Not much to say about Lee as it refuses to stop struggling, but Margot at least is taking advantage of its decent environment quite nicely.
A new area of interest has been added in the East Pacific as a fairly subtle wave has begun to organize relatively quickly, but that’s about all of note currently ongoing in the basin beyond the things already discussed. The remnants of Jova continue to spin down over the open basin well west of Baja California, gradually dissipating into a surface trough and fading away. The Mediterranean system’s damage has been done, with the possibility of a massive death toll from flooding in Libya; it’s not a trackable feature anymore though moisture will likely continue to linger. This storm (unofficially named Daniel) has, remarkably, become the deadliest and costliest Mediterranean tropical-like cyclone in known history. The Atlantic meanwhile is very quiet aside from the active storms and there’s not much else to discuss about it.
The East Pacific wave and Atlantic invests are the only short term development threat on models; these remain mostly quiet in the Pacific through the medium range though an uptick in developing storms looks likely toward the end of the month as the MJO moves back in. Likewise for the Atlantic, the only notable system in the short term is the wave in the eastern Atlantic; after that, development odds seem limited for the next week or two, with the long range GFS spinning up a storm off the southeast coast and also in the eastern MDR. Ensembles suggest further MDR activity can’t be ruled out all the into the end of the month with upper level winds less hostile than they could be for an El Niño year. Recent runs show a subtle low latitude wave nearing the Lesser Antilles south of the hurricane that results from 97/98L, and despite the Niño, that could potentially be worth watching.
Lee has continued to waste multiple days of very favorable conditions, going through endless eyewall replacement and refusing to strengthen. The storm remains symmetrical in all quads with no hint of shear impact, and outflow is tremendous to the north and east, continuing to indicate an environment that would support a much stronger storm than Lee. Pressure is up to around 951mb, with flight level winds at 110kt and SFMRs near 90kt. That only barely suggests a major hurricane at all, significantly weaker than nearly all guidance had it at this point. The culprit this time has been the endless eyewall replacement cycle that’s lasted over a day at this point. A very large outer eyewall developed and closed off yesterday but scraps of the old inner eyewall refuse to dissipate even after a day, hampering any attempt for the outer eyewall to become fully dominant. There is little time now for this to complete as conditions will soon become less favorable but Lee is becoming very large now that the larger core is maintained.
Water temperatures have thus far been very warm below the storm, but that’s expected to change soon as the large storm upwells cooler water from below given the slow movement and larger radius of high winds. That will be made even worse for the storm by moving soon over even cooler waters left behind by Franklin and Idalia. In addition to the rapid decrease in heat content, shear will begin to pick up soon ahead of the incoming trough, which will also be introducing drier air into the circulation. That combination of factors should lead to even faster weakening by Thursday and Friday, perhaps weakening below hurricane force as it loses tropical characteristics over the weekend over even colder water. However regardless of weakening, it’s expected to become very large as it begins to become an extratropical cyclone off New England, spreading impact over a large area.
Lee continues to crawl to the WNW very slowly. Ridging that has built in to the north of the storm has blocked its progress north thus far. However deep troughing over the eastern US is expected to imminently erode away the western edge of this ridging, which will allow the hurricane to begin turning northward tomorrow. Acceleration will probably be slow and gradual with an increase in speed eventually but a fairly slow turn for the next few days. As it gets picked up by the trough later in the week it will probably move faster to the north, on a track that’s likely to take it somewhere between Maine and Nova Scotia. There’s still a spread in modeling, with the track having some degree of uncertainty, but impact in that general area seems inevitable.
The main issue so far has been the continued life threatening rip currents and high seas produced by the storm, affecting most of the western Atlantic including the Lesser and Greater Antilles, Bermuda, the Bahamas, and the East Coast. That will continue through this week. Bermuda has been placed under a tropical storm watch, the first land based watches or warnings issued for Lee thus far, and it’s increasingly likely the hurricane brings significant impact to the island Thursday and Friday. High winds well into tropical storm force are likely, along with torrential rainfall and very dangerous seas. Begin preparation immediately. It’s also likely that the system brings major impact to New England and Atlantic Canada over the weekend. It is still too soon to know where and when exactly, and to what extent, but definitely get ready to prepare for high winds, very high seas, and torrential rainfall.
