If you lived in the Valley of the Sun before 2008, you likely heard a lot about dew point during the monsoon! The reason for the dew point buzz: the dew point rule! Before 2008, the season was defined to have started after Phoenix had a daily dew point average of 55ºF or higher for three consecutive days. After 2008, fixed start and end dates were determined (June 15th - September 30th).

Even though the dew point rule is not the "official" way to determine the start of the monsoon, it is still fun to track! With that said, Monsoon 2021 started using the "unofficial" dew point rule on June 30th. The daily average dew point was 56º on June 30th, 57º on July 1st, and projected to be in the low 60s on July 2nd! Last year it didn't start until July 22nd.

Using this old dew point system, the average start date for Phoenix was July 7th, but it started as early as June 16th in 1925, and it began as late as July 25th in 1987.

Do you use the old dew point rule to determine the start of the monsoon? If so, we might hit the magic three days this coming week! Also, storm chances increase in Phoenix.

The first 100º day of the year signals the unofficial start to summer, and this year, it happened right around 2:15 pm on May 5th.

Using the new 1991-2020 thirty-year averages, the average first 100º+ day occurs on May 2nd. The earliest 100º+ day occurred on March 26th back in 1988, and the latest was in 1913 when it held off until June 18th!

And if you thought this was a little late compared to recent years, you would be right. This is the latest first 100º day since May 21, 2010.

The Valley of the Sun averages 111 days a year with a high of 100º+, so looking at the positive side, only about 110 more days to go!

Get ready for a roller coaster! Afternoon high temperatures in Phoenix on Tuesday are forecast to be in the low to mid-70s before skyrocketing into the 100-102º range Saturday.

If you are wondering how rare it is to go from the 70s to the first 100-degree day of the year, you have come to the right place.

Looking back over the past 125 years (1896 - 2020), there have only been 16 years when the high temperature in Phoenix was in the 70s, four days or less before the first triple-digit high of the year. Drilling down a bit further, since the high this coming Tuesday is right around 74º, there have only been four years where the high has been 74º or less, just four days before the first 100º reading of the year.

The coolest high temperatures preceding the first triple-digit day of the year are as follow:
Four days before: 70º in 1919
Three days before: 70º in 2001
Two days before: 80º in 1961
One day before: 88º in 2003

As for the biggest temperature change four days before the first 100º of the year, the current record stands at 31º in 1919, going from a high of 70º to 101º. This year, the forecast high on Tuesday is 74º, and on Saturday, it is 101º, making it a change of 27º, which would tie as the 4th largest temperature swing on record (when comparing the high on the first 100º+ day with the days preceding it).

Record temperature swings preceding first 100º day:
31º: 1919 (70º to 101º four days before)
30º: 2001 (70º to 100º three days before)
28º: 2001 (72º to 100º four days before)
27º: 1910 (73º to 100º four days before) and this year may tie that!

As you can see, the event that is forecast to happen this year in Phoenix is a rare one!

It was a toasty Easter Sunday in the Valley of the Sun, with Phoenix Sky Harbor hitting 98º. This tied the record high for the day, last set in 1961.

This high also ties as the third warmest Easter on record. The warmest was 1990, coming in with a high of 100º. Second place is 1936 with a high of 99º. And this year is tied with 1930.

Record-setting temperatures were spread across the northern portion of Arizona as well. Cottonwood reached a high of 84º, which broke the record for the day of 82º set back in 1961. Phantom Ranch hit 97º and Seligman reached 92º, which tied the records for the day both set in 1961.

The clouds that moved in later in the afternoon on Sunday were in response to a storm system that will brush the state to the north tomorrow. This system will help to weaken the high allowing temperatures to lower slightly to start the week.

A very strong ridge of high pressure has been anchored over the Southwest US the past few days, resulting in record temperatures across the region.

Phoenix Sky Harbor recorded a high of 97º on Saturday (April 3rd), which tied the record for the day, last set in 1943.

Another interesting fact is the average high temperature so far this month (April 1st-3rd) has been 95.3º, which will go down as the 2nd warmest start to April since records began. The record warmest first three days of April goes to the year 1943, with an average high of 95.7º.

The heat was not confined just to the Valley of the Sun, with Flagstaff coming in with a high of 71º, which also tied the record for the day last set in 1961.

Stay cool!

We all know summer is fast approaching, and the closer it gets, a question always comes up: will this be the coolest day until next Fall? After all, the high temperature today (March 26th) in Phoenix was a nice 66º!

The good news, high temperatures have been 66º or cooler past March 26th before. In 1922, the high temperature on May 10th came in at 65º, which is the record for the latest 66º or cooler high temperature (going from Spring into Summer). Also, just last year, the Valley of the Sun recorded a high of 66º on March 27th.

Ok, now to the bad news, so you may want to stop reading if heat makes you uncomfortable. In the past twenty years (2001-2020), the high has only dropped to 66º or below, seven out of those twenty years. That means there is about a 35% chance Phoenix could see the high drop to 66º or lower before summer. If you are more of a glass-half-empty kind of person, there is a 65% chance that March 26th was indeed the coolest temperature the Valley will see until Fall. Even if it is, it is nowhere near the record earliest last 66º or cooler day, that trophy goes to the year 1972 when the last day was Feb 2nd! So remember, it can always be worse!

The graph at the top of this post shows the twenty and thirty-year rolling averages of the percent of years with a high of 66º or lower along with the number of days with a high of 66º or lower per year.

Phoenix officially hit 80º for the first time this year on Feb 2nd, which is only one day earlier than the 30-year average of Feb 3rd (based on 1981-2010 data). But the comment alway comes up: it feels like it is getting earlier every year.

So... is it? Let's go to the data!

Before we get started, it should be noted that weather observations go back to 1896 in Phoenix. The weather station moved around to a few locations in Central Phoenix from 1896 to 1953. From 1953 onward, it was located at Phoenix Sky Harbor but has moved locations there three times (1953-1994, 1994-2000, and 2000 to present).

Ok, now that we have that out of the way, the graph above depicts the 30-year rolling average, from 1925 to 2021, for the first day Phoenix has recorded 80º+. It also displays the 10-year rolling average to show a bit more detail.

It is quite apparent that yes indeed, based on the 30-year rolling average, the first 80º+ days is getting earlier. In 1925, the average first 80º+ day was Feb 21st, and in 2015 the 30-year rolling average came in at Feb 1st. The 30-year rolling average with this year plugged in comes out to Feb 3rd. (Note: these dates were converted from truncated Julian day)

These data also show that the overall trend is beginning to flatten out, especially compared to the late 1980's into the 1990's.

Will this trend continue? We shall see in the coming years as data comes in.