Friday, April 25, 2014

Tone down the rain hype*

Bad Dad pointed to some gathering clouds this evening and said that the news predicted rain. I'm a believer in the AMS study about the accuracy of TV weather forecasts. They showed that TV weathercasters ALWAYS overpredict rainfall because it keeps viewers glued to their TV sets. Predicting rain leads to higher ratings. Ratings, not forecast skill, determine the success of TV weather personalities.

Who are you going to trust?  The weather reader on TV or the professionals of the National Weather Service at the Oxnard/Los Angeles office? They are predicting gusty winds throughout the southland, but light rain only in Santa Barbara and Ventura counties, with some light snow in the mountains of northern Los Angeles county. I would be thrilled if it actually rains as far south as LAX. But, if it does rain 0.25" or more at our house, I'll bring him coffee in bed tomorrow morning, even in my current hobbled state.

Want to perform your own forecast?  You can cheat a little bit by consulting the output of NOAA/NCEP's Global Forecast System (GFS) as visualized by Unisys.  They are showing no rain in central LA county for the 12Z Fri 25 Apr 14 to 00Z Sat 26 Apr 14.  (Local time during daylight savings is Zulu - 7 hours.)

We might get a whopping 0.01" to 0.05" between 00Z Sat 26 Apr 14 and 12Z Sat 26 Apr 14.
But, seriously, by the beach, it will be more like 0.01" or about as much mist as we get on a typical foggy summer day.

BTW, if you are looking at this type of weather precipitation chart, the rainfall contours represent the amount of rainfall likely to occur since the last chart.  So, if the charts are spaced 12 hours apart, then you are looking at the 12-hour accumulated rainfall prediction ending at the time stamp shown on the chart.  Then the numerical weather prediction (NWP) model resets the rainfall accumulation to zero and adds up the rainfall for each timestep in the next N hours.

If the charts are 3 hours apart, then you are looking at the 3-hour accumulated rainfall since the last chart.

In the mountains, the amount could be locally lower or higher, depending upon whether you are on the windward or leeward (rain shadow) side of the mountain relative to the direction of the storm.

What about the wind forecast?  The California Regional Weather Server shows the jet stream will pass over LA.  That's the same government-provided GFS that Unisys, Accuweather, the Weather Channel (and everybody in the commercial world) uses for their own branded weather charts.
Sure enough, the local radar at LAX showed an uptick in gusty winds.

* Wow, I am humbled.  Or Bad Dad got lucky.  Check out the current radar (circle is location of LAX).
See that band of moderate precipitation? It briefly passed overhead and lead to a highly localized band of serious rain for 10 minutes, as seen in the three-hour actual totals at LADPW rainfall measurement stations.
We just happen to live in the only coastal area where it rained tonight.

But, I think I am still safe.  It didn't quite hit 0.25" and there is nothing major coming down the coast behind this band.

4 comments:

  1. The TV guys always overestimate snow here too, for the same reasons. In the summer, it's thunderstorms.

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  2. San Pedro / RPV had a nice rainfall too. Score one for the TV weather forecasters.

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    1. Yup. I was thinking of only the large-scale storm dynamics. It was a pretty weak storm that had exhausted its moisture by the time it reached us.

      When the cold storm front collided with moist air right above the south bay (mainly near the ocean, the source of the moist air) the water from the moist parcel rained out. It was a highly localized (in both time and space) event.

      Fortunately, I lived too far north from the significant rainfall so I got coffee in bed that morning. ;-)

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  3. That's so interesting! I've got a feeling they do the same here too; rain is predicted, then not a drop. Happens very frequently, in fact more often than not.

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