Frequency of Hurricane Formation (climo)

[Spacecoast]

I was curious about how often hurricanes form.
So....



It looks like, (on average), CAT1+ hurricanes form ~every 14 days (blue linear) trending down slightly since 1950 (n=466).
The red line is a 5 storm moving average, which occasionally dips below 10

The max number is 104 for year 1982 between Alberto (6/3/1982), and Debbie (9/15/1982).

There have been five cases of Hurricanes forming on the same day:
2017: Jose & Katia (9/6/2017)
2012: Leslie & Micheal (9/5/2012)
2010: Shary & Tomas (10/30/2010)
1995: Humberto & Iris (8/23/1995)
1993: Gert & Harvey (9/20/1993)
And on 9/8/1980 three Hurricanes formed: - Earl & Frances & Georges.

Anyway I was trying to figure out how likely it is for 9 Hurricanes to form in the next 60 days.
9 Hurricanes - 1 every 6.5 days throughout Sept and Oct. ....idk, I think this amount of backloading is very rare
8 Hurricanes - 1 every 7.5 days throughout Sept and Oct
7 Hurricanes - 1 every 8.5 days throughout Sept and Oct

The data above includes August and November (the entire season), which probably skews the averages higher.
I will try to breakout frequencies by month specifically for Sept, and Oct.

[Spacecoast]

"Anyway I was trying to figure out how likely it is for 9 Hurricanes to form in the next 60 days. "

1950-2021 data from HURRDAT:
Upon further analysis, it appears that 9 (even 10) H's forming after Sept 1st is not unprecedented, but still uncommon...


Average number of H's (blue) after Sept 1st is four to five.
Min # is 1, Max # is 10.

2005, & 2020 had ten H's after 9/1
1969, 2001, & 2010 had nine H's after 9/1
1999 had eight
1961 had seven

Of course, past performance is not an indicator of future activity, as each season is unique.
I leave the predictions to experts, who forecast 7-9 hurricanes, which is certainly still possible / likely.

On a side note, it's been 333 days since the last Hurricane (Sam) formed on 9/24/2021. That may be some kind of record.

[Spacecoast]

On a side note, it's been 333 days since the last Hurricane (Sam) formed on 9/24/2021. That may be some kind of record.

btw, 333 days without 'cane formation is not a new record (it is in the top 5 inter-seasonal stretches, though).

As far as I can tell, (1950-2021), the record is:

365 days between
Chantal, (which formed on 9/11/1983), and Diana, which formed(9/10/1984).

[Spacecoast]

Post peak update (1950-2021 data from HURRDAT):

In a typical year (7.2 hurricane season), 4 hurricanes would had already formed by Sept 11th

Updating the graph to show number of 'canes formed after Sept 11th.....

Average number of H's (blue) after Sept 11th is three (trending up from two to four).
Min # is 0, Max # is 9.
2010 & 2020 had nine H's after 9/11
2001 & 2005 had eight.

Again, past performance is not an indicator of future activity.
I'm not sure any (reliable) indicator of future activity exists.

[Spacecoast]

Updated numbers for rest of season....

After Oct 1st:
Average number of 'canes = 1.6
Minvalue = 0. Maxvalue = 6 (in 2020)
16 years (22.2%) had zero hurricanes after Oct 1st.


After Oct 15th:
Average number of 'canes = 1.0
Minvalue = 0. Maxvalue = 4 (in 2020)
27 years (37.5%) had zero hurricanes after Oct 15th.


After Nov 1st:
Average number of 'canes = 0.44
Minvalue = 0. Maxvalue = 3 (in 2001)
48 years (66.7%) had zero hurricanes after Nov 1st.

[Teban54]

Updated numbers for rest of season....
https://i.ibb.co/44T923W/numa.jpg
After Oct 1st:
Average number of 'canes = 1.6
Minvalue = 0. Maxvalue = 6 (in 2021)
16 years (22.2%) had zero hurricanes after Oct 1st.

https://i.ibb.co/CvhQ3Nq/numb.jpg
After Oct 15th:
Average number of 'canes = 1.0
Minvalue = 0. Maxvalue = 4 (in 2021)
27 years (37.5%) had zero hurricanes after Oct 15th.

https://i.ibb.co/cvGRDHR/numc.jpg
After Nov 1st:
Average number of 'canes = 0.44
Minvalue = 0. Maxvalue = 3 (in 2002)
48 years (66.7%) had zero hurricanes after Nov 1st.

By "2021", you mean 2020 right?

[Spacecoast]

Last update to this thread...
Although the official end of season is 11/30, Hurricanes forming in Dec / Jan are not unprecedented.


After Nov 15th:
Average number of 'canes = 0.21
Minvalue = 0. Maxvalue = 2 (1962)
58 years (80.6%) had zero hurricanes after Nov 15th.


After Dec 1st:
Average number of 'canes = 0.07
Minvalue = 0. Maxvalue = 1
67 years (93.1%) had zero hurricanes after Dec 1st.

(HURRDAT2 lists 'Alice' forming on Dec 31st, 1954. This seems to be the latest calendar hurricane over last 72 years, (not sure why it was named as an 'A' storm, as there was another Alice named earlier as the 1st storm of 1954). Apparently after 11/30, they start the new season.

A case can be made that the season doesn't really end until mid January.
'Alex' formed 1/14/2016, making it the earliest calendar hurricane over last 72 years, but could be considered last storm of the 2015 season.

[1900hurricane]

Last update to this thread...
Although the official end of season is 11/30, Hurricanes forming in Dec / Jan are not unprecedented.

https://i.ibb.co/zx6G31m/huuraJPG.jpg
After Nov 15th:
Average number of 'canes = 0.21
Minvalue = 0. Maxvalue = 2 (1962)
58 years (80.6%) had zero hurricanes after Nov 15th.

https://i.ibb.co/55fTMSs/huurb.jpg
After Dec 1st:
Average number of 'canes = 0.07
Minvalue = 0. Maxvalue = 1
67 years (93.1%) had zero hurricanes after Dec 1st.

(HURRDAT2 lists 'Alice' forming on Dec 31st, 1954. This seems to be the latest calendar hurricane over last 72 years, (not sure why it was named as an 'A' storm, as there was another Alice named earlier as the 1st storm of 1954). Apparently after 11/30, they start the new season.

A case can be made that the season doesn't really end until mid January.
'Alex' formed 1/14/2016, making it the earliest calendar hurricane over last 72 years, but could be considered last storm of the 2015 season.

Alice wasn't designated operationally until after the calendar changed years. Post storm analysis found it developed earlier however, so the analyzed point of development went forward and crossed the calendar year.