Ground Scouring

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  1. I don't think that we'll get a handle on the dominant center until we see a few more convective maxima.
  2. Can anyone tell, based on the overall structural tilt, eddies within the broad LLC, and convective pattern, where a main center may consolidate overnight and whether it would favor a landfall on Hispaniola? With such a weak system, could frictional convergence over Puerto Rico help transfer energy to the north side of the island and consolidate a center that stays off Hispaniola? (Note: I don't expect that to happen, but I'm still learning.)
  3. Why is the strongest convection continually developing on the south side?
  4. I posted this elsewhere, but it might be worth reading: Despite shear, Erika has some very good upper-level support, with good outflow on the eastern and southern sides. CIMSS analysis shows strong upper-level divergence and ventilation, which, along with low-level convergence and high PWATs, are allowing convection to generate, keeping the system alive as we enter the unfavorable daytime hours, convection-wise. The upper-level "fanning" we are seeing could be an ominous indicator of what Erika could become over the Bahamas in just a few days, depending on its position relative to the upper-level anticyclone. The ECMWF and its ensembles have consistently shown a blocking pattern by days four through seven that would likely force even a fairly strong Erika somewhere over the FL peninsula or the GA coast (between the upper Keys and Savannah). I really don't buy the GFS-based guidance and GEFS members that show either a curve off the East Coast or a track farther north into the Carolinas. I still don't expect Erika to survive, however, given the center relocation and the stronger-than-forecast low-level easterlies that would bring the center over or just south of Puerto Rico and into Hispaniola. I still believe that the combined effects of Hispaniola and continuing short-term shear will kill it. However, if Erika somehow and unexpectedly survives, the western Bahamas and FL/GA will need to prepare for a possible hurricane impact. Even a weaker system tracking over the west side of the FL peninsula would bring heavy rainfall to areas that don't need it (i.e., Tampa Bay).
  5. Based on the latest recon data, the surface center is so far south of the NHC forecast track and is clearly dislocated from the mid-level circulation...with a movement of 280°. Based on the data and the continuing vertical, especially easterly, shear and fast movement, I'm remaining confident that Erika will have an encounter with Hispaniola. The center will likely pass just south of Puerto Rico and over the eastern mountains of the Dominican Republic within another day. Such an outcome would either kill Erika or at most result in a weak, sheared tropical storm heading into South FL by day four, close to the ECMWF deterministic solution. I'd bet on death by Hispaniola for Erika.
  6. The ECMWF and its ensembles have yet to nail down the exact path from day four onward, but have been very consistent in showing a pattern that would likely prevent Erika from curving out to sea. Since Erika, though still disorganized, is a bit deeper than some of the models forecast a day or more ago, it has the potential to become better organized after clearing Puerto Rico and Hispaniola. A system organizing earlier would likely move at least a tad slower from days two to four. By day five, the ECMWF suite shows ridging building in quickly behind a weak shortwave trough that cuts off. A slower-moving Erika, while potentially stronger and situated farther southeast (and thus more directly under the anticyclone, allowing more intensification than the 00Z deterministic ECMWF indicates), would be more susceptible to a turn, but may also allow the ridging to build back in. The pattern by days six and seven, while still uncertain, is consistent with the shorter-term evolution within five days and shows blocking heights over the Ohio Valley. Given all these factors, I'd say that even a stronger Erika, if it survives beyond 36 hours, may well pose a threat from South FL to Savannah. Please feel free to clarify whether I'm wrong. (I will say, however, that the low-level ridging to the north of Erika continues to be a bit stronger than anticipated, thus forcing a dangerously close flirtation with Hispaniola down the line.)
  7. Does it really matter? It would be fish food in that case, and I'm still sticking with my original call for dissipation over the Lesser Antilles.
  8. Erika's dead or close to death. Convection over the past day or so has not succeeded in covering the dominant low-level center, and Erika is entering a more stable and sheared environment nearby. Pulsating convection can only get a weak system so far, and I've already outlined the obstacles to regeneration after day three. I'd say Danny 2.0 or maybe the ghost of Debby 2000--another system forecast to pose a possible hurricane threat to South FL that encountered shear and struck Hispaniola instead.
  9. Question: Why do the HWRF and GFDL models run beyond day three, when their reliability on track and intensity becomes atrocious? Shouldn't their runs be shortened to short-term forecasting?
