September 2010 very busy in the Atlantic Tropics
Early Morning September 25,
September 2010 ties as one of the busiest for the Atlantic Hurricane Season
With the formation of Matthew, September 2010 has experienced 8 tropical cyclones when including Tropical Storm Fiona, which was born in late August but continued into early September. This ties with the 1998, 2002, and 2007 Atlantic hurricane seasons, which also had 8 tropical cyclones in September. We will break this record if we get another storm to form in the Atlantic basin before September 30. Now, for my updated Lisa and Matthew forecasts.
Hurricane Lisa and Tropical Storm Matthew
This is an update to my previous forecasts on Lisa and Matthew.
Figure 1: Latest 700-850 mb CIMSS steering, as of 0000Z September 25, 2010
Figure 1 above is the latest steering in the lower layers of the atmosphere. Because there are several ridge centers, I have marked all of them with Hs. My previous track forecast with Lisa was to the right of the NHC and made Lisa stronger than the NHC forecast. Indeed, Lisa did go to the right of the NHC forecast and got stronger than the NHC forecast, but my previous forecast was not as far to the right and as strong as Lisa got. In fact, Lisa has become a hurricane today, quiet stronger than anticipated.
I thought the big ridge over the open Atlantic would expand eastward to the north of Lisa, thus bending Lisa increasingly to the west with time. Rather, the ridge to the east of Lisa over Africa is now steering Lisa northward, and shows no signs of expanding westward to deflect Lisa any bit to the west in the short-term. You can see in Figure 1 that there is a small ridge due north of Lisa over the Azores. Remember in Figure 1 you are looking at the lower-layer steering currents for weaker, shallower tropical cyclones than Hurricane Lisa. But, Lisa should begin weakening at 25N latitude while it starts moving over waters cooler than 26 deg C and into westerly wind shear. In fact those upper westerly winds begin at 25N and northward in the latest 200 mb streamlines. Thus as it gets weaker and shallower with time, it could get influenced by that lower-level ridging late in the period and bend Lisa increasingly to the west. In fact, computer models diverge around 30N latitude, with some continuing straight north, and other models turning a weaker Lisa due westward. So my projected path in Figure 2 below makes Lisa go due north, with a westward bias in the projected path by the time it nears 30N if Lisa weakens and shallows out, which would allow Lisa to get more deflected to the west by the lower-level ridging to the north.
The intensity forecast reflects the above-mentioned trends, weakening Lisa beginning at 25N latitude from its current 80 mph wind state due to the aforementioned cooler water temps and increasing shear.
Figure 2: My latest Hurricane Lisa forecast
Matthew has made landfall in Central America. Even with the blue trough in Figure 1 being due north of Matthew (which should have weakened the easterly steering currents on the south side of the big ridge in the Atlantic), Matthew still continues to move due west at 15 mph, the same speed and direction it had before the blue trough passed to the north of Matthew. Basically, the blue trough has had no effect on Matthew's track. The gray trough behind the blue trough seems to have more southward influence, so the forecast in Figure 3 shows a northward deflection at the end of the forecast giving credit to the gray trough. My northward deflection is a little more than the current NHC forecast. At the end of the forecast, I weaken Matthew to a remnant low as it should have been inland for quiet some time, especially as the gray trough would slow down Matthew while its over land.
Figure 3: My latest Tropical Storm Matthew forecast
While Lisa Spins its wheels, 95L Expected to be Matthew
Good morning September 23,
The 2010 Atlantic Hurricane season continues to be active, we have a tropical wave in the Caribbean Sea, Invest 95L, very likely to become our next named storm, Matthew. In the meantime, Igor hit Newfoundland harder than expected, and 94L has become Tropical Storm Lisa. Lisa has been caught in poorly-defined steering currents south of a northeastern Atlantic trough, and "has been spinning its wheels" while stalled out west of the Cape Verde Islands.
Igor Pounded Newfoundland
I got it wrong when I forecasted Igor's track on September 20, saying that after Bermuda, Igor would track generally NE into the far northeatern Atlantic Ocean while steered by southern and eastern sides of the green trough (see what I mean by green trough by looking at Figure 1 in the last post). I had some concern that my forecast track was too far to the right late in the period with this post:
"Perhaps I underplay the baroclinic interaction of Igor with the green trough at that timeframe (after 0300Z Sept. 23), which if significant enough, would slow down the green trough as it baroclincally amplifies into a cut-off upper low. The stalling cut-off upper low would then yank Igor much more northward on its east side."
Indeed, I underplayed the baroclinic interation of Igor with the green trough. For the first twenty-four hours of the forecast (up to 0300Z Sept. 21), Igor was following my track forecast, and my forecasted green trough position at 0300Z Sept. 21 was almost 100% accurate. But then, Igor went through baroclinic interaction right after that time, with the green trough stalling out, and Igor turning northward with its center passing just east of Newfoundland. Thus, the north and west sides of the storm hit Newfoundland with heavy rains and gusty winds. A northward hook that early was not expected by the NHC forecast track or I on the 20th, and the NHC had to adjust their track leftwards to account for the earlier than expected northward hook.
Since 0300Z Sept. 21 the green trough has meandered with some slow eastward progress due to baroclinic interaction with Igor, and can still be seen in the CIMSS steering today as shown in Figure 1 below.
Forecasts for Tropical Depression Lisa and Invest 95L
Figure 1: 700-850 mb CIMSS steering layer as of 0300Z Sept. 23
Lisa has been stalled out in a gap of steering currents south of the red trough shown in Figure 1 (note that this red trough has had appearances in earlier Figure 1s in my previous blog posts over the last few days, it helped to turn Igor NW, then turned Julia eastward, and now has caused Lisa to stall out). The red trough has begun to lift out to the northeast, which should allow the massive ridge dominating the Atlantic (seen in Figure 1 as the gigantic clockwise flow) to expand eastward and turn Lisa increasingly to the west.
I drew the computer model consensus on Invest 95L in Figure 1 to see how it compares against the CIMSS steering. You can see the model consensus is in great agreement with the CIMSS steering, following the easterly (east to west) flow on the south side of the aforementioned massive ridge.
Now, for my finalized forecasts on Lisa and 95L
Figure 2: My track and intensity forecast for Tropical Depression Lisa this morning (Sept. 23)
The NHC forecast track has tended to have a leftward (or westward) bias with Lisa while underplaying how much the south side of the red trough in Figure 1 would drag Lisa eastward. My forecast track in Figure 2 gives room for Lisa to continue eastward, norhteastwawrd, or even southeastward (a SE wobble seems to be hinted at in the lastest satellite animation) in the very short term. The centerline of the projected path in Figure 2 is then generally to the right of the NHC forecast to account for how the NHC forecast has had a left bias in Lisa's forecast. In Figure 2, my forecast track with Lisa is valid as long as the storm center doesn't go outside my yellow projected path cone before reaching the green intensity flag.
