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.