Good Evening! The Atlantic is waking up. This wasn't unexpected necessarily, although many seem surprised since it has been quiet this season despite the writing on the wall over a month ago. Some of us were advertising the Atlantic ramping up in early September back during the first few days of August. This wasn't some sort of magic or anything and there were clear indications that the SAL (Saharan Air Layer) would weaken, SST's would increase, and wind shear would be weaker, opening the door for classic Cabo Verde cyclones to organize as tropical waves encountered conditions that are finally conducive for development. To top it all off, climatology alone almost always supports this. We now have one disturbance that the NHC just initiated advisories on just off the coast of Africa with another area of low-pressure that draws interest to the Gulf of Mexico. A parade of easterly waves is set to eject westward as well and as the environment continues to become more favorable, simple probabilities suggest at least some need to pay more attention to possible threats to US interests. Anyone who has followed me for very long at all knows that I prefer to be patient and not jump at the first sign of anything so we're not going to talk about anything impacting the US yet or anything like that because that's just ridiculous. One couldn't even look at the voodoo range of model land right now and pretend to know what the details will look like as the models aren't anywhere near the same page yet. We're going to look at current observations to see how things have come together so far and make some general projections looking forward. THEN we will take a dive into model land. Bottom line: I believe that the Atlantic is rubbing its eyes, stretching, and getting ready to go to work, we'll deal with the details as we can.
The Gulf of Mexico is also deserving of close attention. The environment there isn't very conducive just yet but a few disturbances are meandering around the region and more are possible. They most likely won't overcome the stronger flow aloft right now but come next week we should see the GofM become very favorable as SST's are 30C and shear will weaken significantly. Don't be surprised to see activity develop there sometime after this weekend.
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| Potential Tropical Cyclone Six: Currently its around 13N/17W. Latest analysis indicated 1007mb central pressure with sustained wind of 25kts and gusts of 35kts. This cyclone is expected to track generally WNW while intensifying over the next several days. It is forecast to be a named system within 24hrs or so. It should almost certainly intensify, though the degree of this intensification probably depends a lot on how quickly in gains latitude as it tracks westward. As you'll see shortly, the most favorable conditions exist in a relatively narrow corridor South of 15N until reaching about 25W. If the cyclone gains latitude too quickly then the stronger shear and pocket of cooler SSTs just North of it would likely temper it quite a bit. If it holds steady until it clears that range then it will reach warmer water which also extends to a deeper depth and will follow a similar cycle as the official forecast. |
The above GOES 16 IR image shows the Atlantic and Gulf of Mexico with potential tropical depression six just off of the coast of Africa and another area of interest in the Caribbean. Another tropical wave is West of Psbl TD6 but has little to no organization. Over the last few hours convection has really become more enhanced with Psbl TD6. There's still some messiness to it due to the convective debris but the low-level circulation can be seen trying to tighten up. This should begin to look more "typical" through the weekend as it crosses into warmer water and taps into the environment more.
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| Recent SST analysis shows continued warming across the Atlantic. SST's were slightly cooler through much of the season and this coupled with very dry air off the coast of Africa was extremely detrimental to any easterly waves we saw through the earlier parts of this season but with SST's increasing and the SAL diminishing we've seen an increase in potential and should see this manifest in these waves developing into stronger systems. If a wave comes off of Africa and gains too much latitude too quickly then it could suffer as it hits a pocket of cooler waters but if they hold below 15N then they will most likely organize. |
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| This product shows the depth of the 26C isotherm. The western periphery of the cool pocket we mentioned is visible but once a system clears that then pretty much the entire area is favorable. The other area of concern mentioned was any troughs that develop in the Gulf of Mexico and Caribbean. SST's are 28-30C in those regions and, particularly in the Caribbean, warm water extends to a great depth. Those would pose a notable risk in the fact that there simply isn't as much lead on a storm that develops closer to the U.S. |
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| The dry, dusty air that has helped suppress development this season has broken up a bit with clear areas of weakness evident. If you notice, TD6 has managed to develop in one of these areas where the SAL has broken up. This should be the trend going forward into September. |
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| Current wind shear analysis does show considerable shear across a large portion of the Atlantic and this is a major limiting factor. Forecasts show that this should weaken going into the weekend and ahead however and that is yet another factor to consider. The disturbance in the Caribbean is suffering as a result of this. TD6 is located South of the strongest belt of shear and should manage to shoot the gap and stay in a pretty supportive environment. |
There are countless other factors that come into play and we will no doubt get into those later on. Should we get to the point to start talking impacts and eventually chasing/intercept then we'll look at much more data but I wanted to set the stage for how we got to the Atlantic trying to come alive a little bit after a stagnant season. It seemed likely that all of these things would happen (Weaker SAL, warmer SST's etc) at this point as far back as a month ago and that's why it wasn't a stretch to anticipate an increase in activity by early September.
