What is a 500-year storm?

The ongoing catastrophic rainfall and flooding in Texas as the result of Hurricane Harvey have pushed often misunderstood and misused terms like “500-year storm” into the headlines. Probably the most important thing to keep in mind is that the time period (i.e., 100-year, 500-year, 1000-year) has nothing to do with the amount of time between events!

Instead, the terminology is the result of statistical calculations called “return period”, “return frequency” or “extreme period” analysis. These calculations estimate the probability of an event happening in any given year; and not the interval between similar events. For example, a 100-year storm has 1 chance in 100 of occurring, or one percent probability of happening in any given year.


It should also be noted that a 100-year rain event does not necessarily equate to a 100-year flood. Due to changes in a watershed, like the of filling in of channels or the paving over of permeable areas, the extent of flooding may change for the same amount of rainfall.


Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Email: jnull@ggweather.com  
Web: http://ggweather.com
Twitter: https://twitter.com/ggweather
Facebook: https://www.facebook.com/Golden-Gate-Weather-Services-151100414926621/

 

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Above Normal July with Lots of Hot Days

Above Normal July with Lots of Hot Days

 

July 2017 saw most of the state's mean monthly maxima from 2 to 4 degrees above normal. The exceptions were Redding which was nearly 5 degrees above normal and Eureka 0.8 below normal and San Francisco at just 0.7 above normal.  See http://ggweather.com/calif/jul2017.htm.

Around the SF Bay Area, locales away from the coast saw quite a few days that equaled or topped the 90 degree mark:
Santa Rosa - 13 days (normal 5 days, record 21 days in 1931)
Livermore - 24 days (normal 14 days, record 26 days in 1950)
San Jose - 6 days (normal 4 days, record 12 days in 2006)
Gilroy - 10 days (normal 11 days, record 23 days in 1996)

Farther inland, 100 degree days were equally as popular (?):
Redding - 29 days (normal 13 days, previous record 25 days in 1961)
Sacramento Exec - 9 days (normal 5 days, record 17 days in 1988)
Fresno - 23 days (normal 10 days, record 26 days in 1985)



Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Phone: (408) 379-7500
Email: jnull@ggweather.com  
Web: http://ggweather.com

 

 

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Warmer than Normal July for San Jose


In answer to numerous questions, yes it has been warmer than normal this July in the San Francisco Bay Area. But it is nowhere near record territory. As an intermediate point between the cool coast and hot interior valleys, San Jose works well as a single summertime temperature metric.

Through yesterday, San Jose's mean maximum temperature for the month was 84.3 degrees, or 2.4 degrees above the July normal of 81.9 degrees. It is interesting to note, that the last time San Jose had an above normal July was in 2006, a year when the average maximum was 85.2. The record was 1996 when the mean maxima was 87.2 degrees. Given forecast temperatures of a few degrees above normal for the remainder of the month, this year's numbers will edge up slightly.


Likewise, San Jose has had five days of 90 degrees or greater and will probably add a couple more days by the end month. The normal number of July 90 degree days is four. The record for 90 degree days is 12, set in 2006.  


Jan Null, CCM
Certified Consulting Meteorologist
Golden gate Weather Services
Phone: (408) 379-7500
Email: jan.null@sjsu.edu

 

 

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Too Hot for Dogs Too!

DO NOT walk your dog in the heat! Besides possible heatstroke, just like humans, your dog could likely be scorching the pads on his paws. Veterinarians say that burns can occur when the surface exceeds 125 degrees. To illustrate, I took some measurements of some surface temperatures today at 10:30 am and again at 12:30 pm. And these are still not the hottest time of day! As a rule of thumb (or paw), if you can't stand on it barefoot then don't let your dog walk on it! 

Jan Null, CCM
Golden Gate Weather Services
jnull@ggweather.com

 

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Don't Trust Your Car's (or Bank's) Thermometer

With the warm weather I’ve been hearing about LOTS of really hot outlier temperatures being reported from car thermometers; and their owners wondering why they’re so different from official temperature reports. To clarify, let’s look first at how the official temperatures are measured.