Margot has developed into an impressive hurricane, though not an especially intense one overall. A very compact cold topped CDO contains an eye that’s been obscured at times overnight but becoming more apparent today. This CDO was basically alone yesterday with convection to the northeast but bands have begun to wrap around the center in all quads, giving it a slightly more traditional appearance. Microwave and scatterometer passes have offered other information about the structure of the storm recently. We have confirmation that the wind field has grown tremendously, making Margot a very large storm by gale radius, and a microwave pass showed a possible concentric eyewall structure which implies we may already be looking at an eyewall replacement cycle, somewhat surprising given yesterday’s structure but probably more likely now that banding has increased around the core. The hurricane remains in a reasonably favorable environment, with SSTs still warm enough given the widespread Atlantic warmth and a relative minimum in vertical shear. However tomorrow shear is expected to be on the uptick again, compounded by a continued dry environment and a gradual decrease in SSTs. The forecast peak has come down now that it looks like we’re going to be eyewall cycling, but a little more strengthening is possible today before the environment becomes less favorable. The large storm is likely to wind down fairly slowly and persist into next week.
Margot is wandering north, in the relatively weak flow between ridges nearing the middle latitudes. A bend to the NNW is forecast, and then a gradual deceleration as ridging builds to the north of the storm. That’s where the model agreement ends; a lot of disagreement has come into play regarding the strength of the ridge that is expected to build in to the north of the storm. Some modeling has suggested the ridge is weak enough to allow the storm to eventually escape to the NW and N over the weekend, but some modeling now shows the storm being pushed to the southeast then back to the southwest, probably leading to a clockwise loop. Thus there is huge uncertainty on the future track of the storm. Should be of no land impact either way, though the Azores could be near the edge of the tropical storm wind radios if some of the more eastern tracks come to pass.
The two invests in the eastern Atlantic are more or less being tracked as one system, consisting of 97L to the west as a small low pressure area and 98L to the east as a tropical wave right off the African coast. They have been expected to merge into each other with the wave overtaking the small low to the west, but that’s still a little uncertain; this morning there is little convection or organization whatsoever with the wave while the low pressure area still continues to show concentrated convection and at least a broad low level spin, though the convection is well west of the spin. Models have locked onto mostly 97L, suggesting that perhaps this vorticity could be more in play than the wave, but it matters fairly little overall which one is the bigger contributor to the storm that’s to come as it will probably be a combination of both of them once conditions improve. Either way models are very insistent on a tropical depression forming in the next few days in the central Atlantic. Conditions will favor the development of a tropical storm or hurricane, but where it goes will be the big question – much will depend on whether it feels the weakness in ridging reinforced by Margot or it’s able to track westward under ridging on a path much like Lee’s into next week. There’s still a lot of run to run inconsistencies in the long term track so that’s uncertain but lots of time to watch it.
A tropical wave that was showing very little convection until recently has fired up in the western portion of the East Pacific basin, quickly becoming not only an area of interest but a system showing up on modeling as offering an opportunity for some quick development. A broad low pressure area and scattered convection exist with this feature, showing only minor signs of development but based on modeling seeming likely to wrap up further in reasonably favorable conditions as it heads WNW and attempts to close off a more defined low pressure center. A tropical depression could form over the next several days in the far western East Pacific basin approaching 140°W or in the eastern Central Pacific basin as it’s crossing over, perhaps becoming a tropical storm but staying far from any land. It should be weak and short lived.
A significant cold front sweeping across the country is the big weather story for today. It’s bringing some degree rainfall to a large swath of the central and eastern states, and behind it, surface high pressure and significantly more comfortable cooler and drier weather. The front extends from the Lakes into Texas for the moment with an additional extension into New Mexico; scattered rains are also likely across the southwest, giving a pretty good chunk of the country a chance of rain. Lingering rainfall across New England will probably be swept out soon and further storm systems are likely along the storm train for the Pacific Northwest.
From heat advisories to now frost advisories, fall is definitely in the air – the first frost of the season is expected for parts of MN and WI behind the strong cold front, signs that the long summer is soon to come to an end. We still however have to get through September. The flow around this strong front is producing rip currents and marine hazards for the Lakes region, where rip current statements are up. Rip currents and high swells are also likely to affect the East Coast from Lee. Other than that most national hazards are related to air quality or dense fog.
Deep troughing continues to be evident around the Great Lakes, and it’s easy to see why much cooler weather is funneling into the US with the strong flow around this troughing, strong northerly flow on the west side driving the cold front and a feed of northern Canadian air into the US. It will probably only be a temporary respite from ridging and warm weather but will also serve to steer Lee away from the coast over the coming days. A somewhat more muted pattern lies out west with little ridging present and a more zonal flow. The two week outlook suggests warm and dry weather for much of the country, so enjoy the taste of fall while it lasts.
HNH Weather Notes 
Leave a comment