Possible development off the Southeast US Coast (Is Invest 99L)

[WalterWhite]

2. Western Atlantic:
A non-tropical area of low pressure could form near the southeast
coast of the United States late this week. This system is forecast
to move northward or northwestward and could acquire some
subtropical characteristics if it remains offshore.
* Formation chance through 48 hours...low...near 0 percent.
* Formation chance through 7 days...low...20 percent.

[LarryWx]

This only quite vaguely resembles 1984 (Diana), which originated from the tail end energy on a rather strong cold front going offshore/nontropical which quickly became tropical. But 1984 was an extremely rare storm (100+ year storm?) and it occurred two weeks earlier during heart of the season. I don’t look for anything like that of course and this could easily turn out to be no big deal other than a good rainfall producer for some areas. However, in the extreme, this could be a STD/STS that becomes a TC (especially if it then moves NE offshore). Regardless, it is an interesting situation due to uncertainty.

Late August of 1898 may have been somewhat similar to 1984 that also became a hurricane that came back into the SE coast.

Neither 1898 nor 1984 was during El Niño by the way. I’ll look to see if I can find more. But those are two of the most extreme cases off the top of my head that started from essentially nontropical lows just offshore the SE IS and subsequently came back into the SE as hurricanes.

This is more or less a potential “ridge over troubled waters” setup where a rather strong surface high to the north provides lower level convergence to the south, which leads to lift and then the formation of a sfc low.

[emeraldislenc]

I think Diana formed on Sept 8.

[Teban54]

This only quite vaguely resembles 1984 (Diana), which originated from the tail end energy on a rather strong cold front going offshore/nontropical which quickly became tropical. But 1984 was an extremely rare storm (100+ year storm?) and it occurred two weeks earlier during heart of the season.

I went to look at Diana 1984, and wow, that was such a beauty even with the quality of satellite imagery at that time.

Fortunately, it had kind of a Florence-like stall but further offshore, limiting its impacts. The name Diana wasn't retired after 1984, but it was retired in 1990 for a weaker but more destructive storm hitting Mexico.

[emeraldislenc]

At one time it was a cat 4 storm but it hit as a cat 2.

[WalterWhite]

Western Atlantic:
A non-tropical area of low pressure is forecast to form near the
southeastern coast of the United States late this week. This system
could acquire some subtropical characteristics this weekend if it
remains offshore while it moves slowly northward or northwestward.
* Formation chance through 48 hours...low...near 0 percent.
* Formation chance through 7 days...low...30 percent.

[jconsor]

I think history shows that given a few days (even as little as 36-48 hours) over water off the SE US coast, a system of non-tropical origins with a decent upper-level wind setup can become a hurricane. In addition to Diana, Chris 2018, Arthur 2014, Alex and Gaston 2004, and Bob 1991 are examples. Of course, the majority of such systems remain at tropical/subtropical storm strength.

FYI today's 0Z run of the NASA GEOS-5 model is showing what looks to be a high-end TS or cat 1 hurricane, certainly a more consolidated system than the global models (GFS, GEM, ECMWF) are indicating.

https://fluid.nccs.nasa.gov/wxmaps

I believe the NASA GEOS-5 model is run at 7 km resolution.

This only quite vaguely resembles 1984 (Diana), which originated from the tail end energy on a rather strong cold front going offshore/nontropical which quickly became tropical. But 1984 was an extremely rare storm (100+ year storm?) and it occurred two weeks earlier during heart of the season. I don’t look for anything like that of course and this could easily turn out to be no big deal other than a good rainfall producer for some areas. However, in the extreme, this could be a STD/STS that becomes a TC (especially if it then moves NE offshore). Regardless, it is an interesting situation due to uncertainty.

Late August of 1898 may have been somewhat similar to 1984 that also became a hurricane that came back into the SE coast.

Neither 1898 nor 1984 was during El Niño by the way. I’ll look to see if I can find more. But those are two of the most extreme cases off the top of my head that started from essentially nontropical lows just offshore the SE IS and subsequently came back into the SE as hurricanes.

This is more or less a potential “ridge over troubled waters” setup where a rather strong surface high to the north provides lower level convergence to the south, which leads to lift and then the formation of a sfc low.