I believe Lisa will be a tropical cyclone through the forecast period despite recently weakening to a tropical depression. It has redeveloped some thunderstorms recently, and could regain tropical storm status. At the end of the forecast, Lisa I forecast will be between 30 to 60 mph winds (tropical depression to tropical storm). It'll be on the weaker side if dry air and marginal sea-surface temps get the best of it, it'll be on the higher side if it overcomes the dry air with sufficient storm activity.
Figure 3: Invest 95L (Likely to be Matthew) Forecast
The centerline of the projected path of 95L in Figure 3 is based on the current center fix I made using the latest satellite animation, afterwads making Matthew follow the contour of the steering streamlines in Figure 1. This makes a landfall near the Nicaragua/Honduras border. The computer models get confused as a trough is supposed to erode the west side of the massive ridge, but the models don't know if the weakness from that trough will be enough to turn this storm northward, the model spread is so huge in the western Caribbean, and that is why I widened the cone in Figure 3 at the end of my forecast.
Intensity-wise in Figure 3, I think this will become at least a tropical depression (30 mph is minimal depression) when it passes south of Jamaica. By the time it gets close to the Nicaragau/Honduras border, I am thinking Tropical Storm to Hurricane Matthew of 65 to 95 mph winds. Beyond that, if this system stayed inland in Central America while meandering, it would weaken signficiantly. However, if it was to meander in the western Caribbean open waters with high oceanic heat content, low shear, and good outflow, this storm could blow up. I favor currently the latter scenario as I don't think it is likely to continue straight west and then meander over Central America, the ridge weakness is more likely to draw Matthew northward into the weatern Carbbean Sea. So I bring future Matthew up to 70 to 120 mph at the end of the forecast. This could be too low if Matthew really blows up in favorable conditions. This coudl be too high if indeed Matthew ended up meandering over Central America instead of open water.
After meandering in the western Caribbean Sea, there is an even bigger model spread. Some models in the really long range want to bring Matthew across Florida and than up the east US coast. Others have it threatening the eastern regions of the Gulf of Mexico. Interests in Nicaragua, Honduras, the Yucatan Peninsula, western Cuba, the eastern region of the Gulf coast, Florida and then up and down the east coast should watch 95L. I don't currently have a forecast beyond the western Caribbean Sea in Figure 3 because I don't know which scneario will play out after that time.
After Bermuda, Hurricane Igor Coming to its End
Early Morning September 20,
Hurricane Igor's center passed just west of Bermuda yesterday, and was a category 1 hurricane when it was closest to Bermuda. The two most recent hurricanes to pass so close to Bermuda was Fabian 2003 (cat. 3) and Florence 2006 (cat. 1). It appears Igor impacted Bermuda similar to how Florence 2006 did. Both hurricanes were large, broad cat. 1 hurricanes when they struck Bermuda, and both hurricanes had their center pass just west of Bermuda.
The last time I forecasted Igor was on September 16. In Figure 1 on September 16 (not Figure 1 below), I predicted the red trough would turn Igor northwest, north, and then northeast so that the hurricane's center would pass just east of Bermuda. What actually happened was that the red trough moved quiet faster to the east than I thought it would, thus turning Igor more to the northwest sooner than I predicted on the 16th. After that, I got it wrong in that I thought the red trough would turn Igor NE, but in fact it is the green trough behind it that will now turn Igor NE as the red trough has left Igor far and away behind due to its fast eastward motion. So in my forecast on the 16th, I turned Igor too soon to the NE while trying to make it chase the red trough, and that's why I had Igor's center passing just east of Bermuda. Remember that was wrong, Igor's center passed just west of Bermuda.
Figure 1: Initial draft forecast I made on Igor using 300-850 mb CIMSS steering layer
Usually when I have made an initial draft forecast, I have chosen the 700-850 mb steering map as the background image. What you are supposed to do is pick the appropriate map based on how strong the storm is. But before I did that in previous forecasts, I compared all steering layer maps, and found they showed essentially nearly the same thing. This is not the case tonight in the high-latitudes of the Atlantic, with the lower-level steering layer maps showing the anticyclonic ridges have more northward extent than in the deeper-layer steering maps. As of 11 PM EDT, Igor had a central pressure of 955 mb, and the UW-CIMSS site recommends using the 300 to 850 mb CIMSS steering layer for that kind of storm, which is the background image in Figure 1 above.
Its definite the future of Igor lies with its interaction with the green trough, so forecasting where the green trough will be in the future was critical. For the last three days, the green trough has moved eastward at essentially the same speed, so I kept moving the green trough along at that eastward speed for the entire forecast period.
In Figure 1 between 0300Z Sept. 20 and 0300Z Sept. 21, I had to accelerate Igor's forward speed to keep up with the NHC forward speed guidance. For the first few hours, I kept Igor parallel to the streamlines of the ridge to the east in Figure 1, afterwards bending the forecast track at an angle to the streamlines and more to the northeast by 0300Z Sept. 21 as the south end of the green trough should be near.
I noted that the NHC forecast has a northward hook late in the forecast period as Igor non-tropically interacts with the east side of the trough. So, my goal after 0300Z Sept. 21 was to keep Igor just to the east of the trough axis, and to keep Igor as best in alignment with the NHC foreward speed guidance latitude. Now after 0300Z Sept. 21, I see that Igor is intersecting the trough on the trough's south side where flow is nearly westerly (west to east), so I bent the track even more to the east after that time. I tended to be somewhat south of the NHC forward speed guidance after 0300Z Sept. 21 because I struggle to see how Igor would gain much latitude while stuck on the westerly flow on the south side of the green trough, but I do hook Igor increasingly to the north after 0300Z Sept. 22 to not be too far behind the NHC latitudinal forward speed guidance, and perhaps Igor will be becoming extratropical or be extratropical at that time such that it baroclinically intensifies with the trough, which would support a more northward track by that time.
By keeping Igor just east of the green trough axis in Figure 1, I found out I was nearly in perfect alignment with the NHC forecast longitudinal positions through 0300Z Sept. 23. But between 0300Z Sept. 23 and 0300Z Sept. 24, the NHC bends Igor's track so sharply northward that it doesn't go east at all after 0300Z Sept. 23, which is quiet different in Figure 1 where I kept an eastward component in the green trough's eastward track, which would keep Igor's track having an eastward component thru 0300Z Sept. 24. Perhaps I underplay the baroclinic interaction of Igor with the green trough at that timeframe, which if significant enough, would slow down the green trough as it baroclincally amplifies into a cut-off upper low. The stalling cut-off upper low would then yank Igor much more northward on its east side.