Now let's take a look at a few model products just to see what they're thinking. Based on what we have now though, it's reasonable to say that TD6 will continue to organize as it tracks generally WNW. Intensification is likely, although there is some discrepancy in the degree of this, but it should be a named storm by late Friday.
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| Ensemble spaghetti plots of the possible track. Models show this storm tracking WNW through about 144hrs before it gets captured in between highs and turns North. Divergence regarding exactly where this turn occurs and how sharp it is but that is expected being so many hours out. The consensus is there, observations should be leading you down this thought process as well so I don't have any reason to discount this track as a whole. It will be important to monitor this storm toward the end of the weekend though because if there are any surprises then that's where we would see the first signs of them. |
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| Model Forecasts for Intensity: There is quite a bit more uncertainty regarding how much this system intensifies. The consensus is that it will organize more than it is but the range right now covers everything from barely making it to tropical storm status to a mid Cat 1 hurricane. It's obvious that the mean is tropical storm status by the next 24hrs. I think we see this storm reach TS by 12-18hrs from now. If it does reach hurricane status it still looks like it's 2.5-3 days out before that so we've got the luxury of monitoring it for a bit to see how organized it can get and if it steers clear of the cooler water and stronger shear. If it gains latitude too quickly then it will likely hang out toward the bottom of the model spread here but if it stays fairly steady at its current latitude for the next 40-48hrs then it can hit the warmer open water. Given that it looks like it has already started to really organize and the circulation has tightened up a lot in the last 12hrs, I am expecting a Cat 1 hurricane out of it. The central column of low-pressure seems much less decoupled throughout the vertical and it's certainly on an uptick. I have a hard time seeing it launch North into cooler water. There is high-pressure North and no capture energy to pull it in. |
This next product is the Experimental SHIPS intensity forecast. There's a lot of useful meteorological information within this data so please browse through it and look at some of the things to consider as these are some of what we will discuss in the near-future. Important things to note, this particular output keeps the storm at tropical storm status. It takes it northward just a bit but this is enough to put it into cooler water (from 28C to 26C) with drier air aloft.
* ATLANTIC 2018 SHIPS INTENSITY FORECAST *
* IR SAT DATA PROXY USED, OHC AVAILABLE *
* SIX AL062018 08/30/18 18 UTC *
TIME (HR) 0 6 12 18 24 36 48 60 72 84 96
V (KT) NO LAND 25 29 34 38 43 50 57 62 68 70 74
V (KT) LAND 25 29 34 38 43 50 57 62 68 70 74
V (KT) LGEM 25 27 30 33 35 39 43 47 51 55 58
Storm Type TROP TROP TROP TROP TROP TROP TROP TROP TROP TROP TROP
SHEAR (KT) 16 18 16 13 5 10 12 10 3 5 10
SHEAR ADJ (KT) 4 3 7 8 11 9 0 -4 0 0 5
SHEAR DIR 48 48 50 49 62 89 101 124 186 236 203
SST (C) 28.0 28.2 28.2 28.2 28.1 27.3 26.5 26.0 26.0 26.0 26.1
POT. INT. (KT) 137 140 139 139 138 129 121 115 115 115 115 ADJ. POT. INT. 136 138 135 134 136 129 119 111 110 109 108