[LarryWx]

Thanks, Yaakov.
I found these seven TCs that formed from a nontropical origin that later hit the SE US as an H:

-Arthur 2014: early July; developing weak Nino; cat 2 into NC

-Gaston 2004: late Aug; weak Niño; cat 1 into SC

-Diana of 1984: early to mid Sep; incoming Nina; cat 2 into NC

-Cindy of 1959: early Jul; neutral ENSO; cat 1 into SC

-Yankee Hurricane of 1935: formed from nontropical low E of Bermuda Oct 30 that hit SE FL as a cat 2 H Nov 4th! Warm neutral ENSO

-Storm #5 of 1913: early Oct; formed from nontropical low off NE US and hit SC as cat 1/developing Nino

-Storm #2 of 1898: late Aug; cold neutral; cat 1 into GA


So, a TC that is purely nontropical in origin has resulted in a landfalling H on the SE US coast ~every 18 years on average over the last 125 years. The strongest landfalls were the three cat 2 hits, Arthur of 2014, Diana of 1984, and the Yankee H of 1935. Landfalls: 3 SC, 2 NC, 1 GA, 1 FL

TS landfalls on the SE coast from nontropical origins have occurred much more often than the once/18 years of H landfalls. But that’s intuitive because it typically takes a good bit of time for transition to tropical and there often isn’t a lot of time over water with them developing close to home.

I confirmed the much higher frequency of SE TS landfalls vs H landfalls from a non-tropical origin: 12 in 73 years or ~1 every 6 years on average. So, chances of a nontropical originating TC landfalling on the SE US as a TS are ~3 times higher than landfalling as a H:

2022: Colin

2021: Danny

2015: Ana

2007: Gabrielle

2002: Kyle

1981: Bret

1976: Dottie

1967: Doria

1965: #9

1962: #2

1960: Brenda

1952: #3


To clarify for the readers, the stats I presented are only for those that actually made landfall on the SE US. So, they wouldn’t apply to any that go NE and stay offshore, which may increase the chance for a H vs those that hit the SE.

Based on everything I’ve seen for the current situation along with my analysis of over seven decades of history, I predict that IF this were to ever transition to a TC and IF it then were to landfall on the SE US that it would landfall there as a TS. Also, if it were to landfall there, I predict either NC or SC with further N in SC favored over further S. This doesn’t look to me like a FL or GA landfall though nothing is certain because nothing is ever certain especially this far out.

[LarryWx]

0Z UKMET has a strong TD/weak TS from this that landfalls in NC. The strength of this model for an undeveloped potential storm is nearly always conservatively low. Thus, I’d take its strong TD/weak TS forecast with a grain and figure that a mid grade TS would be just as likely:

NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 126 HOURS
FORECAST POSITION AT T+126 : 30.6N 79.9W

LEAD CENTRAL MAXIMUM WIND
VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS)
-------------- ---- -------- ------------- -------------
1200UTC 23.09.2023 132 31.7N 78.8W 1002 32
0000UTC 24.09.2023 144 34.4N 78.1W 1002 33
1200UTC 24.09.2023 156 37.8N 77.5W 1006 34
0000UTC 25.09.2023 168 40.6N 76.6W 1007 23

[LarryWx]

12Z UKMET: unlike 0Z that forms S of Charleston and then moves NNE into NC, this forms just off NC followed by NNE, NE, and ENE movement to well offshore the NE US/Canada that never allows it to landfall anywhere through hour 168:

NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 120 HOURS
FORECAST POSITION AT T+120 : 34.9N 74.9W

LEAD CENTRAL MAXIMUM WIND
VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS)
-------------- ---- -------- ------------- -------------
1200UTC 23.09.2023 120 34.9N 74.9W 1005 36
0000UTC 24.09.2023 132 37.3N 73.8W 1005 36
1200UTC 24.09.2023 144 38.9N 71.2W 1009 33
0000UTC 25.09.2023 156 39.4N 67.9W 1011 30
1200UTC 25.09.2023 168 39.6N 62.3W 1013 34

[Sciencerocks]

[crownweather]

This only quite vaguely resembles 1984 (Diana), which originated from the tail end energy on a rather strong cold front going offshore/nontropical which quickly became tropical. But 1984 was an extremely rare storm (100+ year storm?) and it occurred two weeks earlier during heart of the season.

