jconsor

Need to keep a close eye on the hybrid low expected to develop off the SE coast and move N/NNW end of this weekend into this weekend. While heavy rains are likely to initially focus well south of the area from eastern NC to Delmarva, they could eventually extend north into portions of the NYC metro area. The region doesn't need any additional heavy rainfall, after several recent flash flood events in early to mid Aug and again the second week of Sep, and another 1-2"+ today.

I think we need to watch the eastern Caribbean in late Sep. Also expecting more NW Caribbean activity than usual in Oct-Nov for an El Nino.

I wrote a detailed thread about the trend toward a sharper trough at 250 mb from the eastern Great Lakes to the Carolinas, and how this could increase the risk of Lee continuing N or NNW toward New England instead of turning NNE into Atlantic Canada.

Need to look at more than just 500 mb heights/winds to get a better idea of Lee's future track. Very important to consider the 250 mb level IMHO. There have been a number of cases in which hurricanes were lured NW toward the SE US/Mid-Atlantic coast by a negative tilt trough or cutoff low at 250 mb, even when the 500 mb trough was relatively weak. It *usually* takes a deep 500 mb trough from near the Great Lakes to the TN Valley or southern Appalachians (570 dm contour down to at least 40N) to pull a hurricane northward from near/east of the Bahamas toward a New England landfall, and also accelerate it enough (forward speed >25 kt) so it maintains hurricane strength despite the cooler waters N of the Gulf Stream. Look at the upper air patterns for 1938, Carol, Edna, Gloria and Bob and you will see such a deep trough. On the other hand, the 1893 NYC hurricane and Irene both had much weaker/further north troughs at 500 mb than the hurricanes mentioned above. The 1944 Great Atlantic hurricane had an initially deep trough over the W. Great Lakes/OH Valley two days before landfall, but it was weakening/lifting out as the hurricane turned N up the East Coast. In both the 1893 and 1944 hurricanes, a sharp trough at 250 mb along with a strong jet streak from the NE US to SE Canada helped to compensate for the lack of a deep trough at 500 mb. Also in Lee's case, a relatively deep/neg tilt 250 mb eastern US trough and 100+ kt jet streak as shown by the GEFS would likely help to compensate. Also important to note - since the frequency of blocking patterns appears to have increased in the past 30 years since the last New England landfall, and there has been substantial warming of SSTs in those 30 years, history may not be as much of a guide as we would hope. In other words, perhaps in the next few decades we will see increasing frequency of slower-moving strong TSs or lower-end hurricanes (i.e similar to Irene, Sandy and Henri) landfalling in the northern mid-Atlantic and New England vs. what was seen up until ~1990.

Combination of stronger ridge nosing down to NW of Lee and upper low to its NE trending stronger and a little closer to Lee is likely the reason we are seeing more ensemble members trend toward a bend to nearly due west between 60-70W longitude.

The upper level pattern across the eastern US and western Atlantic this weekend into next week is far from set in stone. We need to watch for the potential for a trough split, which models often don't handle well until inside 5 days. If the the southern piece of the trough cuts off across the SE US by this weekend, and the ridge over the NW Atlantic/SE Canada is strong enough to resist troughs in the northern stream from progressing eastward much, that could block a typical recurve out to sea. 6Z GEFS and past few GFS runs are now hinting at another trough split mid to late next week from the Great Lakes to the Lower Mississippi/Tennessee Valley. A trough in that position along with an anomalous NW Atlantic ridge has in the past steered many infamous SW Atlantic hurricanes toward a landfall from NC to New England.

2023 is turning out much more active than a typical El Nino so far. No mod-strong Nino has seen as much ACE to date.

Beware of recency bias! The downcasting of the tropical Atlantic happens nearly every year when there is a Jul/early Aug lull in activity. I wouldn't be so sure *all* of the systems next two weeks will be weak and short-lived. I see potential for one or two stronger ones, mainly in the subtropical central Atlantic and GOM/Bahamas area.