Figure 2: My final draft Hurricane Igor forecast I am issuing this early morning
The centerline of my final projected path in Figure 2 directly follows Figure 1 up to 0300Z Sept. 21. This is because Figure 1 and the NHC forecast are in tight agreement up through that time. After that time, I compromise between Figure 1 (which is somewhat south of the NHC forecast track) and the NHC forecast track, making my centerline adjusted south of the NHC forecast track but not as far south as shown in Figure 1. My centerline then goes somewhat east of the NHC forecast after 0300Z Sept. 23 as I don't believe Igor will hook as sharply to the north as the NHC shows at that time. I especially don't like to throw a sharp direction change at what should be a very fast moving storm at that time. Storms that move fast have a harder time turning very sharp (just like when you try to turn a car sharply at high speed, you can't because of the car's inertia). So my turn to the north at the very end of the forecast is not as sharp as the NHC's forecast.
Intensity-wise, I predict Igor will be extratropical (non-tropical) by the time it crosses the vicinity of 45W longitude as shown in Figure 2. This is because Igor by that time will have been very near the green trough axis for hours, giving it time to go from tropical to extratropical during its interaction with the trough.
Elsewhere in the Atlantic tropics, we have decaying Julia to the east of Igor, and tropical wave Invest 94L west of the Cape Verde Islands. I predict at 5 AM EDT, the NHC will issue their final advisory on Julia declaring it a dissipating tropical depression or remnant low. I predict that 94L will eventually become a tropical storm within the next days.
Julia & Karl Change Course, Karl will be a major event!
** 5 AM EDT UPDATE: KARL WAS UPGRADED TO A CATEGORY 3 HURRICANE WITH 120 MPH WINDS AS OF THE 5 O'CLOCK ADVISORY FROM THE NHC. I AM NOW FORECASTING THAT KARL AT LANDFALL WILL BE BETWEEN 125 (CAT. 3) TO 155 MPH (MAX CAT. 4) WINDS WHEN IT MAKES LANDFALL ON THE EAST-FACING BAY OF CAMPECHE MEXICAN COAST LATER TODAY. THE GRAPHIC CREATED IN FIGURE 2 BELOW WAS FINISHED AT 4 AM EDT, JUST BEFORE THE UPDATE FROM THE HURRICANE CENTER. THE FIGURE 2 INTENSITY FORECAST IS ALSO STILL VALID AS OF THIS UPDATE, BUT I HAVE ISSUED THIS UPDATED INTENSITY FORECAST IN CASE KARL GETS EVEN STRONGER THAN WHAT WAS SHOWN IN FIGURE 2. THIS IS A VERY SERIOUS SITUATION, THE LANDFALL SITE WILL EXPERIENCE SEVERE WIND DAMAGE**
Good September 17 morning,
We have witnessed behavior today from both Julia and Karl that have surprised us, a reminder that we still don't know everything about tropical cyclone forecasting. The official NHC (National Hurricane Center) track forecasts and my track forecasts about 24 hours ago got it very wrong with Julia and Karl, with both systems tracking quiet further south than the forecasts. Not only that, Karl is undergoing rapid intensification, and I don't see why it won't be upgraded to a major hurricane (cat. 3 or higher) at any time. Unfortunately, Karl will have tracked further south and be so much stronger than imagined, and will be an unpleasent surprise to whomever gets the landfall in Mexico later today. Karl is a reminder that even when a storm is close to the coast, we can still have errors with where the landfall point will be, and what the landfall intensity will be.
Checkup on Steering, What Happened?
Figure 1: Latest CIMSS steering layers as of 0300Z September 17, 2010.
I encourage you to compare Figure 1 above to Figure 1 in my blog post about 24 hours ago.
First off, the red trough has moved further east than forecast (see where I expected it to be by 0300Z September 1 in yesterday's Figure 1), and the ridge behind the trough (to the west of the trough) built in pretty fast over the central US. This fast and quickly forming ridge is highly likely the reason why Karl turned westward a lot sooner than forecast, causing it to be further south than forecast ultimately.
With Julia, the prominent eastern Atlantic ridge seems to have intensified more than forecast as the purple trough (which is now a purple low over Newfoundland, Canada right now in Figure 1 above) has lifted out to the northeast. Because the ridge is stronger than anyone expected it to be, Julia has turned westward much sooner than expected (a westward bend was expected in the track as discussed in the last post, but not this soon). I am also starting to wonder if the massive circulation of the monster Hurricane Igor just west of Julia may have something to do with the westward turn. I wonder if its tugging the smaller Julia into its larger circulation?
My Adjusted Karl & Julia Forecasts
Figure 2: My updated Karl forecast as of this morning
Karl Track: I expect Karl to continue its due west motion till landfall in Mexico, so I drew my projected path straight west with a rather tight cone around that centerline.
Karl Intensity: Boy, Karl is already at 105 mph as of the 2 AM EDT advisory from the NHC. I forcasted yesterday that Karl would be 80 to 110 mph winds at landfall, but now I have significantly upped that landfall intensity forecast to now 115 mph winds (min. category 3) to 145 mph winds (cat. 4 hurricane). The storm looks more and more impressive with each new satellite image, and this storm could blow up into a very serious hurricane before landfall later today.
Figure 3: My updated Julia forecast as of this morning
Julia Track: The centerline in Figure 3 follows the same philosophy in Figure 3 of my previous blog post, but is quiet a southward adjustment because Julia has been well south of yesterday's forecasts. Notice I don't use all of the centerline, some of it is dashed. The dashed part of the centerline is currently NOT part of my official forecast, but how Julia could track if it survives the immediate term (see my discussion of Julia intensity below to see what I mean by "if it survives the immediate term"). The cone is fairly biased to the left of centerline, giving room for Julia to continue straight west into Igor if that is what will happen. There is not much cone to the right of centerline because it is quiet more likely it'll go left of centerline than right of centerline.
Julia Intensity: I am going bold here, I am going to make Julia weaken significantly (50 mph tropical storm to dissipating system) as it reaches the end of the forecast path in Figure 3. Why? Julia is about to get severely sheared by the upper anticyclonic outflow of the massive Hurricane Igor. I think the anticyclonic outflow will overwhelm Julia, and that's why I think Julia could even dissipate. Its even possible Julia will just go straight west into the massive Igor and get absorbed by Igor.
What about Igor?
Igor has been "behaving" with the previous forecast in my last blog post, see Figure 2 of the last post. Track-wise, Igor has tracked a little bit more to the north in its westward track than I thought it would, and is currently riding the north edge of my projected path cone in Figure 2 of the last post. Intensity-wise, its a moderate category 3 and I still think it will be cat. 3 to cat. 4 as it nears 65W as shown in Figure 2 of the last post. With Igor's center still within the bounds of my forecast cone and forecast strength, not updating my Igor forecast now.
But wait, what about the red trough in Figure 1 above being further east than what was predicted in Figure 1 yesterday? With that east position of the red trough, the NHC official forecast still has Igor passing over Bermuda, but my forecast track centerline in Figure 2 yesterday had it going east of Bermuda. So, I still think Igor's eye will very likely pass east of Bermuda, I want to be east of the hurricane center's official forecast if anything with the red trough further east, and so I keep what I say in Figure 2 yesterday as of now. Its possible that the red trough doesn't become the trough that turns Igor permanently northward. If so, it looks like the red trough will start the turn to the north, with the turn to the north becoming finished off by the green trough currently seen over the central US in Figure 1 of this post (seen above).
