclones as of 10:50 p.m. EDT August 27, 2019. Source: National Hurricane Center

The 2018 hurricane season finished strong, with a total of 16 named storms ending with Hurricane Oscar at the end of October. Four of those storms made landfall in the U.S. Hurricane Michael was a Category 5 hurricane when it made landfall on the Florida Panhandle on October 10.

Track map of all 2018 tropical cylones.

A "normal" tropical storm season (June 1 - November 30), based on data from 1966-2009, is 11 named storms, six of which are hurricanes, with two of those hurricanes Category 3 or greater.

The season tends to ramp up quickly during August and early September, with the peak about September 10.

NOAA's prediction for the tropical storm season (updated 8/8/2019), calls for a likely (70 percent confidence) range of 10 to 17 named storms (winds of 39 mph or higher), of which 5 to 9 could become hurricanes (winds of 74 mph or higher), including 2 to 4 major hurricanes (category 3, 4 or 5) (winds of 111 mph or higher).

All eyes are now on Tropical Storm Dorian, located tonight just east of the Leeward Island and headed toward Puerto Rico. Dorian has not changed much in intensity as of this writing, but is expected to strengthen some before moving across Puerto Rico. The higher terrain of Puerto Rico will cause it to lose some intensity tomorrow before it again emerges over open water and strengthen again late this week. Based in current forecasts Dorian could be approaching Florida by late this weekend. There is a higher than normal uncertainty of the intensity forecast of Dorian due to a large spread in storm model guidance.

"Strengthening" and "weakening" can be somewhat misleading descriptions with tropical systems like this. While winds are what most people tend to think about and focus on, the storm surge, heavy rain, and resultant flooding are what often cause the greatest threats to life and property. You don't need a particularly intense system to produce a lot of rain.

Dorian bears watching. Erin, on the other hand is expected to turn northeast into the Atlantic and weaken to a tropical depression in the next three days. You can find the latest information on Dorian and other tropical systems on the NOAA/NWS National Hurricane Center web site.

Monday, August 12, 2019

The Latest NWS Forecast on Your Smartphone

It looks like an app, it sort of acts like an app, but it's not an app. If you are interested in the latest forecast, advisories, warnings and watches, then a link to the National Weather Service "mobile web" is something you should have on your smart phone. There are literally hundreds of weather apps of one kind or the other, and as usual with something like apps they run the range from just plain bad to very useful. Most of the forecast apps simply run algorithms to display model output data for a day or location, and there is little or no human input. This is my go-to "app" for weather info on my phone.

When you open NWS Mobile Weather, you have a well-organized display that shows the current conditions, the forecast, and other information for your chosen location.

The opening screen for NWS Mobile Weather (left), and the second half of the page seen by scrolling down (right).

In addition to the graphical forecast on the opening page, you can select Detailed Forecast which provides the complete 7-day forecast for the location selected.

The one feature I really like with this is that you can save multiple locations to a list. If you have regular (or non-regular) places you visit you can add them to the list. Tap on the location field and a drop down list of all of your locations is displayed. Select the one you want, and the latest forecast and other data will be displayed for that location. You can also edit this list, deleting locations you no longer want, or change the name of the location to something more descriptive to you, for example "Uncle Bob" instead of "Springfield, (pick your state)".

To install this link on your smartphone follow the instructions found at The instructions differ slightly between iOS and Android phones, but in either case it's just three steps.

As I was putting the finishing touches on this post today I came across a Forbes article written today by Dr. Marshall Shepherd, Director of the University of Georgia’s (UGA) Atmospheric Sciences Program, titled "Why Doesn't The National Weather Service Have A Weather App?"  This provides some explanation why the NWS doesn't have a full-blown app and instead developed this mobile web link. In the article he mentions an app called NWSNow that lists some very nice features, but it appears to be no longer available.

Thursday, August 1, 2019

After a Soggy Spring and Early Summer, Drought May be Creeping Back

After a wet spring and early summer in much of the central and eastern U.S., dryness has become more established in the past three weeks especially in the central U.S. At the same time, drought conditions in the southeast U.S. have diminished in the past two months.

The maps of percent of normal precipitation for the U.S. the sharp contrast in precipitation between the period of May 1 through June 25, and from June 26 through the end of July.

The U.S. Drought Monitor released today is showing some expansion of D0 (Abnormal Dryness) across the central U.S. compared to a month ago, especially across the corn and soybean belt. Corn and soybean planting was delayed several weeks in some areas because of heavy rain and the resulting saturated ground and flooding. The dryness is spreading just as the corn is starting to pollinate in many areas, about two to three weeks later than normal.

The U.S. Drought Monitor for July 30 (l) and June 25 (r)

The rapid transition from wet to dry conditions is dramatically seen in the CoCoRaHS Condition Monitoring Report maps especially in the Midwest. The first map below is the condition map as of today, and the second map shows the conditions as of June 24. Note the moderate to severely wet conditions from Iowa through Illinois, parts of Missouri, and into Ohio as of June 24. In five weeks most of those have changed to normal to moderately dry.