  10. Does anyone know why the August-September TUTT has been so persistent since 2010? I found a study indicating that climate change could, in part, contribute to stronger vertical shear near the Caribbean islands thanks to a more persistent TUTT. Of course, there is plenty of seasonal variability, but I would like to know if the TUTT will finally be less prevalent next season during prime time. Otherwise, systems forming in the MDR would likely struggle near the Lesser Antilles. Additionally, one thing's clear: if the CFS forecast for a large Atlantic warm pool for next year verifies into the heart of the 2016 season, we may see even fewer opportunities for U.S. landfalls, at least with MDR storms. According to this study, a strong AWP correlates with a weaker Bermuda High, allowing for more MDR development, but also more opportunities for those storms to miss the U.S. So we could go another season without major landfalls.
  11. But it's most likely correct. I'd like to see someone make a credible argument against the ECMWF and especially its ensembles. A system like Erika, once an open wave, will have trouble regenerating without a well-defined area of vorticity, unless environmental conditions are absolutely perfect. Even then, organization would be slow at best, resulting in a weak to moderate tropical storm at most moving into South FL. Nevertheless, I've been consistently saying that Erika would eventually open up and move westward into the Greater Antilles, and it remains the most likely scenario now. I'd love to see someone make a decent counterargument that argues for an intact Erika or a regenerating Erika that could reach something stronger than a middling TS.
  12. I posted this elsewhere, but I'll reiterate: "While mid- and upper-level shear has relaxed over the system, the strong low-level easterlies have continued to prevent convection from organizing near the center overnight, during the nighttime convective maximum. The fact that Erika hasn't strengthened will only hurt it over the next few days as it passes near the TUTT axis and continues to deal with stable air, plus less energy for latent heat of condensation thanks to Danny's impact on the sea surface temperature profile. Models don't do well with these weak systems, and given how Erika looks now, its center becoming even more dislocated from the convection (on latest visible imagery) is a death sentence for such a weak system. Unless it can regenerate convection within the next 12 hours or so, Erika will likely weaken to a depression and then open up into a wave near the islands, as the GFS and the ECMWF ensembles have indicated for some time (the fact that the latest operational ECMWF keeps Erika intact doesn't mean much, as it differs significantly from its ensembles). In these cases, trust the mean of the global ensembles--especially that of the ECMWF ensembles--more than individual operational runs. Given that the NHC is still unclear about its intensity forecast even within three days, there is at least an equal chance that Erika will dissipate within the said time frame. I'm willing to place a bet that Erika won't last more than another day and that it may have already weakened to a depression, considering its disorganization and meager convective pattern on satellite. ... As I mentioned earlier, all the strengthening occurs beyond three days, when forecasting errors increase significantly and confidence drops precipitously." The dynamical (12Z) GFDL and HWRF models have continued to lower their intensity expectations within three days, consistent with trends over the past few days. They show Erika basically remaining steady through 72 hours, even hinting at some weakening in the meantime. Furthermore, "Also, some people believe that even if it opens into a wave, ex-Erika can still regenerate in the Bahamas, the eastern Gulf, or the Straits of Florida. There are several major problems with this hypothesis. 1) You would need a well-defined area of vorticity left. If Erika were to degenerate into a dry wave for several days, it would lose much of the convection-driven vorticity needed to sustain a low pressure center. 2) When you're back to square one--with a system having to resume its life cycle from the beginning--you will need favorable conditions in place. If you're dealing with a dried-out wave, you will need MUCH more favorable conditions in place, because you're starting out with less of a system. Currently, none of the reliable models indicates that conditions around the FL peninsula or in the eastern Gulf will be favorable for rapid development, hence the NHC's call for gradual intensification by day five, assuming an intact Erika. 3) A weaker ex-Erika in the short term would move farther south and west with the low-level trade winds, meaning significant land interaction with mountainous Puerto Rico, Hispaniola, and Cuba."