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Updated: 8:58 AM GMT on September 17, 2010
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Igor a Monster! Karl on its way to Hurricane Status
Good action packed September 16 morning,
I will be referring a lot to the previous blog post I made as I update the tropical activity of the Atlantic Ocean today,
Since I last posted, Igor has strengthend into an incredibly large and powerful category 4 hurricane, 93L in the eastern Atlantic rapidly intensified into category 4 Hurricane Julia (which has recently weakened to category 2 as of this writing), and 92L in the Caribbean Sea became Tropical Storm Karl, making landfall across the Yucatan peninsula, and now has just entered the Bay of Campeche as of this writing.
I have issued six forecasts this morning on our three tropical cyclones, three track forecasts, three intensity forecasts.
My Rough Draft Forecasts
Figure 1: My rough draft track forecasts on Karl, Igor, and Julia as of this morning, September 16
When I last posted, the only active tropical cyclone in the Atlantic was Hurricane Igor. If you look at Figure 1 OF MY LAST POST, I had a philosophy that Igor was going to go faster than the NHC forecast, resulting in a "stair step motion" (WNW, NW, then return to WNW travel) as it interacted with the orange trough, and then transient ridge behind the trough (which was positioned over eastern Canada in the last blog post). After the stair step motion, I had Igor turning northward while just southwest of Bermuda on Sept. 16 while interacting with the second trough behind the transient ridge (that second trough was over southern Canada and western Great Lakes in the last blog post).
In reality, it turned out that I shouldn't have moved Igor as fast as I did. Igor did do the stair step motion as predicted (WNW, NW, then back to WNW), but in a different manner. Because Igor moved quiet slower than I thought, what really ended up happening was that both the orange trough and the other trough behind it (the one that was over the Great Lakes and Canada in the previous blog post) caused a general turn from WNW to NW. Note that the purple trough in Figure 1 in this blog post (shown above) was the trough that was over the Great Lakes and Canada in the previous blog post. Igor completed the stair step motion by going back WNW while getting influenced by the big ridge over the eastern US that built behind the purple trough. So now attention turns to the red trough in Figure 1 above for turning Igor ultimately northward (again, I thought the purple trough would do this in the previous blog post, but Igor moved slower and missed the opportunity to turn north with the purple trough).
Before figuring out what the red trough would do, I studied recently how the orange trough (seen in last blog post) and the purple trough behaved. In my last post, I thought the orange trough would go continuously eastward at its constant speed. It indeed did do that at first, but then made a sharp northeastward turn while failing to erode the strong eastern Atlantic ridge. I think the purple trough will do this norheastward turn shortly as well, and in the long term, I think the red trough will follow suit and turn norhteastward sharply as well. Between 0300Z September 15 and 0300Z September 16, the red trough made great headway to the east, but I slowed down the predicted 24-hour displacements of the red trough a bit after 0300Z September 16 as I think the strong eastern US ridge will slow it down as it tries to erode it and push it east. By 0300Z September 21, I bank the trough to the NE as I expect it will fail to erode the eastern Atlantic ridge cell as the orange and purple troughs have failed to do.
In Figure 1 with Karl, I found out that Karl would have to slow down its current WNW motion in order to keep in compliance with the NHC foreward speed guidance. I don't want to repeat my mistake in my last post where I moved Igor too fast because I thought the NHC slowed down the foreward speed of Igor too much. So, I keep in compliance with the NHC guidance with Karl's speed by slowing it down. This also makes sense because the red trough will weaken the steering influence of the eastern US ridge in the next 24 hours anyway. After 0300Z Sept. 17, I turned Karl straight west into Mexico as it begins to get influenced by the anticyclonic ridge currently seen building in NW Mexico. I maintained reduced foreward speed, and found myself in compliance with the NHC foreward speed guidance.
In Figure 1 with Igor, I maintained Igor's current WNW motion thru 0300Z Sept. 18 as I believe the red trough will not have enough southern extent to buckle the motion of Igor norhward thru that time. If you look at the NHC foreward speed guidance for Igor in Figure 1 (which is done by latitude instead of longitude because Igor will be moving primarily north during the forecat period), I have Igor further south than the NHC foreward speed guidance on the 18th. It seems the NHC and computer models hint at some more increased northward motion on the 18th while beginning to feel the red trough, but again I did't think Igor would feel the red trough at all thru the 18th when I made Figure 1.
After 0300Z September 18, I bent Igor increasingly north and then norhteastward as it starts chasing the red trough which I expected would be just north and then northeast of Igor after 0300Z September 20 and 21. Because on the 18th I delayed when Igor would turn north into the red trough, it utlimately had the effect of a sharper turn to the east by the 20th and 21st as I have Igor moving more eastward while dragged by the south side of the red trough rather than moving more northeastward while being on the east side of the red trough. The result of Figure 1 is a track that is left of the NHC early in the period, then to the right of the hurricane center's late in the period.
In Figure 1 with Julia, I began by taking the current NW motion and extrapolating it with time, and I found myself in general agreement with the NHC foreward speed guidance up to 0300Z Sept. 17. As Julia nears the core of the eastern Atlantic ridge, I start bending the path of Julia more WNW after 0300Z Sept. 17 and more parallel with the eastern Atlantic ridge contours in Figure 1. Still found myself in general agreement with NHC foreward speed guidance up to 0300Z Sept. 19. After that time, along 50W longitude, I bend Julia northeastward shaprly and accelerate its foreward speed somewhat as the red trough quickly approaches Julia from the norhtwest. After making the track in Figure 1, it was cool for me to see that I agreed with the NHC forecast philosophy, which also turned Julia sharply NE once it got to 50W. So, I have good confidence in my 5-day red trough position forecasts, because the timing of the red trough positions in Figure 1 are the very reason that I made Julia turn sharply along 50W.
Final Draft Igor Forecast
Figure 2: My final draft Igor forecast, track and intensity, as of this morning (Sept. 16). Note the danger to the island of Bermuda.
This is not a good siutation if you live in Bermuda, Igor has potential to be the worst hurricane to affect Bermuda since Fabian 2003. The centerline of the projected path in Figure 2 is a compromise between the NHC forecast and my unofficial rough draft forecast in Figure 1. I am left of the NHC's track early in the period, and then to the right of the NHC's track late in the period. Early in the period, I did not make my left bias as far to the left as it is in Figure 1. But my right bias late in the period is much closer to the NHC forecast. So only late in the period, the centerline in Figure 2 exactly follows Figure 1. This means I believe the hurricane eye will pass just east of Bermuda, but the hurricane is so large as shown by the satellite view in Figure 2, that impacts with wind and rain will be rather signifcant for Bermuda regardless. With this track, Igor will be producing sea swells and rip currents for the east coast of the US and Atlantic Canada, so watch out if you plan to do water activities along those shores over the next few days, listen to local officials. Even though the weather may be nice over the eastern US and Canada shoers, its important to note that the waters could be dangerous for swimming and boating.