200 MB T (C) -52.9 -53.6 -53.9 -53.4 -52.8 -53.9 -53.0 -53.5 -53.5 -53.7 -53.1 -5
200 MB VXT (C) 0.1 0.2 0.3 0.3 0.4 0.5 0.1 0.1 0.1 0.1 0.2
TH_E DEV (C) 6 4 4 4 4 4 4 4 4 4 6
700-500 MB RH 71 71 73 73 73 71 69 63 55 53 51
MODEL VTX (KT) 14 19 21 23 23 24 24 22 21 19 20
850 MB ENV VOR 87 96 93 102 93 114 152 145 131 111 97
200 MB DIV 99 99 90 88 129 61 18 -14 -25 -4 22
700-850 TADV -7 -12 -12 -7 -9 -3 -5 -2 1 0 14
LAND (KM) 238 341 432 514 600 851 1203 1527 1821 2084 2122
LAT (DEG N) 12.8 13.1 13.3 13.4 13.5 14.4 15.3 16.0 16.8 17.7 19.0
LONG(DEG W) 19.0 20.1 21.2 22.0 22.9 25.4 28.7 31.7 34.4 36.8 39.2
STM SPEED (KT) 11 11 9 8 11 15 16 14 13 13 12
HEAT CONTENT 7 12 14 14 17 5 4 0 0 0 1
FORECAST TRACK FROM OFCI INITIAL HEADING/SPEED (DEG/KT):270/ 12 CX,CY: -11/ 0
T-12 MAX WIND: 20 PRESSURE OF STEERING LEVEL (MB): 500 (MEAN=619)
GOES IR BRIGHTNESS TEMP. STD DEV. 50-200 KM RAD: 18.1 (MEAN=14.5)
% GOES IR PIXELS WITH T < -20 C 50-200 KM RAD: 55.0 (MEAN=65.0)
PRELIM RI PROB (DV .GE. 55 KT IN 48 HR): 7.4
INDIVIDUAL CONTRIBUTIONS TO INTENSITY CHANGE
6 12 18 24 36 48 60 72 84 96 108 120
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SAMPLE MEAN CHANGE 1. 2. 3. 4. 6. 8. 9. 10. 11. 12. 12. 13.
SST POTENTIAL 0. 0. 0. 1. 3. 8. 14. 18. 22. 25. 27. 28.
VERTICAL SHEAR MAG 1. 1. 2. 3. 5. 5. 6. 7. 7. 8. 7. 6.
VERTICAL SHEAR ADJ 0. -1. -1. -2. -3. -4. -4. -3. -2. -2. -2. -2.
VERTICAL SHEAR DIR 0. 1. 2. 3. 5. 7. 8. 9. 7. 6. 5. 4.
PERSISTENCE 1. 1. 1. 1. 1. 0. 0. 0. 0. 0. 0. 0.
200/250 MB TEMP. 0. 0. -1. -1. -1. -1. -1. -1. -1. -1. -1. -1.
THETA_E EXCESS 0. -1. -1. -1. -2. -2. -2. -2. -3. -2. -1. -1.
700-500 MB RH 0. 0. -1. -1. -2. -3. -4. -5. -5. -4. -4. -3.
MODEL VTX TENDENCY 1. 3. 5. 6. 7. 8. 6. 5. 2. 2. 2. 1.
850 MB ENV VORTICITY 0. 1. 1. 1. 2. 3. 4. 5. 6. 6. 7. 7.
200 MB DIVERGENCE 0. 1. 1. 2. 2. 2. 2. 1. 0. 0. 0. 0.
850-700 T ADVEC 0. 0. 0. 0. 0. -1. -1. -1. -1. 0. 0. 0.
ZONAL STORM MOTION 0. 0. 0. 0. 0. 1. 1. 1. 1. 1. 2. 2.
STEERING LEVEL PRES 0. 0. 0. 1. 1. 1. 0. 1. 1. 1. 1. 1.
DAYS FROM CLIM. PEAK 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.
GOES PREDICTORS 0. 0. 0. 0. 0. -1. -1. -2. -2. -2. -2. -2.
OCEAN HEAT CONTENT 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.
RI POTENTIAL 0. 0. 1. 1. 2. 2. 2. 2. 1. 1. 0. 0.
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TOTAL CHANGE 4. 9. 13. 18. 25. 32. 37. 43. 45. 49. 52. 51.
CURRENT MAX WIND (KT): 25. LAT, LON: 12.8 19.0
This post is just the first tropical weather discussion of this season. The Pacific has been absolutely rolling this year but the Atlantic not so much. Now that we've taken a brief look at what has transpired to get us to the point of at least having some activity to discuss the next step is to see how this activity plays out. It seems certain that with a somewhat more favorable environment and a consistent parade of easterly tropical waves that we will see more storms develop. There's no real discussion as far as chasing this particular system right now, or any impact whatsoever, but we're at the point where we have to watch the horizon. No reason to jump just yet. The next step is to see how this system evolves into the weekend and reassess how it's intensifying by the weekend. That's the next move so sit tight. Don't get caught sleeping on tropical weather going forward and don't get caught up in the hype either. Just follow the clues.