I went to look at Diana 1984, and wow, that was such a beauty even with the quality of satellite imagery at that time.
https://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Diana_1984-09-11_2200Z.png/1280px-Diana_1984-09-11_2200Z.png
Fortunately, it had kind of a Florence-like stall but further offshore, limiting its impacts. The name Diana wasn't retired after 1984, but it was retired in 1990 for a weaker but more destructive storm hitting Mexico.

Diana holds a special place in my heart as it's the first tropical cyclone I ever tracked at age 10!! I used the PBS morning show, "AM Weather" & evening national news to help track Diana (We didn't have the Weather Channel or any cable at the time). Gloria, the next year, cemented my lifelong interest & sometimes obsession with tracking hurricanes. Gloria impacted me on the New Hampshire Seacoast with hurricane force wind gusts.

[hurricanetrack]

Mentioned this thread in today's video update.

Diana was the "it" hurricane for me too.



Link: https://youtu.be/2bpxSThq3Fw

[AJC3]

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 PM EDT Mon Sep 18 2023

For the North Atlantic...Caribbean Sea and the Gulf of Mexico:

2. Western Atlantic:
A non-tropical area of low pressure is forecast to form near the
southeastern coast of the United States late this week. This system
could acquire some subtropical characteristics this weekend if it
remains offshore while it moves generally northward.

* Formation chance through 48 hours...low...near 0 percent.
* Formation chance through 7 days...low...30 percent.

[Kazmit]

18z GFS tightens this up as it comes onshore.

[cycloneye]

 https://twitter.com/JimCantore/status/1703935159454613867

[LarryWx]

The 0Z UKMET, unlike the prior two runs, doesn’t turn this into a TC.

[N2FSU]

6z GFS a lot stronger but keeps it offshore:



Sent from my iPhone using Tapatalk

[underthwx]

This is a portion of the 940 am Hartford Ct. forecast discussion. It seems as if there remains a bit of uncertainty, regarding the low that is forecast to develop off the coasts of the Carolinas later this week, that may also affect the weather across the New England region, the NHC currently has a 30% chance of development advertised as of the 8 am tropical outlook.

Rex block evolves this period with below normal heights developing
off the the southeast coast (possible tropical low), while a
positive height anomaly evolves over Hudson Bay. This promotes dry
weather across SNE through at least into the daylight hours of
Saturday, as a 1030+ mb high builds into SNE. This airmass will be
accompanied by PWATs below normal, yielding dew pts in the 40s and
50s, providing warm days but cool nights and very comfortable
humidity. Classic New England fall weather. Given this dry airmass
and 1030+ mb high overhead, followed the cooler MOS guidance to
derive overnight min temps.

Then forecast for Sunday and Monday hinges on the exact position of
the mean ridge over Hudson Bay and downstream position of northern
stream jet. Will the ridge axis remain west of New England,
supporting downstream trough to suppress tropical moisture from
advecting northward into SNE? Ensembles and deterministic guidance
indicate this is a plausible outcome. However at this time range,
will play it probabilistic and follow low chance pops from NBM
for Sat night into early next week. Otherwise, just a fabulous
stretch of fall weather, enjoy!

[LarryWx]

After the 0Z run not turning this into a TC and the prior 12Z having it become a TC but staying offshore, this new 12Z UKMET is back to having a TC, is a bit stronger, and it moves N to a landfall near the NC OB Saturday 9/23 followed by a crawl up the coast to DelMarVa pen/dissipation likely bringing the coastal sections of the mid Atlantic states very heavy rainfall:

NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 78 HOURS
FORECAST POSITION AT T+ 78 : 32.5N 77.0W

LEAD CENTRAL MAXIMUM WIND
VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS)
-------------- ---- -------- ------------- -------------
0000UTC 23.09.2023 84 32.7N 76.4W 1006 47
1200UTC 23.09.2023 96 35.4N 76.1W 1003 42
0000UTC 24.09.2023 108 37.3N 76.3W 1001 40
1200UTC 24.09.2023 120 37.9N 76.2W 1007 33
0000UTC 25.09.2023 132 37.7N 76.1W 1012 32
1200UTC 25.09.2023 144 38.2N 75.7W 1015 32
0000UTC 26.09.2023 156 CEASED TRACKING