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Phoenix: Number of 110º Days Per Year Increasing

05 September 2019 - Posted in Weather Facts by Weather Bus

Phoenix Sky Harbor has recorded 29 days so far this year with a high of 110º or higher; this is now the 4th most Phoenix has recorded since records began in 1896.

So how has the number of 110º or higher days changed throughout this period of record?

For starters, in 1911 there were no 110º+ days, and in 2011 there were 33 days, which currently stands as the record. The current climatological 30-year average (1981-2010) is 18 days per year, but let's take a closer look at the average over time.

Examining the rolling 30-year average from 1896 to 2018, it's easy to see that it is going up. In 1925, the average was just five days per year, and now including this year, the average is 20 days. Even though we see an increasing number of 110º days, there is a piece of good news, and that is that the rate of increase has slowed (more text below graph).

Monsoon 2019: Season-ending Rainfall Scenarios for Phoenix

25 August 2019 - Posted in Monsoon by Weather Bus

Phoenix is in the midst of the fifth driest monsoon on record when looking at rain totals at Sky Harbor from June 15th through Aug 25th. The big question: do we have a chance of ending Monsoon 2019 on a wet note?

The average monsoon total for Phoenix is 2.71", meaning, to get to average, Sky Harbor would need to measure 2.44" of rain before the season ends on Sept 30th. Over the past 123 monsoons, there have only been 11 where 2.44" or more of rain has fallen between Aug 26th-Sept 30th (9% of monsoons).

Just looking at that fact, it's not hard to see that it will take quite a bit to get Monsoon 2019 to normal. But let's break down the numbers and run some different rain scenarios to see just how much (or little) rain we could see by the end of the season!

Average Scenario: First, since there are only 36 days left in Monsoon 2019, time is starting to run out. On average, Phoenix Sky Harbor picks up 0.81" of rain between Aug 26th - Sept 30th. Using just this average, it would put the Monsoon 2019 total at 1.08", which would be 1.63" below average. This total would also place Monsoon 2019 as the 14th driest monsoon on record.

Worst Scenario: There have been nine years in the past 123 monsoons where no rain has fallen from August 26th-Sept 30th. This would mean the Monsoon 2019 total would come in at 0.27", breaking the record for the driest monsoon. The record currently stands at 0.35" set in 1924.

Best Scenario: For those glass half full people, let's switch over to the best-case scenarios. We will start with the "absolute" best case, which is very unlikely to occur. The most amount of rain to fall from Aug 26th-Sept 30th was 5.41" in 1939, followed by 5.11" in 2014 (remember Hurricane Norbert?) If Monsoon 2019 followed in the footsteps of 1939, it would end up with 5.68" and come in as the 12th wettest monsoon on record. Again, the odds are slim, but what I'm saying is, "there is a chance!"

Now for the "middle" best case. Using the top 30 highest end of monsoon rainfall totals from Aug 26th-Sept 30th, the average rainfall comes in at 2.45". Using this average, Monsoon 2019 would come in at 2.72", which would be just 0.01" above average.

Finally, for the situation that is more likely to occur and is considered the "least" best case, we will take the driest 30 monsoons based on rainfall from June 15th-Aug 25th and examine how those monsoons ended. Using these years, the average rainfall to end the monsoon is 0.74", which would bring the Monsoon 2019 total to 1.01" (1.70" below average). Using the same thirty years, the most rainfall to fall between Aug 26th-Sept 30th was 2.15", with the least being 0.07". Using these two values would make the Monsoon 2019 total 2.42" or 0.34" respectively. Both totals are still below average, with the 0.34" coming in as the direst monsoon on record. Again, the current record stands at 0.35" set in 1924.

Overall, it's looking very likely that Monsoon 2019 will end well below average. The table below shows the top five driest monsoons on record based on rainfall from June 15th-Aug 25th (with 1914 and 1993 tying as the driest, and 1994 and 2019 tying as the 5th driest). The sad part of this, looking at the overall monsoon total the top four driest monsoons are on this list.

Pyrocumulus Forms Over Woodbury Fire

17 June 2019 - Posted in Wildfires by Weather Bus

The human-caused Woodbury fire has been burning east of the Phoenix valley since June 8th. During the heat of the day, when relative humidity is the lowest, the fire becomes very active with a noticeable smoke column seen from across the Valley. At times, it may have looked like a thunderstorm was forming at the top of the smoke column. Your eyes are not tricking you; this is called a pyrocumulus.

Let's start with the word itself. "Pyro" is Greek for fire and "cumulus" is Latin for heap (think puffy heap "cumulus clouds"). Put them together, and you get "fire cloud" or pyrocumulus. These types of clouds are also a clear sign that the fire is creating its own weather. So, how do they form?

There are several things to remember when looking at the formation of pyrocumulus. First, a large percentage of what makes up smoke is water vapor. Second, for condensation to form, it needs a condensation nuclei for liquid water to form on. Third, smoke is also made up of tiny particles that are 2.5 microns or less in size. To put this size into scale, a human hair is about 50 microns in diameter, so smoke particles (PM-2.5) are about 20 times smaller than a human hair and make perfect condensation nuclei. And finally, fire is extremely hot. According to the Natural Resources Canada, the average wildfire burns at around 1,400ºF, with an intense fire exceeding 2,000ºF.

A pyrocumulus begins to form when a fire becomes active and produces extremely hot air and smoke. The hot air and smoke begin to quickly rise into the atmosphere, which is also called an updraft. This updraft is the same as you would see with any thunderstorm; however, it is much stronger given the heat from the fire. As the updraft rapidly forces air up, condensation occurs due to all the water vapor in the smoke and the millions of condensation nuclei present. Once all of this comes together, you will visually see the pyrocumulus cloud rise above the fire, reaching heights at times of over 30,000 feet.

These pyrocumulus clouds can also indicate extremely dangerous weather conditions on the ground. As air rapidly rises above the fire into the updraft, the air at the surface has to be replaced. Since air is not coming out of the ground, it has to come in from the side, producing strong and often erratic winds, that only act to increase the intensity of the fire.

Pyrocumulus can get big enough to become full-fledged thunderstorms with all the same threats: lightning, downbursts and rain. The two biggest threats are (1) lightning, which can create new fires and (2) downburst winds that can cause 50+mph winds over the fire.