Intensity-wise in the short term, I think Igor will still be a cat 3/4 oscillating in intensity due to eye wall replacement cycles. So I kept Igor 120 to 150 mph just before it starts turning northward along 65W longitude. After that, Igor may be encountering increasing shear and will be encountering decreasing water temps, and I weaken it to 95 to 125 mph (cat. 1 to extreme case scenario of modest cat. 3) when its either over or closest to Bermuda, and then shortly after passing Bermuda.
Final Draft Julia Forecast
Figure 3: My final draft Julia forecast, track and intensity, as of this morning (Sept. 16).
Track-wise, the centerline of the Julia forecast in Figure 3 pretty much follows Figure 1 dead on, except for early in the period where I am slightly left of Figure 1 because the NHC forecast is somehwat left early in the period of the track in Figure 1.
Intensity-wise, Julia dropped from cat. 4 to 2 over the recent hours due to southerly shear from a neighboring cut-off upper low to its west. As it moves northwest and away from the upper low, shear will drop and Julia will likely stay a hurricane for much of the forecast period. Once it starts turning sharply northeast, I predicted it to weaken and be between 60 mph (tropical storm) to 80 mph (cat. 1 hurricane) at the end of the forecast as shown. That forecasted intensity range at the end of the period may be too generous if shear becomes intense as the red trough seems it will be really close to Julia by that time. As it interacts more with that trough, we could also begin tropical to extratropical (non-tropical) transition by that time as well.
Final Draft Karl Forecast
Figure 4: My final draft Karl forecast, track and intensity, as of this morning (Sept. 16).
Track-wise, I adjusted the centerline of my forecast track in Figure 4 to be slightly left (or further south) of Figure 1 and the NHC forecast because in recent satellite loops, I have seen Karl already bend more toward the west from its previous WNW motion. Tight cone around that centerline due to high confidence.
Intensity-wise, Karl only just hours after moving into the Bay of Campeche is showing a banding-type eye and its winds continue to climb. I think its imminent that Karl will be a hurricane before making its final forecast landfall in east-central Mexico early morning on September 18. I am forecasting 80 to 110 mph max. sustained winds (cat. 1 to maximum cat. 2) at that landfall. That intensity forecast may be too low if Karl pulls of a siginficant rapid intensification episode and perhaps pops up to a cat. 3 (its over really warm waters, and has low shear and good outflow).
Igor Makes It, 92L and 93L Pop Up
Late evening September 11,
In my last blog post titled "Igor's future not clear cut," I stated the possiblity of Igor not surviving the easterly shear during a Fujiwhara interaction and absorption of a neighboring tropical low. Since then, Igor has successfully absorbed the tropical low, and has escaped easterly shear and is now on its way to become upgraded to a hurricane at any time as of this writing. I have now issued a long range track and intensity forecast for Igor now that its surviving. After that, I have some general predictions of 92L and 93L.
Igor Forecast
Figure 1: My rough draft forecast for Igor using CIMSS steering products.
In Figure 1 is only my rough draft forecast I created for Igor, and NOT the final forecast I am issuing tonight. Initially up through 2100Z September 12, I moved Igor westward at its constant speed with an ever so slight northward component, just as it has moved between 0900Z September 11 and 2100Z September 11. What I noticed is that the NHC (National Hurricane Center) predicted that Igor would be along 46.4W longitude by 2100Z September 12 while I have Igor further west than that, around 48W in Figure 1. I believe the NHC forecast underestimates the strength of the ridge currently north of Igor, and Igor would have to slow down its forward speed in the next 24 hours to only be at 46.4W by 2100Z September 12, which I don't think is going to happen due to the strength of the ridge.
To prevent being way too west of the NHC forecast, I slowed down the forward speed of Igor after 2100Z September 12. I also bent the track northwestward between 2100Z September 12 and 2100Z September 13 as I believe the orange trough will begin to influence the steering. Slowing down Igor seems to also be logical at this time too because it will be beyond the influence of the strong ridge. It looks like the orange trough will pass just north of Igor instead of recurving it northeastward, so I bend Igor more westward between 2100Z September 13 and 2100Z September 14 as I expect Igor to become influenced by the ridge behind the trough (this ridge is seen in Figure 1 currently over eastern Canada). However, its not an aggressive west bend nor acceleration in forward speed I make as the ridge doesn't look strong. Finally by 2100Z September 16, I start to bend Igor more toward the north as it becomes influenced by the next trough in the mid-latitude westerlies (this trough is seen in Figure 1 currently over south-central Canada extending into the western Great Lakes). At that time in Figure 1, I have Igor southwest of Bermuda as it starts turning more northward.
Comparing my 2100Z Sept. 16 forecast positon with the NHC's position, I find that I am still well west of the NHC forecast (NHC has it north of the Lesser Antilles by that time along 60.5W, but I have it southwest of Bermuda along 68W). Even after slowing down the foreward speed of Igor after 2100Z September 12, I still can't come into agreement with the slower foreward speed in the NHC forecast. I believe right now the NHC moves Igor too slowly during the forecast period.
Figure 2: My final forecast I am issuing tonight for the future path and intensity of Igor
Figure 2 is the actual, final draft forecast I am issuing tonight on Igor. The centerline of my projected path is a compromise between the NHC forecast as of 2100Z and my rough draft forecast in Figure 1. The centerline in Figure 2 is further to the north and faster than the 2100Z NHC forecast, but not as far north and as fast as in Figure 1 if you look at the details. I then create a tight cone arond the centerline up to 50W longitude due to high confidence, afterwards somewhat widening the cone beyond that with some less certainty.
Intensity-wise, Igor has been able to ward off the dry air to the norhtwest (I wasn't sure how well Igor would do with the dry air initally). Because of this (and the fact there will be low shear and warm waters during the forecast period), I strengthen Igor to 90 to 110 mph winds by the time its in the vicinity of 50W, then to around 120 to 140 mph winds by the time its in the vicinity of 65W.
The track forecast I grade correct and successful if the center of Igor stays within the projected path in Figure 2. The intensity forecast is valid if the max. sustained wind speed of Igor is within the range of the green intensity flags of Figure 2 when it passes through those green intensity flags.
Invest 92L in the Eastern Caribbean Sea, Invest 93L emerging from Africa
We have a tropical disturbance 92L in the eastern Caribbean Sea, and a strong tropical wave (93L) emerging from the west coast of Africa tonight. Both are well-organized and in a very favorable environment of low shear and warm waters to develop. However, because 92L has had a recent decrease in its storms tonight while 93L has maintained itself, I think 93L will become Tropical Storm Julia, and 92L will become Tropical Storm Karl (i.e. 93L will reach tropical storm strength first).