CoCoRaHS Condition Monitoring report maps for the week ending August 1 (top) and ending the week of June 24 (bottom)

This may be the makings of a flash drought from Iowa though central Illinois into Ohio, as well as Michigan. A flash drought is characterized by a relatively short period of warmer temperatures and rapidly decreasing soil moisture. The "flash" refers to the rapid onset of drought, not the duration. In parts of the Midwest almost a month's worth of rain fell in the first week of July, but little since then. The last three weeks of the month were also extremely warm with highs in the 90s and 100s. At the same time, rain became scarce.

For example, at my location in east-central Illinois, I received more than 4 inches of rain the first week of July, close to normal for the entire month. During the last 24 days of the month I received only 0.22 inch. The ground went from being saturated to rock hard with cracks forming by the end of the month.

This is where CoCoRaHS condition monitoring reports can be very helpful. While July precipitation was near normal in my case, it doesn't tell the whole story because the rain largely occurred in a few days. Condition monitoring reports submitted weekly help those monitoring drought and environmental conditions to discern the current state and the impacts of too much or not enough precipitation.

Friday, April 19, 2019

Hail Season is Here

CoCoRaHS' annual Hail Week is coming to a close and will end with "Put out Your Hail Pad Weekend" this weekend. If you have been following this week's Messages of the Day you have seen how to measure hail, report it, and how to make a hail pad. (Mobile app users should select "View message of the day" after submitting you daily observation.) Hail is a fascinating phenomena and there is a lot of information available if you want to learn more about it. The CoCoRaHS Hail page  has some information, and you can find a lot more information at Living With Weather- Hail on the Midwestern Regional Climate Center website.

Compared to the past few years this season is getting off to a slow start in the hail department. Depending on how the rest of April goes the number of national hail reports for the first four months of the year should be about the same as last year, but well below the previous three years.

Hail reports for January through April for this year and the past four years

Normally probabilities for significant hail very low at the end of February and only begin to ramp up in mid-March to early April. Here are the climatological probabilities for significant hail from the the NOAA Storm Prediction Center for mid-April, late May, and August. The center of the high probabilities moves north through April and May, reaching a peak in late May. By early August probabilities are diminished and continue to diminish into early fall.

So far CoCoRaHS observers have submitted hail reports on 72 of 109 days so far this year, about the same as last year.This map is a compilation of hail reports for the year through April 16 from the Storm Prediction Center.

CoCoRaHS has one of the most comprehensive collections of detailed data on hail. While measuring and reporting hail may seem to be secondary to rain and snow, our hail observations provide valuable information not only to the National Weather Service but to others such as the insurance industry. A recent article in the Washington Post noted that Texas has experienced 36 $100 million disasters from severe thunderstorms in the past 25 years. Twenty-nine of these $100 million disasters were from hail!

Measuring hail is a core mission of CoCoRaHS, and the separate hail reports on the CoCoRaHS web site allow you to submit your hail information. There are a few things you need to know before measuring hail, and you can find that information in our "Measuring Hail" training animation. Here is a hail size reference and measuring guide you can download, print, and laminate for use. The rule on the bottom is to scale and fits on a 3x5 card. Make multiple copies and keep one at home, in the car, or at work.

Tuesday, April 9, 2019

The March of the Seasons – Precipitation and Temperature

CoCoRaHS is all about measuring precipitation locally, and contributing to the "big picture" of precipitation across your state and across the country. We get a good idea of how precipitation varies on a daily basis by viewing our observations and those of other CoCoRaHS observers, but it's a little more difficult to get your head around how precipitation varies each day, on average, across the country.

Climatologist Brian Brettschneider is big on maps. He recently compiled an animation of average daily precipitation across all 50 states using PRISM data for the period 1981-2010. He used gridded precipitation data at a resolution of 800 meters. These are the same data that are used to generate precipitation normals for each CoCoRaHS station. The animation is composed of plots of average daily precipitation for each day of the year.

It’s fascinating to watch the expansion north and west of daily average precipitation values in the central U.S. beginning in late January and peaking in early June. The Continental Divide is clearly delineated by the pattern of average daily precipitation values in early June. Low average daily precipitation values start to expand in southwestern Arizona and southern California beginning in late March and early April and continuing to expand north through California through July. The Southwestern Monsoon is evident beginning in late June through early September in Arizona, New Mexico, and western Texas. You can see the effect of Pacific storms in Washington, Oregon, and California in the winter months, and the increase in precipitation along the Gulf coast and Florida during the summer. If you rerun the animation several times and focus on different parts of the map you will notice a number of other interesting features.

Every CoCoRaHS observer can access their PRISM monthly precipitation data through the My Account option on the top line menu after logging in. You can learn more about the CoCoRaHS PRISM Portal on the CoCoRaHS web site.

In addition to the precipitation map Brian compiled a similar map animation showing the daily average temperature through the year.

You can follow Brian Brettschneider on Twitter (@climatologist49) to see some of the fascinating maps he produces. He also has a Facebook page, Alaska Climate Info.