Increased Debris Flow Risks - Resources


With the strong likelihood of significant high-intensity rainfall over parts of North and Central California on Sunday and Monday comes the increased risk of debris flows (and flash flooding) in recently burned areas.

In the aftermath of a wildfire, there is a huge load of material deposited on the ground. Burned vegetation also changes the water balance of the denuded hillsides from wildfires by increasing the soil's water repellency. All of these factors combine for increased runoff of debris downslope.




There are lots of variables, like vegetation type, slope, and rain intensity that ultimately determine the likelihood and impact of debris flows in each individual burn scar.  The USGS produces very comprehensive analyses of these post-fire risks at https://landslides.usgs.gov/hazards/postfire_debrisflow/. However, the basic rules of thumb, mostly used operationally look at rainfall intensity and amount.


Other resources:
https://www.wrh.noaa.gov/lox/hydrology/files/DebrisFlowSurvivalGuide.pdf
https://www.weather.gov/riw/burn_scar_flooding

Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Phone: (650) 712-1876
Email: jnull@ggweather.com
Web: http://ggweather.com
Twitter: @ggweather

 

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Updated Climate Normals Page (1991-2020)

Updated Climate Normals Page (1991-2020)

I have updated the US Climate Normals pages at Golden Gate Weather Services with the new 1991-2020 normals. These pages are designed to give quick, user-friendly access to both the monthly and daily normals for thousands of United States locations. (The normals for the previous periods of 1981-2010 and 1971-2000 are also available)


Please let me know of any comments or corrections.



Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Email: jnull@ggweather.com
Web: http://ggweather.com
Twitter: @ggweather

 

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Shifting Climate - Warmer & Drier Later

For San Francisco, the new climate normal period of 1991-2020 is both drier and warmer than previously (1981-2010). The hottest day of the year was 70.4° on September 24th, and it is now 70.6° and a week later on October 1st.
 

And San Francisco's rainfall season is now slightly (3%) less and delayed by about a week. The new normal date for the first 2" of rain for the season is now November 14th, compared to November 8th.


Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Email: jnull@ggweather.com
Web: http://ggweather.com
Twitter: @ggweather

 

20 Years of Hot Cars and Tragedies

Today, July 24th, marks the bittersweet 20th anniversary of my involvement with Pediatric Vehicular Heatstroke (PVH).  On that sunny 86° summer afternoon in 2001, I got a call from a reporter telling me about the death of a 5-month-old boy inside a hot car in San Jose and asking, “How hot could it have gotten in that car?”.  And so began my 20-year journey of measuring how hot cars get and the tracking of the tragic deaths of children in hot vehicles.

In trying to find the answer to the reporter’s query, I found only a single article and it was for a single 93-degree day in Louisiana. But my scientific curiosity was piqued and during that summer I started tracking temperatures inside vehicles. I was startled at not only how hot it could get but also how rapidly the temperature rose in the car.

The following summer, I did a controlled study where I sampled temperatures in cars over 16 days that ranged in temperature from 72° to 96°.  I was also working on another project with the Stanford University Hospital Emergency Medicine Department and became acquainted with Dr. Catherine McLaren and Dr. James Quinn. They became my co-authors for the article “Heat Stress from Enclosed Vehicles: Moderate Ambient Temperatures Cause Significant Temperature Rise in Enclosed Vehicles”.  This article was published by the American Academy of Pediatrics in 2005 and became the “go-to” article on the topic and has been referenced worldwide.

The article also led to my working with numerous child car safety groups and other organizations, and ultimately to my tracking the instances and circumstances that led to the deaths of children in hot cars. A dedicated website, NoHeatstroke.org was created to give easy access to this research and timely updates when there were PVH tragedies. Through the years, I have spoken at dozens of national conferences, given countless webinars and literally hundreds of interviews to increase awareness and share ideas on
preventing deaths of children in hot cars.

This 20-year milestone is important to acknowledge the children that have died and to continue to raise awareness about children dying in hot cars. If even one child is saved from being left in a hot car, it is more than worth the years of researching these tragic and unnecessary deaths.

