jpeters3

Meteorologist
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Everything posted by jpeters3

  1. It looks like the storm is at least trying to start an ERC right now, and the outer ring of convection is huge. I wonder if the wind field expansion that the models have been predicting is going to result from the completion of this ERC. Most recent MW pass:
  2. Here's the link to the model graphic (and others like it): http://rammb.cira.colostate.edu/products/tc_realtime/storm_model_data.asp?storm_identifier=AL112017
  3. A decrease in pressure gradient is expected due to a decrease in background pressures as the weakness expands and steering ridge erodes. Therefore wind speeds would come down if Irma maintains around 915 to 920 mb pressure. If the eyewall remains intense or reintensifies after a structural change, it would still be capable of producing Cat 5 winds, 180+ just seems unlikely unless it were to have an vigorously low pressure drop to sub 900 mb readings. A potentially larger hurricane might be difficult to pull that off, but not impossible given SSTs, low shear and excellent outflow. Remember though that the part of the pressure gradient in a hurricane that is responsible for the fastest winds is confined to the region right around the core, which may be less sensitive than you think to changes in the pressure of the far environment. My point is that processes like shear or internal restructuring (ERC) are going to be the first order regulators of intensity - not necessarily ambient pressure or storm size (this is just my view). Also, simple hydrostatic arguments can show that the latent heat release (and eye subsidence) is related to the change in pressure between a TC core and its surrounding environment - not the surface pressure value. This means that if the TC maintains the same core temperature when moved to a region of lower ambient pressures, the TC's central pressure should go down accordingly.
  4. It seems like there may be a few of those (at least shear) over the next few days. I suspect that the reason is a combination of ERCs and the low likelihood that a given storm will remain in an environment that supports cat 5 intensity for an extended period of time.
  5. Please don't make pretentious posts like this. Any model guidance is useful (or at least relevant) to this discussion.
  6. Hurricane Ivan (and Allen for that matter) went back and forth from Cat 4 to Cat 5 3 times: https://en.wikipedia.org/wiki/Hurricane_Ivan#/media/File:Ivan_2004_track.png
  7. I'll agree that an expanding wind field is likely once the system begins to interact with the baroclinic zone and starts undergoing ET. Seems to me based on model guidance though that the forecast for decreasing intensity in models is (1) because they generally don't show maintained intensity Cat 5 for whatever dynamical or numerical reasons (they've been showing weakening for a day and a half), and (2) because they are fairly well anti-correlated with vertical shear (which should gradually increase over the next few days). Human forecasts (NHC) have shown decreasing intensity in accordance with model guidance, and the low statistical likelihood that TCs maintain 150 kt + intensity for an extended period of time.
  8. Again, I don't necessarily think a decrease in wind is guaranteed. There have been many large storms (Katrina) a similar latitudes that achieved very strong sustained winds. The main players are going to be internal processes (eyewall replacement cycles), and vertical shear (I suspect there is a bit of Wly or NWly shear right now due to outflow restriction on that side of the storm).
  9. It's worth noting that a northward turn of a TC does not dynamically require that its wind field expand. Likewise, TCs do not conserve angular momentum due to the excessive release of latent heat, so a widening TC does not guarantee that its maximum winds decrease. I suspect that the unexpectedly low winds in model guidance for given pressures is not a result of the TC size - rather inadequate model resolution and/or pressure/wind relationship.
  10. What becomes annoying are the "I don't believe the intensity forecast because there is a blemish in the IR presentation" posts. Sounds like he was just pointing out a feature in the IR.
  11. Agreed. I am interested to see what this would look like after another ERC, given that the core is reasonably wide right now. Would be wild to see a storm with a giant Isabel-esq eye approaching Florida.
  12. What you are pointing out may indeed be a subsidence ring between the outer and inner eyewall. I personally wouldn't go so far as to call the IR presentation "degraded," but based on how things typically go during an ERC, I wouldn't be surprised if the presentation became "less perfect" over the next 12ish hours.
  13. ERC stands for Eyewall Replacement Cycle. Storm's outer rainbands consolidate into a solid ring of convection that surrounds the inner eyewall. Eventually subsidence on the inner edge of the outer ring of convection causes the inner eyewall to erode. Once the inner eyewall has eroded, the outer contracts and becomes the new "inner" eyewall.
  14. In the most recent scans, the echoes look to be weakening a bit in the eastern eyewall (doesn't look like an attenuation issue). Too early to tell whether this is a trend or just a momentary fluctuation. I'd trust the recon obs at this point, which don't show weakening winds or rising pressure.
  15. Settle down folks. My colleagues over here in Monterey are some of the foremost experts in the world on TCs, and they have even beens stumped by some of the behavior of both Irma and Harvey (the half eyewall Harvey had at landfall for instance). Everyone (including NHC) is just making their best guess on what is going on with some of these internal storm processes, based on the available data. There aren't many historic observations of a hurricane of this scale, that has maintained such a high intensity for so long. Yes, it looks like there are now concentric eyewalls. That doesn't necessarily mean the storm is weakening. It may be a sign of an impending weakening cycle (ERCs usually are), but this storm has surprised us many times.
  16. Shear is forecasted to pick up by Sat as the system starts to interact with the midlatitude baroclinic zone: http://rammb.cira.colostate.edu/products/tc_realtime/products/storms/2017AL11/DIAGPLOT/2017AL11_DIAGPLOT_201709061800.png
  17. ERCs are still a poorly understood phenomena. It's hard to actually make them happen in simulations, so that they can be studied in a "controlled" setting. Also, it's rare that such an intense hurricane sits within a region of good radar coverage for so long, so there probably aren't many prolonged observations of this behavior.
  18. Yeah, honestly it's been hard to tell with this particular storm. I've thought an ERC was about to begin several times, but the outer eyewalls seem to have merged with the inner without much disruption. Edit: Agreed - there seems to be no sign of the outer ring impeding the inner, or the storm weakening so who knows...
  19. Starting to me to look like a concentric eyewall - the most defined outer eye wall i've seen since Monday.
  20. It's probably a combination of cold pool/shear interactions and insufficient low-level tilting of vorticity. RAP shows a fairly dramatic increase in low-level shear over the next several hours, however.
  21. Agreed. Give it an hour or two, and these storms will be bad news.
  22. I suspect storms are currently struggling due to somewhat meager low-level shear. This will no longer be a problem once the low-level jet ramps up later.
  23. Goes to show how wind shear can compensate for low CAPE. MLCAPE values in that area were only analyzed at ~ 500 J/Kg by SPC meso analysis.
  24. Check out the 18Z TLH sounding. Some pretty impressive 0-1 km and effective inflow layer SRH, but the numbers aren't quite as astronomical as some of the guidance this morning had predicted. Lacking a little bit in curvature within the lowest 1 km, which may be why these cells aren't becoming widespread tornado producers just yet. http://www.spc.noaa.gov/exper/soundings/17012218_OBS/