Perspectives on the Probability of Precipitation

It is now May 11th and although we got a trace of rain last night at the Salt Lake City International Airport, we have yet to record measurable (.01" or greater) precipitation so far this month.  In fact, the last measurable rain fell on April 16. 

Some rain is possible this week, but not a lead-pipe cinch.  At about 7:45 AM this morning, I pulled up the forecast below for the airport and it shows a 20% chance late Thursday night, a 20% chance Wednesday, and a 20% chance Thursday.

But what does this really mean? 

Some people thing that it means 20% of the area will get precipitation or that it will rain 20% of the time, but neither of those is true.

Instead, the National Weather Service Probability of Precipitation is "the probability that the location in question will receive at least .01" of rain."  A 20% chance would mean there's a 1 in 5 chance that there will be measurable precipitation during the forecast period.

So, Probability of Precipitation tells you nothing about how much it is going to rain. 

Additionally, the forecast above is for a specific location, the Salt Lake City International Airport.  If I switch instead to the University of Utah, the forecast is slightly different.  Note, for example, the higher Probability of Precipitation on Thursday. 

In some instances, that makes sense.  The University of Utah is wetter than the airport. 

There are some aspects of the National Weather Service Forecast Probability of Precipitation that I find confusing.  For instance, when they split the day as they do on Thursday in the forecast above for the University of Utah, 30% is indicated in the left hand side of the icon and 40% in the right hand side.  The text below then says "chance rain" and "chance T-storms."

Does this mean a 30% chance of measurable rain and then a 40% chance of a thunderstorm?  Does it mean a 30% chance of rain in the morning and a 40% chance in the afternoon?

The more detailed accompanying text forecast is below and it does not provide thunderstorm probabilities, but does say that the chance of precipitation is 40%. 

So, it must be 30% chance in the morning and 40% chance in the afternoon, right?  Well, that doesn't make sense statistically.  If you have a 30% chance of something happening in the morning and a 40% chance of it happening in the afternoon, there's actually a 58% chance that it happens at any point during the day. 

Personally, I hope we either end up dry so we can claim some sort of streak or record, or we get a dousing so that the plants can take advantage.  Anything in between would just be a teaser. 

COVID-19, Air Quality, and Climate Change

A lot of people have commented to me that the air has seemed quite clear in recent weeks and I too have noticed.  As reported recently by the media, this anecdotal observation has been put to the test by Dr. Logan Mitchell, a Research Assistant Professor in my department (Atmospheric Sciences).  Although you may have read articles in the Trib or Des News, he has his own summary available here and it's worth a look.  Although it's hard to tease out meteorological influences, a pretty strong case can be made based on his analysis air quality has improved, with the exception of ozone, which Logan discusses in his write up.  A quick note that this work is preliminary and has not yet been peer reviewed.

I have also been asked by several people if the coronavirus crisis might be good news for climate change.  It's a reasonable question, but my response is a pretty firm no.  First, and paraphrasing Logan's comments in his write up on air quality, any improvements in air quality or decreases in carbon emissions are overshadowed by the loss of life and economic hardship that we are currently experiencing.  Second, our society remains highly dependent on fossil fuels and, while CO2 emissions are projected to decrease compared to recent years, they remain very high and we will still see a substantial increase in atmospheric carbon dioxide concentrations this year. 

There have been some news articles stating that this year will feature the biggest "carbon crash" on record.  An example is this article early this week from the BBC, which included the figure below.  Although the forecast for 2020 CO2 emissions is a drop larger than in any previous year, CO2 emissions are so high that it will only lower us to levels near those of about a decade ago.

Source: https://www.bbc.com/news/science-environment-52485712

Thus, on a percentage basis, the drop, which is estimated to be 4-8%, is large compared to anything previous.  However, for the climate system, we're still adding lots of carbon dioxide to the atmosphere. 

Yesterday's Dust Storm

Strong north-northwest winds late yesterday afternoon came accompanied with dust originating likely from southern Idaho and briefly elevated PM2.5 levels over the Salt Lake Valley.

Observations from the Neil Armstrong academy show a clear trough passage in the northern Salt Lake Valley at around 1800 MDT.  The trough passage was accompanied by an abrupt increase in wind speed (red line) and wind gusts (green plus signs) as well as a wind shift from predominantly west-northwest to predominantly northwest-north. 

Source: MesoWest

Accompanying the trough and the stronger winds was dust, with a spike in PM2.5 to just over 45 ug/m3.  PM2.5 concentrations remained above 10 ug/m3 for about 2 hours and 5 ug/m3 for an additional 3 hours. 

Source: MesoWest

Modis satelite imagery suggests that agricultural fields in the Snake River Plain may have been a source for at least some if not much of the dust.  In the image below, you can see the dust in the Snake River Plain northwest of the Great Salt Lake.

Source: MesoWest

I can't rule out contributions from sources in Utah, including around the Great Salt Lake.  The more localized nature of those sources makes them difficult to identify in satellite imagery that was available later in the afternoon.  

H/t to Dr. Derek Mallia and Dr. Maura Hahnenberger for e-mails contributing to this analysis.

Mountain Sports in Corona Times

The Austrian Alpine Club is the largest mountain-sports club in Austria and open to members from around the world.  A primary benefit of members, even if you don't live in Austria, is rescue and travel insurance (important details here). 

