Christmas Eve 2021 will go down in the record books as the wettest since records began in 1896! Phoenix Sky Harbor measured 1.00" of rain, breaking the old record of 0.93" set back in 1944.

This marks only the 13th time measurable rain has fallen on Christmas Eve in Phoenix!

If you like rain, this was certainly a nice present from Mother Nature!

It rained! Around 7:35 pm on December 9th, the rain gauge at Phoenix Sky Harbor measured the first 0.01" since October 11th!

While this dry streak certainly felt like a long one, it is nowhere near the record, which stands at 160 days (no rain from December 30, 1971 through June 6, 1972).

As for rainfall totals with this past storm, Phoenix Sky Harbor officially picked up 0.35", bringing the yearly total to 5.93". The year-to-date average is 6.71", so we still have some work to do before the end of the year.

It might be December, but this afternoon the high was 85º, which is the average high we usually see at the end of October! This high also broke the record for the day, with the old record being 83º set in 1949.

Temperatures at or above 85º in December are very rare. In fact, there have only been three other times since 1896 where the high in December was 85º+. It happened two times in 1980 (86º on December 16th and 85º on December 26th) and one time in 1950 (87º on December 10th).

There is good news on the horizon with slightly cooler temperatures, albeit still above average, moving into the weather picture by the weekend.

On the last day of November in Phoenix, the high was 80º, making it the 25th day during the month with a high of 80º or higher. This breaks the record for the most 80º+ days during the month. The old record was 24 days set back in 1949!

The top years for November include: 25 days: 2021 24 days: 1949 23 days: 2017 21 days: 1932 20 days: 2012, 1981, 1954

As for the least number of days with a high of 80º+ in November, that record is zero and has happened 4 times (1920, 1938, 1957 and 2000).

Also, if you are keeping track, the high of 80º on November 30th was 10º above the average for this time of year and marks the 191st above-average high temperature day so far this year. Some good news, this is better than last year, as at this same time in 2020, we were at 224 days that recorded an above-average high.

The low temperature at Phoenix Sky Harbor bottomed out at 49º during the morning of Sunday, November 28th. This low temperature marks the first time this fall with a low below 50 degrees.

If you thought it was a little late, you would be right; this year ties as the fourth latest on record. The latest on record is December 7th, which occurred in 2017. On the flip side, the earliest on record is September 20th, which occurred in 1965.

So is the first below 50 degree low temperature of the spring season getting later? Looking at data from 1896 to current (graph above), the answer is yes. The 30-year rolling average in 1925 had the first below 50-degree day occurring around October 17th. Using the last 30 years (including this year), the average is around November 13th.

During the early morning hours of November 23rd, cloud cover overhead made temperatures stay warm overnight, with a low at Phoenix Sky Harbor coming in at 64º.

Not only is this 13º above the average low for this time of year, but it also broke the record for the warmest low temperature for November 23rd. The old record was 60º set in 1965 and 1933.

It has been a warm November in Phoenix, with every day (through November 20th) seeing a high of 80º or higher, which has not happened since records began in 1896.

As for the total number of 80º+ days in November, that record stands at 24 and was set in 1949. So far this month, we are at 20 days, which stands as the 4th most for the month of November.

Rankings:
24 days in 1949
23 days in 2017
21 days in 1932
20 days in 2012, 1984, 1954 (now 2021)

With the 80s likely to continue through Monday, this November will likely make it into at least the number three spot.

Monsoon 2021 has been active, and now the main question to ask is: will August be active in Phoenix since July was?

There is no way around it; this is a very complex question as weather forecasting, even sometimes out just a few hours, is tricky during monsoon, but here we will dive into the numbers and see what they say regarding the odds of an active August. Warning: this might get a little number heavy, so read at your own risk! This analysis is also just a quick free time on the weekend so just "nerding out" type look.

Let's first look at a simple rainfall comparison of if you have a wet July what happens in August (don't worry, we will dig deeper).