It is 92L that looks like its a big threat. Do you see that ridge over the central US in Figure 1? That should keep 92L steadily moving west-northwest across the Caribbean Sea, and the Caribbean also has some of the warmest waters right now in the Atlantic basin (and remember 92L will be under low shear too). And because 92L will be near land masses over the next few days, its going to be more of a threat to land areas than Igor will be in the next few days. Watch out for this one if you have interests in the central and western Caribbean Sea.
Igor's Future Not Clear Cut
Tropical Storm Igor has formed in the eastern Atlantic Ocean near the Cape Verde Islands from a strong tropical wave disturbance that rolled off of Africa. However, Igor is having a rough start. To see what I mean, lets first review the synoptic history of Igor:
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Tropical Storm Igor
Dates: September 8 to Present
Maximum winds: 45 mph
Minimum pressure: 1005 mb
Synoptic situation: Igor originated from the sixth in a series of strong tropical waves that emerged into the Atlantic tropics from western Africa in late August and early September. On September 6, the aforementioned tropical wave entered the Atlantic with an expansive but disorganized area of storm activity. A study of satellite animation suggests that a rather strong storm cluster toward the western portion of the wave on the 7th led to a low pressure spin with well-organized storm bands by early on the 8th, located just south of the Cape Verde Islands. Persistent organization on satellite imagery coupled with a satellite scan of winds suggested that the low had quickly become Tropical Storm Igor by that late morning.
As Igor came into being, a rather strong storm flare off the coasts of southwestern Mauritania and northwestern Senegal developed, perhaps from the remaining eastern portion of the tropical wave. This led to the rapid formation of yet another tropical low pressure spin just northeast of Tropical Storm Igor later on the 8th. Subtropical ridging to the north began steering the tropical storm westward, with the forward speed slowing to a stall early on the 9th as the tropical storm was tugged into a Fujiwhara interaction** with the neighboring tropical low to the northeast. Storm relative easterly shear over Igor increased as the forward speed had fallen behind upper easterly wind speed induced by the deep-layered ridge and the outflow of the tropical low.
Color-enhanced infrared satellite image of Tropical Storm Igor in the eastern Atlantic tropics on September 8 next to a tropical low pressure center to the northeast it was interacting with.
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** A Fujiwhara interaction is an interaction when two low pressure spins get really close to each other. Because the flow around a low pressure spin is cyclonic (counter-clockwise in the northern hemisphere), the two low pressure spins orbit each other cyclonically in a Fujiwhara interaction.
In summary, the problem with Igor is it has struggled during an interaction with another tropical low pressure center located just to its northeast. If Igor was much stronger than the tropical low, it would have no problem getting rid of it and becoming the dominant system (just like recently how much stronger Hurricane Earl was able to destroy weaker Fiona with wind shear produced by its outflow). Two things could kill Igor (1) Easterly shear, which is caused because Igor has stalled around while doing the above mentioned Fujiwhara interaction. Stalling in this location is bad for Igor because the deep-layered ridge to the north is producing a steady stream of upper easterly winds. If you have upper easterly winds over a stationary tropical storm, then the storm clouds get sheared westward away from the tropical storm center, which would kill the tropical storm. (2) As Igor tries to absorb (or "eat") the other low pressure today, it may itself degenerate into a broad area of low pressure that lacks a well-defined center. This is because the minimum pressure value of Igor is not much lower than the minimum pressure of the other low, so the two lows might just melt together into a disorganized system. If Igor was much stronger, than its much lower minimum pressure would have an easier time sucking up the other weaker low pressure.
In lieu of all this, I am predicting right now that Igor in the short-term will weaken to a tropical depression or dissipate.
After that, its possible (although I am not officially predicting this now) that the depression or remnants of Igor could strengthen into a much stronger system down the road. Granted that we will have warm waters and low shear in the future with this system, the only reason that wouldn't happen is because of lots of dry air currently seen in water vapor imagery between 40W to 65W, and between 15N and 25N.
Another interesting thought. Lets say scenario (2) above happens in a manner that Igor's remnant low and the other low melt together into a disorganized system. I wonder if that disorganized system were to then organize and develop into a tropical cyclone, would that tropical cyclone be renamed Igor? Its possible that it wouldn't be renamed Igor because technically that new tropical cyclone is a product of Igor's low mixing with another low. It'll be interesting if that scenario ever happened.
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Updated: 10:35 AM GMT on September 09, 2010
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Hermine "Pulling an Humberto?"
Afternoon September 6,
Tropical Storm Hermine has managed to rapidly form and develop in the southwestern Gulf of Mexico from the remnants of eastern Pacific Tropical Depression 11-E. There has been a "tropical wave drought" in the Gulf and eastern Pacific regions recently, and most tropical cyclones in this part of the world develop due to tropical waves originating from Africa. The "tropical wave drought" (or lack of tropical waves) in this region has been due to the fact that the most recent string of wave disturbances from Africa have developed into Danielle, Earl, Fiona, and Gaston in that order, and so far neither of those systems have made there way toward the eastern Pacific or Gulf regions (were still watching the remnants of Gaston, we'll see if in the very long range if Gaston makes it to the Gulf, too early to tell). In my synoptic history of 11-E/Hermine below, you can see how this system managed to form from the ITCZ (intertropical convergence zone) instead of an African tropical wave.
Below my 11-E/Hermine synoptic history below, I have also posted my synoptic history of Humberto I wrote about 3 years ago. Like Hermine, Humberto was a rapidly developing September tropical storm in the Gulf of Mexico. Humberto was so impressive in that it went from tropical depression to hurricane in the same day (Sept. 12, 2007) and took residents in High Island, Texas by surprise do to its rapid development off the coast (they not expecting hurricane conditions when the system was just a tropical depression early in the day). Hermine's recent rapid development today makes us ask, could Hermine be "pulling off an Humberto?" However, unlike Humberto which hit the east Texas coast, Hermine is headed for northeast Mexico just south of the Texas/Mexico border. Small, rapidly developing storms like Humberto and Hermine have small wind fields, so the worst winds will be tightly confined to the center. Hermine though will be producing rains at and away from the center, and will be the third tropical cyclone to affect the Rio Grande region near the Texas/Mexico border this season, after Alex and TD 2 had done so back-to-back in June. Lets hope that Hermine doesn't causing another flood in this region after Alex & TD 2 has already caused problems in the area early this season.
I am forecasting that Hermine's center will make landfall later this evening in northeastern Mexico while exhibiting maximum sustained winds ranging from 65 mph (strong tropical storm) to as strong as 80 mph winds (cat. 1 hurricane). If Hermine reaches hurricane strength tonight, it will only be the second Atlantic tropical cyclone on record to go from tropical depression to hurricane in the same day while so close to land, a feat only achieved by Humberto to date. Hermine is running out of time to do this as it has just recently flattened out in intensification, not strengthening beyond 60 mph winds between 1 PM and 4 PM CDT. However, the central pressure has dropped from 995 to 992 mb between those hours, so I think it'll strengthen at least a little more before tonight's landfall.
