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October tropical cyclones:
The strong tropical wave - ‘98-L’ - was upgraded Wed. afternoon to tropical depression #18 then to tropical storm “Sam” Thursday morning & to a hurricane early Friday (avg. date for the 7th Atlantic hurricane is Nov. 16), so Sam was a hurricane for 10+ days before finally losing tropical characeteristics over cooler water & under increasing shear while merging with a front. The large extratropical ocean storm will swirl not too far from Iceland for a good part of this week. The last NHC advisory was issued early Tue.
Ultimately... the end result - Sams’ track - was driven by September’s re-adjustment of the upper level flow across the Northern Hemisphere which included a pretty persistent trough the last several weeks over & near the Eastern U.S. & Western Atlantic essentially protecting the U.S. from any Atlantic storms.
A strong tropical wave that moved off the coast of Africa Tue. was upgraded to tropical depression #20 Wed. morning & to “Victor” Wed. afternoon. The good news: - thanks to the Bermuda High sitting so far to the north & east over the Atlantic - Victor turned sharply northward as it rapidly organized becoming a victim of strong southerly & southwesterly shear & then dry air. The last NHC advisory was issued Monday.
Long range forecast models are showing the potential for some low pressure to develop over the Western/SW Atlantic this week. We’ll have to watch this for possible tropical development. Early indications are that this low will just meander east of Fl. &/or the Carolina’s with any development to be very slow & mainly offshore of the U.S.
We’ll also need to monitor parts of the Caribbean &/or Gulf of Mexico in the longer range, but there’s nothing “cooking” for the moment.
An unfavorable MJO phase is forecast to slowly change over the next 2-3 weeks. There is a lot of “sinking” (brown lines) air over the Atlantic Basin which doesn’t usually favor a good deal of tropical development (there can be exceptions!). *But* the rising air (green lines) will likely overspread the Atlantic as we move through Oct. leading to a potentially active 2nd to the last month of the Atlantic hurricane season. During this evolution, we’ll need to monitor the Caribbean, Gulf of Mexico & SW Atlantic where we’re already getting some occasional - albeit inconsistent - hints of tropical development from long range models.
Ocean temps. remain “fit” to help maintain tropical cyclones.
Sea surface temps. across the Atlantic are now near to above avg. across much of the basin (2nd image below) & - even more importantly - deep oceanic heat content (which helped “feed” Ida & Sam) is impressive & the “equivalent oceanic heat content” - namely depth averaged temperature in the upper 300 m (~984 feet) - is even more impressive all the way from Africa to the Gulf of Mexico. Such an ocean water temp. pattern is conducive to long track deep tropical Atlantic tropical cyclones & can lead to a more favored regime for rapid intensification cycles. From an AMS research paper in ‘08 Mainelli, DeMaria, Shay, Goni: “Results show that for a large sample of Atlantic storms, the OHC variations have a small but positive impact on the intensity forecasts. However, for intense storms, the effect of the OHC is much more significant, suggestive of its importance on rapid intensification. The OHC input improved the average intensity errors of the SHIPS forecasts by up to 5% for all cases from the category 5 storms, and up to 20% for individual storms, with the maximum improvement for the 72–96-h forecasts. The statistical results obtained indicate that the OHC only becomes important when it has values much larger than that required to support a tropical cyclone.” More recent research continues to indicate similar correlations.
Saharan dust. Dry air - yellow/orange/red/pink. Widespread dust is indicative of dry air that can impede the development of tropical cyclones. However, sometimes “wanna’ be” waves will just wait until they get to the other side of the plume then try to develop if everything else happens to be favorable. In my personal opinion, way too much is made about the presence of Saharan dust & how it relates to tropical cyclones.
2021 names..... “Wanda” is the next & last name on the Atlantic list (names are picked at random by the World Meteorological Organization... repeat every 6 years... historic storms are retired (Florence & Michael in ’18... Dorian in ’19 & Laura, Eta & Iota in ‘20). Last year - 2020 - had a record 30 named storms. The WMO decided beginning this year that the Greek alphabet will be no longer used & instead there will be a supplemental list of names if the first list is exhausted (has only happened twice - 2005 & 2020). More on the history of naming tropical cyclones * here *.
Mid & upper level wind shear (enemy of tropical cyclones) analysis (CIMMS). The red lines indicate strong shear:
Water vapor imagery (dark blue indicates dry air):
Deep oceanic heat content continues to increase across the Gulf, Caribbean & deep tropical Atlantic & has become pretty impressive from the Central/NW Caribbean into the Gulf of Mexico:
Sea surface temp. anomalies:
SE U.S. surface map:
Surface analysis centered on the tropical Atlantic:
Surface analysis of the Gulf:
GFS wave forecast at 48 & 72 hours (2 & 3 days):
Atlantic Basin wave period forecast for 24, 48 & 72 hours respectively:
The East Pacific:
West Pacific IR satellite:
Global tropical activity:
Cox Media Group