They recently issued some recommendations for mountain sports that are worth a look.  I often wonder if the 2 meter rule (converted to 6 feet in the U.S.) is appropriate for all activities (it isn't) and if mountain sports require any specific precautions (some do, some don't).  I'm including a translated version of their web page below for your consideration.  If one has not already been developed, I wonder if a similar info sheet for Utah might be useful.

A quick comment on the situation in Austria.  After imposing strict stay-at-home orders, they have really squashed the curve and appear to be doing better than most European nations.  Daily confirmed COVID-19 cases peaked at 1,141 on March 27th and dropped dramatically thereafter.  Over the past week, they've reported 12 to 68 cases per day. 

Source: ourworldindata.org.

Below is a 3-day running average of per-capita confirmed cases, showing that their peak and current case rates are much lower than in several other European countries and much lower than the United States. 

Source: ourworldindata.org.

However, if this makes Austria seem like a good option for vacation, good luck.  More here. 

A Record Dry April and Early May in Salt Lake

Chances are that you are aware that April was the driest on record at the Salt Lake City Airport.  The start of May hasn't been any better, with only a trace of precipitation through the first four days.

Looking to the immediate future, the NAEFS ensemble has only one member out of 42 producing any precipitation at the Salt Lake City Airport through the 9th of May.  After that, about half produce measurable precipitation on the 10th or 11th. 

We'll have to see if that pans out, but lets assume we remain dry through the 9th (Saturday).  If this were to happen, the rainfall since 1 March, the start of meteorological spring, would be only 1.79 inches and would rate as the 4th driest 1 March - 9 May on record.  

Source: NOAA Regional Climate Centers

#2 and #3 aren't much drier with 1.70 (1939) and 1.78 (1956) inches respectively.  The all time driest March 1 to May 9, however, is 1934 with only 0.93 inches.  Ah, the good old days!

Of course most of the precipitation we've had this spring fell in March.  If we look at precipitation from 1 April - 9 May, assuming forecasts hold, it would be the driest such period on record, with only 0.26 inches.  

Source: NOAA Regional Climate Centers

#2 would be 1934 with 0.45 inches.

1934 was a remarkably dry spring, with only 0.93 inches for March through May.  It will remain in the record books even after 2020 due to the precipitation we received in March.  To beat 1934 for April-May, we have to observe less than 0.19" of precipitation this month.  Let's hope that doesn't happen. 

May the 4th Be with You

C3PO: "Sir, the possibility of successfully navigating an asteroid field is approximately 3,720 to 1."

Han Solo: "Never tell me the odds."  

As Benjamin Franklin once wrote, "in this world nothing can be said to be certain, except death and taxes."  Movie characters like Han Solo ignore the odds because it makes for a good story (although he would probably tell you the odds of navigating the asteroid field, while low, were better than dealing with the Imperial tie fighters).

Meteorologists deal with statistics daily.  The atmosphere cannot be predicted with absolute certainty and every forecast needs to consider probabilities.  There are times when something is virtually certain, such as no rain when you are under a ridge and the atmosphere is stable and very dry, but more often than not, there is uncertainty and uncertainty that has major public safety and economic ramifications.

Exhibit one for meteorologists are tropical cyclones, which in the Atlantic and northeast Pacific Oceans are referred to as tropical depressions, storms, or hurricanes depending on the maximum sustained wind speed.  A 2003 study concluded that it costs anywhere from $1 million to $50 million to evacuate coastal counties in North Carolina, numbers that have surely increased with inflation and increasing development and densification of coastal areas over the past two decades.  This is one reason why storm track and intensity forecasts are so important. 

However, when a tropical storm is approaching, the actions taken by us can reduce risk to their lives and property, but do nothing to alter the strength and path of the storm (note: I'm talking about short-term forecasts here - I'm not dealing with how anthropogenic climate change may be altering tropical cyclone strength and statistics).   The strength and path of the storm are a response to physics, not human action. 

Which brings us to the novel coronavirus.  This is a totally different beast because the actions we take or don't take do "alter the storm," if I may speak metaphorically.   Good hygiene, distancing, mask wearing, and cleaning frequently touched surfaces greatly reduce the odds of transmission and the rate of infection.  They change the statistics of the storm and reduce severity.  On the other hand, when we don't take these actions, the statistics of coronavirus spread are not only unforgiving, but also stealthy due to the delay between spread and detection. 

I suppose my repeating of these facts is getting old, but how else to fight misinformation, such as the idea that sunlight protects you from coronavirus.  It doesn't.

A summary of UV effects on the coronavirus produced by the National Academies of Sciences, Engineering and Medicine is available here.  The bottom line is that the type and intensity of ultraviolet radiation that reaches the Earth's surface is not sufficient to kill viruses quickly.  Their conclusions:

• "Going outside on a sunny day will not quickly break down coronaviruses on skin. But it can give you a sunburn if you are not wearing sunscreen!"

• "Some viruses are seasonal and spread more slowly in the summer. This is probably due to warmer temperatures, higher humidity, and changes in human behaviors—not because it is sunnier in the summer."

• "The best ways to protect yourself from COVID-19 are to wash your hands, keep your distance from other people, and clean and disinfect frequently touched surfaces."