  13. Classification as a TS won't change the impacts to the Lesser Antilles, which will be the same whether the system is Erika or a low pressure area along a sharp trough axis. I wouldn't upgrade something on the basis of ambiguous evidence, as ASCAT showed a very elongated circulation that arguably wasn't closed. Furthermore, 98L has failed to generate convection sufficiently close to its alleged center for an upgrade. Additionally, 98L will face even worse conditions beginning in another day, so if it's struggling now (thanks to easterly shear in the mid to upper levels, plus stable air to its north), it will really struggle against the TUTT near the islands, no matter how broad a system it is. I wouldn't waste an upgrade on something that may not even develop and certainly, in my view, won't evolve beyond a minimal tropical storm (40-45 kt) at most. If someone disagrees with me and the global models (ECMWF especially, but also GFS), then I'd like to hear the reasoning. I didn't disagree about the science; I mentioned the fact that ASCAT and satellite data seemed insufficient to upgrade.
  14. From a chasing standpoint, nothing is more frustrating is seeing the ECMWF and its ensembles shift westward, showing ridging strong enough to drive 98L into FL, and then drop down a nice TUTT that moves in tandem with 98L, shearing it apart into an open wave that tracks into FL. If you're not dealing with a persistent, fishy East Coast trough, then you're dealing with shear. Grrr. I'm still sticking to my earlier contention that this system won't survive beyond day three, even if it does manage to become a depression or weak storm over the next 36 hours.
  15. Given that 98L is embedded in a strong, low-level, easterly trade surge, it will be hard pressed to develop until the gradient relaxes near the Lesser Antilles in about four days. A young system simply can't consolidate when it's moving at 15-20 kt. Additionally, the easterlies are bringing a very stable air mass to the north of 98L that will persist over the next several days. The operational ECMWF, by showing some development before then, differs sharply from its ensembles, most of which show little or no development or even weakening. By the time 98L reaches the islands, moreover, it will running into the persistent TUTT axis that has bedeviled many systems this season (and indeed in most of the past four seasons). I have strong doubts that 98L will survive the hostile conditions that it is facing, which are worse than those that Danny faced, as Danny at least did not have to deal with strong easterlies undercutting its vertical stacking, allowing it to become a major hurricane before succumbing to the TUTT.
  16. The GFDL and HWRF both are poor at best when forecasting upper-level conditions, even in the short to medium range. Anyway, even if Danny were an organized storm in the Bahamas, the persistent westerly mid-level flow around 30°N east of Florida would ensure that it would curve away from the East Coast. I'm quite convinced that Danny will open up into a wave or weaken to a depression before reaching the islands and moving west with the low-level flow into Hispaniola, consistent with the ECMWF and its ensembles' forecasts over the past several days. Still, Danny has put on quite a show and shown the limitations of satellite estimates when dealing with compact but intense, deepening systems.
  17. Danny likely peaked at ~105-110 kt at about 15Z, since the recon data came as Danny's satellite presentation deteriorated. Convection has warmed, the southern eyewall has opened up, the eye has become clouded, and the western side of the CDO has become eroded, while flight-level winds suggest that Danny is tilted from SW to NE. These trends are clear indications that strong westerly shear is finally affecting Danny, causing the system to become elongated. For once, I agree with the (12Z) HWRF, which quickly brings Danny down almost as rapidly as it spun up, showing a strong tropical storm in 36 hours and a depression reaching the Leewards later. Small systems like Danny often deepen and then peter out more rapidly than the NHC anticipates. I expect that there won't be much left of Danny in two days. Even though Danny intensified more than I, the NHC, or any of the dynamical models expected, it won't make much difference, given the very hostile conditions and drier air ahead of such a tiny cyclone. The GFDL's re-strengthening of a weak system near Puerto Rico makes no sense, given the collapse of Danny's inner core and the very dry air surrounding the cyclone, even assuming, as the Devil's advocate, that shear eventually relaxes.
  18. If Danny were a more typical-sized cyclone and had another two days or so, it would potentially have enough time to become a substantial hurricane and form a large anticyclone overhead, potentially counteracting the moderate to strong TUTT-generated westerlies lurking to its northwest. However, Danny has less than 24 hours to go before it encounters those westerlies (strong vertical shear), thereby halting significant intensification in its tracks. You can already see signs of shear impinging on outflow in the western semicircle. Given the small size of the storm, I think that Danny will likely peak sometime later today or overnight before weakening rapidly starting tomorrow. The NHC shows a peak around 12Z tomorrow, based on a blend of the dynamic and statistical guidance, which I think is too late, given that the ECMWF already shows less favorable conditions and signs of weakening overnight.