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Tropical Storm Hermine
(Eastern Pacific Tropical Depression Eleven-E)
Dates: September 5 to Present (September 3 to 4)
Maximum winds: 60 mph (35 mph)
Minimum pressure: 992 mb (1005 mb)
Synoptic situation: Tropical depression Eleven-E of the 2010 Eastern Pacific Hurricane Season developed from enhanced storm activity along the ITCZ (intertropical convergence zone) that began on September 2 to the south of the Isthmus of Tehuentepec of southeastern Mexico. A strong upper trough over the United States was located at the longitude of the disturbance on this day, and it seems plausible that relatively higher pressures south of the upper trough perhaps allowed for anticyclonic upper ridging than enhanced the outflow over the area such that the ITCZ storms were regionally enhanced. This led to a broad east-west area of low pressure along the ITCZ on September 3 that rapidly organized into tropical depression Eleven-E later that day. The depression drifted northward into the low-level ridge weakness associated with the United States upper trough, making landfall across the Isthmus of Tehuentepec early on September 4 and weakening into a broad remnant low. The northern side of the remnant low produced strong storm clusters across the Bay of Campeche later that day, with the storm activity organizing about the remnant low as it moved into the western portion of the Bay early on September 5. As the center of circulation became better defined while continuing northward into the southwestern Gulf of Mexico, it was upgraded to tropical depression ten during the late evening.
A subtropical ridge to the north was building in the wake of the United States upper trough, which allowed for an increasingly westward component to the northward track of the depression on the 6th. Over warm waters and under low shear, tropical depression ten briskly intensified into Tropical Storm Hermine during the early morning, and was already a strong tropical storm by the afternoon hours.
Visible satellite image of intensifying Tropical Storm Hermine taken during the afternoon hours of September 6
Warnings: At the formation of tropical depression Eleven-E close to the south Pacific-facing coast of Mexico on September 3, a tropical storm warning was immediately issued from Boca de Pijijiapan to Puerto Angel. Warnings were dropped after the depression made landfall and dissipated into a remnant low early on the 4th.
A tropical storm warning was immediately issued for the east Gulf-facing coast of Mexico, from Tampico northward to the mouth of the Rio Grande River, when tropical depression ten formed during the late evening of September 5. The warning was extended northward to Baffin Bay, Texas early on the 6th when the depression intensified to Tropical Storm Hermine. With Hermine intensifying quickly during the morning hours, a hurricane watch was issued from Baffin Bay, Texas southward to Rio San Fernando, Mexico. The tropical storm warning was discontinued south of La Cruz, Mexico and extended northward to Port O’ Connor, Texas as Hermine had tracked a little more toward the north than initially forecast.
Impacts: Hermine has yet to make landfall as of this writing. No major impacts so far, but Hermine is likely to produce a tight area of high winds at landfall tonight, and produce rainfall in and south of the Rio Grande Region inland after landfall.
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Hurricane Humberto
Dates: September 12 to 13, 2007
Maximum winds: 85 mph (Category 1)
Minimum pressure: 985 mb
Synoptic situation: The precursor to this cyclone was the same decaying frontal boundary that produced the pre-Gabrielle surface low. The southern portion of the front decayed into a surface trough over Florida. The surface trough moved westward with an associated broad storm cluster across the Yucatan and southern Gulf of Mexico on the 8th through the 10th. The trough consolidated as a weak surface low just southeast of Texas on the 11th, and conditions were generally favorable with warm waters and low shear under a deep-layered ridge to the north dominating the southern United States. A large frontal system moving into the southern United States eroded the ridging to the north except for a small upper ridge directly over the surface low that provided a favorable outflow environment for storms to develop over the surface low. The storm activity quickly organized around the center, and tropical depression nine formed just off of the Texas coast by late morning on the 12th.
The depression quickly became Tropical Storm Humberto in the early afternoon while developing in the favorable outflow aloft. Low-level ridging to the east was driving the new storm northward into the Texas coast. Humberto developed unusually fast, forming a tight core of hurricane winds just before landfall in southeastern Texas. Humberto continued strengthening till landfall, reaching a peak of 85 mph winds while the center crossed just east of High Island very early on the 13th. This was the first time on record for a cyclone so close to landfall to develop from a tropical depression to a hurricane in the same day.
The large frontal system just north of Humberto stalled, and the upper ridge that had been over Humberto had been pushed further south into the Gulf while expanding. Hurricane Humberto made an eastward turn on the north side of the upper ridge and just south of the front. The hurricane finally weakened to an inland tropical storm in southwestern Louisiana by the late morning of September 13. During the afternoon, Humberto quickly weakened to a dissipating tropical depression moving eastward across Louisiana, and the National Hurricane Center issued its final advisory.
The remnant low of Humberto soon merged with the stalled front to the north on the 14th, and the low tracked eastward into Mississippi on the north side of the expanding Gulf ridge to the south. While moving across northern Alabama, the remnant low became indistinct along the frontal zone later on the 14th.
Tropical Storm Humberto forms off of the Texas coast on September 12. The above visible satellite image was taken during the early afternoon. Humberto became a hurricane that evening.
Warnings: A tropical storm warning was immediately required as tropical depression nine was declared just off of the Texas coast. The warning was issued in eastern Texas and parts of southwestern Louisiana. Tropical storm watches extended further east into Louisiana. This watch was upgraded to a warning hours later. Humberto’s record development before landfall was not anticipated, and a small hurricane warning for the tight hurricane wind core was issued as Humberto was upgraded. This only provided a couple of hours warning time, far short of the ideal 24 hours for a warning. As tropical storm and hurricane conditions cleared from the coast after landfall, all coastal warnings were removed.
Impacts: The most notable impacts from Humberto occurred in a small swath in the southeastern corner of Texas and the southwestern corner of Louisiana as the hurricane had a tight core. The peak storm surge was measured at 4.87 ft, with a 2 to 4 ft average near the landfall site. Hurricane force winds occurred in a tight area near the landfall site, with some notable wind damage in High Island, Texas that was initially assessed to be from a tornado. The highest rainfall total from the storm was 14.13 at East Bay Bayou, Texas, and a narrow swath of 3 to 5 inches spreads from this area into central Louisiana. As a remnant low, Humberto produced locally heavy rains across the southeastern United States. The total damage from Humberto is estimated at $50 million, with costs coming from flood damage and also winds damaging trees and power lines.
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East Coast Earl, My Goodness Gaston!
Afternoon Folks,
The Atlantic tropical cyclone outbreak continues with Hurricane Earl set to affect a large swath of the east coast, Tropical Storm Fiona potentially affecing Bermuda, and Tropical Storm Gaston forming out in the eastern Atlantic, yet another tropical storm!
Due to schoolwork, I haven't had much time to make a detailed analysis on Fiona. Interests in Bermuda should make preparations for tropical storm conditions as they are currently under tropical storm advisories. I am currently forecasting that Fiona will be at a strength of 30 mph winds (tropical depression) to 45 mph winds (tropical storm) when it passes near Bermuda. This is because I expect northerly wind shear from Hurricane Earl's anticyclonic outflow will continue to induce gradual weakening to Fiona.