 

Sincerely,
Jan


Jan Null, CCM
Adjunct Professor of Meteorology
San Jose State University
Email: jan.null@sjsu.edu  
Web: https://noheatstroke.org 
Twitter: https://twitter.com/ggweather
Facebook: https://www.facebook.com/noheatstroke/

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Driest California Average Rainfall on Record

  

The just ended California rainfall season has ended with the driest average statewide rainfall in the 126 seasons on record going back to 1895. And the 2-season average is the second driest with just 60% of normal. Similarly, half of California's 10 hydrologic regions saw their driest on record for the 2020-2021 season. The two-season numbers were only slightly better, primarily in the southern half of the state. [Data Source: Western Region Climate Center (https://wrcc.dri.edu/)]





 






Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Phone: (650) 712-1876
Email: jnull@ggweather.com
Web: http://ggweather.com
Twitter: @ggweather
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2020-2021 Rainfall Season Recap and Historical Context


 

Wednesday will mark the end of what has been a dismally dry 2020-2021 rainfall season across California with many stations logging one of their top ten driest seasons on record. And even more significant are some of the two-season totals which are also among the driest, especially in the critical watersheds represented by the Sierra Nevada Indices. And most troubling in terms of the Sierra Nevada are extremely dry three and four-year totals.


Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Email: jnull@ggweather.com
Web: http://ggweather.com
Twitter: @ggweather
 








 

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A Drier California Seen in New Normals


 

The differences in the new (1991-2020) rainfall normals in California show a noticeably drier state when compared to the previous period (1981-2010). On average, precipitation for stations in Northern California decreased by 8%, while Central and Southern California stations decreased by 6% and 12% respectively. On a slightly brighter note, in the Northern Sierra Nevada, there was essentially no change, but the Central and Southern Sierra normals fell by 3% and 5%.

 



11111

 

 

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The (Not So) Elusive Green Flash

 

As a meteorologist, I have long been aware of the Green Flash and would always try see it when I watched the sunset over the ocean. But I never did; and most of the colleagues and friends that I speak to haven’t either! That has all changed since moving to the coast about a year ago. Now that I frequently photograph sunsets, I probably capture it at least 50% of the time! So, what has changed?   

The biggest difference is that I switched from trying to “see” the Green Flash to photographing it! When most people are trying to see it, they are staring into a bright yellow-orange-red ball, and in doing so their irises dilate down to the size of pinheads and their color vision is degraded; making it almost impossible to ever see the green flash with the naked eye.

There are actually two different basic types of Green Flash.  The first occurs as a relatively bright flash of an emerald-green color right after the upper disk of the sun has dipped below the horizon. This “classic” type is the one most often seen when a person has not been watching the bright sun during the setting process. It’s the result of warm air overlying the colder ocean and the effects of an inferior-mirage display of the sun and the actual disk of the sun interacting. (For a great technical explanation see https://www.atoptics.co.uk/atoptics/gfimform.htm).



The second variety of Green Flash appears as a small detached element of green light above the disk of the sun. During the course of the sun setting, there are often multiple occurrences of these “green wiggles” of light. This type of flash happens when there is a temperature inversion (i.e., cooler air near the surface and warmer aloft) resulting in a mock-mirage. (See https://www.atoptics.co.uk/atoptics/gfmmform.htm)
 


And as I found out recently, bright light from the full moon can also be the source of a Lunar Green Flash! The image below was taken November 2020 as the full moon set next to Pigeon Point Lighthouse on the San Mateo County, CA, coast. And it wasn’t until I got home and looked at the image on a monitor that I noticed the tiny wiggle of a rare Lunar Green Flash.
 

For a deeper dive into the topic check out what I consider the two definitive resources that I have found on the topic. The first is Les Cowley’s Atmospheric Optics pages (https://www.atoptics.co.uk/), which is a great go-to source for not only the Green Flash, but also Rainbows, Halos, Rays and dozens of other visual phenomena. The second is Andrew Young’s Green Flash pages (https://aty.sdsu.edu/). Enjoy and happy Green Flash hunting.

Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Email: jnull@ggweather.com
Web: http://ggweather.com
Twitter: @ggweather

 

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California Rainfall Update

 

The rainfall for the bulk of the last two seasons is now the second driest in 170 years of record for San Francisco. The 21-month total rainfall for the period from July 1, 2019 through March 31, 2021 is just  20.46". This is 45% of the normal rainfall for that period; a deficit of 24.52", which is more than one full rainfall season's total of 23.65".  The only drier period was 1975-77 with a total of 18.53".




California Percent of Normal Rainfall (July 1 to March 31, 2021)


Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Email: jnull@ggweather.com
Web: http://ggweather.com
Twitter: @ggweather

 

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