There are four general guidelines for accurately measuring air temperatures.
- The thermometer must be outside of direct sunlight., otherwise you are measuring the sun’s intense shortwave energy and not the air temperature.
- It should also be in a well ventilated environment that’s representative of surrounding air, not in an enclosed area.
- Official temperatures are taken at about 5 feet above a natural surface and not a heat-retaining surface like concrete or asphalt. There can be as much as a 25 degree difference between air right above the ground and that at 5 feet.

- And measurements should also be far enough away from other heat emitting sources like walls and vehicles as to not be biased.

Conversely, the thermometers on most cars violate just about every one of the above guidelines. They are usually located under the front grillwork or adjacent body panel, only a foot or two above the highway or parking lot. Especially when a car has been sitting it is measuring the air heated by the parking surface plus any residual heat being given off by the vehicle itself. Even when a car is moving the temperature readings are upwardly biased by the heat given off by the roadway and the vehicle. It’s been my experience that car outside temperatures at startup on a warm day can be more than 10 degrees to toasty, and even while they seem generally to be at least 5 degrees too warm.


Similarly, the readings from locations like bank thermometers and some backyard thermometers don’t do a good job of accurately capturing the air temperature for some of the same reasons.

So when you next start up your car and see a dashboard outside temperature reading of 120° you can take solace in the fact that it’s probably only 105°!

Jan Null, CCM
Golden Gate Weather Services

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California April Weather and Rainfall Season Summary

April was generally wetter than normal across California, except south of the Tehachapi's; while there were above normal temps near the coast and cooler readings inland.  See California Key City Climatology.

The seasonal rainfall continues to be mostly well above normal through the end of April, especially in the important Northern and Central Sierra Nevada watersheds. See California Rainfall Season to Date  and


The California Precipitation Snapshot has also been updated. 

Jan Null, CCM
Golden Gate Weather Services
jnull@ggweather.com

 

 

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NOT the end of the rain for the season

No, it's NOT yet the End of the Rainfall Season!!

Despite proclamations by several individuals, the most recent rainfall does NOT mark the end of California's rainfall season. Yes, the amounts drop off significantly as we transition into late spring and summer, but climatologically it would be quite unusual not to have rain in May and June. For example, the last time that San Francisco had zero rain for the months of May and June was in 1978!

Below are the normal rainfall amounts and the normal number of days of rain across California for May and June.


Jan Null, CCM
Golden Gate Weather Services
http://ggweather.com
jnull@ggweather.com

 

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A Look Back at the CPC 2016-17 Winter Outlooks


It's time to take a quick look back at the good, the bad and the ugly of the CPC Winter Precipitation and Temperature Outlooks from this past winter. Because of the probabilistic nature of this outlooks this review, like previous reviews, is very subjective. But at least in the West, when their sub-headline for the just finished winter was "Drought expected to persist in California" followed the previous winter's busted above normal El Niño forecast, the efficacy of the product certainly needed scrutiny. See or details CPC Winter 2016-17 Seasonal and Monthly Precipitation and Temperature Outlooks in Review:




Jan Null, CCM
Golden Gate Weather Services
jnull@ggweather.com

  

 

Posted

California 2nd Rainiest Jul-Feb

 

Statewide, the first eight months of the current rainfall season were the 2nd wettest on record in California going back to 1895-96. This season's 28.03" statewide was just short of the 1968-69 record of 28.30". Looking at the individual climate divisions, the Sacramento drainage (Clim Div 2) and the Northeast Interior (Clim Div 3) had their wettest ever, while the San Joaquin drainage (Clim Div 5) had their 3rd wettest. Neither of the Southern California climate divisions (i.e., Clim Div 6 and 7) were in the top 10.    



Jan Null, CCM
Golden Gate Weather Services
jnull@ggweather.com

 

 

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