  19. Downstream influence from typhoons Goni and Atsani would indicate a mean weakness off the East Coast in about five to seven days as Danny nears the area, suggesting than an intact Danny would curve east of the mainland United States...correct? (As an aside, does anyone know what happened to "Hart"/nineinchnails with his good overviews of downstream patterns over upcoming weeks? Feel free to send me a message.) As an aside, the GFS has been consistently intensifying Danny too quickly and, as I have mentioned, apparently underestimated the impact of dry air on the structural organization of the cyclone. The fact that the GFS shows less impact from vertical shear in three days and beyond is related to the model's showing a much deeper Danny building a robust anticyclone aloft. That won't happen, making the ECMWF solution more realistic. I'd go with 60-65 kt tops for Danny's peak, though 50-55 kt may be more plausible, given the possibility that the dry air won't mix out as Danny leaves the ITCZ.
  20. Which operational model has a more realistic handling of the low-level ridging north of Danny through day five: the ECMWF or the GFS? Why? (My personal guess is a blend of the ECMWF and GFS, since the ECMWF, through its bias, likely makes Goni and Atsani too deep by day five, thereby making a "wavier" N Hemisphere pattern that causes a more elongated subtropical ridge, even though the ECMWF has a superior handling of the overall pattern.) How the models handle the ridge would certainly affect whether Danny is a Caribbean cruiser or goes over/north of the Leewards, Puerto Rico, and Hispaniola. It would also affect how much dry air Danny would potentially ingest, given that the GFS indicates a more moist mid-level environment outside the ITCZ than does the ECMWF. Personally, I already believe that the 0300Z (08/18) NHC forecast for 90 kt in five days is far too high...I would go with 60-65 kt as a compromise between the most bullish (statistical) and bearish (dynamical) guidance, given the likely influence of dry air. Danny could easily end up even a bit weaker than this if the ECMWF forecast verifies, with its stronger low-level trades serving to increase vertical wind shear--a detrimental trend for sub-hurricane systems nearing the islands. I could be wrong, but when comparing the ECMWF to the GFS, I would give the ECMWF a nod when looking at the influence of environmental conditions on developing systems. Given its superior convective scheme, the ECMWF may better capture the effect of mid-level dry air on the developing core of a weak to moderate tropical storm. Once Danny gradually leaves the ITCZ over the next few days, it could really begin to struggle against much lower relative humidity to its northwest. The NHC mentions short-term rapid intensification or slower deepening due to dry air as the two options. I would suggest a third: that Danny fails to develop beyond a mid-range tropical storm (50-55 kt) and then unravels into an open wave as it encounters stronger low-level easterlies near the islands in five days. We've seen other systems similar to Danny form in the MDR and then fail to intensify as much as expected, thanks to the unexpectedly large impact of dry air on the inner core of the system. I can't recall the last time I've seen a TC strengthen significantly in an environment as dry as Danny's. Plus, the spread between the statistical and dynamical intensity guidance has widened over time. As far as Danny's only reaching moderate TS status before dissipating, I wouldn’t necessarily go with that solution, but I would give it a 40% chance of occurring, with the highest chance (50%) that Danny slowly intensifies to 60-65 kt by the end of the NHC forecast period. 10% would be other solutions.
  21. What are the odds of the TUTT splitting and backing away from Danny as it nears the NE Caribbean in five days and just beyond? Models seem pretty unanimous in that the TUTT will be a problem by that time, so what evolution in the overall pattern could make that change? Windspeed wrote: So you expect shear to actually decrease on or after day five as Danny approaches the NE Caribbean?
  22. Does anyone have a way to contact HM? I have some questions for him regarding ENSO and am unsure as to who would know how best to reach him. If appropriate, send me a private message. Thanks!
  23. Blanca's clearly been having problems with extensive outer convective banding on its south side over the past few days. The extreme upper-level divergence in the environment is facilitating outer convection that is choking off inflow to the core, hence the relatively warm CDO around the eye.
  24. SOI might say otherwise. Recent trends show a steady, possibly significant + trend, reflected in the 30- and 90-day values. Daily values have skyrocketed to more than +15 as well, and the overall frequency of negative dailies has steadily dropped.
  25. Season: 6 / 2 / 0 (accounting for Ana having formed) June: 1 / 0 / 0 July: 1 / 0 / 0 August: 2 / 1 / 0 September: 1 / 1 / 0 October-November: 0 / 0 / 0 For good measure, I do not expect any U.S. hurricane landfalls this season.