Using rainfall data from Phoenix: (data range 1896 to 2020)
Since 1896, July has measured above-average rainfall 48 times. Out of those 48 times, a total of 27 times (56%), August measured below-average precipitation, and 21 times (44%) rainfall was above average following a wet July.

Using the Phoenix Rainfall Index: (data 1990 - 2020)
Since 1990, July has measured above-average precip 15 times. Out of those 15 times, a total of eight times (53%) August has measured below-average precipitation and seven times (47%) rainfall has been above average following a wet July.

This simple comparison shows there is a very slight "lean" toward coming in below average in August if July is wet, but not much better odds than just flipping a coin, so let's see if we can get a bit better.

Bring in ENSO!

For the second comparison, let's bring El Niño-Southern Oscillation (ENSO) into the picture to see if it help us to get past just flipping a coin.

For this comparison, we will cut Phoenix Sky Habor data down to 1950 to 2020 as ENSO data only goes back to 1950. We will also not use the Phoenix Rainfall Index since data only go back to 1990, so it severely limits the ability to have a good comparison.

The first task was to break rainfall data into two categories: (1) above average July rainfall, followed by an above-average August, and (2) above average July rainfall, followed by a below-average August. We then compared those groups with the three-month average ENSO 3.4 anomaly, using a Kruskal-Wallis test, and some interesting points emerged.

Looking at ENSO from March through September in three-month blocks showed that only the ENSO 3.4 three-month average of April, May, and June (AMJ) was statistically significant at the 95% confidence interval. So we will focus our attention there.

On average, the ENSO 3.4 AMJ anomaly for the years in which there was a wet July but a below-average August came out to -0.33. When both July and August saw above-average rainfall, the average ENSO 3.4 anomaly was 0.08.

This year, the ENSO 3.4 AMJ anomaly was -0.50, which, based on the above-mentioned statistics, would tilt our odds fairly heavily toward a below-average August.

Furthermore, looking at the 16 years where there was a wet July and a dry August, seven of them (43%) had an ENSO 3.4 AMJ anomaly of -0.5 or less, with 13 of the 16 years (81%) having a negative anomaly. Three out of the 16 years (19%) had an ENSO 3.4 AMJ anomaly of zero or greater.

Meanwhile, looking at the 12 years there was a wet July and August, only two (16%) had an ENSO 3.4 anomaly of -0.5 or less, with six out of 12 (50%) having a negative ENSO 3.4 anomaly. The other six (50%) had an ENSO 3.4 anomaly of zero or greater.

To bring the Phoenix Rainfall Index back into the picture, as it really does tell a great story of rainfall in the Valley, there have only been 15 times since 1990 that there has been an above-average July, so again, data points are spares... Looking at all the years August measured below average rain, with an above-average July, the average ENSO 3.4 AMJ anomaly came in at -0.28. The ENSO 3.4 anomaly when above-average August rainfall occurred after a wet July came in at -0.03. Even with sparse data points, it is nice to see that this shows a similar pattern as the Phoenix Sky Harbor data.

The final thoughts!

So, what does all this mean? The simple comparison of just rainfall leans slightly toward a drier August after a wet July. Since this year the April, May, and June ENSO 3.4 anomaly was -0.50, this also statically shows that odds are higher that Phoenix will have a below-average August. Based on this, I would go with the assumption that it could certainly be less active this August compared to July.

This also makes sense, as all forecasts are trending toward La Nina, which could limit the potential for a more active East Pacific Hurricane Season, which limits the odds of copious moisture surging into Arizona from these storms.

With all of this said, it is still VERY important to be ready for rapidly changing weather throughout monsoon. This analysis is simply looking at the numbers and by no means is a day-by-day forecast. Severe thunderstorms with very heavy rainfall are certainly still possible! It only takes one or two good storms to hit the right location to make the record books show a very wet monsoon (or in this case August).

Bottom line, there are some hints that point toward a below-average August but remember it is all about location, location, location!

If you made it to the bottom of this: congratulations!