During my last post a couple of days ago, I projected Earl had an opporutinity to make category 5 status. An eye wall replacement cycle and shear from the upper low discussed last time weakened Earl to category 3 instead as it passed near the Bahamas. Then, Earl moved away from the upper low and did not wrap in dry air, and it was able to strengthen to category 4 last night as it charged toward the east coast. Thankfully, Earl has now begun weakening, and is back down to category 3. Now, what is to be expected of Earl & Gaston?
My Rough Draft Forecasts Using CIMSS Reasoning
In Figure 1 are my forecasts I made early this morning (at around 1116Z, or 7:16 AM EDT) using the CIMSS steering layer products. These ARE NOT my final forecasts I am issuing this morning, but these rough draft forecasts help me see if I should make my forecast have some right or left bias in the track from the hurricane center's forecast track.
Figure 1: Rough Draft track forecasts I made for Hurricane Earl and Tropical Storm Gaston I made this morning.
Lets begin with the Earl reasoning in Figure 1 first. The long awaited collapse of the eastern US ridge in advance of an upper trough and associated surface cold front has occurred, only a small ridge remains now over western North Carolina. This has allowed Earl to curve northward in a weakness between that ridge and the ridge centered at 30N 60W in the above CIMSS map. When I was making this graphic this morning, satellite animation showed Earl was beginning a track increasingly to the north, and it seemed it was going to end up going straight north along 75W, that's what I did in Figure 1. I kept moving Earl northward, and then bend Earl northeastward at the latitude of the NC/VA border, and kept Earl moving at the same foreward speed. What I found was that I kept in generally alignment with the NHC foreward speed guidance when I kept Earl at its current foreward speed, then I gave it some acceleration as it moved into Nova Scotia to continue to keep up with the NHC foreward speed guidance.
I bent Earl NE at the NC/VA border latitude based on the curvature of the steering ridge centered at 30N 60W. My eastward bend is a little sharper and just slightly to the right of the NHC forecast late in the period because I see a lot of straight westerly flow (flow from west to east) on the north side of the steering ridge. As I moved Earl into Nova Scotia, I bent the track ever so slightly to the left as there is a lot of discussion of the upper trough/cold front system should begin digigng into the circulation of Earl and make it extratropical.
With Gaston, its south of a ridge weakness generated by the red trough shown in Figure 1, which has softened the steering influence of the ridging, thus making Gaston move slowly westward. Believe it or not, this red trough is the same trough system that has absorbed the remnants of Danielle. Anyway, I took the slow foreward speed of Gaston between 0300Z and 0900Z September 2, and moved it at the same exact foreward speed initially through 0900Z September 3 (and I found out that I was in general agreement with the NHC foreward speed guidance). Between 0900Z September 3 and 0900Z September 4, I created some slight acceleration as it beginst to feel the strong ridge at 30N 60W, and I agreed with the NHC foreward speed guidance again. After that, I decided to accelerate Gaston somewhat faster to the west and keep it slighlty faster than the NHC foreward speed guidance. This is because I feel Gaston will swiftly accelerate westward as it gets influenced by the really strong ridge at 30N 60W. Plus, Fiona and Earl have recently accelerated westward in that same region while influenced by the same strong ridge cell.
Few Hours Later, My Final Forecasts
I decided to wait before issuing my final forecasts, and now am issuing my final draft Earl & Gaston forecasts this afternoon. Why the wait? I wanted to see if Earl & Gaston would begin to follow the trends in Figure 1 in the short term.
Figure 2: My final draft Earl track and intensity forecast this afternoon. Areas along and east of the red line are where I expect possible power outages from wind gusts.
Earl: I have high confidence in my forecast track in Figure 1 because Earl has been following that track almost exactly so far. So, I used that track as the centerline of my projected path in Figure 2, with a tight cone around that centerline due to high confidence. In mph, you can see what maximum sustained wind intensity I expect in Earl's center. Based on the current radius of tropical storm force winds and assuming that Earl follows the centerline of my projected path, I drew a red line where I expect power outages to be possible due to wind gusts. If you are ALONG or EAST of the red line, that's where I expect that to occur. I bend the red line closer into the projected path center as Earl should be weakening and having a shrinking wind field as a result. Based on the red line, power outage risks well includes Cape Hatteras NC, Norfolk VA, most of Maryland EAST of the Chesapeake Bay and all of Delaware, southern half of New Jersey, barely including New York City, and Boston, Massachusetts and the Cape Cod area. The eastern tip of Maine and then much of Nova Scotia is under that power outage risk late in the period. Over course, rough waves and rip currents are a big problem up and down the east coast. There will be heavy rain, but flooding risk is not worst case scenario as Earl is moving fast.
Figure 3: My final draft Gaston forecast this afternoon
After making the graphic in Figure 1, Gaston had some northward motion perhaps due to the weakness induced by the red trough to its north. Notice I had Gaston moving straight west in Figure 1, but in my final draft in Figure 3 above, I decided to give some room for a northward motion in the short term. Once it escapes the ridge weakenss, I gave Gaston some slight west-southwest (and I mean very very slight southward bend) and then straight west track just to the south of 15 N latitude as it comes under the influence of the strong ridge at 30N 60W. My projected path center late in the period is south of the NHC forecast, which keeps Gaston just north of 15 N latitude, but I think the ridge influence will be stronger than the NHC forecast is showing (that's why I keep Gaston just south of 15 N). Around that center line, I created a somewhat wide cone in the immediate short term in case Gaston tracks more north than I thought due to the ridge weakness, or if Gaston reforms a tighter center to the north or south because right now its a broad circulation center. I then kept the cone tighter later.
Intensity wise, Gaston has suffered from dry air, and weakened to a tropical depression. Midway through the forecast around 48W longitude, I have Gaston perhaps beginning to strengthen to 50 to 80 mph winds. As it nears the Lesser Antilles, I have it around 75 to 105 mph winds. This forecast intensity may be way too generous if dry air kills off Gaston. This forecast inensity may be too low if Gaston mixes out dry air more than expected and strengthens faster. This is a low-confidence intensity forecast.
Summary
I have issued five forecasts. First, Fiona I expect will be at a strength of 30 mph winds (tropical depression) to 45 mph winds (tropical storm) when it passes near Bermuda. I then issued two track and two intensity forecasts in Figures 2 and 3 for Earl & Gaston. My track forecasts are wrong if the storm centers leave the yellow cone. My intensity forecasts are wrong if the storm is stronger or weaker when its in the vicinity of the green intensity flags in those Figures.
Listen to local officials concerning evacuations. If you are going to be riding out the storm in a power outage risk area (i.e. Bermuda due to Fiona, or due to Earl along the immediate east coast of North America from Cape Fear, NC northward and across the Canadian Maritimes, and if you are along or east of the red line in Figure 2 if you are inland), click here for a supply list of things you should have.