I am skeptical of the idea that the main period of favorability for Atlantic TC development will be compressed into a week to 10 day period at the end of Aug/very beginning of Sep, like some extended guidance is hinting at. Overall, there are indications of a more active than usual September. I explain why in the thread below:

The system near the Bahamas has maintained healthy outflow aloft and some mid-level spin on satellite for at least 24 hours, though convection has waxed and waned. I wrote about the potential for this system (or a few others trailing it) to take advantage of favorable light shear in the Gulf of Mexico next week and possibly become "surprise" TCs.

Some important context re: the ECMWF seasonal outlook. Firstly, as others have pointed out, the Aug outlook is for Sep onward. So if we were to have an active Aug, that would be not be reflected in the ECMWF outlook. I go through some other important points to keep in mind in this thread:

The Euro weeklies almost never show any chance for hurricanes (>5% on those maps) in my experience. More relevant to look at tropical depression *anomaly* probability and TS probability.

Regarding the Bahamas/FL system... yes, pressures are high and this will need some time to "cook" before developing. However, it going just inland next day or two is not a guarantee against a TD or weak TS developing. We have seen such systems develop even with little model support a few times in the recent past. There is another brief window for development early next week as it heads back offshore and passes near the Gulf Stream:

A thread about the East Atlantic system. I think the it has a decent chance for development (I would say 50-60% within 7 days). Don't be fooled by recency bias just because 95L encounted hostile conditions and didn't develop. The atmospheric state has changed substantially in a way that favors development more for this wave than it did for 95L.

Don't sleep on 95L. It looks sheared and convectively anemic at the present time, but could gain renewed energy in the W. Caribbean.

Be careful using the CFS for assessing how favorable/unfavorable the Atlantic will be for tropical activity in the subseasonal to seasonal timeframe (2 weeks to several months out). Its recent performance has been awful. Copernicus multi-model ensemble and ECMWF weeklies are more reliable tools.

Thread about the next potential MDR/central Atlantic system. Of course it's very early and the system isn't expected to form for another 5-7 days, but I though it's worth discussing potential track. I'm not so sure this will form as early as ensembles imply. Given the indications of a weakening E US trough/building W Atl ridge first week of Aug, this could work against a clean recurve out to sea.

Need to watch for possible development near the SE US coast late this week into this weekend.

While the tropical Atlantic will likely remain relatively quiet until at least the last third of the month, the overall atmospheric and oceanic pattern in the month tends to be strongly correlated to the level of activity during the rest of the season and can provide some very important clues to assess the reliability of seasonal guidance. There has been consistency in the guidance on easterly upper-level wind anomalies prevailing much of Jul across much of the Caribbean and tropical central Atlantic, along with significant westerly anomalies at 850 mb in the MDR (as was forecast by the Copernicus multi-model ensemble - see Alex Boreham's excellent site here: https://cyclonicwx.com/models/c3s/atl/). If this verifies, it would be a strong indicator of an active season.

Be careful using the EPS weeklies beyond 2-3 weeks to gauge favorability of the overall pattern for Atlantic tropical activity. They have been biased toward too much -VP over the E. Pacific and +VP over Indian Ocean, apparently playing catchup to the lingering impacts of the very warm SST in the tropical W. Pacific and the -PDO, as well as the influence of the strong WAM and very warm tropical Atlantic.

It is unlikely that in the face of significant easterly anomalies in the central Pacific through mid-Jul, the waning subsurface warmth will be enough to foster any further warming in the Nino 3.4 and 4 regions. If anything, I would expect some slight cooling.

https://www.atmos.albany.edu/student/ventrice/real_time/AAM.png Dr. Gensini's site (https://atlas.niu.edu/gwo/) has CFS GWO forecasts at bottom

IMHO, We need to look beyond the Central Pacific for indicators on the trajectory of this El Nino event:

14 storms 8 hurricanes 4 major hurricanes